01-Smart Plug vs. 1938

SmartPlug vs. 1938

In today’s day and age it is far and few between that I come across products that:

#1 Do as they say.

#2 Are built to an exceptional standard.

#3 Redefine what great is.

The SmartPlug is one of these products.

I got a call from this owner one day saying his shore power cord looked “charred” and no surprise it certainly was. I knew exactly how to fix and remedy his problem, a new SmartPlug inlet and cord set..

Unfortunately this call was in February, in Maine and the boat is stored in the water because this hearty owner sails year round. Due to the fact that he sails year round he keeps his boat heated with electricity to prevent it from freezing up. I had previously warned him that we would want to address his aging shore connection and cord, to which he just shrugged and put it off.

A few months later my suggestion became an emergency

Pictured here, in the snow, is a 50′ X 30A SmartPlug cord set and a 30A X 125V AC inlet.

You can purchase a Smartplug kit from the MHT Amazon store:

Buy Smartplug Products – MHT Amazon Store

No Mincing Of Words…

This is an ordinary day for me and it brings to mind that saying, a picture tells a thousand words…

Rant Warning:


The regulatory & certification organizations that over see this, such as UL, NEC, NFPA, NMMA & the ABYC should all be ashamed that they continue to allow this standard to continue in the marine environment.

There I said it.. Whew….. So why do I dislike it so much? Lets examine this further…

  • When this plug standard began life it was back in 1938. I can’t imagine that Harvey Hubbell III ever intended this to be used on boats, inches from the ocean, back when it was conceived..
  • In 1938 houses had cloth covered wire and no safety ground for 120V outlets
  • In 1938 we did not even have seat belts
  • In 1938 GFCI outlets were not even a thought in an inventors mind
  • In 1938 lead paint was the rage and asbestos was in everyday use.
  • In 1938 there were NO SAFETY STANDARDS for electrical safety on docks or boats.
  • In 1938 safety testing for marine use never even existed.

How many other critical electrical items do you use in your house or boat that have remained virtually unchanged since 1938..?

Suffice it to say we have been grandfathered into a substandard and marginally adequate plug & socket combination for marine shore power cord/plug/receptacle use. This twist-lock standard would likely never gain approvals under today’s stringent electrical safety standards but we have it due to grandfathering. Safe? Read on and decide for yourself…

02-Smart Plug vs. 1938

03-Smart Plug vs. 1938

High Resistance Is Your Enemy

“But RC I have a circuit breaker, aren’t I protected?”


With this old, antiquated left over 1938 NEMA L5-30 standard there is approximately 20 times less surface contact area than there is in a modern SmartPlug. Yes, you read that correctly, 20X less contact area!

On boats we are in a corrosive environment, that also moves. Add just a little corrosion, some torquing & pulling on the plug and stack all that onto the minimal contact area and we develop heat.

Heat is generated at points of high resistance, in an electrical circuit, when pushing current. Please understand & comprehend that you can easily start a fire with high resistance and never trip a circuit breaker.

As we can see in this photo the wire was not only corroded, due to the marine environment and the age, but it also suffered due to the high resistance. The wire jacket itself was melted, became brittle, and then the jacket fractured.

“But RC what actually causes these failures?”

There are a number of factors that all conspire to lead to a shore inlet melt down or fire:

  • Poor design
  • Arcing
  • Corrosion
  • Lack of support
  • Wear & loosening of the spring metal inside the sockets
  • Vessel movement
  • Locking rings that fail or are not used
  • Lack of water / moisture seals
  • Age
  • Pulling too much current through the shore power system


Untinned Wire

Here’s a prime example of where tinned marine wire may have bought this owner a bit more time. A new run of 10/3 triplex tinned marine wire was in order. The black oxidation on this wire can also lead to or create high resistance. When in doubt, it’s best to also replace the feed wire from the new AC inlet to the AC panel.

If your wire run between the AC shore inlet to the AC main breaker, at the AC panel, is more than 10′ wire feet, not as the crow flies, then you will want to install a second 30A or 50A double pole breaker.

A main AC breaker on a boat needs to interrupt both the AC WHITE/NEUTRAL & AC BLACK/HOT wires, and be located quite closely to the AC shore inlet (10′ or less). Single pole main breakers (hot only) are not used in the marine AC systems despite many builders doing so in the 60’s 70’s & 80’s.

04-Smart Plug vs. 1938

05-Smart Plug vs. 1938

“Why Do I Want A Smart Plug?”

Over the years I have seen far too many examples of situations like this. I suspect a large number of the 55% of boat fires that are electrical in nature (source ABYC & USCG data) are the result of this antiquated and unsafe twist-lock standard we currently use.

Of those 55% of boat fires nearly 54% of that number is caused by “shore power“. This means that nearly 25% of all boat fires start in the shore cord/power system. Ouch!

Read on and I will dig much deeper into the why’s & how’s…

Need More Convincing?

When word got out, in my Hall of Fail article, that I was a huge fan and proponent of the SmartPlug pictures came flying in from all over, pictures of burned twist-lock plugs. I am certainly not alone in my dislike for this antiquated & unsafe standard.

I have far too many images of burned twist-lock plugs to show them all so I chose this one to be a good representation.

Photo courtesy: A. Mazon

06-Smart Plug vs. 1938

07-Smart Plug vs. 1938

Early Signs of A Potential Fire

If you know what you are looking for the early signs of high resistance can be pretty obvious. Look at the pin closest to you and you can easily see the melted plastic migrating up from the base. There are also signs of arcing and burning on the pin itself.

If your your cord looks like this it is time for a replacement!

It’s important to understand that I am not alone in my feelings about shore power safety. The quote below comes straight from ACE Insurance one of the nations largest underwriters of marine insurance. ACE understand these problems because they pay the claims!

Begin Quote:


“Each vessel’s connection devices shall be inspected annually. Annual inspections shall be carried out between the end of the cruising season and the beginning of cold weather, when most boats have returned to their moorings, but before they have begun to present winter heating loads to the Marina. Any boat that enters the marina under a new Moorage Agreement during the winter heating season shall be inspected within 15 days of its entry into the Marina.

A major reason for this inspection routine is to document the condition of each boat’s Inlet Receptacle, the device on the boat’s exterior to which the shore power cord is connected, and the condition of the mating end of the cord itself. This cord-to-boat connection is subject to wear-and-tear damage from the activity of connecting and disconnecting the cord, and it is also subject to damage from wet winter weather, and it is a major source of fire in marinas. If the connection between the cord and the boat is even slightly damaged, it will create heat that can eventually result in fire.

End Quote:

A NEMA L5-30 Socket

In order to dig deeper than we’ve ever seen before, into this antiquated standard, I bought a NEMA L5-30 socket. This is the standard used for 30A shore power on boats here in the USA and elsewhere.

In order to make these points clearer I chose to destroy this socket by cutting it open to give a full look into why they are so dangerous.

SITE PLUG: Here is a prime example of where your support comes in handy. I paid for this socket out of my own pocket. Support for this site through my donate button (end of this article) help keep this site FREE and the in-depth brutally straight forward articles coming your way. Keep in mind that I don’t have advertisers I need to worry about offending so I can actually be honest in my writing.. Do your part and help support MarineHowTo.com.

08-Smart Plug vs. 1938

09-Smart Plug vs. 1938

NEMA L5-30

Other than a yellow color, and some tinning of the socket contacts, a NEMA L5-30 is a NEMA L5-30.. They are built to the NEMA L5-30 standard whether you buy marine or RV sockets they must all work together.

Some readers have suggested that the problem is not the design, but rather how boat owners improperly use these cords and is because they don’t use the locking rings.

Do you see any locking rings on this dock pedestal? Why are there no locking rings at the dock end?

While many boats owners do improperly use these cord sets, and this too can lead to high resistance, many of them still fail and burn up even when being properly used with the locking rings properly used and installed.The failure modes are well beyond just one cause and most of it starts with the design.

Lots of these locking rings fail within a few months or just weeks and quite often just fall off and disappear entirely. Others owners simply cross thread them and ruin them.

Should the ring falling off or cross threading even be possible if this is so critical to the performance of the cord set? In my opinion the answer is no…

These failures are a combination of a poor design that is not well suited to the application and sometimes no option to use the cord-set properly even if an owner wanted to.

I Cut The Plug Open And Exposed The Actual Sockets

My goal here was to show exactly why or how this standard can result in so many boat or marina fires. In this picture we can see the contact area between pin and socket. Not too much is there? The green headed screw is the grounding/Earth terminal but they all have about the same contact surface area.

10-Smart Plug vs. 1938

11-Smart Plug vs. 1938

The Locking Dimple

If you click this image you will be able to see the locking dimple on the female socket that locks into the hole on the pins of the male plug. Why is this important?

Socket Riding On Spot Dimple For Contact Area

Please do yourself a favor and blow this image up. This is the result of just 1-2mm’s of movement and the dimple riding up and out of the hole in the pin..

Let’s go over some of the major issues with this standard that lead to safety issues.

1- Round plugs need to twist in order to lock into place. To facilitate this locking feature dimples are used to create the detente so you know the socket is in place.

2- Due to the fact that this standard relies on twisting to lock the plug into the socket the fit of the round plug into the female socket is more sloppy than it should or could be and allows movement between socket and plug. Shore cords weigh a lot and the cord alone can cause strain to the plug & socket, forget waves, boat movement or people stepping or tripping over the shore cord etc…. It is very rare that a shore cord is properly supported to strain relieve the plug & socket.

3- We are trying to use this on boats and boats move continuously….

4- Even just 1-2 millimeters of movement between the plug & socket can cause the dimple to ride up out of the hole and create even less contact area than this already poor design has.

5- There are no seals on this plug standard thus it is very easy for corrosion to set in on the pins & sockets and create yet even more resistance.

6- The socket clamping mechanism/spring clamping feature is made from material just 1mm thick.. It does not take much pressure or offset torquing to loosen a 1mm spring clamp and cause it to provide even less pressure for good contact. Over time the female spring mechanism tires, becomes weaker and the plug fits loosely and no longer snaps or clicks into the detentes…

12-Smart Plug vs. 1938

13-Smart Plug vs. 1938

We All Know Boats Don’t Move……Right..?

I sure wish I could say I never see this, but I see it with regular frequency.. Scary stuff for sure.

Ask yourself how a boat-owner locks a shore cord like this into place to prevent this occurrence….? Do you by chance see an operational locking ring …?

Actual Contact Area NEMA L5-30 Socket

I needed a good way to show the actual contact surface area of the clamping socket on a NEMA L5-30. I grabbed some sheet metal the same thickness as the plug pin and inserted the socket over it.

I then slid it back and forth thus creating a corresponding mark on the sheet metal showing the actual contact area. The results were, well, shocking..

It should be noted that this was not the dimple side but the non-dimpled side.. Do you really want to run 30A through that?

14-Smart Plug vs. 1938

15-Smart Plug vs. 1938

I Was Being Kind Here

In the shop, shooting these photos, I realized my glasses were in the house. When I blew up the picture I realized I had been very kind to the width of the contact surface area. Still 1.31 mm is pretty minimal for 30A of current, on a moving boat where corrosion is a huge player.

My Computer Seems To Be Taking Over

Really, I am not uploading these, my computer is just so full of them it keeps puking them out… (grin)

16-Smart Plug vs. 1938

17-Smart Plug vs. 1938

McCotter’s Marina Fire

During the course of my research I came across the McCotter’s Marina fire that happened in January of 2011 in North Carolina. Results on google were inconclusive other than to say the cause of the fire was “electrical“. Frustrated with the investigation follow up I could find on-line I reached out to the Bunyan, NC fire department to see if they had any more information, they did.

What I was told was that the “exact cause” can’t be 100% pin pointed, and they rarely are. Their best guess and best information came directly from an eye witness who had stepped off his boat and saw the fire engulfed on the outside of another boat at the shore power inlet.. D’oh….!

The individual I spoke with at Bunyan Fire Department asked not to be identified because they were unsure if the damage numbers on boats, in dollars was accurate. The individual I spoke at length with was directly involved in the investigation and very confident about the eye witness and the perceived or assumed cause of this massive fire and where it originated.

This fire at McCotter’s Marina was estimated at approximately 23 million dollars in damage and included a Bertram 68, with a single boat value into the millions. This fire also turned McCotter’s into a SUPERFUND SITE for clean up. Over 20 boats and the marina were destroyed all on what is believed to be a shore power cord/inlet. Think this stuff is not serious? Think again!

Image Courtesy: Beaufort County Now

This is an actual video that was posted on YouTube of the McCotter’s fire. Think about this the next time you need a shore power cord.

Okay, Back On Track….

The contact area that was scored into the sheet metal was just about the same width as a US dime..

18-Smart Plug vs. 1938

19-Smart Plug vs. 1938

But Wait There’s More…..

Never being satisfied with what should be enough to show readers what I am trying to say, I pushed on.

This time I aligned the socket with a straight edge and moved it in and out to score another line onto the sheet metal. I wanted to see if this motion created any more contact area or width. It would only be fair to show both.

HOLY COW !!!!!!

Are you beginning to see the reason why so many of these plugs and sockets lead to melt downs, boat fires or entire marina’s burning down? The entire contact area here is but a 1.3mm X 2.7mm and 1.3mm X 2.4mm area often trying to handle 30A of current.

Smart Plug 30A
Back of Pin Contact = 16.4mm X 8.5mm = 139.4 sq mm
Front of Pin Contact = 6mm X 8.5mm = 51 sq mm
Total Contact Area = 190.4 sq mm

Twist-Lock 30A
Back of Pin Contact = 1.3mm X 2.7mm = 3.51 sq mm
Front of Pin Contact = 1.3mm X 2.4mm = 3.12 sq mm
Total Contact area with dimple locked in place = 6.63 sq mm

IMPORTANT: Due to the curved shape of the twist-lock measurements are as near as can be made without going to impressions on transfer paper. The Smart Plug pins are flat thus measurements are far easier. Still the difference in contact surface area is huge.

20-Smart Plug vs. 1938

Why Is There So Little Contact Area..?

Remember this is called the “twist-lock” or “HUBBELLOCK®” standard. It requires both inserting & twisting.

Because of this dual stage insertion method the socket receptacle has to be shaped to accept both forms of engagement, insert and then the twist.. The two mating contours needed to allow insert & twist create a very small point of final engagement.

Insert Contour

With the last two images you can easily see why the two actions required for connecting this plug, insert & twist, leaves us with very, very, very little contact surface area.

22-Smart Plug vs. 1938

23-Smart Plug vs. 1938

The Spring Mechanism

I very often come across twist-lock sockets where the detente or dimple & hole really don’t feel as if they have meshed up or actually locked into one another. I have learned over the years that this is due to the clamping mechanism in the female socket weakening over time and getting loose.

This weakening is likely exacerbated by movement in the cord and plug as the boat moves. It may also be due to the rather thin metal used in the clamping mechanism of the socket?

As you can see here the metal used to create the clamping force on the male pin is a mere 1mm thick in the NEMA L5-30 socket…

The Male Pins Are Thicker

Here I’m measuring the pin thickness of a NEMA L5-30 “marine grade plug” made by Marinco. It measures about 1.77mm thick.

24-Smart Plug vs. 1938

25-Smart Plug vs. 1938

An RV/Home Depot L5-30

Here is a different brand of NEMA L5-30, a non marine grade version, and we can see that the male pin is virtually the exact same thickness. Why is this? Because marine or not these are all built to the same standard.

The only differences, when it comes to contact area & clamping force, are usually in the tinning of the terminals on marine plugs & receptacles.

Let’s Examine The SmartPlug

This is the back of a SmartPlug 30A inlet. This is the original style plug which has now been updated, and further enhanced for ease of installation, but the basics are the same.

In order to wire the AC shore inlet you will need to first remove the rear cover plate. This is a simple one screw deal. Once the plate is removed the inlet can be wired.

This new SmartPlug 30A inlet is purposely designed to use the same mounting & hole pattern as the twist-lock standard 30A marine inlet. It will fit the old location regardless of whether the original inlet was plastic or metal. This makes the installation & conversion to a SmartPlug simple.

26-Smart Plug vs. 1938

27-Smart Plug vs. 1938

Smart Plug Inlet

Here we are looking at the back of the 120V SmartPlug inlet with the cover plate removed.
You can easily see that GREEN/GROUNDING/EARTH is the bottom hole, BLACK/HOT is the right hole and NEUTRAL/WHITE is the left hole. Everything is clearly coded for a US AC wire color standard installation.


The SmartPlug makes installations as easy as it possibly can be. Even the clamping screws are color coded to the corresponding wire colors. This is the clamping screw for the AC BLACK/HOT wire.
In this photo we are looking at the side of the 30A shore power inlet.

IMPORTANT: If you are uncomfortable with AC wiring, and do not understand the differences between HOT, NEUTRAL and GROUND, please hire a competent marine electrician to do this install. AC wiring and wiring mistakes can kill you..

28-Smart Plug vs. 1938

29-Smart Plug vs. 1938


All the clamping screws on the Smart Plug inlet are color coded to make it nearly fool proof. Please notice that I said nearly.

NOTE: New versions of the 30A SmartPlug inlet will begin shipping with the new set screw design you will see below. There is no problem with this design but the new design makes installation even easier. The guys at SmartPlug are constantly looking to make this product the absolute best it can be and they do listen to the trade and DIY customers, when it comes to feedback.


The silver screw is your AC 120V WHITE/NEUTRAL wire.

I apologize for the tape residue but some of these pics were taken in 11F weather and the tape did not want to come off cleanly.

30-Smart Plug vs. 1938

31-Smart Plug vs. 1938

Flat Straight Blades

One of the reasons the SmartPlug can have so much more metal to metal contact surface area, between the male pins and the female socket, is because this is a single-action plug. This means it only has to insert but does not have to twist as well. The shaping of the female socket to accept the twisting results in considerably less surface area contact.

The other reason there can be more contact area, is because the pins are flat not curved. This makes it much easier for spring tension to create a longer & wider mating area, as you’ll see below.

NOTE: At this point in time SmartPlug’s are for the boat end only. This is the end that sees the most abuse and a 50% fix is better than no fix at all.. SmartPlug now has approvals (the UL testing standards were physically re-written to accommodate the SmartPlug) for the dock pedestal end, a long and arduous process, and once they finalize the UL testing they will have dock pedestal products available too.

Oh My, Another One

Apparently my computer has a mind of its own today and these images just keep seeping from my image database… Sorry, I will try not to let it happen again.. (wink)

32-Smart Plug vs. 1938

33-Smart Plug vs. 1938

A Very Sung Fit

The SmartPlug only inserts one way, straight in, and the male and female are unidirectional in shape thus no mistakes.

I have literally seen NEMA L5-30’s forced in and inserted incorrectly. This can create a very dangerous situation. This type of situation can’t physically happen with the SmartPlug.

The positive locking mechanism is also considerably more robust. Because it does not involve fine threads, twisting and the plug can’t physically wiggle, once inserted, the locking mechanism actually lasts for more than a few weeks before self destructing.

A feature I really do like is the LED indicator light found on the pre-made cord sets. The field install plugs do not have the LED indicator. They tried, but to make them waterproof was proving too expensive on a field install plug. Factory made cord sets have a blue LED in the boat end of the cord.

Overload the main breaker and trip it? One look in the cockpit, or at the boat end of the cord, saves you a trip to the dock pedestal.


In my experience nearly everything about the SmartPlug is extremely well thought out and constructed specifically for the marine environment. This 30A inlet is highly polished cast, not stamped, 316L stainless steel. If you break the cover off a Smart Plug inlet you’re also going to have a nice hole in your boat… (wink). The gasket for keeping it water tight is also very well designed and engineered.

34-Smart Plug vs. 1938

35-Smart Plug vs. 1938

Inside Construction

This is what it looks like inside the female SmartPlug. The receptacle is robust and offers massive contact area when compared to the circa 1938 twist-lock standard.

Notice how robust the spring clamping mechanism is and the up turned guide points which make entry of the male pins seamless and easy..

NOTE: This is a 50A female SmartPlug inlet not the 30A version.

Innovative Designs

The guys at Smart Plug like to think outside the box and come up with very well designed equipment. This is their new set screw design for clamping the inlet wire.

One of the issues with some inlet designs is the resulting damaging & molesting of the wire strands when clamping them.  You also can’t easily drive a standard set screw directly into finely stranded wire without fracturing it. This is also against the ABYC safety standards.

To combat these issues the guys at SmartPlug came up with a floating & rotating head for their set screw design. This prevents fracturing the wire strands and creates incredible clamping force on the wire to create a good electrical connection.

I experimented with these set screws in my shop to see how they performed.. I clamped a 10GA UL1426 wire under one of the clamp screws 35 times. After 35 inserts and removals only two very small wire strands broke. This is pretty incredible when you consider you will only ever do it once or perhaps twice at the most.

36-Smart Plug vs. 1938

37-Smart Plug vs. 1938

Side View Of Set Screw Head

Here you can see the design and floating nature of the set screw head. Even under quite a bit of torque the head continued to spin in the set screw and not destroy the wire.

These Were Brand New Plugs

When I called SmartPlug, during the research phase of this article, I spend many hours or sometimes weeks researching before I delve into a detailed article, I asked about independent testing and temp scanning.

They told me that Cascade Thermal Imaging, Inc. of Redmond Washington had in fact conducted some independent, third part thermal imaging tests on the Smart Plug.

I was then able to convince them to send me this thermal image of two brand new plugs & inlets, both 30A. One is the circa 1938 NEMA L5-30 twist-lock standard, most boaters are using today, and the other is a SmartPlug.

These plugs and inlets were both 30A versions run at 28A or 93% of the face value rating. They were loaded to 28A for 1.5 hours at an ambient room temp of 72F..

I think the image speaks for itself and represents pretty much what I see in the real world. When you look at the temp spread you can see a 23F rise in temperature, from the ambient air temp, on the circa 1938 NEMA L5-30 plug standard. Loaded identically, the SmartPlug yields just a 4F rise from ambient room temp.

I am playing devils advocate here, but there may be differences in the plug casings and how they transmit heat? My gut instinct is that I suspect 1.5 hours should be sufficient to get any heating at the plug/socket juncture to the surface of the plug.

Keep in mind that a 30A NEMA L5-30 circuit should never be loaded to this level for long duration’s. This was 28A in a controlled testing environment with brand new specimens. Even at this 93% of rating level the Smart Plug handles the current in a far better manner. I suspect if the NEMA L5-30 standard were tested today, to today’s standards, it would have never gained approvals.

I regularly measure plug temps with my infrared thermometer and also see this in the real world. Here in Maine we do have a fair number of boats that winter over, many of which use electric heat.

I will say it again, portable electric heaters can be VERY, VERY dangerous even on well maintained cord sets and inlets because they draw immense current. Please be careful using electric heaters.

The max continuous level at which you want to load any shore power circuit is 80%. Keep in mind this is when they are new and with clean tight connections. As the connections age corrosion can set in and this can create high resistance, especially with the highly susceptible 1938 twist-lock standard.

I generally don’t recommend loading a NEMA L5-30 30A circuit to more than 70% of its face value rating, especially in the marine environment. This means approx 21A on a 30A circuit. If you want to push it to 80%, and feel safe about that, the number is 24A. As a marine electrician I do not feel safe about that with the 1938 NEMA L5-30 plug standard.

WARNING: If you want to push beyond a 21A load on a 30A twist-lock you are doing so at your own risk.

38-Smart Plug vs. 1938

39-Smart Plug vs. 1938

To Be Fair

I examined rather closely the NEMA L5-30 socket and pin design and now want to show the SmartPlug 30A pin and socket.

What jumps out to me is the substantial contact area made by the SmartPlug pin and socket. Not only can the clamping spring be longer but because the pin and socket are flat, or insert straight, there is a $hit load more contact area.

Once plugged in and locked a SmartPlug 30A or 50A male/female socket combo can not move side to side or up and down. This was a major part of the design criteria for this system..

This essentially means that no torquing or unintended loading of the spring clamping mechanism, beyond the design criteria, can occur with the SmartPlug. It does not take an engineer to see that the spring clamp can remain tighter, and within design spec for far longer, with the SmartPlug, than it can with a twist-lock. It can remain in spec far longer than a twist-lock because the twist-lock allows for odd angles, twisting, unintended off center loading, torquing and expansion/stretching of the clamp mechanism all due to its left over 1938 design.

Sure, in 1938 perhaps the twist-lock plug was the cats meow for industry and factories. Today, in 2016 we can certainly do better.

Field Install Plug & Inlet

Here we are looking at the 30A field install/retrofit plug and a 30A inlet. There are four points of positive locking between the inlet and the plug:

1- Right & left push lock connectors which are similar to the locking mechanisms found in cordless drill batteries. They are SS and lock into very thick and robust notches in the sides of the inlet

2- The front portion of the lid, nearest the hinge, positively locks into the plug body.

3- The back portion of the lid locks positively into the plug body.


Rather than creating my own video SmartPlug has already done this:

40-Smart Plug vs. 1938

41-Smart Plug vs. 1938

Keep It Dry

One of the biggest pit-falls of the circa 1938 twist-lock design is that it’s nearly impossible to gasket and keep it dry. This can be a critically important consideration when installed just feet from a salt water environment.

Corrosion is a direct contributor to high resistance. High resistance is a direct contributor to boat fires!

Due to a design, that included for the projected environmental concerns, the SmartPlug system is considerably better sealed in order to keep it as dry and corrosion free as is possible.

The Lid Also Has Gasket

Here is the lid gasket. When not in use this gasket keeps the pins drier..

This image also shows just how thick and robust the plastic of the plug body is. This is not an accident. The robust nature of the inlet is part of the design. It needs to be robust to handle the marine environment and loads imposed by a moving boat. It also creates a nice wide gasket mating surface and a very solid area for the positive locking devices to lock into.

42-Smart Plug vs. 1938

43-Smart Plug vs. 1938

Holy Flaming Tamales

Darn, another one got by me…

Another Safety Feature

When Ken, the owner of SmartPlug, set out to design this product he wanted to incorporate a thermal-breaker into the inlet body to prevent melt downs, boat and marina fires due to high resistance. It was not good enough to just design a better plug & inlet, he wanted to design a truly safe product. Should be simple right?


It seems that no thermal breaker existed that could work in this type of application.

  • Problem number one, design and build a thermal protection system for the SmartPlug 30A inlet.
  • Problem number two find a company that can make it to SmartPlug’s stringent specifications and quality standards. They finally settled on a design for the thermal protection system and then found a company to manufacture it. This company is not Chinese, as one may assume, but rather a German company where labor costs are quite high.

It should be noted that the vast majority of this system is made right here in America and I like that.

  • The tinned wire used in the cord sets is made here in America
  • The thermoplastics used in the SmartPlug are all made in America
  • Assembly is all done here in America with US labor
  • The thermal breaker is made in Germany using comparable wages to US workers.
  • The stainless used in the SmartPlug does come from China but only out of necessity. If US Made, the product would sit in US shelves because it could not be built affordable and could not be competitive. Ken tried his hardest to manufacture it all here in the US which is more than most companies will even attempt these days.

The Additional Safety Feature:

Inside the body of the 30A inlet is an epoxy potted thermal breaker that will trip at 200F. This is below the melting point of the wire jacket. Once the inlet has cooled it will automatically re-set at 110F so you don’t lose bilge pumps etc..

No other shore cord system, in the world, incorporates this innovative safety feature. It will prevent the high resistance melt down images you have already seen so many of here and could save lives, entire marinas and boats.

IMPORTANT: There are some in the industry who feel an auto-resetting breaker, of any kind including thermal, is not safe. This mind-set & origin stems from breakers re-setting into a dead short, and I agree 150% with this. However resetting automatically into a dead short is not the same as a high resistance thermal shut down.


Up until now we have had no shore power thermal breakers that trip on high resistance / temperature just on overload of the circuit or an imbalance such as an ELCI (basically a whole boat GFCI)..

The argument the opponents (most likely the manufacturers who don’t care about safety) propose is this:

Boat owner’s Darrell & his other brother Darrell awake in the middle of the night to no boat power.. Darrell & Darrell find no electricity at the AC panel. Darrell #1 opens the AC panel at the same exact moment the thermal breaker re-sets and gets zapped.

While, in theory, this could possibly happen, it is about as likely as Rosie O’Donnell beating Lance Armstrong in The Tour de France. At least Darrell & Darrell were still alive to open the AC panel & not cooked medium rare in their sleep

Follow me for a moment please;

The same few folks (ABYC members & even instructors) & competitors of the SmartPlug arguing against the thermal-breaker in the SmartPlug 30A inlet have zero issues with auto-transfer inverters which can create the same exact Darrell & Darrell scenario they paint above, and use, to argue against the innovative and significantly safer SmartPlug..

Once such competitor actually argues out one side of their mouth against the thermal-switch then out the other sells piles & piles of auto-transfer inverters…. Blatant hypocrisy? You bet…

Follow me again please;

Darrell #1 goes to the dock pedestal and physically unplugs the shore power cord. He yells down to Darrell #2 “Okay-dokey it’s safe now.” Darrell #2 opens the AC panel and ZAP………!!!!!!!!!!!!!

Oops, the inverter transferred AUTOMATICALLY without requiring a manual-reset to produce 120V AC.…… D’oh.

It is insanity to argue against the thermal-breaker in the Smart Plug 30A without also arguing against auto-transfer inverters, especially if they want to use the Darrell & Darrell argument….

Like anything common sense should rule the day, but too often common sense is being waved bye-bye to out the back window of the school bus.. We now have a safer mouse trap & a better mouse trap but it will always get hung up with second guessers & direct competitors proposing poorly thought out & grossly hypocritical supposition like I just outlined. You will have to decide what is safer for your vessel.

EDIT: SmartPlug has decided to offer a version of the 30A inlet without a thermal breaker to be in compliance with the ABYC standards against auto-resetting. Now if we could just get all those inverter manufacturers to… Aw $hit, I give up………

44-Smart Plug vs. 1938

45-Smart Plug vs. 1938

So How Do You Get A Smart Plug?

SmartPlug sales have taken off dramatically and demand is high, and for good reason. That said the product is out there and is available.

WARNING: I will be honest here and say that I much prefer a new cord set and inlet as compared to the retrofit kit shown here. We don’t and won’t install replacement cord ends for customers.

If your old yellow cord is in good condition, then replacing the boat end is a choice you’ll need to make on your own. We’ve seen far too many examples of failed replacement cord ends to recommend this practice.  The safest practice is to do the job correctly and buy a new cord set and inlet.

Where do you buy a SmartPlug?

The crew at Sailboatowners.com (LINK) are really great. They have good prices, great customer service. They are also in close proximity to the SmartPlug facilities which means they have great access to inventory.

Lots of other chandlers have the SmartPlug too  such as Hamilton Marine, Defender and West Marine.

A Shill….?

Rant Warning:

It recently came to my attention that some, within an organization I support fully, namely the ABYC, have told people that this article is a direct marketing piece for SmartPlug and that I am a shill for SmartPlug? If this level of immaturity keeps up I will begin naming names and I will call you out. Enough with the lies already, please grow up..

This is flat out bovine dung and a 150% LIE!

Lets stop the BS here and now. I find this level of immaturity, by people who should know better, utterly disgusting and the lowest form of low. Grow up!

FACT: I make no compensation from SmartPlug, I do not sell it on this site and I have specifically & purposely not become a direct dealer so as to not impede or negatively impact the importance of this article. I believe in the dire seriousness of these issues 150%.

FACT: This article was written by me and of my own volition. I did not need any prompting and am only doing what 2000+ other good marine electricians should have done long ago. By that I mean calling out the sheer safety concerns of a left over, grandfathered & arguably dangerous standard from 1938 vs. today’s technology designed from the ground up to minimize the exact issues the twist lock standard is fraught with.

FACT: SmartPlug had nothing to do with this article other than to offer heat sensing images and answer about 1000 pestering questions from me. I wrote this from my own physical experiences and from what I firmly believe, as a marine electrician, is safer.

FACT: I receive nothing from SmartPlug, zero, zilch, NADA. I buy my Smart Plug products through regular wholesale or retail organizations such as Hamilton Marine, Sailboatowners.com, Defender etc.. I don’t even buy direct from SmartPlug. I do this purposely and by design to eliminate any potential for being called a shill.

FACT: The only compensation I’ve made from this article has been from the “DONATE” button in this article. This site is 100% FREE and reader supported, not industry supported in any form shape or manner.. To date I have made less than it cost me to write this article due to the cost of buying the supplies etc. then destroying them. (EDIT: 2/24/16 I have now made $245 in net reader donations on this article after paying off all the parts and pieces used or destroyed in the article.)

FACT: I personally believe in this product and SmartPlug had nothing to do with my writing of this article. Twist-Lock failures, marina fires and the safety of my fellow boaters are what drive my motivation.

FACT: What have I received from Smart Plug? Exactly one warranty replacement, a 30A 50′ shore cord for a customer who had one fade to a paler shade of orange. This was a very early cord and they subsequently added more UV inhibitors in later production runs. The customer benefited, not me.

Rant off… (grin)


Good luck & happy boating!!

46-Smart Plug vs. 1938

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