Shore power plug keeps burning out...

Bill Roosa · Feb 1, 2009

Heat from resistance

Hi Levin
I've been doing the live aboard lifestyle also and don't have that problem so it can be corrected. We just have to home in on the variables.

Two possible causes of heat are
a) overloading the circuit
b) poor, loose, corroded connections that result in voltage drop across the plug and heat.

A 30 amp service (and I'm going to get flamed but engineering is engineering and I believe in safety factors not derating) should be able to handle 30*120=3600 watts. Folks will tell you that you should de-rate that to 85% or 3060 watts etc for safety but my belief is that the circuit is designed to handle 30 amps continuously and could physically handle 110-120% of that. That is the way we do it at the Ohio State university. Tell the customer "that there thing is a 30 amp service, you can get 30 amps out of it" not "that is a 30 amp service you can only get 25 amps out of it." The latter being a good way to get into a law suit IMHO. Truth in advertising.

So what we have is either a maintenance problem or a load management problem.

Before we begin and I dig a hole that you can't get out of, how comfortable are you at working around live 120 volt circuits? and do you have a multimeter?
Also we are gong to need a list of the AC loads you have on the boat.

sailingdog · Feb 1, 2009

Are you talking about the end that goes into the power post. If so, then the problem is likely that the socket on the post is loose and you're getting a bad connection—leading to arcing and excessive heat. If this is the case, you need to fix it, because it is a serious fire hazard.

Auspicious · Feb 1, 2009

I would look first for a loose connection or corrosion at the connector that is failing. The problem could by at the connections on the back-side of the boat connector. If you are using the same cord over and over it could possibly be a problem in the cord, but again it will be loose or corroded connections.

If you borrow or buy a clamp on ammeter you can measure the current through the cord just before the connector which will be a useful bit of information in tracking things down. Frankly, if it is simply an over-current problem I'd expect you to see damage at the connectors on both ends of the cord.

Bill Roosa said:
A 30 amp service (and I'm going to get flamed but engineering is engineering and I believe in safety factors not derating) should be able to handle 30*120=3600 watts. Folks will tell you that you should de-rate that to 85% or 3060 watts etc for safety but my belief is that the circuit is designed to handle 30 amps continuously and could physically handle 110-120% of that.

We went to different schools. *grin* I'd design for 25% safety factor for sustained load and 50% surge with a slow-blow breaker in the pylon. I have no idea what factors Marinco and their competition use.

Shell · Feb 1, 2009

It definitely a resistance problem. The load you are using on the boat is exceeding what the 3 wires connecting to the shore power recepticale on your boat can handle. They call them a 30 amp plug but it unlikely even 30 amp is even reaching the boat. (Sorry, Bill Rossa, but I don't believe when they say 30 amp it is 30 amp). Maybe in labratory settings. Moisture, slight corrosion or a loose cord connection can create the resistance. Heats the 3 wires to the piont where they will react with the surrounding moist air, slowly corroding, creating more resitance and then finally burning out. Barring the fact that maybe Hunter is buying inferior wiring from China, I would add another 30 amp service to the boat or for a tempory solution run a heavy duty extension cord to take some of the load off of you 30 amp service directly to your power pedistal and plug in one of you heavy load applaincances in that. Make sure not to overload that cord.
At our marina, with many liveaboards in the winter, heaters, coffee makers, almost every new person burns out the wires behind the recepticale. Luckily no fires.

I hope this can help. Good luck!

Bill Roosa · Feb 1, 2009

I would note that you said almost everybody...

The laws of the universe do not allow "almost". It either works that way or it don't. So if someone is getting 30 amps out of a 30 amp service (I do with the standard setup you would expect, no fancy stuff) then everybody can and it is either a case of

a) maintenance is not being preformed often enough or
b) you are overloading the circuit in a continuous fashion.
c) defective equipment or equipment not meeting standard engineering design

The latter being where I get flamed on.

I would grab a 30 amp CB and then calculate the wire size that can safely handle 30*1.1 (110%) or 33 amps continuously. There will not be a wire that handles that exactly so we go up the the next larger one. This is called fail safe design. The CB is the safety device so it will trip BEFORE I overload the wire. No amount of overload will cause a fire because I can't get enough current flowing to heat up the wire. A similar though pattern will drive your choice of connectors and such. You don't just "use what is avalable," you design it so it works. Always keeping in mind that everything must be able to handle continuously 110% of the CB rating.
Then it is just a matter of checking the connections once in a while for corrosion, looseness (use screws not quick connects on main power circuits) to insure you have a system is up to snuff and seaworthy.

This is standard engineering practice and is taught at every university in the world.

Mike B · Feb 1, 2009

"No amount of overload will cause a fire because I can't get enough current flowing to heat up the wire. "

Bill, I agree with most of what you've written except for the part about the fire. In too many cases I've seen a circuit load up and burn out the weakest portion. You put 30 amps across a marignal connection and you'll have more than enough to start a fire, but not trip the breaker. This is especially true at shore cord connections as they're of the push and turn to lock variety. You just can't get the same tight connection as if you screwed them down like you would in a breaker panel.
Mike

Ross · Feb 1, 2009

30 Amps calls for number 8 wire. If a circuit is drawing a full 30 amps for a prolonged time a small surge may trip the breaker. But often the load rating is defined as inductive, resistive or tungsten. The start up load from an electric furnace is quite different from the start up load from a bank of incandecent lamps.
In this case the problem is most likely in the connections. Are the wire to outlet/inlet connections properly torqued?

Bill Roosa · Feb 1, 2009

See what I mean

Auspicious was paying attention in safety factor class.

The ABYC probably has some numbers for surge and continuous use safety factors on AC circuits. Anybody KNOW what they are?

I used the 110% because I'm a Civil engineer. We don't do surge, we build stuff that, by design, does not move.
Auspicious has a, IMHO, better attack to it since he is accounting for two states that you find in electrical systems, surge and continuous with slow blow CB.
I'll bet he is shrink wrapping all his connections to help slow down the corrosion too.

ITMaster · Feb 1, 2009

I Have Seen This Happen

As stated by the other replies, this is a combination of factors.

First the assumption of the amp load needs consideration with safety factors calculated in. However, eliminate the assumption and conduct some measurements. Find out the voltage coming in, which if it isn't 120 and is only 110 or even 109 or lower, then that will drive up the amp load. As Dog said, you have something loose that is the cause of the heat that is burning up the connectors. Check the connection at the panel from the bus bar to the breaker and make sure that the connection and the screw to the lug is tight. We don't realize that there is energy flowing constantly through these connections and with time, the lugs will loosen up and cause an increase of heat and an increase of amp load and a reduction of power and wattage availability.

My solution and what we had the person do at out marina is this:

First, ensure that the Shore supply connector from the dock is properly wired and that the black wire goes to the connector that will eventually be the same as the black wire inside the boat. Do the same for the white and green. This ensures proper polarity. Make certain the screws from the wire coming from the panel are tight to the connector at the dock. Coat these connections with Dielectric compound and all connections from the source to the end of the circuit.

Do the same for the connector going to the shore power plug that you plug into the dock. Again, use Dielectric compound on the terminals that you screw the wires to the plug.

Thirdly, I would do the same for the connector that you plug the wire into the boat at. Again, Dielectric compound.

You should have a poliarity indicator on the boat at the panel, ensure it doesn't indicate a polarity fault. You will need to follow all wiring from the source to the end at your panel. You can do this with your eyes or with a multi meter.

I would replace all the connectors starting at the dock and ending at the boat. Coat all the male prongs that you plug into the female connectors with Dielectric compound. Use the kind that have a screw on locking collar device to make sure that the twist lock connection is tight and stays tight.

I would use Marinco connectors in lieu of Hubble. They cost more, yet IMHO are far superior for a marine application over the Hubble. Also, make sure that all the wires are of a marine grade nature and not standard house hold solid copper or aluminum wires. Expect to spend about $150 on this. When you leave your slip, pull your plug out and put their cheap crappy plug back in if you are a tightwad. If you are a good citizen, leave it for the next person, like I would. You wouldn't want to knowingly be a contributor of someone's boat burning to the water line like yours can if you don't do something fairly quick.

Do the math and make sure your loads are within what you have available. If you are running electric heaters, a microwave, a water heater, tv, electric blanket, you are more than likely drawing too much current for a 30 amp service and may want to either step up to a 50 amp service or add a second 30 amp supply.

AC Volts x Watts = Amps

Your Hot water heater more than likely is a 1500 watt element. Your electric space heater is more than likely a 1500 watt device. Your microwave is more than likely a 1000 to 1200 watt device. Coffee pot, electric (1200 to 1500 Watts)? Your battery charger is another high wattage appliance. You will be amazed when you go through your on board electric devices and conduct an inventory. I find it common to see where someone has tried to put a combined load on a circuit in excess of 30 amps, both at the dock and in commercial applications.

Good luck, let us know what you find.

Mike B · Feb 1, 2009

Re: I Have Seen This Happen

Just a minor correction, AMPs is Watts divided by Volts or A=P/E.

ITMaster · Feb 1, 2009

Absolutely Mike, brain fart on my part. Thank you for catching that for me

That's what I get for not editing my post more thoroughly. Usually my students catch me also. When I do the math and it comes out screwy, I usually find my problem is in the formula 9 out of 10 times.

Thanks again, Tom

Mike B · Feb 1, 2009

Tom, I fugured most would know it was bass ackwards but I didn't know if Levin would also. I agree he needs to make a list of his loads but in reality he can only run one or two high wattage devices at a time or he'll trip the breaker. Other than this year we typically spend most weekends on the boat, even in winter. There's no way to run a space heater, and coffee pot or hairdryer without tripping a 30 amp breaker. On the last boat I tied in an outlet on the air conditioner circuit and used it for a space heater. When it got so cold you couldn't run the AC/Heat unit I'd plug in the heater and we could use a coffee pot or hair dryer w/o problems. I removed the outlet and wiring when we traded the boat in. I was afraid the whoever the new owner would be wouldn't know not to use both the AC and outlet at the same time.

As for Levin's problem I think he's got some good advice. Replace the connections with the best quality available. Make sure they're always properly secured by twisting the plug and tightening the locking ring. Check all his connections to the circuit for corrosion and tight connections. Checking the voltage at the dock post as it's not uncommon to find they don't meet specs. And of course understand what loads his running to make sure he's not constantly maxing out the circuit. All basic, common sense due diligence.
Mike

Warren Milberg · Feb 1, 2009

Lots of good advice above. To me, it certainly sounds like your boat is drawing more amps than your shore power can continuously supply. To me, that means that you and your boat are going to be toast if the situation is not corrected. While it is certainly a good idea to make sure your shore power connection, shore power cord, and on-board connections are bright and tight, the first thing you would seem to need to do is get an exact idea of how much power your boat is drawing, to include big-amp start up devices like water heaters, air conditioners, electric pumps, and so forth. My guess is that you are drawing more amps than your boat and/or marina can handle. I seriously dobut that any aging marina can deliver exactly 30 amps (or 50 amps) or whatever their shore power posts claim to be rated at, continuously. Same comment re aging boats and their wiring systems. Age=impedence=heat. You've got the potential here for a very serious problem that would seem to require serious attention, quickly.

newly anonymous · Feb 1, 2009

I had this problem

I started melting shorepower plugs shortly after making a passage downwind through a two-day gale. Apparently, salt water had gotten into the thru-hull shorepower connector on my transom. With 40 knot winds and 14' following seas, that can happen fairly easily.

The solution is fairly simple. Re-read ITmaster's post, and do everything he suggests, especially the part about the dielectric compound.

Don S/V ILLusion · Feb 1, 2009

This seems to be a fairly common complaint - about corrosion/discoloration around these plugs.
In addition to what has already been said, keep in mind the overall quality of these cables and their connectors has degraded compared with a decade ago during which time no one, to my memory, ever complained about this problem. Certainly try all the suggestions mentioned previously but it might be telling for someone who is about to trash a bad connector to open it up for examination.

Franklin · Feb 1, 2009

Levin, I'm a live-aboard and have to tell ya that when I'm running on just one 30 amp wire, the breaker's flip because I draw too much for that. I'm assuming you are too, just not flipping the breaker. Get another 30 amp connection and power cord.

If that isn't something easy, then just keep the water heater turned off. 30 minutes before you need hot water, then turn the heater/AC off. The water heater will keep the water in it hot for a long time...like 12 hours, so get it hot in the morning and only use it when you have to.

Franklin · Feb 1, 2009

Oh yeah....and don't use a hair dryer

e sea goin · Feb 1, 2009

tripping breaker, over loaded circuits

Use the amp calculation, I (amps) = P/E. However, check the incoming voltage with a meter, don't assume that it is 120 volts. When I checked my voltage it was 105 volts, a lower voltage will increase the amount of current flow. If your cords are too long, you will have a voltage drop, or the incoming voltage from the marina may be low due to voltage drop. 1500 watts at 120 volts is 12.5 amps, 1500 watts at 100 volts is 15 amps. It starts to add up.

Levin said:
Hello everyone...
Ok, so I have a question and hope that someone here has experienced something similar. I'm a live aboard and I own a 2000 model year Hunter 340. Ever since I bought the boat I have had a problem with the shore power plug. The problem is that over time it seems to "burn out" (best way I can think to describe it). Basically the connectors on the boat that the shore power cord plugs into turn black and appear to slowly fry over time. This doesn't happen all of a sudden from a fire or something, seems to be slow and insidious, but it does happen. I replaced the original plug and the shore power cord I was using (which in all honesty came with the boat when I bought it and looked like it had seen better days) about a year ago hoping that the problem was one of the two of these and that replacing both of them would solve the difficulty. No such luck... Now a little over a year later the new plug is doing the same thing and I'm lost for a reason.

Does this just happen over time? Is this because I'm a live-aboard and use more electricity than others who don't live on their boat? Can the humidity in the air around my marina cause this over time? I'm good about making sure the plug never falls in the water and that it's well screwed down when attached... but still I seem to be getting this problem. Any ideas?

Thanks for whatever thoughts you might have... I'm likely going to end up replacing this plug for a second time and I would like to prevent doing this a third.... so any input would be greatly appreciated.

Take care,
-Levin

Steve Dion · Feb 1, 2009

Levin:

All of the above seems to be true. I would suggest that you replace the AC line been your panel & plug as your first fix. Be sure that the NEW cord is securly fastened to the NEW plug in the boat with the collar properly tightened.

We often trip breakers either at on board breaker or at the service panel, but have not had the type of problems you are experiencing. I would also suggest that you clean the plug & recepticle every month or so to prevent corrosion build up in your connections.

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