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Date: 17 August 2023. 50 Amp 125/250 Volt Power Plug Wiring How-To (4-wires)


This all started aboard a neighbor's large motor yacht. The boat is a beauty. She requires a 50amp dock power plug, which is split into two legs for the boat's circuits. A problem has been isolated to one of those 125 volt circuits. First though I had to understand how the power in that plug worked. These are my observations along with the testing procedure I utilized.

Please Note: I am NOT an expert. I can follow directions and know the first rule of power:

MAKE CERTAIN no imbecile plugs the boat back in to "help".

*DISCONNECT totally. That specifically means turning off the breaker to the power source. When working in a boat I unplug the vessel, and then verify the inverter is turned off. Next, if the power is from a dock where a bozo (aka a "helpful" idiot) might walk by and plug the boat back in, I bring the end aboard the boat. This may sound like overkill, however please remember that electrocution is a Very Bad Thing.

On Amazon I was able to find a cool diagram which is definitely
helpful in identifying the differences between two types of 50A power plugs.


The previous chart did confirm that I was indeed dealing with a 125/250 Volt power line.

On the boat there is a receptacle for the 50 amp power cord. Please note the Ground in the lower quadrant, left side of the inlet.

The two blades with "L" shaped bends are positives. Each leg (blade)
carries 125 volts, totaling 250 volts on the 50 amp 125/250 volt power plug.

When wiring the plug, the ground goes onto the side. Each of the 125v wires connects to a bent prong. The final wire, a neutral, attaches to the last flat slot.

Here is how those wires look from the front side.

To test I pulled out my multimeter. That is when I ran into a problem. My multimeter prongs did not reach far enough into 50 amp plug to make a connection with the positive inside the plug. I had a solution however.

I scavenged a couple of leftover segments of 10AWG wire and cut two pieces. When doing wiring I almost always have one short length and the other is longer. This way the ends cannot possibly connect. As you can see, the white is nearly twice as long as the black wire. I shoved the prongs from my multimeter into the wire, then inserted in the 50 amp plug.

It would be wise to tape over the exposed metal where the multimeter prongs connect to my bridge wires.

During testing the exposed metal where the multimeter prongs connected to the bridge wires were covered in electric tape.


HOT , two "L"s, each 125 volts
, flat slot,
and, GROUND is green

50 Amp Power Cord Testing:

First plug in your cord to the power source. Using the extension, shove one end in the NEUTRAL slot. That is the flat one in the power cord. Next, take the other extension and insert it into one of the two HOT sides. Those, on 50 amp plugs, are the "L" shaped slots. Your multimeter should read 125 volts.

Finally test the other HOT side. As long as you have 125 volts in each slot, all is well with the power and cord to your vessel.

Please note this differs from 30 amp plugs. 50's are a whole 'nother level of complication. On 30's, the ground is the "L" shaped slot.


Now that we have power to the vessel, the troubleshooting continues.

As a quick peek at the future, here is the problem:

More on this mess shortly...


Free Advice: In addition to a multimeter, it is a good idea to have a couple of short lengths of 10 gauge wire for those times when you need to make a bridge.

Affiliate link

Multimeter DT830B LCD Digital Voltmeter Ammeter Ohm multimeter



Tomorrow I shall show more about the problem on the 50 amp boat. We know where the power stops. That is the Problem, and hopefully one of you can help with a viable solution. See you tomorrow, and thanks for reading.

Have you ever rewired a power plug?
And, does your boat use a 30 or 50 amp power cord, or even 100 amps?

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