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Date: 23 August 2023. Troubleshooting a 50A Power Leg Problem.


This article concerns 50 amp 125/250 volt power. During the troubleshooting phase of this project first we ascertained that full power reached the boat via the power cord. That involved testing power at the breaker, power pole, and finally at the plug which connects to the vessel. I described the plug testing/wiring done in the 50 Amp 125/250 Volt Power Plug Wiring How-To (4-wires) article. We now have 125/250 volts to the boat.

One month ago all was well aboard the vessel. Her power systems were functioning perfectly. As all boat owners know however, while we are out on the aft deck relaxing with a cup of coffee or tea stuff inside is breaking.


Troubleshooting continued. There is a 50 amp 125/250 power cord that runs from the power post to the vessel. Hatteras has multiple power inlets on both port and starboard *amidships.

*amidships: the middle of the boat, outbound by the hull (versus dead center)

← Inlets on port side

Starboard side inlets

↑ click to enlarge ↑


Similar fittings are found on both port and starboard sides of this vessel.
50A 125/250V inlet
Under the round cover:

Opened, male inlet for power (with the prongs)

power cord plugs into the boat


The female (has holes) plug attaches to the male (with prongs) and then is twisted to secure in place. This type is called a Twist Lock and is common on marine power cords. Marinco has been the most popular brand for decades.



Marinco Power Cord - 30 amps, 50'

For the curious, here are two power cords by Marinco. The 30A cord is significantly less costly  than the 50A power cord. Both are 50' in length.

Affiliate Links

Marinco Power Cord - 50 amps, 50'

These are the labeled port side inlets.


This picture appears innocent enough. Troubleshooting continued.

The situation is that inside the boat we have only a single leg of 125 volts
of power available, not the two that are live (hot/powered) inside that 50 Amp cord.


Troubleshooting continued. We peeked at the wires behind the inlets.

The forward inlet that we are utilizing has four wires on the back side.

Further aft, the second 50A 125/250 labeled power inlet has just three wires attached to the backside:

The missing wire attaches to one of the L-shaped prongs on the front.

After pulling the bundle of wires behind that inlet I could not find the missing black 8 gauge wire. It is not there. AND, that black is from an "L-shaped" prong. On 50 amp 125/250 volt cords the two L-shapes are both hot/powered. Each is 125 volts. So, the fact that there is no black wire running down to the panel MAY be an issue. Is it?


Trouble shooting involves going from known to unknown, attempting to isolate exactly where the problem lies. Like Bruce Van Sant in his book Tricks of the Trades, our family practiced the replace parts mode of repair. Thus finding out what exactly to purchase is important. Frankly I am out of my league when it comes to AC power, especially the 50A variety.

This is the boat Daddy built:

My own copy of Tricks of the Trades is worn and frayed. That is a sign of a good book. Affiliate Link ↑

We have plugged into both the aft 125/250V inlet on the port side and the 125/250V inlet on the starboard side. Our panel voltage meter shows the same for all three 50A 125/250V inlets on this vessel. I am confused. Shouldn't all 125/250V inlets have four wires attached to the back. AND, plugging into all three outlets (two port, one starboard) result in the same bad news below, i.e. just one leg of 125 volt power arrives at the main panel.

THIS is the point where I should have looked at
the book describing the boat's wiring... But I didn't.



Instead of consulting the Power Connections paperwork for the vessel, I continued to troubleshoot the power situation. Fundamental errors were made.

#1) I believed the Marinco power inlet markings on the plug inlets



#2) I tried to understand why there were only three wires instead of the expected four.

Port Side aft 50A 125/250 inlet:
Starboard Side 50A 125/250 inlet:

We tried all configurations of plugging in the Shore Power cord into every labeled 50A 125/250V inlet. Below, at the panel we still saw just one 125V leg with power. Argh!!!


More testing ensued. One of the boat guys saw power
at the unused inlets when the boat was plugged in.


We plugged the 50A 125/250V cord into the forward inlet on the port side. That one has 4 wires.

Port 30A/125V inlet Aft Port 50A 125/250V inlet Starboard 50A 125/250V inlet Starboard 30A/125V inlet
Multimeter shows: 116V 0 / 123V 19V / 123V 116V

We plugged in the 50A 125/250V cord into the aft inlet on the port side. That one has 3 wires.

Port 30A/125V inlet Forward Port 50A 125/250V inlet with 4 wires Starboard 50A 125/250V inlet Starboard 30A/125V inlet
Multimeter shows: 116V trickle / 123V 0 / 123V 122V
The power is in the lines though not where we want
it to be, i.e. at that second power distribution panel.

Then I had the brilliant idea (Spoiler Alert: it wasn't!) to take apart the 50A 125/250V aft inlet on the port side. I wanted to switch the Red (hot) wire to that prong where the black belonged. I was so happy to have something that might work... except it would not. Here is why: The red wire runs down to the switch that functions perfectly. Merely switching the red to the black prong will change nothing. It will still end up at the exact same place/switch down below.

Part of problem solving is determining what is not
the problem. So far I am excelling at that component.

We clearly have multiple inlets for 50A 125/250V power. One appears to be wired
properly. Two are not. None provide power to the second leg of the electric panels below.

I finally figured out the problem is not the wiring. The Power Connections page is illuminating. (pun intended)

Tomorrow I will detail why there are 3-wire inlets labeled 50A 125/250V.


This is what the dock crew has been dealing with of late. As I wrote this article I did include our testing procedures. Though nothing was successful in solving the issue at hand, knowing what is not wrong is helpful. I do hope my failures will be enlightening to those of you who might face similar problems. I actually should have read the flipping book. A lot of time was wasted pondering when the answers were right where Sparky (the author) detailed them so many years ago.

Please be smarter than I was. And thanks for reading.

Do you have the books (shop repair manuals, wiring diagrams) for your vessel?
And, have you ever dealt with similar wiring on a boat you've worked aboard?

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