Let's Name the Electrical Problem...

[letsgetsailing3]

Old, unfamiliar boat. Tested shore power, and found a "reverse polarity", with either the "hot" or "neutral" to ground producing 120V. "Hot" to "neutral" (two pronged) produces zero volts.

Consistent finding on all boat outlets.

What does this mean, and how would I find the problem?

[Mycroft]

When you say that you "tested shore power", it doesn't sound like you tested the power at the shore power post.

Test the post and if it's right, then test the next connection after the power inlet. Perhaps the inlet is wired wrong.

What's after the inlet? Iso transformer? Breaker?

[Fiveslide]

Do you have other circuits for stuff like the waterheater, battery charger and such? Have you tested those circuits? If you test them and the problem shows up there as well then I would switch off all AC circuits but one and then test them one at a time. That might help narrow down which circuit is the problem.

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[Mycroft]

Good point by Fiveslide. My water heater (outside the charger/inverter) is sharing a neutral with my port side receptacle circuit (inside the inverter). Not good. I need to run a new circuit to get them separated.

[sailvayu]

When doing any trouble shooting always start at the source and work your way through one step at a time. Start by checking the dock power, the problem could be there after all. Then check the power cord end, then check the power in at the back of the panel and so on. Doing this you will hopefully quickly locate the problem. If all the outlets are the same and the lights are lit on the panel it would be a good guess it is the dock or shore cord. I once pluged into a 120 volt outlet on a dock and got 240 volts. Pays to check.

[letsgetsailing3]

Thanks for the responses. I'll start going through circuits.

I don't think it's the dock, but might as well start there and work my way through.

The solar power controllers appear to be pretty corroded, so I'll check there, but that shouldn't cross the 110 v circuit, I wouldn't think.

[Terra Nova]

Originally Posted by letsgetsailing3

...The solar power controllers appear to be pretty corroded, so I'll check there, but that shouldn't cross the 110 v circuit, I wouldn't think.

Hire a pro.

[leftbrainstuff]

I like to start with a schematic. One i draw myself. If there are any obvious issues they'll stand out.

Then i'll start taking some measurements. For AC starting at the shore power source is a good place to start. Record the readings on the schematic. I also like to record wiring color, connection and termination details and sizing as i go. It's surprising how boat wiring gets messed about with over time. Especially when less than competent hacks have done something unexpected.

Often when troublshooting you lose sight of the big picture. The schematic gives you that helicopter view. It also gives you a head start for next time.

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[Lake-Effect]

Originally Posted by letsgetsailing3

Tested shore power, and found a "reverse polarity", with either the "hot" or "neutral" to ground producing 120V. "Hot" to "neutral" (two pronged) produces zero volts.

Consistent finding on all boat outlets.

What does this mean?

Serious, f&@#ing dangerous fault in AC system.

How would I find the problem?

It's always hard to gauge the competence of someone posing such a question. Forgive me, the title and contents of this post haven't given me that confidence. Could you describe your electrical experience?

It could be as simple as a miswired shore-power cord, or it could be the tip-off of a horribly miswired boat.

Troubleshooting the described problem is fairly straightforward. Any decent book on marine wiring will show you the expected wiring and suggest where to make measurements.

I'm a bit concerned that you might be trying to fix someone else's boat. Lacking any further knowledge of your skillset, the advice would be - check the obvious (pedestel, shore-power-cord, AC inlet on boat, back of AC panel, then, if the problem isn't located, call a pro.

[sparrowhawk1]

Originally Posted by sailvayu

When doing any trouble shooting always start at the source and work your way through one step at a time. Start by checking the dock power, the problem could be there after all. Then check the power cord end, then check the power in at the back of the panel and so on. Doing this you will hopefully quickly locate the problem. If all the outlets are the same and the lights are lit on the panel it would be a good guess it is the dock or shore cord. I once pluged into a 120 volt outlet on a dock and got 240 volts. Pays to check.

Could not have said it better and it's amazing how many marinas are wired improperly

[colemj]

Are you sure that is a 110V outlet and not a 3-wire 220V?

Mark

[smac999]

Originally Posted by letsgetsailing3

Old, unfamiliar boat. Tested shore power, and found a "reverse polarity", with either the "hot" or "neutral" to ground producing 120V. "Hot" to "neutral" (two pronged) produces zero volts.

Consistent finding on all boat outlets.

What does this mean, and how would I find the problem?

that is not a "reverse polarity" that is a big screw up. get a electrician if you are not familiar

actually that means your ground is probably hot. and your H is probably ground. so your whole boat is probably hot including your engine block, and feeding ac into the water. unplug it now and don't plug it back in until it's fixed.

[DotDun]

Originally Posted by letsgetsailing3

Old, unfamiliar boat. Tested shore power, and found a "reverse polarity", with either the "hot" or "neutral" to ground producing 120V. "Hot" to "neutral" (two pronged) produces zero volts.

Consistent finding on all boat outlets.

What does this mean, and how would I find the problem?

If you tested with the boat plugged in, it's probably an open neutral between the boat and post (assuming it's good at the post). With boat plugged in, circuits on and an open neutral, the neutral will test hot wrt ground.

[exMaggieDrum]

Like just said - get a pro if you don't understand how to wire AC on a boat. Similar but different to a house. If you have an inverter - check the wiring for the input and output. There is too much to describe to you in a post well. Neutrals are not "bonded" to the ground on a boat except at a generator of AC (inverter, genset). If on shore power - the neutral is grounded at the top of the dock - to real ground - and NOT on the boat. Inverter/chargers combos have a switch that bonds neutral to ground when shore power is not input to the charger side. The bond is disconnected when shore power (or gent set AC input) is off. Also, there should be one and only one, connection from the AC side to the main DC negative post/bar. But I would check to make sure the engine is not "hot" - extremely dangerous to you, anyone in the water tens of feet away (e.g. divers cleaning boats), plus it will eat up the zincs (and boats) around you. It only takes milliamps in the water to kill.

I worked on a boat (one time) for a gentleman who had reverse polarity in his forward berth. I started at the shore power post and worked forward to the outlets (like suggested above). When I got to the panel, I found that it had been "owner-wised" where hot was white and neutral was black. He insisted that this was correct. He had wired the boat himself. I could not convince him that this was backwards so I just accepted it and went on to see if I could help. I found the inverter miswired according to this scheme. He put white to white and black to black. Not good. I backed off the boat and wouldn't return since he wouldn't allow a complete inspection of the AC wiring. He also would not let me inspect the junction box that connected the berth outlets to the rest of the system. I am not sure why he wouldn't but I suspect he had stuff hidden there that he didn't want me to see. I hope he doesn't kill himself or burn up the boat.

If you don't know what you are doing get a pro.

[jeepbluetj]

Open neutral or reversed hot/ground. Both are dangerous. (reversed hot/ground is real dangerous)

The "hot" pin on 120v NEMA is the smaller pin, both in the NEMA-15 and the NEMA-30 twist lock. There should be no confusion what should be hot on a receptacle, it's polarized.