Voltage Drop or Reverse Polarity

[oceaneyes]

Hello everyone,

I have an interesting situation that I'd like to get some advice and opinions on.

The reverse polarity light on my panel illuminates if I have two out of three breakers turned on and drawing power (AC/Heat, Hot Water, or Stove). The light only comes on at my home dock - not at transient marinas. The reverse polarity light only shows on Shore Power - not when I use the generator.

I think my question is - will the reverse polarity light illuminate if there's voltage drop, or only when there is true reverse polarity? My pier is about 130 feet so voltage drop is quite possible. I believe we ran 8 gauge wire. I tested the outlet on my pier and it's all wired correctly.

Thank you!

 

[Wee Venture]

The reverse polarity light is measuring (with a small light) the voltage potential between the neutral (white) and ground (green) conductors. This should be zero, because they are connected together at the shore power source. If you had reverse polarity, with the white wire hot, then you would have full voltage between the white and ground, causing the light to glow. The current through the light is designed to be very small and not enough to trip the ELCI ground fault breaker.

It’s just a guess, but what may be happening is there is a big enough voltage drop along the neutral wire when you turn on all your high current items that the neutral at the point of your boat panel is a high enough potential above ground to make the light glow. This is quite feasible with #8 wire along a 130’ pier, plus whatever distance back to the mains panel.

John

 

[oceaneyes]

Thanks John. So there isn’t a safety issue per se but increasing the wire gauge should fix the issue?

 

[Wee Venture]

You’re welcome. I don’t think it’s a safety issue because you have clearly established that your power to the boat is wired correctly, and also you are not tripping your ELCI breaker. It would be an interesting experiment to measure the AC line voltage (hot to neutral) at your main shore power receptacle with your boat unplugged. Then measure it at some receptacle on your boat with all of the devices on those large circuit breakers drawing power. The difference will be the voltage drop over your long cable out to the end of the dock. And half of that, theoretically, will be the drop due to your neutral side which is what is getting compared to ground by your polarity light. If your overall drop looks to be pretty large, say 5% or more, you might want to consider running a larger cable out to the boat. You also want to be careful of running AC motors when you have a low voltage. If you do get an unexpectedly large voltage drop between boat and shore power panel, it would be good to check for loose connections which can heat up and cause trouble.

John

 

[knotflying]

Can you try plugging in to a different source of power? Sounds like it is your dock power. Maybe buy one of those reverse polarity testers at Home Depot. They are pretty cheap. This will determine if the issue is with your dock wiring. I have also found that if one plugs into a ground fault outlet sometimes you also get a reverse polarity.