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Thursday, January 18, 2001

Voltage Difference on Two Legs of 50-amp Service

I received an email from Bob S. recently, regarding some strange AC voltage measurements at his RV resort. I've asked electrical guru, Mike Sokol, to weigh in on this one. Here's Bob's question, followed by Mike's insightful response. Gary

My motorhome is hooked up to a 50-amp circuit in our RV Resort. Everything works great until the afternoon, when the temperature gets above 85-degrees. I have a built-in voltage protector with a digital readout. In the morning I get 118 volts on both sides (legs) of the circuit. In the afternoon it reads 118 volts on one side and 103 volts on the other. This causes my power to shutoff momentary during the afternoon. Sometimes more then once. To prevent damage I shut of the air conditioner. I use propane for refrigerator and hot water. What would cause the low voltage on one side of the 50-amp circuit? Bob S.

Hi Bob! Here's what I suspect is happening. As you probably already know, in a 240-volt service, there are two hot legs with 120-volts each. Let's call them Hot-1 and Hot-2 for this discussion. Now, your 50-amp/240-volt coach is hooked up to both of these legs, and is probably balanced internally. That is, the manufacturer has wired the RV appliances so that half of the AC loads are connected to Hot-1, and the other half of the loads are connected to Hot-2.

Now as current is drawn through a wire, there will always be some sort of  voltage drop. Most of the time it's only a few volts, but as a wire is extended throughout the campground, each additional site will cause a few more volts of drop and it can easily reach 10 or more volts total drop.

(Please read this article on NoShockZone.org <
http://NoShockZone.org>  for a  review if you've forgotten the basics: http://www.noshockzone.org/rv-electrical-safety-part-vi-voltage-drop/).

Now, as mentioned before, your coach is probably pulling equal current on both the Hot-1 and Hot-2 wires, so when there's a voltage drop it should be pretty much be equal between the two legs. However, not every coach will be 50-amp coaches. 30-amp/120-volt service being very common as well. Since they're only drawing current on one of the hot legs supplied throughout the campground, that's the only leg that will have a voltage drop. In a properly wired campground system, the  electricians will alternate the 30-amp service connection. So the first pedestal should have its 30-amp circuit connected to Hot-1, and the second pedestal should have its 30-camp circuit connected to Hot-2, the third  pedestal connected to Hot-1, and so on down the line...

I'm guessing  that at your particular campground, the electrician has wired every single pedestal in the same line with the 30-amp outlets wired to the same hot leg. So when all the air conditioners get running at the same time, all those 30-amp hookups are drawing down one of the hot legs a lot more than it should, while the other hot leg is going along for the ride. The solution is that the electrician inspects each pedestal and swaps the Hot-1 and Hot-2 legs, which alternates the 30-amp circuit breaker so the load is equally shared. So there you go! Mike

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