Forum Discussion
myredracer
Sep 16, 2016Explorer II
j-d wrote:
I learned via another OP that a pair of electric heaters is very taxing on the 30A system, and it often shows up where the flexible shore tie splices into the hard wire of the coach.
By code calculation, two 1500 watt heaters operating simultaneously at 120 volts (and NOTHING else running) would require a 31.25 amp breaker. There is no such thing of course and you'd have to go to the next size up which is 35 amps but the main breaker in the converter panel is 30 amps and is the one in the pedestal. You'll also have the converter drawing a couple of amps or so.
The reason a 30 amp breaker did not trip is probably because of voltage drop and the heaters aren't running at rated wattage. Heater output varies as the square of the voltage. For ex. at 115 volts wattage would be 1378 and at 110 would be 1260. That is for voltage measured right at the heater's terminals inside the unit and the drop would include the heater's cord and all wiring back the the CG main power source. You probably won't get exactly 120 volts at the heater element very often. A voltage of 118 volts at the heater terminals would let you just squeak by and not trip a 30 amp breaker, but that's ignoring any other loads like the converter.
There is the code required 80 percent safety derating actor that is required, but that covers fixed/hardwired loads and the NEC doesn't apply to portable plug-in appliances. Interestingly, here in Canada we are allowed to operate fixed heater circuits at 100 percent of the wire (but the breaker must be derated by 80 percent). We can have 1800 watts on #14 wire (at 120 volts).
The worst thing that could happen is one of the 30 amp breakers will trip if running 2 x 1500 watt heaters. However I'd be concerned about how good the shore power plug to pedestal receptacle connection is. Depending on the floor areas you have and how good the insulation is, two heaters could be running together for hours at a time. If there is resistance in the plug/recept. contact surfaces, you'll get overheating and potentially a meltdown. In a perfect world you should be able to draw 30 amps all day long but pedestals are not perfect and some older ones are really nasty looking and asking for trouble.
A 1500 and a 1000 watt max. heater *might* work (if running continuous) and I don't think I would go more than that to allow for something else to run. Ideally heaters would cycle on/off but they probably won't. We have 1750 watts of permanently installed heaters and it works fine for the approx. 200 sq. ft we have, which is about 9 watts per sq. ft. In comparison, a wood frame bldg., 6 watts/sq/ ft. was a rule of thumb that was typically used here in BC.
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