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Monday, October 19, 2009

Four for Two-fer

If both of my air conditioners can run with the generator doesn't that mean the wiring can handle them both? Why can't you just replace the 30-amp cord with a 50-amp and run both A/Cs from the shore power also?
Marcia, (Ft. Walton Beach, FL)

In an earlier column you talked about the 30/50-amp shore power option, but you didn't explain why an RV that can run two air conditioners on generator power cannot run both when on shore power.
Larry, (Victorville, CA)

I have a1994 Coachmen with a 30-amp cord. I have a 7kw generator with two air conditioning units. Running on the generator I can use both A/C units. With it plugged into a 30-amp outlet I can only use one air conditioner at a time. Can this be changed maybe to a 50-amp cord?
Mark, (Hollywood, MD)

I have a Class A motorhome that has two air conditioners on the roof and is wired for 30-amp service. When we go to campgrounds and plug in, only the front A/C will operate. The rear one will not come on. I also wired a 30-amp circuit at my house and it will only run the front A/C. Why will they not both run together when plugged into land power? They both run on the generator.
Rodney, (Arlington, TX)  

Okay gang, even though logic says it should be possible to just install a 50-amp cord in place of a stock 30-amp and run two air conditioners simultaneously, the issue is circuit protection. To comply with RV codes and standards, each roof air conditioner must be wired through its own, dedicated 20-amp circuit breaker. In a typical 30-amp system, when on shore power, there is only one 20-amp air conditioning circuit available. Some RVs are equipped with a switch (either manual or automatic) that will allow a choice as to which air conditioner is to run off that shore power. But with only one, 20-amp breaker available via shore power, only one air conditioner can be safely operated. 

Properly sized generators, however, can power both A/Cs at the same time because the generator has a high enough output above the 40-amps required. In some installations, the second circuit breaker is located on the generator itself. With each air conditioner being protected by its own 20-amp breaker, it would be a mathematical impossibility to use a 30-amp shoreline cord. Two times 20 is 40, right? It’s only a 30-amp main circuit on the panelboard. Replacing the 30-amp with a 50-amp cord is simply not feasible either. The 50-amp cord has four conductors since it’s bringing in two separate legs of 120-volts AC from the campground pedestal (not 240-volts). A 50-amp RV has two legs of 120-volts each powering the various circuits, plus a neutral and a ground conductor. The 120-volt appliances and circuits inside the coach are split between each leg of 120-volts along with their respective circuit breakers. One A/C is wired to one leg and the other A/C wired to the other leg of incoming voltage. 

It is possible, though it’s probably cost prohibitive, to have the coach retrofitted and rewired properly and converted to a “legal” 50-amp system. But what about adding a second shoreline cord rated at 20-amps and wire one of the air conditioners to that second power cord? Most campgrounds have a 30 and a 20-amp receptacle in the same pedestal, right? Such a set-up would not be code compliant. In years past it was permissible for an RV to have two power cords provided they each their own panelboard distribution box and were electrically isolated from one another. But this changed with the 1996 National Electric Code.

However, there are aftermarket products available that will allow you to run both A/Cs automatically but only one at a time. One cycles, then the other. They are never running at the same time but alternate to keep the motorhome evenly cooled. Such a product is probably the wiser alternative.


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