Either Versus Or
Dear RV Doctor:
I have a 1991 Damon Ultrasport on a Chevrolet chassis. I am looking for a detailed wiring diagram for the two switches that control the two air conditioners via the generator and the shore power.
Glen Smith
Via E-Mail
I’m assuming the motorhome has a 30-amp shore-power cord along with those two roof air conditioners, right? And an onboard generator large enough to run both air conditioners at the same time? This setup was quite common across many brands and models of motorhomes back when your coach was built.
The two switches you mention are commonly referred to as “either/or” switches. In other words, when on shore power, you can run “either” the front air conditioner “or” the rear air conditioner, but not both at the same time. It’s simply an issue of mathematics, and it remained so until motorhome builders started installing one smaller air conditioner, thereby reducing the electrical requirement and the need for those two switches you mention, and ultimately began moving to 50-amp service.
Mathematics? Yep, here’s the explanation. A 30-amp shore-power connection supplies a maximum of 30 amps that can be used by the RV. When your motorhome was built, most roof air conditioners were rated at 13,500 Btu each, which required a dedicated 20-amp circuit breaker for each unit. In order to maintain code compliancy, it was mathematically impossible to power two 20-amp circuits (40 amps) from a single 30-amp connection.
Confusion spread, because it was possible to power both air conditioners from the onboard generator. Common sense would seem to indicate that there should be more power coming from the grid and the shore cord than from the onboard generator. However, the output of the RV generator back then typically allowed for a true 240-volt configuration whereby one of the air conditioners (usually the rear unit) was hardwired directly to the generator. This left enough current to power the rest of the coach, including the front air conditioner.
Today, sophisticated energy management systems can monitor the temperature inside the motorhome and allow power to flow to the most-needed air conditioner at that snapshot in time. Devices exist that allow an alternating sequence between the two air conditioners. Sadly, there also are some illegal, noncompliant accessories available in the aftermarket that conscientious RVers should stay away from. Some are downright scary, considering it’s the 120-volt-AC system being altered or manipulated! Exercise caution and look for RV-approved products for aftermarket applications.
The simplest method manufacturers utilized to manually switch between air conditioners was by installing these “either/or” switches. So, you have two power sources — 30-amp shore power and likely a 54-amp (or larger) onboard generator, plus the two air conditioners. By manipulating two switches, the RV owner could choose which air conditioner to power while connected to shore power and also be able to position the switches while on generator power in order to run literally everything in the motorhome.
The wiring configuration for a set of “either/or” switches can be somewhat confusing, especially for someone not trained in RV electrical systems. And it gets more confusing when you have to first discern which type of three-way switch was used. If you or someone you know will be accessing these switches, I must caution you to first disable the RV generator by removing one of its battery cables, thereby prohibiting its ability to crank or start. Also, if you’ve installed an inverter in the intervening years, disable it completely. Finally, be sure the motorhome is not plugged in to any shore power. The wiring diagram included here shows the secret to those twin “either/or” switches.
Plastic Versus Brass
Dear RV Doctor:
I have a question regarding your May 2012 article in Family Motor Coaching about water heater maintenance (“RV Water Heater Maintenance,” page 40). I have an Atwood water heater in my 2006 Fleetwood Southwind, and your article says not to replace the plastic drain plug with a brass one. Why not? I’ve tried replacing it with another plastic plug, but they leak. Also, if you replace with plastic, do you wrap plumber’s tape around the threads of the plastic plug? Thanks for your help!
Dave Tanis, F375335
Carver, Massachusetts
The reason Atwood owners manuals do not recommend replacing the plastic drain plug with a brass plug on an Atwood water heater might be based on the design intent. Some consider the plastic plug to be a redundant safety device should the pressure-and-temperature (P&T) relief valve fail. As we know, when water is heated inside a confined space, it expands and the pressure increases. Precautions are put into place to eliminate the possibility of an exploding inner tank filled with extremely hot water. Normally, the P&T valve protects against that; however, if that component fails to open when the temperature or the pressure within exceeds the preset limits, the tank still has the potential to rupture — and cause serious harm, I might add.
It is thought that the plastic plug will “blow” off if the P&T valve fails to open, though I’ve personally never seen this happen . . . yet. The predominant thought is that the plastic in the plug will warp and distort enough under pressure and high temperature that eventually it will fail before the tank has a chance to rupture. The plug is a specific type of plastic molded to specific dimensions, so it is paramount that it be replaced with a genuine Atwood plug rather than just any ole plastic plug purchased at a big-box store.
Thread sealant is never required on a plastic fitting, since pipe threads are tapered and will seal when installed properly. Obviously, overtorquing the plug may damage the threads, so care is required. That’s probably why the common aftermarket plug you installed leaked. The Atwood plastic plug should not leak. Also, electrolysis can cause metal plugs to seize tightly to the threads in the aluminum tank, making it nigh impossible to remove them for draining purposes. Hope this helps!
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