AC Wiring—Introduction
In our previous series (BCM February 2008), we showed the circuits for 15, 30 and 50 Amp AC service. Just as you must decide how big a generator to use, you must also decide what kind of service is necessary. Bigger is not always better, and you may end up paying for wiring that isn't really required.
Should we really consider 15 Amp service? A single water heater can load that circuit to its limit. Wiring, in any case, has to be big enough to carry the minimum load. So, if the generator is rated at 50 Amps, should your campsite hookup be just 15? Perhaps there's much more to consider.

Consider the Generator First
For argument's sake, let's assume you've selected a 120 Volt generator that can provide, a continuous 54 Amps. Just because you have more than 30 Amps available doesn't mean that you have to wire your coach with 50 Amp service. Previous articles illustrated that 50 Amps is delivered on two hot wires, each of which is 120 VAC referred to as a neutral wire. The hot wires are also out of phase, so that between the hot legs there is 240 VAC available. Each hot leg is good for 25 Amps ... 50 Amps total.
The limitation in the campsite service is really due to the AC plug and mating connector Obviously we can have wires big enough to carry as many Amp as we want, but high current connectors get to be very expensive, and are not generally available.
The main point to grasping the fact that some internal wiring must be sized for the generator, but there is no need to match the generator rating from the shore hookup. With a 54 Amp generator, the wiring-from the generator all the way to the main distribution point-should be wired for about 60 Amps. We'll get into wire gauges later.

Back to Shore Power
Should we consider 15 Amp service from the public utilities, As mentioned, a single water heater can load a 15 Amp circuit, so if you plan to spend extensive time hooked up, then you probably won't be happy with only 15 Amps. On the other hand, you're building a coach to stay away from RV parks, and one only needs to keep a battery charger and a few lights going when you rig is parked, then 15 Amp service may be appropriate.
What about 30 Amp service Two 16,000 BTU/hour air conditioners can trip a 30 Amp breaker, although some models ma just sneak in under the limit Obviously there's no room left for the water heater and the TV. If you're using a couple of smaller air conditioners, and are willing to turn off the water heater when both are operating, the you may be happy with 30 Am service.
Thirty Amp service is more readily available than 50 Amp service. And, there is the possibility of picking up a couple of plug-ins to get more than 30 Amps onboard.
With 50 Amp service, you can run the two air conditioners and not have to shut the water heater off. It will cost more to wire 50 Amp service, and it's also a little harder to manage. With many campsites unequipped for SO Amp service, you may find yourself wired for 50 Amps, but operating most of the time on 30 Amp plug-ins.

Wiring for 30 Amp Service
Figure 1 shows one way that a generator can be wired with the 30 Amp service. (I was lazy and didn't want to go through the process of scaling down a symbol for the 30 Amp plug, so please forgive its size).
AC power from the generator and from the 30 Amp plug is wired to a selector switch. Note that both hot and neutral are switched. As mentioned, the wires from the generator must be sized for the generator capacity.
Wires from the AC Inlet only need to be rated for 30 Amps. The ship/shore selector switch must be rated for 60 Amps to accommodate the generator.
Once past the selector switch, a main breaker is found. This is used to protect wiring between it and distribution breakers. Note that the wires split after the main breaker with some wires going to the inverter/charger and a second set going to breakers for the jacuzzi, welder, and battery charger. These devices would | not typically be operated from the inverter, and they could overload the breaker in the inverter.
Inverter/chargers have an internal transfer relay that switches incoming AC power through to the output if AC is present, or switches the inverter produced AC to the output. The transfer relays are typically rated for 30 Amps, and some inverter/chargers have a 30 Amp breaker to enforce the rating. For that reason, it is a good practice to split the AC after the main breaker into two circuits, one for inverter operated devices, and the other circuit for appliances that won't be operated from the inverter.

Centralized Control/Breakers
It is wise to have a single| location where both the AC main | breaker and the DC main breaker | are located. Should an emergency arise where you need to | shut off power in a hurry, having the two main breakers near one | another could save the day. Remember, however, that the function of a breaker is to protect the wiring smoke, so breakers should go as close to the power source as practical.
Does it make sense to have a control panel with all the breakers located there? From a convenient point of view, probably not. Either a breaker panel or a fuse box should be used for all wires as they leave the main breaker, unless the wires are rated above the main breaker. Individual circuits that are wired with smaller wires, than the main wires need to be protected at the source of power, that is, immediately after the main breaker. It is for this reason that boats and some RVs have a central control panel with a multitude of breakers for each of the individual circuits.

Wiring for 50 Amp Service
Wiring for 50 Amp service is more complex than for 30 Amp service, that’s a subject for next later.
We recently stopped at Uncle Mervin’s and Aunt Mildred’s to beg for her mustard pickle recipe. Uncle Mervin pestered us with questions about Dr. Watt and Sherlock Ohms. Apparently, there have been a couple of suspicious characters snooping around his 1946 Farmall generator. Aunt Mildred thinks it may be some investigators from the Department of Health, looking into reports of huge magnetic fields in the neighborhood.
Uncle Mervin assures her that all they’ll find are huge corn fields. We can’t think of a reason that Dr. Watt and Sherlock Ohms would be interested in Uncle Mervin’s generator, but his description did sound like the dastardly duo. We’ll keep you informed.