AC Electrical Knowledge

AC Electrical Systems

The Berkshire AC electrical system consists of three possible sources of 120VAC, the distribution panel which includes the circuit breakers (NOT fuses) and a variety of users and outlets.

Sources of 120VAC power:

  • Shore Power
  • Generator Power
  • Inverter Power

Shore Power: Via an external cord we can connect to an outlet, preferably a 50A outlet like we find at a campground. The 50A male plug has four prongs. One ground, one neutral and two “hots”. The voltage between the two hots is 240VAC. The voltage between each hot and neutral is 120VAC. Although available, there is nothing on our coaches that runs on 240V. We use the two 120V legs to power what is in effect a split AC system on our coach. Half the users are on one leg, half are on the other. Keeping it balanced is the objective, so the there is enough power for everyone. The two legs are 50A EACH, so we have a total of 100A available. That’s 100A x 120VAC = 12,000 W, or 12 KiloWatts. (KW) If we do not have 50A but we do have 30A available we use a converter cable, also known as a “dog bone”. The 30A plug has three prongs: A ground, a neutral and one hot 120VAC. The one hot leg is then connected to both sides of the electrical system in our coach. Note that we now have 30A x 120VAC = 3600W….. a LOT less than the 50A service! This requires awareness of how much we draw, we need to manage our load as 3600W is NOT enough to power everything we have. Lastly, sometimes we only have a regular outlet available, usually 15A. (At home in the driveway for instance). With yet another converter plug we can step the 30A plug down to this. We will have 15 x 120 = 1800W available. That’s not a lot.

Generator Power: Using a diesel motor we power a generator which “generates” electricity. Depending on model coach we have an Onan QD (Quiet Diesel) 6000, 8000 or 10,000. The number stands for the Wattage it generates. If you have an 8000, that gives you 8000W / 120VAC = 66A Less than a 50A service (remember that was in fact 100A) but more than a 30A service.

Inverter Power: An inverter is an electrical device which turns DC into AC. Using the energy stored in our House Batteries in 12VDC format (more on that later) it “makes” 120VAC. This 120VAC comes in two types: Modified Sine and Pure Sine. Without getting too deep it pays to know that Modified Sine is a crude, dirty, form of AC and some delicate electronics don’t like it. It also often doesn’t play nice with GFI’s (more on those later) After mid 2017 all coaches came with Pure Sine inverters, which deliver clean AC. Inverters also come in sizes. Most coaches came with 2000W inverters, some of the larger newer coaches came with 3000W inverters. In the same box as the inverter is the battery charger. (more on that later too). In fact we refer to the box as the “Inverter/Charger” (I/C). All coaches are equipped with I/C’s manufactured by Magnum Energy. Located in an outside compartment you can get the model number off it and find the Wattage and whether it is Pure or Modified on the Magnum website. They all look like this, either grey or white:


We interface with the I/C through the remote, which is located upstairs and looks like this:


I delve deeper into the remote in this writeup. Also, I STRONGLY suggest reading the manual on it. If you don’t have the paper version you can download it from the Magnum website.

Transfer Switch

Two of the three sources of power come into the distribution system through the “Transfer Switch”. It’s a metal grey box, about 7 or so inches square, also located in an outside compartment usually where the shore power cord comes into the coach. Its job is to decide whether Shore or Generator power are connected to the coach. You do NOT want two different sources of AC on the coach at the same time. Way over simplified, the reason is that they would not be “in phase” and the electrons running back and forth would start running into each other and that is bad. So one or the other.

If the Generator is not running and you connect shore power you will clearly hear the relay (fancy word for an automatic electrical switch generally designed to handle large loads) CLICK as it lets the AC onto the coach. By the same token if the shore power is not connected and you start the Generator you will hear the relay CLICK when the 120VAC from the Generator comes on line.

If BOTH the Generator is running AND shore power is connected the Generator gets the right of way. If you are on shore power and then START the Generator the transfer switch will flip over to the Generator once it comes up to speed. If you then shut the Generator down, it will flip back to shore power. It is important to know that if you are running a lot of high power users (A/C’s, heaters, microwave) the transfer switch flips over with a high current flowing through the contacts inside it. This can cause arcing, and may lead to premature failure. Minimize your AC load when connecting/disconnecting generator or shore power or when switching between the two.

Finally a word on the quality of shore power. The internet is full of discussion on protection against surges (through an EMS, Electrical Management System) or against low voltage through a booster (Most often the Hughes Autoformer) Newer models of the latter do BOTH. I have one, I recommend it. Voltages spikes are bad for your electronics, so is low voltage. The Autoformer boosts the voltage. Know that the amount of energy available (Wattage) stays the same so according to the formula W=VxA the Amperage available suffers…. Some newer high end coaches come with EMS installed.

Distribution and Protection.

Once the power is through the transfer switch it goes to the distribution and protection panel, or as we call it the AC breaker panel. (AC has breakers, DC has Fuses). A breaker is more correctly called an over current protection device. It is rated for a certain amperage. If the current running through the breaker exceeds its rated amperage it “pops”, and “breaks the circuit”. So the two inbound supply legs from the transfer switch go through a 50A breaker each. Far left of your panel. Inside the panel are Bus Bars where other breakers, properly rated for their circuit, clamp onto, to distribute, and protect the various circuits. Like an A/C, or the Microwave, or the Kitchen Outlets. The later will have another layer of protection added: The Ground Fault Interrupt. (GFI) An over current is the result of a short. (direct path to neutral/ground for ALL the current on the hot leg). THAT much current running unopposed will cause a LOT of heat….. to the point of starting a fire. So, too much current, the breaker pops and the short is broken. But it doesn’t take that much current to injure, or kill, a human being. If even a little electricity finds its way to ground THROUGH a person, in a wet environment for instance, it can do serious neurological damage. So the GFI measures the outgoing and returning current and if it finds electricity “missing”, presumably through a person to ground, it pops. You will find this protection in bathrooms, near kitchen sinks and on outside outlets. GFIs come in two forms: A circuit breaker with an extra button, or a normal breaker and then a GFI outlet. More regular outlets may be daisy chained down stream of a GFI outlet, all protected by the first one. On my 38A there is a GFI outlet in the mid bathroom, and down stream of it are the two cockpit outlets, and the outlet in the forward right outside compartment. Popped GFI’s are one of the top reasons stuff stops working in your motorhome. Know which outlet are GFI protected and where the GFI is…. when they don’t work reset the appropriate GFI. They do, however go bad, so sometimes they need replacing. They may also be doing their job and signaling a fault.

Now we need to talk about the Inverter/Charger, again. As I described earlier, it can make 120V AC when there is no other available to power limited items. There is no point in doing this when there is Shore or Generator power available. When there is it is sent to the I/C through it’s own breaker (so labeled). It uses part of the AC for the charger to power the battery charger, and the rest of it it “Passes Through”, back to the breaker panel where it goes to a few more breakers that power certain items, items that can also be powered by the Inverter when there is NO Generator or Shore power available.

There is a more in-depth description of the AC distribution/protection panel HERE.

So that’s the AC system. Three sources, distribution and protection of the circuits with users, either individually when large, or in groups when smaller. Depending on he amount of AC power available we can run more or less users. Awareness of how much your AC users need (their Wattage) is important when less than 50A (100A in fact) is available. When no Generator or Shore power is available the Inverter can use Battery power to power a select number of items. On the older coaches a few outlets, on the newer coaches a few more, the microwave and the fridge. When on the road, the alternator will charge the house batteries to keep the inverter going, within certain limits. Don’t run the convection oven for three hours cooking a roast….. or can we? Mine is about 1600W, less than the max 2000W the inverter could provide……theoretically at 12VDC (which is what the inverter gets for power when driving) that’s 130A…. The alternator is 150A….. But the coach needs (quite a bit) of 12V for the drive train too, so while on paper it could work in reality it doesn’t.