By Jerry Whiteaker 96 U270 36′ 12/19/09
Many of the recent posts about diodes, battery isolators, etc are mainly due to a lack of knowledge about electricity and electrical circuits. Although humans have known about electricity for some time, it is still mysterious, as we can’t see it except in lightning or an electrical arc when it is traveling through air or a vacuum. Most of us know that under the right conditions it can kill us or at least give a nasty shock. Many of us did not get training in electricity as we pursued other careers in fields unrelated to electricity. However, we now live in a wonderful age where a computer connected to the Internet is really the equivalent of a huge library easily accessible at our fingertips. Most anything we don’t know we can look up rapidly. Whether it is accurate or not is another thing, so it pays to check more than one website about the same topic.
The typical FT MH is especially confusing as it has (IMHO) 2 ac systems with 3 sources of power and 2 dc systems with 3 sources of power. Many will say that there is only one ac system which is split into 2 sections and that is also true. The sources of ac power are shore, generator, and inverter. The generator generally speaking can power everything, but you can overload it if you turn enough stuff on, at which point hopefully some circuit breakers will pop before it burns up. Generators are usually rated in watts or thousands of watts. If yours is 8000 watts or 8K that is all it will produce. Shore power may or may not be able to power everything depending on which type of receptacle you are plugged into.
15 amps may power the battery charger and that is about it. 30 amps and you should be able to power the battery charger and 1 air conditioner maybe 2; it gets a little iffy with 2 air conditioners especially if the voltage is 115 or below. 50 amps are best and will power everything in the coach and you may have a hard time turning on enough stuff to overpower it.
Volts x Amps = Watts. 120 x 15 = 1800 watts, not much. 120 x 30 = 3600 watts, a little better. The 50 amp receptacle has two 120 vac circuits, each delivering 6000 watts, giving us 12,000 watts available, which is almost 3 times the power available than a 30 amp shore power. That’s why it’s so critical to managing power when on 30 amp as it’s not just a little less it’s 1/3 the power.
You can load up each leg with 50 amps each of 120-volt load. But remember that it is an unbalanced load if one leg is carrying more than the other. When the load is “equal” or close to it, the neutral leg carries no load. If one leg has 49 amps and the other 1 the neutral is carrying 50 amps. That is why our neutral leg is the same size as the hot legs. Whereas in a 240-volt circuit the neutral can be smaller as it never carries any load as the hot legs draw equally
If we put a voltmeter between the hot wires of these 2 circuits we will get a reading of 240 volts. So yes we have 240 volts going into the MH, but it only gets as far as the main ac circuit breaker panel. Inside the panel, half of the circuit breakers are connected to one circuit and half are connected to the other circuit. The breakers alternate circuits from top to bottom rather than left or right. There really isn’t anything installed that uses 240 vacs although there could be, it would only function when plugged into 50 amps.
Well, when I am plugged into a 30 amp receptacle which is one 120 v circuit everything is powered, how is that possible? In order to plug into the 30 amp receptacle, we have to use a 30 amp adapter on the 50 amp cable. Inside the adapter, the one 120v circuit hot wire is connected to both hot wires of the 50 amp cable and everything works.
We do not want both the generator and shore power to power the ac system at the same time as this would result in a fried generator, there is an automatic transfer relay sometimes called transfer switch which feeds power to the main ac CB panel from either the generator or the shore power cable. The CB’s in the main panel power the big items; air conditioners, refrigerator, etc and an ac subpanel. The subpanel powers the rest of the ac circuits. The circuits on the subpanel can also be powered by the inverter, which gets its power from the house battery.
The inverter and charger is one device that can do one or the other but not both at the same time. It either charges the house battery’s or discharges them to make 120 vacs. If 120 vac is available from either shore power or generator it will charge the house battery. If 120 vac is not available the inverter will only work if it is turned on. You can leave it on all the time as it reverts to battery charge with another source of 120 vacs. If you inadvertently lose the outside 120v ac you could also discharge your house battery, so I think it is a bad idea to leave it on.
Now for the 2 dc systems. One is the house system, which powers all of the lights inside the MH and the porch light outside. Also provides power for the refrigerator control circuit and interior light, but not the heating element (ac) which makes it cool, also the LP gas water heater control, exhaust fans, water pump, aux air compressor, leveling system, generator starter and controls, and other things of a similar nature.
The other dc system is the chassis system; powers the outside lights, instrument panel, transmission, and engine control circuits and devices, engine starter, items associated with driving the motor home.
Both dc systems are powered by battery banks or when the engine is running by the alternator. If the generator is running or shore power is available the inverter/charger via the charge function will power the house dc system and the chassis system if the boost switch is on. The dc systems are referred to as 12 volts as that is the battery voltage; most of the time unless the battery is the only source of power, the voltage is higher 13 or 14 volts. If the voltage exceeds 14.5 something needs to be corrected before the batteries are damaged. That is how the coach comes from FT. If one adds solar panels or a wind generator, then the dc systems have another source of power. Some folks install electric refrigerators that use either 120 vacs or 12 VDC as a source of power for cooling.
A standard diode is a 2 terminal electrical device that allows current to flow in only one direction. They are built in many sizes and are rated for the amps and volts they are expected to handle. Diodes are built in many different ways so they can perform other functions such as a light-emitting diode, light sensing, voltage control, etc. Standard diodes can be combined together to operate as rectifiers for changing ac to dc or as battery isolators.
Our battery isolator has 2 large diodes inside an aluminum housing with fins. Current flowing through these diodes produces heat and the fins help get rid of it. Both diodes are connected to the center post leading to the alternator so that current can only flow to the batteries and each related dc system. The other end of the diode is connected to a battery terminal on the isolator. When the alternator is not running or producing power, power cannot flow from one battery bank to the other. This prevents one battery bank from discharging the other. However if the boost switch is on when it shouldn’t be, it is possible to deplete both.
A solenoid is really just a coil of wire used to generate a magnetic field. What we have been posting about and calling solenoids, myself included, are really relays, an electrically controlled switch that requires another switch to operate it. Why would anyone want to do that, why not just use one switch? It usually has to do with the amount of current or amps flowing through the switch. Low voltage circuits as in 12v require large electric cables to handle large currents. The large cable is expensive and long runs have an undesirable voltage drop. The coil of wire in a relay (solenoid) does not require much current to operate the switch inside. It is much cheaper and better electrically to have a remote small switch and small wiring to operate the big switch handling the large current near the motor or whatever is using it.
This is mainly for the newbies or anyone having difficulty with the electrical systems. I hope it helps. I’m not an electrical engineer, just what I have picked up along the way. Still confused, Google the terms: diode, bridge rectifier, solenoid, etc. Jerry Whiteaker 96 U270 36′ 12/19/09
I want to add something to the original article that is not entirely correct. I said that the Main Circuit Breaker panel feeds the subpanel. This is only partially correct. While they are located very close together there is no direct electrical connection. The main panel has a circuit breaker that sends 120vac to the inverter/charger which sends it back to the subpanel. With 120 vacs available the inverter/charger is working as a charger only and it converts some of the currents into the proper dc voltage to charge the house batteries through separate wiring to the batteries. As long as it is doing this there is no danger of running down the house battery from having too many interior lights and other equipment on. If 120v is not available at the inverter/charger, the inverter can be turned on if desired and the inverter/charger will draw 12 VDC currents from the house batteries, invert it to 120 vacs and send it to the subpanel only. The subpanel sends 120 vacs to the microwave and several 120vac outlets forward of the bedroom. The TV’s are plugged into 2 of these outlets unless it is a TV designed to work on 12 VDC. It is very possible to deplete the house battery with this condition, especially with the microwave, hairdryer or an electric heater in use. Best to start the generator if you are going to do any of the above for very long.
There is one other device that sometimes causes confusion and problems. It is the GFCI (Ground Fault Circuit Interrupter). Its sole purpose is to prevent electrocution if you should accidentally contact a 120 v circuit. It has the ability to detect this condition and kill the power before you receive a lethal dose. You may get a shock though. GFCI’s are normally installed in kitchens, bathrooms, and other wet locations. They look a lot like a normal 120-volt outlet, except that there are 2 small buttons; test and reset. It is a normal procedure to have several outlets on one circuit. One GFCI installed at the first outlet from the Circuit Breaker panel can protect all the other outlets downstream. On my 96 U270, the GFCI is located in the bathroom. If it trips, all outlets downstream lose power. GFCI’s sometimes seem to trip for no apparent reason. If you don’t know it there, the first reaction is to reset the circuit breaker which doesn’t help and leads one to believe there is a greater problem maybe even to the point of calling an electrician. Electricians make a lot of money resetting GFCIs. Sometimes GFCI’s fail and have to be replaced. In our MH’s make sure you replace it with one rated at 20 amps, not the cheaper 15 amp version. That circuit is rated at 20 amps and all the current being used downstream is flowing through it.
One other thing of a general nature. Circuit breakers are electrical devices that sense too much current flowing and a trip like a switch to off. Their primary purpose is to protect the wiring connected to it and only secondary protect the device that is using the current. Wiring of a size rated to carry 20 amps should be connected to a 20 amp CB and so forth. The excess current flowing in a wire as from a short to ground can cause the wire to heat up to the point of melting the insulation and starting a fire. A fuse is also used for the same purpose but has to be replaced. The trouble starts when after replacing the fuse several times, someone decides to use a larger fuse.
All of the above is primarily for my 96 U270. Most other Foretravels are probably similar, but there could be differences. Jerry Whiteaker 96 U270 36′