Electrical 101 -Motorhome
Contents
1. Formulas
2. Generator
3. Shore power (hook up)
4. Battery
5. Alternator
6. Inverter/charger
7. Transfer switch
8. Power routing & panel
9. Charging Batteries
10. Solar
11. By the numbers
1. Formulas
Figuring power is not very hard. Here are a some examples:
volts x amps = watts
watts / volts = amps
120 volts AC x 10 amps = 1200 watts
1200 watts / 120 volts = 10 Amps
12 volts DC x 10 Amps = 120 watts
12 volts DC x 100 Amps = 1200 watts
30 amps x 120 volts AC = 3600 watts
50 amps x 240 volts AC =12000 watts
5000 watt (generator) / 120 volts AC = 41.66 Amps
8000 watt (generator) / 120 volts AC = 66.66 Amps
A 1000 watt hair dryer that operates on 120 volts AC will draw 8.33 amps and if operating through an Inverter will draw 83.33 amps from a 12 volt battery (slightly more as an inverter is about 95% efficient)
Notes: We call a battery 12 volts to designate a size vs 6 volt but a 12 battery operates between 12 volts in the low side to about 14.2 volts on the high side. Divide by 2 for a 6 volt battery. Typically under load it runs between 12.2 volts and 12.6 volts. A battery being charged will run from 12.8 to 14.2 volts depending on state of charge. The alternator will generally produce 13.8 volts all the time while running. AC voltage also has an acceptable range, generally from 110 - 125 volts
2. Generator
Built in Generators run from about 4,000 - 12,000 watts. They generally supply power on 2 legs, each at 120 volts AC. Measuring voltage across both legs will measure 240 Volts AC. Some smaller units may only produce power on a single leg or pole. A two leg/pole generator will have two breakers or one switch with 2 poles. Balancing the load is important. Here is why. Lets take a typical 8000 watt (8KW) generator. At 8KW it can produce 66.66 Amps at 120 Volts but it is split into two legs/poles each at 33.33 amps. Such a unit will likely have a pair of 35 amp breakers or a single switch that has 2 poles. The two legs are isolated from each other. Power is split up to balance the load so it is hard to exceed the ratings on either leg.
Generally a RV with one air conditioner has a 4,500 watt generator, two air conditioners, a 8,000 watt generator and three air conditioners a 12,000 watt generator.
3. Shore Power - Hooked Up
Typically RV parks have either 30 amp or 50 amp service (will also have a 30 amp socket) and sometimes there is a standard 120 volt plug too. The sockets for each type of service are different. A 50 amp service means there are 2 - 50 amp legs, each at 120 VAC. The total wattage for a 30 amp service is 3,600 watts - 50 amp service is 12,000 watts. There are adapters that will allow a 50 amp pug to plug into a 30 amp service but be warned, you only get 30 amps from that service. Your RV was designed to use more power, that is why you have a 50 amp plug. You must mange power consumption so you do not exceed the breaker limits on the service box. If on a 30 amps service typically only run one air conditioner. Other high power items will include an electric water heater, microwave, hair dryer, electric room heater and washer dryer. If you just plugged in, the battery charger may draw as much as 1,400 watts. Many people will also buy a surge protector. The better ones will actually disconnect when the voltage is high or low. They will also help protect against spikes. Some will also make sure the wiring configuration is correct and give you a indicator that everything is good before you plug in.
Tip - turn off high powered items when you plug or unplug. This will save arcing on the plug contacts and transfer switch and use the breaker switch on the service box to switch on and off after your connected.
Some people want to plug in at the storage location but only have a 120 volt source many feet away. This helps to keep the RV charged. Be sure to use a heavy duty cord capable of supplying power at a distance. Be sure all your high power items are off and this will work just fine.
4. Battery
The batteries give us mobility but have limitations that must be observed. Most RV have the typical flooded batteries. They are less expensive and provide a good amount of total power but need periodic attention. Sealed AGM batteries are maintenance free and that is the attraction for many people but do cost more and they do not provide as much power for the same physical size. Lithium is another alternative, they are light weight, charge fast and provide more power but will cost a lot. They also require a special charger.
There are two battery set ups on a RV, one to start and run the engine (chassis) and the other to run the "house". The engine battery is usually fairly simple, one battery. The house battery set is a different story. Larger RV's may have as many as 8 batteries. Batteries are arranged in series and/or parallel or in a combination. Typically a 4 battery set has 2 - 6 volt batteries in series to make 12 volts and then 2 pairs will be connected together. I highly recommend using 6 volt batteries, they will give you more power. Here is how to figure the ratings on the batteries. You will need to find the manufactures numbers to figure your set. Here is a typical example. Golf car battery GC2 are ratted at 230 amp hours (AH) for a 6 volt battery. Placing 2 - 6 volt batteries in series get you 12 volts at 230 AH. If you add second set the the AH doubles to 460AH, add a third set then you have 690 AH.
So what does 460AH mean? By the book is means the battery set will deliver power at a rate of 20 amps per hour for 23 hours (460/20=23) and it will be dead at the end. This is also equivalent to 240 watts running for 23 hours.. Now to be participle we do not use the battery that way. We may a have couple lights on or are using a lot of power for a microwave. Also I call it the 50% rule. Basically the usable amount of battery is about 50% because you never want to run a battery dead, it performs worse at low voltage, As the battery ages there is less capacity and your never sure how much power is used or left in the battery. Bigger is better. In general the RV is designed to operate for 24-48 hours without the need to recharge batteries.
BATTERIES MUST BE ACTIVELY MANAGED. Please see my post
http://www.irv2.com/forums/f258/why-...ng-458688.html for more information.
The RV power is always on. I say this because a number of items are directly connected to the batteries. They may include the inverter, propane and CO2 detectors, dash radio, wireless key entry or alarm systems, TV amplifiers, lighter plugs and USB chargers. Some things may be connected but draw no power until activated like door steps, baggage compartment lights. Even though you have disconnect switches you need to see how they work and check for these parasitic loads. In a few weeks a stored RV can kill a battery. Either physically disconnect the battery or consider a solar set up if your not plugged in. Buy yourself a clamp tester to measure current at the battery, you may be surprised at the results.
5. Alternator
Nothing special here. Your RV probably has a larger alternator designed not only to recharge the engine battery but also the house battery. But beware it still puts out only 13.8 volts. A good charger will put out more voltage and and allow the battery to charge faster. Running the engine just to charge house batteries is not advised because because at idle the alternator may not provide its rated current. Charging may take many many hours.
6. Inverter / Charger
Most RV's have a combination inverter and charger. Usually they are in 2,000 - 3000 watt range and can charge a battery at 100 amps. They are designed to provide a limited power supply at 120 VAC. Typical uses are for microwave, residential refrigerators, hair dryer, TVs, computers and other small misc items. They will not power your air conditioner, washer dryer, water heater or LP furnace. Inverters come in two flavors; true sine wave (just like home) or a modified sine wave. Both generally work just fine but if given a choice always buy a true sine wave inverter..
An inverter takes a 12 DC battery and converts it to 120VAC. They put a huge load on a battery. Example; a 1200 watt microwave oven will draw 10 amps at 120 vac but the inverter will draw 100 amps from the battery. Think 10 times, a 2 amp gizmo at 120 volts will draw 20 amps at 12 volts. In just couple of hours you could use up a 230 AH battery. Remember you have other things that need power like lights, etc I like to plan to use things like the microwave when I am charging my batteries with the generator. The inverter should be used sparingly.
The charger is an important part of your electrical systems because most things run from 12 volts. The charger will provide a charge for your batteries but will also act a as a power supply for everything that runs on battery. It is highly used when plugged in. It will also charge when your running the generator. The output is limited. For example a 2000 watt inverter/charger will only supply 100 amps DC or about 1,300 watts. The 2000 watt ratting is the total power it may produce for inverting.
7. Transfer switch
You may have 2 switches. The main switch is more obvious. It may hum when your plugged into shore power and may be located near the cord end on the coach. It switches power between the generator and shore power and usually the default is connecting the generator. When energized by shore power it switches. The second switch is built into the inverter. When 120 vac is present from the first transfer switch (via shore power or generator) it feeds through the inverter. But when the power goes away the inverter is activated so all the items on that circuit are still supplied with AC power.
8. Power routing & panel
What makes the RV unique is the 4 sources of power and how they interact. The RV also has a special need to segregate certain loads under certain conditions. Lets start at the load side. This description covers most mid to large RV's, the small units may not have inverters. Some appliance are dual powered but all rely on 12 volts (except residential refrigerators). Typically they are Norcold or Dometic refrigerators, water heaters and some furnaces. These items switch from 120 vac to LP when 120 vac is not present. We assume when 120 vac is not present your not running the generator or hooked up. We also recognize the air conditioner will not be able to operate on battery so you must be running the generator or be hooked up. Last some appliance draw too much power to operate on a inverter.
To mange there various loads they are separated into to groups. The breaker box is split internally to provide a "main" and "sub panel". You may have second breaker box that will be a sub panel. The main will power the air conditioners, water heater, washer dryer, fridge (Norcold or Dometic) and inverter/charger. Notice they are high current and/or dual source appliances. The sub side is technically a sub panel that operates all the wall sockets, microwave and a residential fridge if you have one.The source of the sub panel is the output line of the inverter. When the inverter gets AC power on it's input then that power flows through and it is passive. When no AC input power is present then the inverter activates and operates off of the battery. Notice the sub side are lower power items, wall sockets and includes a residential fridge if you have one. This way you get AC power when the generator is off or your not plugged it. The Main is also split like a house panel is because you have a 2 leg/pole source. The loads like AC units (120 volt) will be one each leg to balance the load. Nothing operates on 220 like in a house.
The house battery powers all the 12 volt systems to include, lights, controllers for the water heater, fridge (Norcold and Dometic), leveling systems, TV antenna amplifiers, wireless key systems, alarms, retractable steps, fans, water pump, furnace, aqua hot, awnings and slide systems. It also powers the inverter if you have one.
9. Charging Batteries
There are two set of batteries and two ways to charge them. The battery systems are also tied together in two ways.When the engine is running and the alternator is generating power the chassis battery gets charged. In larger RVs a solenoid will active sensing a running engine and connect the two battery set together. In smaller RVs and isolation device will allow the charge of both batteries. This does a good job of charging the house battery while the RV is traveling. This same solenoid may also be part of a boost or bypass circuit that may help you start the engine when the engine battery is dead. Smaller RV's that use a battery isolator will have a separate solenoid for the boost circuit.
When either plugged in or when the generator is running the inverter/charger charges the house battery. These are usually sophisticated 3 stage chargers made to operate all the time when your plugged in. A separate small charger provides a trickle charge to the chassis battery. It works when the house battery is at a higher voltage (charged), then allows some current to flow to the chassis battery. When the batteries fail to charge each other the usual cause is the solenoid or the small charger.
10. Solar
Solar is a great option but it has its limits. Panels are flat on the roof so they never see the sun at a perpendicular direction and trees at RV parks may shade you. Basically solar is a battery charger and connects to the house battery set. It is great way to keep your batteries charged when in storage assuming your parked outside and not covered. They will augment your batteries but rarely are the panels and battery set is big enough to go completely off grid when you consider cloudy days. It is very hard to run an air conditioner from batteries.
11. By the numbers
Every device or appliance will be tagged with the voltage and current ratings. This one is hard to find so I will provide it. A typical 15,000 BTU air conditioner may draw about 13-15 amps at 120 volts AC. Know the start current can be as high as 55 amps. This is called the LRA. This is significant because when it starts you hit your source with a high surge current. Try not to operate on the edge of your rated breakers or sources. Most breakers are rated for continuous duty at 80%, above they get hot. One of the best things you can do to reduce your load is switch to LEDs for all lighting.
Note: This article was designed to cover most mortorhome configurations. Trailers have similar systems but do not have the engine charging systems and many do not have built in generators or inverters.
Linear Mode
