Installing an Inverter


Inverters are simple enough devices. You put 12 volts DC in and you get 120 volts AC out, right? All you have to do is run some battery wires to the inverter and plug in your "stuff" to the inverter's outputs. Well, that might be over simplifying things a bit. That may work for a small portable unit of small capacity but as the capacity increases, so does the complexity of the installation. Needless to say, the first thing you should is read the installation manual that came with your inverter. It will tell you the exact requirements needed for your situation.

DC Wire Sizes:

The larger the inverter, the more power it puts out. And, as the output amps go up, so do the input amps. Wire and battery cables have the ability to pass a limited amount of current. The smaller the wire diameter, or gauge, the smaller the current rating. Increasing the cable length also decreases the ability to handle that current. Therefore the key is to spec your inverter's DC wiring to the proper diameter and keep the cable lengths as short as possible. We know that a #12 automotive wire can handle 20 amps if the distance is not excessive. We also know that a #10 wire can handle 30 amps under the same conditions.

A 300 watt inverter will output 300 watts, which is equal to 2.5 amps (300 watts divided by 120 volts = 2.5 amps). However, it will consume 300 watts of battery power in order to make this output, which is equal to 25 amps (300 watts divided by 12 volts = 25 amps). Actually, because inverters are not 100% efficient there is some loss in this so you'll want to allow 10% extra when calculating this. The best way to adjust for this is to use 10 volts rather than 12 volts when making your calculations. Therefore 300 watts divided by 10 volts means that we will be passing 30 amps of battery current in order to get our 300 watts output.

Now this is a very small inverter. You might be able to run a TV off of it but it better not be too large of a TV and you can forget about VCRs and surround sound systems. For that you'll need a bigger inverter - more like 450 watts or better. Of course when you go to a larger inverter you'll also need larger wire sizes and if this is a small unit that is located in an overhead compartment next to the TV you'll have issues trying to fish a # 8 or # 6 wire up there (plus a second wire of equal size to provide an adequate ground or return path to the batteries). That's why 300 -450 watts is the largest that you will see in this application. If you'd like to run a 1,000 watt inverter someplace you'll have to run some serious battery cables to it and that requires a location down low in the coach. Once you get to this level the jump is generally made to a large 2,000 watt or greater inverter/charger.

There's a number of reasons why the larger size inverters are located in a basement compartment. The biggest is that they draw amps - lots of amps. This requires a fairly heavy battery cable. Also, as the distance from the battery increases, so does the cable diameter. By keeping short cables you can help keep the diameter down to a more tolerable wire gauge.

Environmental Concerns:

All of the electronic circuitry inside the inverter also throws off heat. The small 300 watt models aren't too bad but they can still add heat to an enclosed cabinet so you may need to add ventilation or keep the cabinet door open when running these. The larger basement models have even greater heat requirements. These models will have thermostatically controlled fans that will kick in every now and then to help cool the unit. This means that you will need to mount the inverter into a very large basement storage area or a ventilated compartment. These fans can generate a bit of noise so you don't want the inverter located in the main living area. The basement location will help to minimize the fan noise.

Also, the sensitive electronics in them don't like anything but dry temperate air. Placing them in a compartment that is exposed to the elements is not a good idea. The moisture will cause problems with the inverter. Locate it in a dry area. Also, do not place it in the same compartment as the batteries. Keep it close to the batteries but not in the same compartment. The hydrogen gassing during charging and the corrosive vapors won't do your inverter any good if they get sucked in through the inverter's cooling fans.

AC Output Routing:

Now that you have the DC side figured out, how are you going to connect your AC devices to the inverter? If you have a small inverter there is generally an electrical receptacle on the back. You can plug in your TV directly to it. If you have more things to run, a small power distribution strip may be required.

If you have a larger inverter it will undoubtedly be hard wired. You will have to wire in a non-metallic shielded wire (boat cable works best) to the devices that you want to use. This can be done in a number of ways. My 2004 Allegro Bus originally came with a Xantrex Freedom 458 inverter. This is a 2,000 watt modified sine wave inverter. It had a 30 amp transfer switch rating and two output circuit breakers on the inverter - a 15 amp and a 20 amp. The 20 amp ran to the outlet where my microwave/convection oven plugged into and the 15 amp fed a select number of outlets in the coach, most notably the front electronics equipment. The inverter was fed by a single #10 non-metallic shielded cable connected to a 30 amp breaker in the main breaker panel. The inverter's output left via a #12 cable to the microwave circuit and a #14 cable for the 15 amp outlet circuit. This system allowed inverter operation of the microwave as well as a few select outlets while driving.

I decided to install a new inverter and upgrade my system to power more outlets. I purchased an Xantrex RS2000 true sine wave inverter/charger to replace my Freedom 458. The RS2000 also had a 30 amp transfer switch rating but it had a single common unfused 30 amp output rather than two separate 15 and 20 amp breakered circuits. I was able to use the same #10 wire and 30 amp circuit from the main panel to power the inverter. I then ran a new #10 output wire from the inverter to a sub-panel. This sub-panel had 4 circuit breakers so I was able to relocated some more of the existing non-inverter circuits over to this sub-panel. I left the large draw circuits such as refrigerator, electric water heater, washer-dryer, engine block heater, and air conditioners on the non-inverter fed main panel because they would tax the batteries too much if run on the inverter.

Overcurrent Protection:

You will need to provide overcurrent protection for both the AC and DC sides of the inverter. Running a small inverter isn't so bad. You just figure out how many amps it will draw and what wire gauge is needed then install the proper size fuse. Your owner's manual should give you those requirements. If it's a larger inverter you'll probably need to mount a large 300 amp fuse near the batteries. These fuses will protect the DC wiring and the inverter from any overload damage.

If you have a larger inverter it will normally be fed AC power from the main breaker panel. It will pass this power through whenever possible and will create it's own inverted power from the batteries when the AC input power is not present. This AC input power will need to be carried on the proper wire gauge and circuit breaker size. Most large inverters have a 30 amp transfer switch so #10 wiring and a 30 amp breaker are required. If you have a small 300 watt inverter you may not have any input power so that's not an issue. If you do, it's probably just a normal cord that plugs into an existing outlet.

AC output power will depend upon how the inverter is designed. Some inverters are designed to be their own distribution system so they will have a pair of breakers and output connections right on the inverter. Other inverters pass through the entire 30 amps that is input to them without any circuit protection. These inverters are designed to feed a sub-panel. Because the AC input power from the main panel is already protected by a 30 amp breaker it won't need any output breakers until you get to the sub-panel. Once at the sub-panel, each output circuit will have it's own dedicated breaker that is properly sized for the wire gauge on that circuit.


Submitted by Mark Quasius - 2/25/06

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