Inverter Connection

I'm not an electrical contractor, so always consult your local electrical code to check if the situation sketched below applies to you!

Here is a bit on how to safely install an inverter in a residence.

First of all, not every inverter is meant to be hardwired into a house, and you should make sure that the inverter you wish to install is both UL (or CSA for canadians or whatever your local code is) approved AND suitable for wiring into a domestic application.

As a rule of thumb, if your inverter has a few sockets coming out of it and cost $250 or less new it is probably not suitable for domestic use, plain and simple.

That does not mean it can't be done with a cheap inverter, it means it's not meant to be done that way, that it won't be 'code' if you do that, and that if your insurance would ever catch you out rigging something like that and there is a fire that you'll probably lose it all. A good sign too is an inverter that needs it's ground and neutral to remain separated, if you would connect such an inverter to a service panel it would simply self destruct because of the bonding screw in the panel.

So, the purpose of this little writeup is to make sure you do it safe, or not at all. Either use an extension cord and a cheap inverter, just plug in the few consumers that you need, or go all out and do a hardwired system, which will cost more, but will be much more convenient.

Professional inverters come in several flavours, the most important distinctions are sinewave vs. modified sine wave. Modified sine wave is a bit of a misnomer, the waveform coming out of those inverters really is pretty ugly, and most appliances do not like it (though they may work). Some, most notably jet pumps and such do not work at all, and some may even break !

The second big difference between one inverter and the next is that some inverters are capable of being used as 'grid tie' machines, which means that they can backfeed excess power into the grid, which will save you money if you are connected to the grid. If you are 'off grid' then this is not a concern, but if you plan to be connected to the grid with your installation long term then now would be the time to make sure you have the right equipment.

Grid-tie, sine-wave inverters are not cheap, they run several thousand dollars.

Non-grid tie, modified sinewave inverters for domestic use still aren't cheap, but a lot cheaper than those that are sinewave and/or grid tie (modified sinewave inverters as a rule are not grid-tie).

To safely connect your inverter to the house wiring, make sure to follow local code convetions. Important things to pay attention to are grounding, disconnects, lightning protection and breakers.

Well walk through the system coming from the dc side, going through the inverter, to the service panel.

So, we start at the battery, the + pole of the battery (red wire) will go to a DC rated breaker, that should trip at slightly more than your inverter can consume. In my case (a 48 V system, using a trace sinewave inverter) that means that I'm using a 175 A DC rated breaker. That DC rating is very important, it makes sure that the breaker will actually break when needed, and not weld itself shut with the arc voltage from trying to break the DC current.

The - pole from the battery will go to the 'low' side of the optional shunt (if you use one make sure the shunt can handle full inverter current + some margin!), use a black wire, the high side of the shunt gets bonded to the frame of the disconnect box, and has another wire leading to the - side of the inverter. From the bonding point there should be a #6, bare wire to a properly installed 10' long ground rod.

The wires, that carry battery current have to be sized properly, look at a wiring table like this one to find the right size.

The 'load' side of the DC breaker goes to the '+' input on the inverter. The grounding stud on the inverter should be connected to the ground bonding point in the disconnected box.

Any other solar or wind that you want to connect should have their breakers feed off the load side of the battery disconnect (red wires), their - poles should be connected to the 'high' side of the shunt (black wires) and their grounds (bare wires) should meet at the bonding point.

The inverter is now hooked up and can be put through it's paces, but it's not feeding anything yet.

After testing it (check all the wiring once more first and check polarity before attempting to switch anything on !!) you are now ready to hook up the AC side of things.

The AC side of the inverter has to go through it's own disconnect switch if the inverter is not housed next to the house service panel (for instance when it is in a battery shed). From this disconnect - if installed - a cable goes to the service panel, where it is connected to live, neutral and ground. If you have a 240 V panel and only a 120V inverter you may want to bridge from the one 'live' to the other so that both sides of the panel are fed. YOu will not be able to use 240 V loads on a panel like that, but it will allow you to use the whole panel rather than only the left or the right half.

In a grid connected system you still have to wire up the 'incoming' connection, that which used to go to the service panel to the appropriate lugs on the inverter, and some utilities require you to install a lockout switch for their linemen to make sure that your inverter will not backfeed the wiring during maintenance. Do not rely on your inverters automatic shutdown, ask the utility or wiring inspector if they require a lockout to be present or if the inverters shutdown mechanism is good enough, and TEST THAT THE INVERTER REALLY ISLANDS WHEN REQUIRED TO DO SO, in other words, if the line voltage goes 'off' your inverter should take itself offline automatically.

We have seen at least one case of a grid connected inverter that happily kept on running with the line disconnected and that did not make the inspector very happy...

Finally, if you have a generator in your equation you'll have to connect that to the appropriate terminals on the inverter too, again, sometimes an additional disconnect is required. Even if it's not required it is good practice to do so.