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Old 12-30-2012, 11:15 PM   #1
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120 Volt Sub Panel - For Solar Inverter Backup and Generator Backup


I am trying to find a safe way that meets code to hook up specific circuits in my home to a backup power source for Solar and Generator Backup. I have about 3 days worth of battery bank in which I would hook up a 2500 Watt Inverter. The batteries are kept trickle charged by a small 40 watt panel just for emergency use. After that was exhausted I have a small 3000 watt Generator that I would hook up to power the home as well as charge the battery bank. Both of these only have 120 Volt power output however.

Here is my current plan



So this plan allows me to keep everything running as a normal 240V sub-panel when connected to the grid. When I switch the breakers over and disconnect the main feed and turn the Generator circuit on, it will feed BUS B directly for the generator, and BUS A will be fed from the other pole of that same breaker via another 15 amp single pole breaker on BUS B. It is using the NEC Approved Interlock from Square D. As per a short conversation with the electrical inspector in my area. I have found it is legal to make a 120 Volt only panel as long as you label it 120 volt only. At first I was just going to tie the other pole into Bus B directly, but found out that if the wire came loose from the Generator breaker and shorted there is no protection except for the 50 amp breaker feeding the panel, so I would have to use 6 Gauge wire and you can't find a 15 amp breaker that holds 6 AWG, so that is the purpose of using a second breaker.

Does this seem like a safe way to do this? Does it look legal. Any comments or suggestions would be appreciated. I want to see what you think before I spend the 40 dollars on an application fee to have the work approved and inspected by the local inspectors office.

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Old 12-31-2012, 01:12 PM   #2
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120 Volt Sub Panel - For Solar Inverter Backup and Generator Backup


I don't think it is going to fly. You are not making a 120V panel that you can label permanently.
You are trying to make a 240V panel into a temporary 120V panel, when you want to.

What make and model inverter are you planning on ? A lot of the less expensive inverters are not listed for connection to residential wiring.

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Old 01-01-2013, 01:27 AM   #3
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120 Volt Sub Panel - For Solar Inverter Backup and Generator Backup


I have a Samlex 24V to 120V inverter that is designed and rated to connect to a load distribution center, it is a True Sine with an Isolated (From Battery Negative) neutral. It was around $850, so definitely not a cheap one. Would a better solution be to just bring 120 Volts into the Panel from the Main and make it 120. My thought was I would like to balance the load in my panel, but I can move the breakers that remain in my other panel around so that it all remains balanced.

I am trying to do this on the Up and Up and correctly. The last thing I want is a fire, for my families safety and second the insurance company may refuse to pay. So I want it all signed off on by the inspector.

Last edited by tmulkey; 01-01-2013 at 01:31 AM.
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Old 01-01-2013, 06:29 AM   #4
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120 Volt Sub Panel - For Solar Inverter Backup and Generator Backup


Feed the subpanel with 240 volts but put all of the breakers on one of the busses. In the main panel put more of the breakers on the bus whose leg is not used by the subpanel so when utility power is in use the load of the main panel breakers and subpanel breakers combined is not badly unbalanced between the two legs given the positions of the breakers in the subpanel.

This should meet the approval of any inspector.

For three possible sources of power to the subpanel -- utility, generator, batteries -- you need two regular transfer switches (or one three position transfer switch) between main panel and subpanel. For two transfer switches you might have the switch nearest the subpanel select utility/main panel power versus alternative power and the other switch select between generator power and battery/inverter power.
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Old 01-01-2013, 09:25 AM   #5
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120 Volt Sub Panel - For Solar Inverter Backup and Generator Backup


I will use that scenario with balancing the breakers seems to eliminate a lot fo problems. Do I really need an extra transfer switch? My plan was to have a Male Twist lock receptacle for the generator and then use it to plug into the inverter power as well. Both the Generator and Inverter have a NEMA 5-15R on them and I was planning to make a cord that had a 5-15P on one end and a female twist lock on the other end.

This way I could just plug in either the Generator or Inverter outside. The battery bank and Inverter are located outside as well in a small shed. They are only kept trickle charged by the solar panel for emergency power, not regular daily use.
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Old 01-01-2013, 09:28 AM   #6
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120 Volt Sub Panel - For Solar Inverter Backup and Generator Backup


You will need a mechanical way to prevent back feed into the utility. If you had stuff you wanted to back up in the sub panel all on one side you could maybe have an interlock that switched from the main panel feeding it to your generator and inverter feeding it.
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Old 01-01-2013, 09:51 AM   #7
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120 Volt Sub Panel - For Solar Inverter Backup and Generator Backup


I would jumper the two busses in the subpanel together and feed it 120 volts from either the main or your backup power source(s). Balance the load like AllanJ said, slap a 120 volt label on it and you will be good to go.
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Old 01-01-2013, 10:57 AM   #8
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120 Volt Sub Panel - For Solar Inverter Backup and Generator Backup


Yes there will be a mechanical Interlock on it, It is made by Square D and NEC approved.

I like the idea of a 120V pannel. I would hate for someone to put 240 breaker in there and then under Generator it only has one leg powered. Not sure it that would be good for most appliances, plus I would worry about an appliance then partially feeding power through, say like a heating element or something, it and making the other buss partially hot.

The mecanical interlock must use a 2 pole breaker as the primary, but I can just feed one pole of it with with 120 from my main panel, and then do a bus tie in the panel, so it is all 120.
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Old 01-01-2013, 11:01 AM   #9
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120 Volt Sub Panel - For Solar Inverter Backup and Generator Backup


I had a local builder the other day, say that i may want to draw up 3 different ways to do it, and submit them all with the application and let the permit office choose which way the would like me to do it. He seemed to think also since there is 240 coming in, I could not label it 120 V. The sticker would leave someone to believe there is only 120V in it all the time and they wouldn't know they were working with 240 under normal operation.

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