120V off grid

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Old 02-16-17, 07:58 PM
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120V off grid

Is there any code violations in using the two legs of a 240v sub panel to act as a main and sub panel for a 120v off grid system within the same enclosure?

120v 30amp generator supplying leg 1 of panel via back fed 30amp breaker. All branch circuits that will only have power when generator is running being fed off this leg.

Leg 1 also feeding leg two with 30 amp breaker via an inverter/charger that is capable of passing 30 amps of AC power through it or can switch to inverter mode on loss of AC power supply from leg 1 and invert 8.3 amps of power supply to leg two.

Leg 2 would supply branch circuits that will have power while generator is running and when generator is off via inverter.
 
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Old 02-17-17, 02:51 AM
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Welcome to the forums! Basically you can't back feed any panel without proper precautions. You must have a lockout or transfer switching system in place to use a generator. Now, we don't know if you have power to your structure as it sits, or if it is a cabin in the woods without municipal power.
 
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Old 02-17-17, 10:25 AM
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As far as I know you cannot have two "input" feeds to a panel without a mechanical interlock system which guarantees that only one feed can be active at any given time AND you can deenergize the whole panel in an OFF position which disconnects both feeds.

I think what you may need is a three position (LINE - OFF - GEN) switch before the panel which allows you to fully switch between the incoming power sources. A prefab generator transfer panel may also be useful here if you want to move individual circuits between the different power sources.
 
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Old 02-17-17, 10:59 AM
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Thanks for the replies. I'll try clarify my prospective system as best I can.

1. Cabin will have no grid connection

2. Inverter that is between leg 1 and leg 2 has an internal switch that will only allow pass through of power to leg 2 or inverter on loss of supply power from leg 1.

This will basically be a panel/sub panel set up with a UPS system between the main and sub. I just figured because my system is only 120v single phase I could integrate the main and sub within one enclosure utilizing both legs of a 240v two phase panel.
 
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Old 02-17-17, 02:15 PM
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If you have a loss of power on leg 1, there will be nothing to send to leg 2. Nothing there to invert. The panel is single phase, not two phase. You won't be able to derive 240 volts from 120 volts.
 
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Old 02-17-17, 04:46 PM
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I think the OP means a battery powered inverter. So when leg 1 failed.... the inverter would come on and feed leg 2.

I read this last night and couldn't formulate a plan. I don't feel comfortable with your plan.
There is a problem with two possible sources (L1 and the inverter) in one box as Ben mentioned.

I'm giving you my opinion.
It sounds like since there is no grid there is no inspection but safety must always come first.
 
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Old 02-17-17, 04:58 PM
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After posting, I thought about the UPS kicking in. Unless there is some switch to separate the UPS from the generator, I don't think it will work, either. Safety is utmost important. It was mentioned earlier that a positive Generac type switch gear with Gen-off-line would be the safest way to do it, although it won't be automatic, and I believe that is what you are looking for.
 
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Old 02-17-17, 05:49 PM
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The switch inside the UPS is such that it would only allow either AC feed flow through, or the inverter to feed its output, never both at the same time. Is there some other method that would allow a UPS to automatically provide power upon power failure?

The inverter internal switch gear also bonds the neutral while in inverter mode so when it cuts ties with the failed AC input and provides inverted power, the neutral of the fed sub panel remains bonded. The only thing I would be doing out of the ordinary is having a panel and sub panel within the same enclosure. Maybe I'll just use a second panel for the sub panel to eliminate the confusion even though over half of each panel will remain unused with my 120v distribution system.

I wish I could post a picture as I'm sure it would alleviate a lot of the confusion of what my intention would be.
 
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Old 02-17-17, 06:07 PM
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Old 02-17-17, 10:40 PM
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L1 is frist leg of 240v panel. L2 is second leg of 240v panel. Inverter is rated to pass through30amps of 120v power but has an internal switch that will cut ties with AC power supply and invert 8.3 amps of power from a deep cycle battery bank.

I'll attach more picks in next post if the internal switch gear of the inverter.
 
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Old 02-17-17, 10:44 PM
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Internal switchgear of inverter. Shows how it bonds neutral when its inverter mode so the sub panel neutral remains bonded when it cuts ties with AC supply source.
 
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Old 02-18-17, 03:36 AM
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We aren't sure what brand/style panel you are using, but the usual configuration of the hot buses are as shown. They alternate from side to side with the legs. Have you determined yours to be different?

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Old 02-18-17, 05:01 AM
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Actually panels are like this:

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Is the issue that the UPS cannot carry the entire load of the cabin and you will have some loads drop off when the generator drops?
 
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Old 02-18-17, 05:47 AM
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I drew the panel like that for simplicity of the drawing. I wanted loads dropping off with generator so the kids wouldn't decide to run the toaster without it running and draw down the batteries or tripping out the inverter requiring me to reset it possibly in the dark with the lights going out. My only question is if there was a rule against using the two legs of a panel to act as a main and sub panel for a strictly 120v distribution system.
 
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Old 02-18-17, 06:40 AM
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With the difficulty of trying to explain what I want to do, I've decided to just run the entire power supply from the generator through the inverter on the way to the cabin just for simplicity for the permitting, inspector, or for anyone else potentially dealing with the system in the future.

Generator -> 30amp fused disconnect to act as main panel with bonded neutral -> inverter, which will pass through the full 30 amps of generator power or invert 8.3amps for all down stream branch circuits to share -> sub panel with all branch circuits and breakers.

Basically the same as my original intention but with the main and sub panel separated and no branch circuits coming off the main panel before it feeds the sub.

I'm sure after a few times of tripping out the inverter everyone will learn to start the generator before using higher draw loads.
 
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Old 02-18-17, 07:12 AM
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That might be a question for the AHJ. In my opinion I think your first plan it is a bad idea.

You will basically have two sources on one panel. Even though the UPS output is derived from the generator when it is running, the power/voltage is still going through the electronics of the UPS and there could be some hazardous difference between the two sources.

You could install two smaller panels, one off the generator, one off the inverter. Still wire them using your first basic idea, they would just be separated. If your worried about taking up too much space I see no reason they couldn't be stacked in the same stud space, nippled together, and have the bottom panel circuits pass through the top panel. 6/12 or 12/24 circuit panels are only $25 - $40.

Edit - Your 2nd plan would likely be the easiest. Although 2 panels would give you the most flexibility.
 
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Old 02-18-17, 08:39 AM
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Thank you, your explanation was very helpful and you seem to clearly understand what I was attempting to do. I will separate the panels and decide if I'm still want to break some circuits off the UPS system or not. Thanks again.
 
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Old 02-19-17, 12:50 PM
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New plan. Thoughts?

1. Gen hooked to 30amp fused disconnect where I will bond the neutral. Ground to earth will be provided from this location as well. Second fused hot switch will be empty and just holding a spare fuse for this 120v system.

2. Fused disconnect feeding inverter which is rated to pass through 30 amps. When inverter loses the AC supply power it flips to inverter mode which disconnects from AC hot and neutral and re bonds it's neutral to ground internally so the sub panel remains bonded.

3. Sub panel will be back fed via 30amp breaker. Will use 10g wire to jumper the two legs together if I need more than the three remaining slots on the one side for branch breakers.

4. Battery bank will have a switch and a fuse on the positive lead to inverter. Inverter chassis will be grounded to battery negative post and is also common to the AC ground point on the inverter so will be grounded to earth along with the AC.
 
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Old 02-20-17, 07:27 AM
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I think the new plan looks good. If you're having this inspected be prepared for the inspector to possibly want an additional disconnect or two in the mix depending on where everything in mounted. For example some areas require a disco on the DC batt/solar connection to the inverter, and the inspector may want all of the "service disconnects" grouped or at least within line-of-sight. It's a little tough to say exactly how they'll choose to apply codes given that you're off grid and have an unconventional installation, so I'm just anticipating what you might be asked.

If you wanted the ability to separate your "high availability" circuits and your "generator-only" circuits, I'd replace that fused disconnect with a small panel -- say 8 spaces, even if you only use the one breaker for the generator and one breaker for the inverter now, you can always move circuits there that should only have generator power.
 
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