Backfeeding breakers on a generator panel

sub3marathonman · January 2009

I'm trying to figure out if the setup I'm thinking about would be NEC code compliant.

I understand that backfeeding the main panel is limited to 20% of the panel rating, so that a 200 amp service can have a maximum 40 amp backfeed breaker.

However, what I can't find is information about feeding into a generator panel that is on a transfer switch. In my opinion, if you are "backfeeding" into that panel only when the power isn't on, wouldn't it be logical that you could backfeed any amount up to the maximum generator panel rating? And, the only way that power would even get to the generator panel would be by switching the manual transfer switch away from grid power over to backup power.

I just can't find any information or documentation on this situation though, so I was hoping somebody here could help.

BB. · January 2009

Re: Backfeeding breakers on a generator panel

I am having a bit of a difficult time understanding your connections...

My suggestion, is to draw a simple 1-line block diagram showing how your circuit is wired and where the power sources/consumers are.

Basically, from my understanding, you need to trace back all power sources (AC Line, Generator, Grid Tied, etc.) sources of power and for a commercial installation, none of those places should total up more than the rating of the breaker panel/bus bars. For a residential system, none of those points should add up to more than 120% of the box/bus bar rating.

And, if your system is a Grid Tied Inverter, I would be very careful that it not be connected at the same time as when the generator set is up and running (unless you know what you are doing and willing to take the risks of possibly feeding energy back into your genset--which most probably will not like).

For a standard transfer switch system (as I understand them--not a professional here)--A GT inverter should be connected to the mains side (along with the "AC Mains"), the genset to the "Gen" side, and the protected load to the Transfer Switch output.

If you have a sub panel for the generator / transfer switch connection (or the transfer switch includes and internal sub panel)... For example it is a 50 amp panel, with a 30 amp AC Mains Feed and, because it is handy, you connect your 30 amp GT inverter, with 30 amp breaker, feed here, and also connect to a 30 amp transfer switch (with 30 amp branch breaker)... Note, if you transfer switch does not have a 30 amp breaker, then you have a 30a+30a=60a feed--would need appropriate wire/bus bars/breaker added to protect transfer switch and its feed wiring.

The input to the transfer switch is unidirectional (Load only), but the 30 amps AC mains and 30 amp GT inverter can both supply energy to a common bus point. Even though that common bus point is protected by a 30 amp breaker to the transfer switch--it will still be a 60 amp supply to the bus bar. Commercial is 100% of 50 amps--too high. 120%*50a=60amps, within rating.

The above is my unofficial understanding of your application, I do not have an NEC code book, and I do not have a large amount of experience in this area--just my 2 cents on how to break down the problem.

As always, contact a licensed professional electrician and/or building inspector in your area for "proper" interpretation and review of your system to ensure safety.

-Bill

sub3marathonman · January 2009

Re: Backfeeding breakers on a generator panel

The XW inverter is set up to be grid connected when the grid is up. For that situation it is connected to the 200 amp main panel with a maximum 40 amp backfed breaker, as NEC requires (120%).

However, when the grid is down the XW inverter will still be able to supply power. It will supply that power not on the line connected to the main panel, but a separate line to a subpanel (that I call a generator subpanel). That subpanel is usually fed power from the main panel, at 100 amps maximum, when the grid is on, but there is a manual transfer switch to disconnect from the main panel and receive power from the XW inverter. It is impossible for the XW inverter to supply power to the subpanel while the subpanel simultaneously gets power from the main panel, because of the manual transfer switch.

So therefore, in my opinion, the subpanel, when disconnected from the grid, should be able to be supplied with up to 100 amps by any power source or combination of power sources. So the NEC 120% rule, which would otherwise limit the connection of the XW inverter to a maximum backfed 20 amp circuit, should not apply to this subpanel, but I can't find the rules for this particular situation covered in the NEC book.

BB. · January 2009

Re: Backfeeding breakers on a generator panel

I don't see any problem with your reasoning...

The XW inverter should be turned off and the internal battery charger running (as needed)--so there should be no back feed issues from the XW Inverter to the generator connection (no adding of sources).

The Generator is never on when the AC power mains are connected--so there is no back feed there either.

If you have an inspector to convince (and to double check your own work), doing a line drawing / block diagram (possibly three different versions, AC Mains, XW Backup, Gen Backup) would "prove" that there is no 120% issue except for the branch circuit/breaker box to XW feed/breaker point (when the XW is operating in Grid Tied mode).

-Bill

sub3marathonman · January 2009

Re: Backfeeding breakers on a generator panel

OK, that does seem like a good idea with the different drawings depending on the situation of on-grid or off-grid. Thanks.

BB. · January 2009

Re: Backfeeding breakers on a generator panel

Or the same basic drawing with the path of of current flow highlighted.

-Bill

Backfeeding breakers on a generator panel

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