NEC requirements for a back-fed subpanel

I'm installing a 7000 watt solar array. My inverter specs a 40A back fed breaker. The solar array is going on a garage located 350 feet away from a 200A main panel. We are going to put a 100A subpanel in the garage.

My question has to do with the NEC's requirement of the "sum of the amps ratings of the overcurrent devices shall not exceed 120 percent of the rating of the busbar or conductor." -690.62B

My inverter requires a 40 amp back-fed breaker, which means the main breaker in the garage subpanel can't be larger then 80A (40A + 80A = 120A or 120% of the subpanel busbar rating). Is this correct? This is confusing me because it is a subpanel fed from a main panel, and I'm back-feeding the subpanel. Where do these 80A breakers go? Is there a 80A main breaker in the subpanel as well as in the main panel?

Anyone have a schematic of a solar subpanel set-up?



  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: NEC requirements for a back-fed subpanel

    yes, what you say is true and at first i didn't grasp that either, but the problem is when your household draws the max of the panel through the subpanel's main breaker that there is still more unchecked power available due to the pv system at 40a for example. the main breaker of the subpanel would not pop when you start exceeding the sub's buss ratings as that power is now shuffled in from the pvs causing an overload to the buss's rated current as the sum of both sources are upon it. your choices are to either go with an 80a main breaker or buy another panel of a higher rating to accommodate both currents on the buss. too bad we couldn't just swap the buss itself, but the nec approves of things in their own way. you will need another panel.

    i don't have a drawing, but perhaps somebody else has one. if it was for you to picture what was going on for the higher buss rating, i do hope i had explained that well enough for you to visualize it for it's the total current going through the buss and not the sub's main breaker.
  • fiddleheadfiddlehead Solar Expert Posts: 29
    Re: NEC requirements for a back-fed subpanel

    I wonder about sizing the wiring to the garage for 100 amps, but feeding it from the main panel with an 80 amp breaker. I would have to read more thoroughly through the code. Also, why not feed the garage with an even smaller breaker ( 70 or 60 amp)? Unless there is a need to have that much power in the garage, I would feed it with something smaller.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: NEC requirements for a back-fed subpanel

    yes, you could go to 80a or smaller like 60-70a if you do not need that much power into the garage and thusly save yourself the expense of having to change the subpanel to a higher rated panel. a 60a main breaker would in essence add to the 40a rated from the pvs to give you the exact 100a rating of the subpanel so if you do not need anything over 60a in the garage go for this as higher amps over 60a involve the panel's over-rating on the buss.
  • System2System2 Posts: 6,290 admin
    Re: NEC requirements for a back-fed subpanel

    John Wiles has a great write up.

    Check out the examples.
  • PgovetomPgovetom Solar Expert Posts: 30
    Re: NEC requirements for a back-fed subpanel

    Check this document out:

    You have encountered one of the funky problems. You need a 200A panel if you want to backfeed 40A. John Wiles and the NEC are concerned that in the future, if you buy a panel rated at say 100A, then some person you sell your home to might fill the panel right up to the rating. Even though you know not to do that or it will overload your bus bars heating them, the future is their concern. So there logic says you can take 120% of your panel rating and subtract the rating and that is your backfeed limit.

    If you want to feed 40A then (Panelsize x 1.2 ) - Panelsize = 40A

    If you solve for Panelsize then you get 200A - YUCK a big one.

    That is the rule. The 20% is for your backfeed while 100% is for the current usage AND the future.

    So without de-ratings:

    100A panel allows 20A = 4800W x .8 = 3840W with 80% derating

    200A panel allows 40A = 9600W x .8 - 7680W

    300A panel allows 60A = 14,400

    400A panel allows 80A = 19,200

    Its also important to determine if the 40A, if backfed into your main panel, will overload its bus bars. Just because you have remoted the backfed panel as a sub-panel and meet its 120% bu bar requirement, this is a grey area because simply moving to a sub-panel doesn't avoid the spirit of the 120% rule for your main panel. The same 40A could overload your main panel bus bars if someone in the future filled it to capacity.

    If your main panel is filled to it maximum in the future, then that 40A being injected from the sub could overheat your main panel bus bars WITHOUT tripping your main breakers which is the normal warning you are at the limit. Those extra 40A can be flowing up into your main panel ( say its 200A) and 240A is flowing on the bars and the main breakers are still happy.

    If your main panel is a 200A panel, then you would meet the 120% rule in both your 200A main plus your 200A sub. Neither of them will ever have more than 120% current flowing if someone in the future goes to both panels full rating.
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