Backfeed Rule Cheating ?

According to the NEC, a backfed system is limited by the 20% in the 120% rule.

If one has a 200A panel, then 200 x 1.2 = 240A and so 240-200= 40A is the backfed limit.

A 200A panel is pretty large for a typical home yet is limited to 40A x 240V = 9600W which is 7680W after derating. That is lousy.

You can upgrade the panel to 225 or 300A but thats a lot of expense and effort.

You can do a line side tap but some utilities don't allow it. Some panels like Combos have lots of room for the tap but its on the utility si8de and some say NO. Doing a lineside tap that is legal can be tough since utilities can say no, panels don't always have room or way of doing it so it can send one back to installing a new panel anyway. Local inspectors don't always understand lineside taps and can pass the buck to th utility who says no.

Or one can cheat a little.

You can buy a 200, 225 or 300 small panel and use it as a sub panel and use its busbar rating and NOT the main panel. A simple cheap 225A sub panel allows 225 x 1.2 = 270A ---- 270-225 = 45A

Or using a 300A sub panel allows 300x1.2=360 ---- 360 - 300 = 60A.

Or one could use say 2 200A subpanels off the main if possible and the rating becomes 40A x 2 = 80A.

Even though not exceeding the sub panel bus bar 120% rule meets the strict code, the original problem forseen by the NEC of future additions creeping up to the maximum and overloading the main bus bars is not really avoided.

So are there any opinions out there about using the sub panel bus bar rating to avoid having to change the main if the lineside path gets blocked by the utility, local inspector or physical limitations?
:confused:

Comments

  • loreleclorelec Solar Expert Posts: 200 ✭✭
    Re: Backfeed Rule Cheating ?

    I'm not sure NEC allows you to backfeed an entire panel. If it does, you're still going to have to backfeed that subpanel into your main, and you will be subject to the exact same restrictions that apply to backfeeding your PV system directly. In either case, you can't exceed 120% of the main's busbar rating (despite what the rating of your subpanel might be).

    Marc
  • newenergynewenergy Solar Expert Posts: 291 ✭✭✭✭✭✭
    Re: Backfeed Rule Cheating ?

    Seems to me that lorelec is right.

    If the main bus bar is rated for 200A, you are limited to 240A in feeders, but that could be a 175A main and then you could use a 60A (65A if it were available) for solar.

    So you could lower the size of the main breaker.

    Right?

    I've done it before anyway and the inspector agreed that it was ok.
  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Backfeed Rule Cheating ?

    Yes, lorelec is right.

    If you've got a 200a rated main panel then no matter what size your sub-panel is, you still can't use more than a 40a breaker to connect it to the main panel *if it's a back fed panel*.

    I've been thinking maybe the thing to do would be to move 40a of load from the main to the sub, then if the solar is putting out, that 40a of load can be handled by the solar, while still backfeeding up to 40a to the main.

    And if the solar isn't feeding, then the 40a sub-panel load will draw off the main.

    Then even a 100a sub-panel, with two incoming 30a from the solar and an incoming 40a from the main would be at exactly 100a on the supply side, and only at 80a on the load - 40 for the sub-load and 40 backfeed to the main panel.

    This would keep the sub-panel at 100% on the supply side (could even take it up to 120% using a pair of 40a from the solar), and 80% on the load side (40a to the local loads and 40a to the main panel), while keeping the main panel at 120% on the supply side and "somewhere under 125%" on the load side.


    But I'm tired right now and maybe not thinking clearly. :)
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Backfeed Rule Cheating ?
    Seems to me that lorelec is right.

    If the main bus bar is rated for 200A, you are limited to 240A in feeders, but that could be a 175A main and then you could use a 60A (65A if it were available) for solar.

    So you could lower the size of the main breaker.

    Right?

    I've done it before anyway and the inspector agreed that it was ok.
    Correct, per 2008 NEC 690.64(B)(2), Point of Connection / Load Side / Bus or Conductor Rating (in part):
    The sum of the ampere ratings of overcurrent devices in circuits supplying power to a bus bar or conductor shall not exceed 120% of the rating of the bus bar or conductor.
    HTH,
    Jim / crewzer
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