Can I backfeed 2-phase 240V through one 1" knockout?

System
System Posts: 2,511 admin
in Grid Tie and Grid Interactive Systems #1
An electrician told me I can install a 2-pole 240V 20A 1" breaker:
THQP220.jpg
for the back feed from a SunnyBoy3000.

But, then we would only be back feeding into one phase (120V) of the grid.
It's been a long time since I took physics, but this doesn't seem possible to me.
We currently only have one 1" knockout available, and he is trying to avoid giving me another.

Anybody care to weigh in on this?

thx
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Comments

  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #2
    Re: Can I backfeed 2-phase 240V through one 1" knockout?

    The picture you are showing is a GE dual breaker used for 240v feed. The toggle handles are tied together with a metal pin so if one side over-current pops, both will open. It will feed both L1 & L2 legs. The bus bars in the panel alternate between L1 & L2 for each single breaker slot.

    Slight correction, the picture is half width breaker. You have to select a spot to get both L1&L2 feeds. At half breaker width, panel, from top-down is L1,L2,L2,L1,L1,L2,L2,...

    I think there is an interference key between dual half breakers and panel that prevents you from putting it in an L1,L1 or L2,L2 position in the panel.
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  • System2
    System2 Posts: 6,290 admin
    #3
    Re: Can I backfeed 2-phase 240V through one 1" knockout?

    Thanks for the reply.

    I don't think it would be considered "half width," since it is a 1" breaker, but, without seeing the back of this breaker I can't picture how it could possible hit both L1 and L2 because when I look in the panel I can see only one bar behind that one 1" knockout.
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #4
    Re: Can I backfeed 2-phase 240V through one 1" knockout?

    The half width breakers use the 'U' shaped wings off the main leg connection. The full width breaker connect is to the center, which is a stronger connection rated at higher currrent.

    They sell half widths up to 50 amps but I would not use half widths for more then 40 amps. (you only need 20 amp breakers for sunboy 3000).
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #5
    Re: Can I backfeed 2-phase 240V through one 1" knockout?
    psablo wrote: »
    Thanks for the reply.

    I don't think it would be considered "half width," since it is a 1" breaker, but, without seeing the back of this breaker I can't picture how it could possible hit both L1 and L2 because when I look in the panel I can see only one bar behind that one 1" knockout.

    When you say 'knock out' you are referring to front panel. If you only see one bar you are looking at L1-L1 or L2-L2 spot. You need to move down or up another knockout and that 240v breaker will strattle between the two. If this is the only remaining two breaker slots unused then you can reposition the adjacent 120v breaker two half slots to make a space for the dual half width 240v breaker.

    This will free up the other phase half slot. The movement of the one 120v breaker will take it from L1 to L2 side.

    If you end up with half a front panel knockout open you can get half width plastic front panel cover inserts at Home Depot or Loew's that snap into the exposed front panel half slot.
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #6
    Re: Can I backfeed 2-phase 240V through one 1" knockout?

    Maybe a picture will help. Unfortunately I didn't have a picture showing the unpopulated panel bus bar.

    The labels on breakers red and black squares represent the L1,L2 phase.

    Down the right side notice the half width 240v breakers for range, clothes drier, and hot water heat. Notice the first half breaker below the full width breakers at top is a 120v feed (marked 'outside lights'). You can not put the first half width 240v breaker directly on next position below the full width breakers. That would be two black, same phase. Also the center connection post, used for a full width breaker connection, would prevent you from snapping in a 240v dual half width breaker.

    I think the confusion is you are strickly relating front panel knockout to a given breaker position. All the half width 240v breakers strattle the front panel knockout edges. Notice at the bottom of right side there is a 120v breaker consuming the last half position of last front panel knockout. If that 120v breaker was not needed then a half width plastic cover would be snapped into cover panel at that position.
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  • System2
    System2 Posts: 6,290 admin
    #7
    Re: Can I backfeed 2-phase 240V through one 1" knockout?

    Ok, I get it now, yes I was equating front panel to panel bar.
    So, I need to move every breaker one half step so the 1" 240V spans both lines, meaning I will need two 1/2" breakers to allow for the half spacing:
    1/2-1-1-1-1/2-1-1 hey that's Phrygian.

    I appreciate your help RCinFLA
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #8
    Re: Can I backfeed 2-phase 240V through one 1" knockout?

    Not every breaker, just one 120v breaker.

    Not sure what you mean by '1/2-1-1-1-1/2-1-1'. If it means you have 120v full 1" width breakers you can alway change one of them to half inch breaker.
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  • System2
    System2 Posts: 6,290 admin
    #9
    Re: Can I backfeed 2-phase 240V through one 1" knockout?

    yes, a 2 pole means 2 phases, normally still called, single phase. even though there's really 2. You don't even need a 1" KO. unless your PV sys. is over 60k. 1/2" is fine-2-#10s = 60 Amps
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