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Thread: load side tap, Ampacity of subpanel

  1. #1
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    load side tap, Ampacity of subpanel

    Been a loooong time since i did a loadside tap.

    Existing system:
    150A main breaker in MBO MSP (no busbar)...... feeds #1/0 CU feeder conductors ......which feed a MLO (no main breaker, nor opportunity for one) 125A rated s.p.

    I plan to tie in 40A of PV onto the above #1/0 feeder with a (N) 2p60 fused disco and #6 conductors.

    If above s.p. were replaced with 200A rated load center, are we good?
    Last edited by Zee; 09-18-17 at 11:20 AM.

  2. #2
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    If the new 200A sub has a 175A main breaker then yes, in my opinion. Otherwise no, because the 1/0 isn't rated for 190A.

    Another option: put in a 200A rated sub instead of your disco, and put breakers in it for the existing sub (125A) and solar. It's not a tap anymore, but is likely also doable.

  3. #3
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    Could you do a line side tap at the MSP? Probably enough room. Grouping service disconnects would be easier and you’d be within 10’.
    The hardest faults to clear are mine

  4. #4
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    Quote Originally Posted by electricalist View Post
    Could you do a line side tap at the MSP? Probably enough room. Grouping service disconnects would be easier and you’d be within 10’.
    good idea, but not possible. no room on bus between meter and main breaker. also gets into voiding mfctr warranty or whatever. basically tampering with original equipment which the AHJ doesn't like.

  5. #5
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    Quote Originally Posted by Zee View Post
    Been a loooong time since i did a loadside tap.

    Existing system:
    150A main breaker in MBO MSP (no busbar)...... feeds #1/0 CU feeder conductors ......which feed a MLO (no main breaker, nor opportunity for one) 125A rated s.p.

    I plan to tie in 40A of PV onto the above #1/0 feeder with a (N) 2p60 fused disco and #6 conductors.

    If above s.p. were replaced with 200A rated load center, are we good?
    It's 705.12(B)(2) 1 & 2 that covers this. Basically, anything on the load side of the feeder tap has to be rated for 150+40*1.25= 200A. That includes the existing feeder and the sub-panel. In this case, your problem area is the #1/0 conductor since it's not rated for 200A.

  6. #6
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    Quote Originally Posted by pv_n00b View Post
    It's 705.12(B)(2) 1 & 2 that covers this. Basically, anything on the load side of the feeder tap has to be rated for 150+40*1.25= 200A. That includes the existing feeder and the sub-panel. In this case, your problem area is the #1/0 conductor since it's not rated for 200A.
    That was changed in the 2014 NEC. It's now in 705.12(D)(2)(1)(a) or (b). (a) says what you are saying, but (b) says that an OCPD on the load side of the connection rated not greater than the ampacity of the feeder will suffice. Pick (a) or (b).

  7. #7
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    Quote Originally Posted by ggunn View Post
    That was changed in the 2014 NEC. It's now in 705.12(D)(2)(1)(a) or (b). (a) says what you are saying, but (b) says that an OCPD on the load side of the connection rated not greater than the ampacity of the feeder will suffice. Pick (a) or (b).
    Pv_noob was quoting the 2017 NEC. 705.12(B) and (C) were removed so the old (D) became (B). The content relevant to this thread didn't change though, iirc.

  8. #8
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    Quote Originally Posted by ggunn View Post
    That was changed in the 2014 NEC. It's now in 705.12(D)(2)(1)(a) or (b). (a) says what you are saying, but (b) says that an OCPD on the load side of the connection rated not greater than the ampacity of the feeder will suffice. Pick (a) or (b).
    OP did not state an NEC version preference so I use the latest version, 2017. If the OP puts a 150A main breaker in the new load center then the feeder conductor is good. So add a main breaker or replace feeder, whichever is cheaper.

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