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Thread: Sub-panel, MLO or Main-Breaker requirements

  1. #1
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    Sub-panel, MLO or Main-Breaker requirements

    Regarding sub-panels, I'm looking for code references that talk about whether the sub panel needs to be a MLO or a Main Breaker?

    I know typically a sub is protected at its source by a breaker sized to protect the feeder that feeds the sub. Also I know the sub is sized for the load of the ckts that originate from it, which is used to decide the feeder size (125% of continuous + 100% non-continuous).

    What code reference tells me if/when it's necessary to have a main breaker in the sub?
    I would assume it's related to treating the main breaker in the sub as a disconnect, because the feeder is already protected. So, if you need to do work on the branch circuits you can disconnect power at the sub (example, re-arrange breakers to add or delete ckts, etc) rather than go all the way back to the main panel to disconnect power to the sub.

    So maybe a matter of convenience? Maybe it's a design issue that the code doesn't specifically address? Or am I missing something?

    Thanks for feedback!

  2. #2
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    What your looking for is requirements to when a disconnect is required. (no more that six hand movements to disconnect the supply)

    As long as the feeder with correct over-current protection for a panel with- in the same structure, than no main would be required.

    Secondary tap rules and feeder tap rules may expand single disconnect requirements.

  3. #3
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    If the sub-panel is within the same structure then a MLO is permissible (no main or disconnecting means at the panel required) because the sub-panel feeder it protected at the source.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  4. #4
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    One could argue a main being a means of lockout w/in sight , or a combination component for AIC proximal to the serving Xformer , but either stance is a dicey debate......~RJ~

  5. #5
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    6 switch rule

    Yeah, my main scenario that I'm thinking about is a sub panel within the same structure, so I don't think the 6 switch rule would apply.

    If your 4-wire feeder went to another building then the 6 switch rule would apply.
    At that point you'd have to to address grounding, etc.

    I guess my question was regarding the same structure. So then it would be a design scenario, and not a code issue as long as the feeder was properly protected and the branch ckts in the sub were properly protected?

    Thanks!

  6. #6
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    Quote Originally Posted by sw_ross View Post
    Yeah, my main scenario that I'm thinking about is a sub panel within the same structure, so I don't think the 6 switch rule would apply.

    If your 4-wire feeder went to another building then the 6 switch rule would apply.
    At that point you'd have to to address grounding, etc.

    I guess my question was regarding the same structure. So then it would be a design scenario, and not a code issue as long as the feeder was properly protected and the branch ckts in the sub were properly protected?

    Thanks!
    Well almost, there has been discussion here where the overcurrent at the main panel protected the feeder but the panel was rated less than the feeder in those cases the sub- panel would still need over current protection, usually a main breaker

  7. #7
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    I did a summary list of this a little while back, but i couldnt pull it up. Ill try to duplicate it. Note a main breaker is never required to be part of the panelbaord assembly, it could be in a separate enclosure next to it, but a mian breaker is usually the simplest and cheapest way to go.

    Main breaker needed if:

    1. a disconnecting means is required and there are more than 6 branch breakers, or the panel requires a main breaker even if there are 2-6 branch breakers (heard of those on this forum, never seen one). This would apply for a service, separate structure, or SDS.

    2. To comply with 408.36 when taps, transformers, or a larger feeder/OCPD than panel rating is used.

    3. To meet conductor overload protection requirements for taps or transformer secondary conductors

    4. To meet transformer protection requirements (450.3)

    5. Possibly installed or modified to meet one of the provisions in 705.12(D)(2) (PV applications)


    That is ll I can think of. IF anyone comes up with any more feel free to add to the list. Finally, a note on practice. Fairly often I see main breakers installed where they are not required. I assume some of these are lack of knowledge and the specifier was just "playing it safe", but others the designer perhaps just thought it was "good design" to have a main breaker installed for perceived convenience.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  8. #8
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    Quote Originally Posted by electrofelon View Post

    That is ll I can think of. IF anyone comes up with any more feel free to add to the list. Finally, a note on practice. Fairly often I see main breakers installed where they are not required. I assume some of these are lack of knowledge and the specifier was just "playing it safe", but others the designer perhaps just thought it was "good design" to have a main breaker installed for perceived convenience.
    Convenience & less stock
    They (panels w/mains) can go just about anywhere , not a huge $ dif

    ~RJ~

  9. #9
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    Quote Originally Posted by romex jockey View Post
    Convenience & less stock
    They (panels w/mains) can go just about anywhere , not a huge $ dif

    ~RJ~
    For Resi stuff, yes indeed. I almost always use those homeline 100A main breaker panels for subs. But with commercial panelboards, adding a main can be a significant cost. I finished a job where 4 - 250 amp panelboards with main breakers had been installed but were not needed. I was to finish the service which was a MLO with 6 mains. I pulled the main breakers out of the subs and used them in the MDP, saved the owner $1600.

    I thought of a few more for the list. One is kinda nitpicky, but the other is potentially quite important:

    Add to #3: service entrance conductors where this a single set.

    A new one: Where fault current exceeds 10k, a main breaker could be used to get a series rating with the standard 10k branch breakers.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  10. #10
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    Quote Originally Posted by electrofelon View Post
    I finished a job where 4 - 250 amp panelboards with main breakers had been installed but were not needed. I was to finish the service which was a MLO with 6 mains. I pulled the main breakers out of the subs and used them in the MDP, saved the owner $1600.
    So you ordered an MDP with connector kits only but no breakers and pulled the Mains out of the subpanels to put in the MDP, then, installed lug kits back on the (4) 250 amp panels and blank covers or different dead fronts to cover the holes where the mains used to be?

    JAP>

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