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Thread: sub panels when main panel is full

  1. #1
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    sub panels when main panel is full

    What section would apply to tapping off the lugs on the line side of a resdential main breaker panel, to go to a main breaker sub panel

    I run into this a lot, is it code?...... was it code?
    Master Electrician

  2. #2
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    Re: sub panels when main panel is full

    I assume there is no circuit breaker before the main you are tapping from. In other words, you are tapping off the line side of the service disconnect.

    I think this would be a violation of 230.70(1), or 230.94, or 230.82

    Steve

  3. #3
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    Re: sub panels when main panel is full

    If the conection is made in a code compliant manner, not just extra wires jamed under the lugs.

    And you can prove the total load will not excced the rating of the service conductors.

    And you treat this panel as a service panel in regards to bonding and grounding.

    And the panels are grouped

    This exception would allow it.

    230.90(A)(1)Exception No. 3: Two to six circuit breakers or sets of fuses shall be permitted as the overcurrent device to provide the overload protection. The sum of the ratings of the circuit breakers or fuses shall be permitted to exceed the ampacity of the service conductors, provided the calculated load does not exceed the ampacity of the service conductors.

  4. #4
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    Re: sub panels when main panel is full

    I'm learning something new here. I have always assumed that this exception was for disconnects installed on the outside of a building (like meter stacks) or at least for disconnects bused together in a switchboard. I didn't know you could use this to parallel two panelboards.

    Do the wires to the second breaker have to be sized the same as the service entrance conductors? Are they still considered service entrance conductors? Or are they just sized for the load on the second breaker?

  5. #5
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    Re: sub panels when main panel is full

    If the connection is made in a code compliant manner, not just extra wires jammed under the lugs.
    Bob, I'm just curious as to how you would/could make this code compliant. I believe Jes25 is talking about an existing residential installation with a main breaker panel. We've had this discussion before where in a new installation (usually commercial) there are feed-through lugs to a second or third tub but I don't believe he has that situation here or the ability to do so.

    The only way I can see doing this with 2 main breaker panels is to pull out the main feeders, install a trough and bug on and install 2 new sets of service conductors to each panel. But why go through all that trouble when you can pull out 2 breakers from the original panel and reinstall them into a new "main lug" panel with a 2-pole plug-in main in the original panel ?

    Phil

  6. #6
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    Re: sub panels when main panel is full

    I have to agree with goldstar. Remove two breakers from the main panel, Install a two pole breaker in the existing panel to feed the new panel and then reroute the two circuits from the existing into the new panel.

  7. #7
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    Re: sub panels when main panel is full

    With Bob's method, I could run a 400A service and install (2) 200A panels. This would be very useful, but it would be unlikely to find a 200A MCB to put in the first panel that would land 2 sets of 500's.

    But a 200A service and (2) 200A panels might be possible. I think I have seen 200A breakers rated for (2) sets of 3/0. This could also be done with a subfeed breaker. But the two mains might help isolate loads. For example, a HVAC load would be less likely to cause lights to dim if it was fed this way.

    Steve

  8. #8
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    Re: sub panels when main panel is full

    Originally posted by goldstar:
    Bob, I'm just curious as to how you would/could make this code compliant. I believe Jes25 is talking about an existing residential installation with a main breaker panel.
    This is what I think, first this is not how I would choose to do it, I was only answering the original question.

    What section would apply to tapping off the lugs on the line side of a residential main breaker panel, to go to a main breaker sub panel

    I run into this a lot, is it code?...... was it code?
    There is no reason a single house can not have up to six service disconnects.

    You have an existing service panel, nipple another panel right beside it. Tap the service conductors in the existing panel with split bolts, H-Taps, or whatever you like, run these tapped conductors to the main breaker in the new panel. Ground and bond this new panel now you have 2 service disconnects and a new empty panel.


    Do the wires to the second breaker have to be sized the same as the service entrance conductors?
    No, size the wires by the breaker they are feeding.

    Using 230.90(A)(1)Exception No. 3 we might be adding a second 100 amp panel to a existing 100 amp service, all the conductors will be 100 amp conductors, even though the original service conductors will now have 200 amps of overcurrent protection.

    Are they still considered service entrance conductors?
    Yes.

    Here is a common (around here) application of that exception.



    200 amp service conductors in the riser, six 100 amp main breaker panels in the basement.

  9. #9
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    Re: sub panels when main panel is full

    Ok, so the way I am understanding it, a 60a panel off the 200a main panel fed with number 6's is code, as long as you are not cramming more wires under a lug not rated for it.
    Master Electrician

  10. #10
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    Re: sub panels when main panel is full

    Yes if that 6o amp panel has a 60 amp or less main breaker to protect the panel bus from overload.

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