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Thread: Wire size to breaker size

  1. #1
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    Wire size to breaker size

    I have a tendency to size the conductors to the size of the breakers. Is there anywhere in the NEC that makes me do it this way or can I size the wire to the load of the circuit and then go up the next breaker size. I do this for street lighting and cameras along the highways. The reason I tend to size to the breaker size is for future expansion. Thanks in advance guys

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    Start with a review of Section 240.4 of the NEC and move on from there to the specific circuit type section (feeder, branch circuit, etc.) and/or equipment type for specific requirements.
    Bryan P. Holland, MCP

  3. #3
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    Quote Originally Posted by magicman76 View Post
    I have a tendency to size the conductors to the size of the breakers. Is there anywhere in the NEC that makes me do it this way or can I size the wire to the load of the circuit and then go up the next breaker size. I do this for street lighting and cameras along the highways. The reason I tend to size to the breaker size is for future expansion. Thanks in advance guys
    You have me fascinated as usually the conductors used for these applications are very large compared to the actual load on them due to voltage drop considerations.

    We might use 3 AWG copper to feed circuit of site lighting due to voltage drop but the actual load may be 15 to 25 amps.

    We would never use a 100 amp breaker to supply it based on the use of the 3 AWG.

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    Like Iwire, I've also used rather large conductors for powering site & parking lot lights due to voltage drop. Largest I've put on a 20-a circuit is #0.

    You may also violate the listing of the load if you upsize the breaker.

    If an item has a nameplate that says "Max OCD 20amps" and you run 10s or 8s to it for voltage drop purposes, you still only use a 20a breaker maximum.

    For example, the lighting in this auditorium was all run in #8, but they're all 20a circuits.

    Last edited by 480sparky; 12-19-08 at 11:27 AM.

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    I guess what my question really is do you size your conductors to the breaker size or the load? I understand that you might have a bigger AWG then what your breaker is capable of handling due to voltage drop but can it be done the other way around.
    Last edited by magicman76; 12-19-08 at 11:35 AM.

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    Quote Originally Posted by magicman76 View Post
    I guess what my question really is do you size your conductors to the breaker size or the load? I understand that you might have a bigger AWG then what your breaker is capable of handling due to voltage drop but can it be done the other way.
    Are you asking if it's OK to install a larger breaker just because you have larger wires installed for voltage drop?

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    Quote Originally Posted by 480sparky View Post
    Are you asking if it's OK to install a larger breaker just because you have larger wires installed for voltage drop?
    Now that you mention it, yes I'm curious on that.

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    Quote Originally Posted by magicman76 View Post
    Now that you mention it, yes I'm curious on that.
    210.20 has your answer.

    I'm still not clear on your original quesion, though.

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    Quote Originally Posted by 480sparky View Post
    210.20 has your answer.

    I'm still not clear on your original quesion, though.
    Thanks, I guess my question is can I run a #10 AWG (35A) to a 50 amp breaker or would it have to be a #8 (50A). In other words lets say there is an existing 50 amp breaker and my load is 28 amps can I size my wire to the load or to the breaker size?

  10. #10
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    Quote Originally Posted by magicman76 View Post
    Thanks, I guess my question is can I run a #10 AWG (35A) to a 50 amp breaker or would it have to be a #8 (50A). In other words lets say there is an existing 50 amp breaker and my load is 30 amps can I size my wire to the load or to the breaker size?
    Your wire size would be based on the load, then the breaker would be sized to both the load and the wire size.
    With a few exceptions, #10 is limited to a 30a OCD. 240.4(D).

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