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Thread: Can a CB be too large if all devices it feeds have their own protection?

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    Can a CB be too large if all devices it feeds have their own protection?

    I design electrical controls for large conveyor systems. In our power distribution, 480VAC 3PH, we feed up to 4 units (1.7 FLA) with a 45A breaker (this 45A CB scheme has been passed down for years and I don't know the origins of it). Each unit has their own local protection. We size our conductors for voltage drop. In my mind there is no issue because the 45A CB protects the conductor and since each unit has its own local CB, there isn't a risk of it shorting out pulling 45A.

    I have searched all over to find out if there is anything in NEC that says that a CB can be too large in this case. Can someone please help me put this question to bed?

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    It sounds like you are doing “taps”. Look at NEC 240.21

    Details are all important, but that should answer your questions.

    eta: You didn’t give us enough information to say Yes or No on your installations.
    Tom
    TBLO

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    What you have is a feeder. It can pretty much be any size you want. 45 amps is an odd rating for a circuit breaker. However generally speaking you are not allowed to size the conductors for voltage drop. You have to size them according to the load. Normally the smallest single conductor you can use for Motors would be number 14 no matter what the load or voltage drop is.
    Bob

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    We don't know what you mean by large. I think a 480v 200 HP is large. 1500' is getting close to large in distance.

    WAG that your feeder with a 45 amp breaker has #6 cu. Evenly distributed 1.7 amps loads at 100, 200, 300, and 400 will give you about .3% VD total.

    Calculating just for VD with #14 cu would give you a VD of about 1.9%. You would not be able to use the 45 amp breaker. It would need to be a 15.
    Tom
    TBLO

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    Quote Originally Posted by ptonsparky View Post
    We don't know what you mean by large. I think a 480v 200 HP is large.
    That's a good one, how about a 20,000Hp fan motor at 13.8kV.
    "Just because you're paranoid, doesn't mean they're not out to get you"

  6. #6
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    Quote Originally Posted by kingpb View Post
    That's a good one, how about a 20,000Hp fan motor at 13.8kV.
    Probably ranks up there.
    Tom
    TBLO

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