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Thread: Sizing protection for 3/0 THHW

  1. #11
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    Quote Originally Posted by roger View Post
    Your situation doesn't sound like it's for a dwelling unit and even if it was it wouldn't fly. From what you are saying, your 225 A OCPD is to big.

    Roger
    Roger I taught too OCPD was too high for this conductor a specially it feed panel not individual motor. On the other hand if was done many years back maybe back then was good and up to cod. Now from all input I hear I probably never was correct.


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  2. #12
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    Quote Originally Posted by Cow View Post
    Do you know how to do branch and feeder calcs?

    Determine what this feeder serves, do a quick calc, and see where your calculated load ends up. If it's well south of 200 amps, fuse the run at 200 amps, extend the wire to the new disconnect location and be done with it.
    That panel is energizing lab years ago now this area is empty we want to keep it for future use. So if years ago it was in use and feeder was sufficient now I will install 200A disco and 200A fuses and will be as new with correct protection.


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  3. #13
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    Quote Originally Posted by Malywr View Post
    Roger I taught too OCPD was too high for this conductor a specially it feed panel not individual motor. On the other hand if was done many years back maybe back then was good and up to cod. Now from all input I hear I probably never was correct.
    That is correct, it never was per code however, if this was an industrial application there are some provisions for "Engineering Supervision" but that is probably a long shot.

    Roger
    Moderator

  4. #14
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    Quote Originally Posted by jumper View Post
    I thought we were talking about the OCPD/breaker rating and 3/0 CU conductors?
    The OP said he's relocating a "safety disconnect", it would likely need to be a 400 amp disconnect if the upstream protection is 225 amps.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  5. #15
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    Yes it is industrial 3ph, 208V

    I guess I relocate OCPD to 6,6’ of the floor install new disco 200A with 200A fuses and change building diagrams documentation from 225A to 200A

    Now since I am thinking about that violation maybe oven is over protected too. Monday I will need to check conductor size and fuse size. Also tap is approximately 25’ so suppose to be 1/3 of the feeder and I don’t think it is.
    Question if it is same situation OCPD is to high I should correct it or can leave it alone because I am not touching that one tap


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  6. #16
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    Quote Originally Posted by Malywr View Post
    Roger I taught too OCPD was too high for this conductor a specially it feed panel not individual motor. On the other hand if was done many years back maybe back then was good and up to cod. Now from all input I hear I probably never was correct.


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    75C ampacity of 3/0 has been 200 amps for a very long time AFAIK, maybe at least for however long it has been since they started using thermoplastic insulation.
    I live for today, I'm just a day behind.

  7. #17
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    Quote Originally Posted by Malywr View Post
    Yes it is industrial 3ph, 208V

    Now since I am thinking about that violation maybe oven is over protected too. Monday I will need to check conductor size and fuse size. Also tap is approximately 25’ so suppose to be 1/3 of the feeder and I don’t think it is.
    Question if it is same situation OCPD is to high I should correct it or can leave it alone because I am not touching that one tap


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    Ok so today I checked oven tap it is approximately 15’. Size #3. 200A disconnect and 125A fuses
    So this one is in violation too. Tap size 240.21.B.2. Supposed to be 1/3 of feeder OCPD which is 500/3=166.7A tap should be size #2/0 and oven protection has to be 100A inside 200A disconnect
    Please correct me and explain because I am confused why someone would protect #3 wire with 125A fuses and what should I do I am not touching oven tap should I fix it ? At least install 100A fuses or don’t touch it?


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