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Thread: copper feeder size for 3-phase 4-wire 225 amp OCP use 3/0 or 4/0?

  1. #1
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    copper feeder size for 3-phase 4-wire 225 amp OCP use 3/0 or 4/0?

    3/0 XHHW is rated 200 amps @75c.
    4/0 XHHW is rated 230 amps @75c.

    per NEC 240.4B & 240.6 is it allowable to use 3/0 on a 225A ckt bkr? (for a non-continuous rated load).

    my colleague says you cannot use 3/0 since there is an OCD rating of 200A so 3/0 cannot exceed this established OCD rating. the argument is that the next OCD rating of 225A must use 4/0 since 3/0 cannot go up to the next higher rating because it is locked in at 200A.

    this is a big deal since there is an existing 2 inch conduit that can be utilized and the smaller 3/0 conductor would help the installation. (this feeder is not continuously rated.)

  2. #2
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    Unfortunately I have to agree with your colleague per 240.2(B)(2). Is it less cost effective to replace the 225A OCPD with a 200A?

  3. #3
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    Quote Originally Posted by chase15 View Post
    3/0 XHHW is rated 200 amps @75c.
    4/0 XHHW is rated 230 amps @75c.

    per NEC 240.4B & 240.6 is it allowable to use 3/0 on a 225A ckt bkr? (for a non-continuous rated load).

    my colleague says you cannot use 3/0 since there is an OCD rating of 200A so 3/0 cannot exceed this established OCD rating. the argument is that the next OCD rating of 225A must use 4/0 since 3/0 cannot go up to the next higher rating because it is locked in at 200A.

    this is a big deal since there is an existing 2 inch conduit that can be utilized and the smaller 3/0 conductor would help the installation. (this feeder is not continuously rated.)
    I agree with your colleague. 240.4(B) (2) states that to use the next size up rule the ampacity of the conductors must not correspond to a standard sized breaker or fuse. 3/0 copper has an ampaciity of 200 amps which is a standard sized breaker.

    Chris

  4. #4
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    I agree with the others but will add that we run 4-4/0 and an EGC in 2" all the time.

  5. #5
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    Quote Originally Posted by iwire View Post
    I agree with the others but will add that we run 4-4/0 and an EGC in 2" all the time.
    Can you also fit a #6 EGC in there?

  6. #6
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    XHHW is dual listed I'll assume that this will not help since the breakers are @75c?
    = : )

  7. #7
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    Quote Originally Posted by skeshesh View Post
    Can you also fit a #6 EGC in there?
    Depending on the conduit. If it is emt no egc is required however a #6 egc will fit. PVC is another story-- won't get a #6 in there and be compliant. The op may be able to drop the size of the neutral if there are a lot of 3 phase load.

  8. #8
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    With the numbers I'm using, THHN wires, (4) 4/0, and (1) #6 ground, I get that it barely fits in rigid steel conduit.

    For EMT, I get a fill of 40.04%, with the limit being 40%.

    Are we allowed to round off the .04% , or does a maximum fill of 40% mean just that??

  9. #9
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    Quote Originally Posted by steve66 View Post
    With the numbers I'm using, THHN wires, (4) 4/0, and (1) #6 ground, I get that it barely fits in rigid steel conduit.

    For EMT, I get a fill of 40.04%, with the limit being 40%.

    Are we allowed to round off the .04% , or does a maximum fill of 40% mean just that??
    That's why I brought it up. One of our senior engineers told me he specified 2" (emt) on a project before where client's internal specs required EGC wire. The contractor refused to pull due to the fill ratio and inspector agreed with him. Apparently they argued over it for a while but the rounding down got shut down at every point by the inspector and his higher-ups. I guess it was one of the "expensive lessons to learn" (every experienced engineer, technician or contractor seems to have a few of those!). But I bet you could get away with it if you know the inspectors in the area and/or if they are not very strict.

  10. #10
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    Quote Originally Posted by steve66 View Post
    With the numbers I'm using, THHN wires, (4) 4/0, and (1) #6 ground, I get that it barely fits in rigid steel conduit.

    For EMT, I get a fill of 40.04%, with the limit being 40%.

    Are we allowed to round off the .04% , or does a maximum fill of 40% mean just that??
    Op is using xxhw

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