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Thread: Wire size, 3ph Delta, 120/240

  1. #1
    DIY Senior Member Homeownerinburb's Avatar
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    Default Wire size, 3ph Delta, 120/240

    Hey, anyone want to tell me the de-rating?

    The engineer has decided for reasons that I am not privy to, to spec a 125a 3pole on the switch gear to feed the sub panel I hope to be installing next week.

    He had originally drawn it with a 100a 3pole, and spec'd #1 cable.

    I'm going to need to pull 1/0, yes?

    I don't work a lot of three phase. Happily the switch gear is less than a year old, and there is a blank open for the new breaker.

    Not like some Frankenstein equipment in some damp basement.

  2. #2
    Electrician ActionDave's Avatar
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    If you have less than three current carrying conductors in a conduit then you may not need to derate. For 3phase delta you would only have three. If you are under '11 code then there is a whole new set of rules concerning temperatures.

    310.15 is still the place to look.

  3. #3
    Electrical Contractor/Instructor jwelectric's Avatar
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    A #1 copper THW conductor is good for 130 amps so I see no problem.

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    DIY Senior Member Homeownerinburb's Avatar
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    Quote Originally Posted by jwelectric View Post
    A #1 copper THW conductor is good for 130 amps so I see no problem.
    Three hots and a neutral (four conductors) does not demand a derating?

    I'm not complaining, the pull will be by hand, down 50' of 2", probably 3 x 90 degrees. With a #6 ground.
    Last edited by Homeownerinburb; 07-11-2012 at 08:18 AM.

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    Electrical Contractor/Instructor jwelectric's Avatar
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    145*.80=116 (use the 90 degree column for derating)
    240.4(B) next highest OCPD = 125 amp
    I see no problem
    But it was a good question, it shows you are thinking

  6. #6
    DIY Senior Member Homeownerinburb's Avatar
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    Quote Originally Posted by jwelectric View Post
    145*.80=116 (use the 90 degree column for derating)
    240.4(B) next highest OCPD = 125 amp
    I see no problem
    But it was a good question, it shows you are thinking
    I'll tell you one thing for sure. I don't want to buy 200' of #1, get the businesses in the building to understand that they are going to lose power over night, call the alarm Co and tell them the same, get some burly guy (likely the plumber I will be working across from on this one) to show up at 7pm, help me tug that crap thru, only to have the inspector at rough inspection ask me why I was keeping my head in my ass, and tell me that I had to do all that again, plus buy 200' of 1/0 on my own dime. A mistake like that and I'd pretty much be doing the rest of the damned job for free.

    I'd sooner put in some backstab outlets!

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    Electrician ActionDave's Avatar
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    Quote Originally Posted by Homeownerinburb View Post
    Three hots and a neutral (four conductors) does not demand a derating?
    .
    The neutral does not count as a CCC for derating if it only carries the imbalance of the load.

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    DIY Senior Member Homeownerinburb's Avatar
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    Quote Originally Posted by ActionDave View Post
    The neutral does not count as a CCC for derating if it only carries the imbalance of the load.
    I keep forgetting. I do so little 3 phase.

  9. #9
    General Engineering Contractor ballvalve's Avatar
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    I have 3 phase, 3 hots and a neutral . One wire is 277 or so volts to N - the other 2 wires are 240 volts to each other. Those 2 to neutral are 120 volts. This I assume is the 4 wire [ ground] 120-240 delta configuration. The POCO warns me not to try and start more than a 20 HP motor without a soft starter.

    Its a great system as one need not have the burden of transformers.

    But my question is why would the feed wires not be down sized as in all of the feeds to the 3 phase motors? Certainly the "stinger" high voltage wire could be smaller.....

  10. #10
    DIY Senior Member Homeownerinburb's Avatar
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    Quote Originally Posted by ballvalve View Post
    One wire is 277 or so volts to N
    Are you sure that it is not 208 to neutral?

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    Electrician ActionDave's Avatar
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    Quote Originally Posted by Homeownerinburb View Post
    Are you sure that it is not 208 to neutral?
    Couldn't be anything else.

  12. #12
    Electrician ActionDave's Avatar
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    Quote Originally Posted by ballvalve View Post
    I have 3 phase, 3 hots and a neutral . One wire is 277 or so volts to N - the other 2 wires are 240 volts to each other. Those 2 to neutral are 120 volts. This I assume is the 4 wire [ ground] 120-240 delta configuration. The POCO warns me not to try and start more than a 20 HP motor without a soft starter.

    Its a great system as one need not have the burden of transformers.

    But my question is why would the feed wires not be down sized as in all of the feeds to the 3 phase motors? Certainly the "stinger" high voltage wire could be smaller.....
    In a Delta service all three hots read 240V phase to phase.

  13. #13
    DIY Senior Member Homeownerinburb's Avatar
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    Quote Originally Posted by ActionDave View Post
    Couldn't be anything else.
    That's what I had thought.

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    DIY Senior Member Homeownerinburb's Avatar
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    Quote Originally Posted by ActionDave View Post
    In a Delta service all three hots read 240V phase to phase.
    What color code is most common in your experience with this arrangement?

    Black, Red, and Orange for the wild leg?

  15. #15
    Electrician ActionDave's Avatar
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    Black, orange, blue- three phase delta.

    Black, red, blue- three phase wye.

    I have heard that in California purple is used for the high leg.

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