Wire size, 3ph Delta, 120/240

Discussion in 'Electrical Forum discussion & Blog' started by Homeownerinburb, Jul 10, 2012.

  1. Homeownerinburb

    Homeownerinburb New Member

    Messages:
    525
    Location:
    Los Angeles, CA USA
    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. ActionDave

    ActionDave Electrician

    Messages:
    363
    Location:
    Colorado
    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. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,566
    Location:
    North Carolina
    A #1 copper THW conductor is good for 130 amps so I see no problem.
  4. Homeownerinburb

    Homeownerinburb New Member

    Messages:
    525
    Location:
    Los Angeles, CA USA
    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: Jul 11, 2012
  5. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,566
    Location:
    North Carolina
    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. Homeownerinburb

    Homeownerinburb New Member

    Messages:
    525
    Location:
    Los Angeles, CA USA
    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!
  7. ActionDave

    ActionDave Electrician

    Messages:
    363
    Location:
    Colorado
    The neutral does not count as a CCC for derating if it only carries the imbalance of the load.
  8. Homeownerinburb

    Homeownerinburb New Member

    Messages:
    525
    Location:
    Los Angeles, CA USA
    I keep forgetting. I do so little 3 phase.
  9. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    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. Homeownerinburb

    Homeownerinburb New Member

    Messages:
    525
    Location:
    Los Angeles, CA USA
    Are you sure that it is not 208 to neutral?
  11. ActionDave

    ActionDave Electrician

    Messages:
    363
    Location:
    Colorado
    Couldn't be anything else.
  12. ActionDave

    ActionDave Electrician

    Messages:
    363
    Location:
    Colorado
    In a Delta service all three hots read 240V phase to phase.
  13. Homeownerinburb

    Homeownerinburb New Member

    Messages:
    525
    Location:
    Los Angeles, CA USA
    That's what I had thought.
  14. Homeownerinburb

    Homeownerinburb New Member

    Messages:
    525
    Location:
    Los Angeles, CA USA
    What color code is most common in your experience with this arrangement?

    Black, Red, and Orange for the wild leg?
  15. ActionDave

    ActionDave Electrician

    Messages:
    363
    Location:
    Colorado
    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.
  16. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,566
    Location:
    North Carolina
    The high leg will be the square root of the voltage from phase A to C squared minus A to N squared
    240 squared minus 120 squared then find the square root of this
    57600 – 14400 = 43200 squared root = 207.846 volts on the high leg

    The only color code required by the NEC is that the high leg be orange.
  17. Homeownerinburb

    Homeownerinburb New Member

    Messages:
    525
    Location:
    Los Angeles, CA USA
    The engineer tells me this is delta.

    When I walked the job last week, I did not get to see any 240v work, but there were some 120v faceplates not installed, and I saw red and black.

    I think I will go with black, orange and red. It seems to be the standard that the building is wired to and seems common enough that any electrician with an active brain should understand it.
  18. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    This is a small shop system. One ground, one neutral. There is a 'stinger' wire that is only used with the other 240 volt to ground wires to feed three phase appliances. Its perhaps 270v to ground. You get 240 volts between the other 2 wires and 120 volts from either of those 2 wires to ground. Its sort of a 'fake' three phase because it uses just 2 transformers. One must be careful on placing breakers to balance the loads. We had one set of transformers blow up already and cause quite a fire. This is why they advise you to try and stay inside 15 HP motors, we can run 30 HP+ but not start them at the same time.

    There is NO 208v available in the panel. Its a great system.

    And again, why not downside feed wire size to the panel considering one has 3 hot wires? Especially one at 270 volts?
    Last edited: Jul 16, 2012
  19. Homeownerinburb

    Homeownerinburb New Member

    Messages:
    525
    Location:
    Los Angeles, CA USA
    You have three phase OPEN delta (two transformers), which never when used correctly delivers 208 or 270.

    Yes, balancing is difficult and you will probably never achieve it completely. The wild leg will always get less use than the other two phases, because the other two phases are the only source of 120v to the neutral. Even if you do not use the two low legs to produce 240v, they will always be working harder than the wild leg, if you have any 120v loads.

    You cannot use the wild leg to neutral to achieve 208v because then you would be running thru one winding on one of the transformers and half way thru another winding on the other transformer. It is very untidy.....
  20. ActionDave

    ActionDave Electrician

    Messages:
    363
    Location:
    Colorado
    It's called an Open Delta. It is real three phase power; nothing fake about it. At one time it was a very common system and is still used today.

    You don't have 270V anywhere. That is impossible. Depending on your supply voltage you could have 214V high leg to ground.

    I suppose the stinger leg could be smaller. I don't know if it is allowed by code or not. JW are you out there?
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