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Thread: 500 MCM Current Ratings

  1. #1
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    500 MCM Current Ratings

    When I started the trade as an apprentice in 1973 and for years, I was taught that 500 MCM could beused for 400 amps and multiples of 400 amps.

    3-500 MCM were used for a 1200 amp feed but now 3-600 MCM cables must be used.

    Were code changes made, or was I always taught wrong?

  2. #2
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    You were taught wrong, but it was close, and the difference may or may not change the actual installation requirements.

    A 500 MCM copper conductor is good for 380 amps. If the load is 380 amps or less, then you can use a 500 MCM. If the load is 381 amps or higher, you cannot use a 500 MCM. If you have not calculated the load, and you plan to use a 400 amp overcurrent device to protect a 400 amp panel, then a 500 MCM will not be sufficient. The point is that you must first determine the load, that you next select a conductor that has at least that much ampacity, and finally you select the overcurrent setpoint to protect the conductor.

    That said, if you are allowed to use a 500 MCM, then you can protect it with a 400 amp overcurrent device (the next higher standard size). You can also protect a pair of parallel 500 MCM sets with an 800 amp breaker (again, the next higher standard size). But with three or more sets of parallel conductors, the overcurrent device will be rated over 800 amps. At that point, you can no longer use the next higher standard size, you must have an overcurrent trip setting that is no higher than the conductor ampacity. Reference 240.4(C).

    Welcome to the forum.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  3. #3
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    Thanks for the fast response. I have researched some older NEC editions also. The Article 240 references have little change. I did find that the temperature limitations in 110.14 (C) were not part of the code requiremnts until the 1993 edition. 500 MCM, THHN amperage could have been based on the 90 degree C rating of 405 amps until then. This would then explain why there are differences today. What do you think?

  4. #4
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    Quote Originally Posted by Bodine9200 View Post
    I did find that the temperature limitations in 110.14 (C) were not part of the code requiremnts until the 1993 edition. 500 MCM, THHN amperage could have been based on the 90 degree C rating of 405 amps until then. This would then explain why there are differences today. What do you think?
    No, you were still taught wrong.:smile:

    Since I started back in the 70's 500kciml has been rated as 380A (for connection to non-ANSI protective devices). I know we would round up to the next higher rating up to 1200A, I don't remember going higher. Sometime around 1990, the limit rounding to 800A came into effect.
    Just because you can, doesn't mean you should.

  5. #5
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    Smile

    Jim, I remember using a set of 500 kcmil for each multiple of 400 amperes of service as long as the calculated load was equal to or less than 380 amperes. I started into the electric utility business in 1969 and became a master electrician in 1974. Some time after that is when 240.4 was revamped, I think (this is from memory). :smile:
    Charlie Eldridge, Indianapolis, Utility Power Guy
    Responses based on the 2011 NEC, unless stated otherwise.

  6. #6
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    Quote Originally Posted by charlie View Post
    Jim, I remember using a set of 500 kcmil for each multiple of 400 amperes of service as long as the calculated load was equal to or less than 380 amperes. I started into the electric utility business in 1969 and became a master electrician in 1974. Some time after that is when 240.4 was revamped, I think (this is from memory). :smile:
    This sounds similar to what I think I remember. But as I tell my wife; I didn't forget, I just remembered wrong.:rolleyes:
    But service and feeder conductors are different animals.
    Just because you can, doesn't mean you should.

  7. #7
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    Quote Originally Posted by jim dungar View Post
    No, you were still taught wrong.:smile:

    Since I started back in the 70's 500kciml has been rated as 380A (for connection to non-ANSI protective devices). I know we would round up to the next higher rating up to 1200A, I don't remember going higher. Sometime around 1990, the limit rounding to 800A came into effect.
    Jim,
    The prohibition on rounding up to the next larger size for OCPDs 800 amps and larger was in the 1978 code as Exception #1 to 240-3. That is the oldest code I have so I don't know how long it has been a code rule.
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  8. #8
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    Quote Originally Posted by don_resqcapt19 View Post
    Jim,
    The prohibition on rounding up to the next larger size for OCPDs 800 amps and larger was in the 1978 code as Exception #1 to 240-3. That is the oldest code I have so I don't know how long it has been a code rule.
    I will have to see if I have some old Wisconsin Codes,it may have been a local allowance.
    Just because you can, doesn't mean you should.

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