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Thread: Service Neutral Grounded Conductor Ampacity?

  1. #1
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    Service Neutral Grounded Conductor Ampacity?

    Got a test question not sure how to answer: "A 120/240-volt, 200-ampere service requires a neutral with a minimum ampacity of:".

    Answers to choose from are: 140 160 175 or 200 amps.

    At first I thought well 230.42 Exception says" Grounded conductors not connected to a OC device shall be permitted to be sized at 100% of non and continuous load so that would be 200 amps. However...

    230.42C says "The grounded conductor shall not be smaller than required by 250.24C which says it can't be smaller than the GEC in table 250.66. Table 250.66 gives wire sizes based on the Largest ungrounded service conductor, but it does not chart minimum ampacities and the question doesn't say the type of cable or even if it's residential. So how do I determine the "minimum ampacity" of the service neutral? And then there is 220.61 which says the service neutral load shall be the max unbalanced load between any one ungrounded conductor and neutral, so wouldn't that be 100 amps in a 200 amp service?

    What am I missing?

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    Quote Originally Posted by hitehm View Post
    ...And then there is 220.61 which says the service neutral load shall be the max unbalanced load between any one ungrounded conductor and neutral, so wouldn't that be 100 amps in a 200 amp service?

    What am I missing?
    The 200 A in that service could be 200A from hot-to-hot or hot-to-neutral, though it would be decidedly rare for that latter to occur. You'd have to purposely power everything on one side and not the other, and many panels many not even have that many loads, but they CAN. Even a randomly distributed panel is likely to never see a load near that unbalanced.

    I'm not addressing the full context of the test question, just that principle.

    Look at how a 20 A single-pole breaker and a 20 A double-pole breaker are both 20 A, but one is hot-to-hot, and the other is hot-to-neutral. Same amperage at different voltage.

    Now put ten 20 A single pole breakers on the same buss, and load them up. You are going to have 200 A from hot-to-neutral on the feeders of that 200 A, 240 V panel. Again, this would almost have to be done on purpose to ever see it in real life.

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    Quote Originally Posted by MAC702 View Post

    Now put ten 20 A single pole breakers on the same buss, and load them up. You are going to have 200 A from hot-to-neutral on the feeders of that 200 A, 240 V panel. Again, this would almost have to be done on purpose to ever see it in real life.
    And would also mean the neutral is carrying the entire load, i.e. no imbalance.
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    Quote Originally Posted by hitehm View Post
    Got a test question not sure how to answer: "A 120/240-volt, 200-ampere service requires a neutral with a minimum ampacity of:".

    Answers to choose from are: 140 160 175 or 200 amps.

    At first I thought well 230.42 Exception says" Grounded conductors not connected to a OC device shall be permitted to be sized at 100% of non and continuous load so that would be 200 amps. However...

    230.42C says "The grounded conductor shall not be smaller than required by 250.24C which says it can't be smaller than the GEC in table 250.66. Table 250.66 gives wire sizes based on the Largest ungrounded service conductor, but it does not chart minimum ampacities and the question doesn't say the type of cable or even if it's residential. So how do I determine the "minimum ampacity" of the service neutral? And then there is 220.61 which says the service neutral load shall be the max unbalanced load between any one ungrounded conductor and neutral, so wouldn't that be 100 amps in a 200 amp service?

    What am I missing?
    The question is impossible to answer.
    Once in a while you get shown the light
    In the strangest of places if you look at it right. Robert Hunter

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    Quote Originally Posted by ActionDave View Post
    And would also mean the neutral is carrying the entire load, i.e. no imbalance.
    Or a perfect imbalance?

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    Quote Originally Posted by MAC702 View Post
    Or a perfect imbalance?
    Perfect example of an oxymoron.
    Once in a while you get shown the light
    In the strangest of places if you look at it right. Robert Hunter

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    Quote Originally Posted by ActionDave View Post
    The question is impossible to answer.
    +1
    I will have achieved my life's goal if I die with a smile on my face.

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    Thanks for replies

    Update on this: The writer has the correct answer as 140 amps, which means he's incorrectly interpreting the 70% demand factor rule on the service neutral load, which only applies the the portion of the unbalance load "in excess of 200 amps". Although the question is a flawed, it does bring up a good question though. How do you size the "ampacity" of the service neutral when knowing the service amperage size (not conductor size)? Do you:
    A: size it exactly the same as the ungrounded service conductors
    B: If you know the conductor type (thhn, thwn etc) do you use table 310.15b7 for the service conductor size and then apply that to table 250.66 for the neutral wire size and then go BACK to 310.15b7 for the ampacity using that wire size? (that seems nutty)
    or C: Something other?

    What is the minimum info needed to answer his flawed question?

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    Quote Originally Posted by hitehm View Post
    Got a test question not sure how to answer: "A 120/240-volt, 200-ampere service requires a neutral with a minimum ampacity of:".

    Answers to choose from are: 140 160 175 or 200 amps.

    At first I thought well 230.42 Exception says" Grounded conductors not connected to a OC device shall be permitted to be sized at 100% of non and continuous load so that would be 200 amps. However...

    ...
    ... And then there is 220.61 which says the service neutral load shall be the max unbalanced load between any one ungrounded conductor and neutral, so wouldn't that be 100 amps in a 200 amp service?

    What am I missing?
    It looks like you are falling into a common misconception. A 200A service is not 100A on each leg. It is 200A on each leg. 200A at 240V.



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  10. #10
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    Quote Originally Posted by hitehm View Post
    ...What is the minimum info needed to answer his flawed question?
    You need to know the calculated neutral load under 220.61 to reduce the neutral conductor size to less than the ungrounded conductors.
    I will have achieved my life's goal if I die with a smile on my face.

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