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Neutral cable and EGC selection advises needed

Question asked by Rene Immerkr on Mar 1, 2018
Latest reply on Mar 1, 2018 by Nick Sasso

Hi all

Hope some of you can help me on this topic regarding selection of the right cable sizes for US market.

 

1)

I am sizing a neutral conductor for a system expected to have high harmonic content due to non-linear loads.

I this case 1,73 times phase current. I use 1,25 rated CB's due to continous load. 4 wire system at 40 deg Celsius.

Let's say nominal phase current is 100A.

Neutral is then supposed, in worst case, to be sized for:  100A x 1,25 x 1,73 / 0,8 / 0,91 = 297A

0,91 is reduction factor and 0,8 is adjustment factor.

 

This gives me a very large neutral conductor compared to phase conductor.

Am I allowed to leave out any of the correction factors or do I have any other oppotunities to reduce neutral without violating any rules?

 

2)

Regarding EGC size selection I am using Table 250.122.Table 250.122

Marked in red box, the NEC states that in some cases the EGC will have to be sized bigger, than suggested by Table 250.122.

As I see this, this means that EGC has to be size to make sure of correct operation of OPCD. Can anyone come up with an example of this, since I haven't been able to come up with any cases where EGC has to be sized bigger than stated above.

 

Thanks in advance.

 

\René

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