Current calculations for non-linear loads

[LMAO]

If you have loads with a lot of harmonics (like VFDs); how would you calculate the load in order to find minimum cable ampacity? Should you find the RMS of all currents at all frequencies? I don't see any guideline regarding harmonics in NEC except that it considers the neutral to be current carrying.
Thanks

 

[G._S._Ohm]

Should you find the RMS of all currents at all frequencies?

A true RMS ammeter with 50 kHz bandwidth will probably register almost all harmonics because the amplitude of the higher harmonics keeps dropping off. Even a 5 kHz bandwidth may be enough. The bandwidth of a thermal RMS meter is probably several mHz.

Good question.

 

[LMAO]

A true RMS ammeter with 50 kHz bandwidth will probably register almost all harmonics because the amplitude of the higher harmonics keeps dropping off. Even a 5 kHz bandwidth may be enough. The bandwidth of a thermal RMS meter is probably several mHz.

Good question.

You are correct; but what I was really asking was: AFTER you calculate all harmonics and find the total RMS current, should you use that to calculate your cable ampacity and size the cable?
So here is my confusion: say I have a 1200kVA VFD at 480V; my current would be 1445A and 1445*1.25=1806. Should I use 1806A as the ampacity of cable feeding the VFD? Or should I add up all the harmonics (mostly at 5th, 7th, 11th and 13th harmonics) to the 1806A? NEC does not really say anything about this.

 

[gar]

110325-1928 EDT

LMAO:

You have to find out what 1200 KVA means. What are the standards for defining this value? Is it based on RMS current at full load under worst conditions, or something else.

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