Capacitor Bank Cable Sizing

[pristava]

For the calculation of the 200kVA Capacitor Bank Cable Size
I started by :
Amp = 1.35 x 200 kVA / 1.73 x 600 Volts
After that, that Amp I multiplied by 1.25

Some people working with me is is trying to say that we don't need a another multiplication (by 1.25), what I can not accept.


Please comment.

 

[wareagle]

NEC 460.8 only requires 135% of the rated capacitor current.

 

[LionelHutz]

The NEC (and CEC) requirement is 1.35 times the rated current.

Where did you get the extra 1.25x factor?

 

[pristava]

Most capacitors are designed to operate at 135% of their kvar ratings. I am finding for that situation 1.25 factor must be provided.
Also, some manufacturers suggested cable sizes for their capacitor banks. I re-calculated their cables and I found that 1.25 factor is included, othewise cables are oversized.

 

[rbulsara]

warengle is correct. If the nameplate of the capacitor reads 200kvar, 200 it is. You do not need 1.25 on top of 1.35 to meet the code.

If units are designed to operate at higher capactiy but not intended to run at that capacity, you need not over size cables. NEC takes that (135% capacity) into account andthat is why it asks for 1.35 factor and not 1.25 with normal loads.

After all you did not select 200kvar rating, because you needed 270kvar, or did you?

 

[LionelHutz]

A 200kVAR cap draws about 192A at 600VAC. A 270kVAR cap would draw about 260A at 600VAC. If you take 192A times 1.35 then you get 260A. This seems to be where the 135% value for cable size comes from.

In reality, you'd have to apply 697VAC @ 60hz to achieve 270kVAR which would cause a current draw of 224A.

The capacitor is designed to operate at 200kVAR. The 135% designed in capacity is to allow for some tolerance in the operation so that if you apply a voltage greater than 600VAC the capacitor won't fail. It can also be to allow for some harmonics in the system without failing. Afterall, the power system won't supply a perfect 600VAC, 60hz.


The 3 continuous ratings of a typical power system capacitor are;
135% KVA
110% Voltage
180% Current