Unbalanced Feeder Calculation

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xguard

Senior Member
Location
Baton Rouge, LA
GIVEN:

208/120 3 phase panel

L1 - 60,000 VA
L2 - 63,000 VA
L3 - 54,000 VA

QUESTION: To determine the current for sizing the feeder conductors and feeder ocpd, that supplies a 3 phase panel with MCB, which approach is correct (hopefully one of the options is correct)?

(60,000 VA +63,000 VA +54,000 VA)/(1.73 x 208 V) = 491 AMPS

or would the following be necessary to take into account the higher load on L2:

(63,000 VA X 3 ) / (1.73 x 208) = 525 AMPS

NOTE: Assuming multipliers for continuous versus non continous loads have been taken into account in VA values. Also I realize in this example the difference may or may not affect conductor or OCPD sizing.

Thank you for any help!
 
Both are "correct," in that they provide results that you can use to select the wire sizes and breaker ratings. I would choose the second method, because I prefer to be conservative in my design choices. If that results in a higher cost, and if the client wants me to find a cheaper solution, I would work on balancing the loads a bit better.
 
charlie b said:
Both are "correct," in that they provide results that you can use to select the wire sizes and breaker ratings. I would choose the second method, because I prefer to be conservative in my design choices. If that results in a higher cost, and if the client wants me to find a cheaper solution, I would work on balancing the loads a bit better.
Click to expand...

Not to mention that some variant of Murphy's Law will ensure that furture load additions will only exacerbate the unbalance, not correct it.
 
charlie b said:
Both are "correct," in that they provide results that you can use to select the wire sizes and breaker ratings. I would choose the second method, because I prefer to be conservative in my design choices. If that results in a higher cost, and if the client wants me to find a cheaper solution, I would work on balancing the loads a bit better.
Click to expand...
I think the first method is incorrect. It could result in choosing a wire of ampacity less than 500A (60,000VA ? 120) connected to L1 or 525A (63,000VA ? 120) connected to L2.


All sizing determinations require the ampacity to be not less than the calculated load current.
 
Smart $ said:
I think the first method is incorrect. It could result in choosing a wire of ampacity less than 500A (60,000VA ? 120) connected to L1 or 525A (63,000VA ? 120) connected to L2.


All sizing determinations require the ampacity to be not less than the calculated load current.
Click to expand...

I agree. I don't see how you can use a method that results in a wires ampacity being less than what is actually required.
 
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