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Current Unbalance Due To 1Phase Loading

Current Unbalance Due To 1Phase Loading

Current Unbalance Due To 1Phase Loading

(OP)
In a 3 phase 3 wire system, the voltages are measured at 242VAC. The currents are measured at 20A, 18A and 7A. If these are single phase line-to-line loads, in the absence of a panel schedule, can we say that the power consumption will be 1.732 x (20A + 18A + 7A)x 242V ?

It is also observed that the current unbalance due to actual loading is severe 100%-[(7/20)x 100%] = 65% unbalance. Is there a way to avoid this much of unbalance regardless of load switching arrangement or is it a fact that if the panelboard circuits were properly balanced, then there is no way of any unacceptable current unbalance regardless of how much loads are switched on?

RE: Current Unbalance Due To 1Phase Loading

You need to think about your formula a bit more.  What would it look like if all three currents were the same, and you only wrote that number once?

RE: Current Unbalance Due To 1Phase Loading

(OP)
Thanks for the correction, for this case then,the three phase apparent power can be safely estimated using the highest current on the phase (1.732 x 242V x 20A).

My question is if there is another way of making the computations adding up the total power per phase in this unbalanced condition?

RE: Current Unbalance Due To 1Phase Loading

It really depends on what you are trying to accomplish.  If you want to estimate a utility bill, the average current would probably be fine.  If you are trying to figure out the capacity of the system in use then the maximum current would be better.

RE: Current Unbalance Due To 1Phase Loading

(OP)
Thanks david for the technical advise.

RE: Current Unbalance Due To 1Phase Loading

Um, sorry to disagree with y'all, but you have the 1.732 factor on top, and it should be on the bottom.  If 242 is the L-L voltage, then the total power would be (20A + 18A + 7A)x 242V/(1.732).

Your sum of (20+18+7) = 45A = 3 TIMES the average phse current.  The average phase current = 45/3=15A.  If you use 45, then you need to divide by 1.732; if you use 1/3 x 45 = 15, then the 1.732 goes on top:

P=IV sqrt(3)
P=15x242x1.732 = 6.3kVA
P=45x242/1.732 = 6.3kVA
P=45x242x1.732/3 = 6.3kVA
P=(20A + 18A + 7A)x242/1.732 = 6.3kVA.


RE: Current Unbalance Due To 1Phase Loading

peebee, your formulas are correct.  nightfox1925 could have modified his original formula to put the 1.732 on the bottom as in your fourth equation; or he could have just used a single current as he did in his second post and the 1.732 is correctly on the top there, as in your first equation.

RE: Current Unbalance Due To 1Phase Loading

Depends on "where" you are measuring the power consumption.... If you want to look upstream of the xmfr then you may be in for a suprise especially if dealing with bad pf's and/or non-linear loads.  Also, is the measured value a fundemental current measurement or does it include harmonic currents as well?

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