Your calcs need to be with 208 not 120. And your protecting the conductors feeding the other panel. So size at 1.25 percent of conductors. Screw the total load. Your ocpd cares about the conductor not what your putting on it.This is my first time calculating a breaker for a three-phase source.
I have a fuse panel (we’ll call it FP #1) supplying another fuse panel (FP #2) with three phase power.
The output of FP #1 is fitted with a 50 A breaker. FP #2 is connected to 80 loads (unbalanced) which total the following:
Apparent Power: 12000 VA
Voltage: 120 V
Frequency: 60 Hz
Full Load Current (FLC):
- Phase AB = 38 A continuous
- Phase BC = 36 A continuous
- Phase CA = 31 A continuous
I calculated the FLC for each phase to be:
- A = 59.9 A
- B = 64.1 A
- C = 58.1 A
I feel like someone has put a 50 A breaker on FP #1 based on the maximum line-to-line current of FP #2 (38 A * 1.25 = 47.5). Am I correct in thinking that a three-phase breaker should be rated based on the average of the phase currents (59.9 + 64.1 + 58.1)/3 = 60.7A; 60.7A * 1.25 = 80 A breaker on a 80 A+ rated cable.
Thanks!
That is what I would use for balanced loads; I'm under the impression that you can't use that for unbalanced loads?12000/(208x1.732) hint hint
For phase A:Give out the data (homework) you used to calculate this and we can show you how to do it right.
And I thought I knew something. Now even apprentice calcs look like gibberish to meFor phase A:
x=0.866*(AB + CA)
y=-0.5*AB + 0.5*CA
z=x + yj
|z|=(x+y) ^ 1/2
Current in Line 1 (Phase A) = 59.9A
This process is useful for unbalanced loads. For balanced loads I'd simply sum the currents, multiply by 3^1/2, and divide by 3. This value would then be used to find the circuit breaker rating (times FLC by 1.25 if continuous).
Hope this helps you understand what I'm trying to wrap my head around.
I would round to the 80-amp breaker instead of the 85-amp breaker as far as the calculations go. If I was installing #4, I would use the 85-amp breaker.I'm looking at how to calculate a breaker for a three-phase circuit.
Given the following Full Load Currents (FLC):
- Phase AB = 38 A continuous
- Phase BC = 36 A continuous
- Phase CA = 31 A continuous
I calculated the FLC for each phase to be:
- A = 59.9 A
- B = 64.1 A
- C = 58.1 A
I'm wondering how three-phase breakers work. 64.1A x 1.25 = 80.125A. Does this mean a 85A breaker would be ideal for protecting this circuit as it covers our highest load (phase B)?
Thanks!
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