3 Phase power......refresher

[dwalker]

This is probably elementary but, I need a quick refersher on 3 phase power. We have a control panel, which contains motor starters and heaters, that is fed by 3 phase 240VAC and I need to size the incoming overcurrent protection. There are two 3-phase heaters and a dozen or so single phase heaters that are connected to different legs of the 3-phase lines. I can figure out the load calculation for each load: three phase loads: A = kW/(V*1.732) single phase loads: A = kW/V The three phase loads can just be added up, but what about the single phase loads? Let's say I have 5 loads on phase A-B, 4 loads on A-C, and 4 loads on B-C what is the correct way to determine the load on the three phase system? Thanks in advance. -D

[BobLfoot]

It may be old school and somebody will chime in I'm wrong, but I alsways do three calculations. A Phase A, Phase B and Phase C calculation. The calculation is 3 Phase Load plus all Singles for that phase. An example follows: 3P#1 - 5A 3P#2 - 7A 3P#3 - 10A A-B#1 - 2A A-B#2 - 3A A-C#1 - 2A A-C#2 - 3A A-C#3 - 4A C-B#1 - 2A C-B#2 - 3A C-B#3 - 4A C-B#4 - 5A Phase A = 3P#1 + 3P#2 + 3P#3 + A-B#1 + A-B#2 +A-C#1 + A-C#2 +A-C#3 = 5A + 7A + 10A + 2A + 3A + 2A + 3A + 4A = 22A + 14A = 36A Phase B = 3P#1 + 3P#2 + 3P#3 + A-B#1 + A-B#2 + C -B#1 + C-B#2 + C-B#3 + C-B#4 = 5A + 7A + 10A + 2A + 3A + 2A + 3A + 4A + 5A = 22A + 19A = 41A Phase C = 3P#1 + 3P#2 + 3P#3 + A-C#1 + A-C#2 + A-C#3 + C -B#1 + C-B#2 + C-B#3 + C-B#4 = 5A + 7A + 10A + 2A + 3A + 4A +2A + 3A + 4A + 5A = 22A + 23A = 45A

[dwalker]

Makes sense....I think.... So when using the above calculations I then size the conducter to feed the panel, and select the breaker size to protect those conductors based on NFPA79 13.5.4. So, using above #'s, I assume the 3P devices are motors and the rest are heaters: Phase A = 3P#1 + 3P#2 + (3P#3 x 1.25) + (A-B#1 + A-B#2 +A-C#1 + A-C#2 +A-C#3) = 5A + 7A + (10A x 1.25) + ((2A + 3A + 2A + 3A + 4A) x 1.25) = 24.5A + 17.5A = 42A Phase B = 3P#1 + 3P#2 + (3P#3 x 1.25) + ((A-B#1 + A-B#2 + C -B#1 + C-B#2 + C-B#3 + C-B#4)x 1.25) = 5A + 7A + (10A x 1.25) + ((2A + 3A + 2A + 3A + 4A + 5A) x 1.25) = 24.5A + 23.75A = 48.25A Phase C = 3P#1 + 3P#2 + (3P#3 x 1.25) + ((A-C#1 + A-C#2 + A-C#3 + C -B#1 + C-B#2 + C-B#3 + C-B#4) x 1.25) = 5A + 7A + (10A x 1.25) + (2A + 3A + 4A +2A + 3A + 4A + 5A) x 1.25) = 24.5A + 28.75A = 53.25A So this would lead me to select a 60A feed. Am I on the right track here or did I diverge way to much? Thanks for your help. -D

[paulengr]

Calculate as you already did. Do not forget that your VOLTAGE number will be different if you have any line-neutral loads. Total the balanced 3-phase loads. Then for each leg, add all the single phase loads that are attached. This gives you total current draw on each leg. Buy your wire so that it is symmetrical (use the worst case). You are also missing power factor in your numbers. If you can assume that things are reasonably balanced, then you can outright use total kw and figure it out with the 3-phase calculation. That's what is typically done. Partly because if a feeder is long enough it tends to balance anyways (inductive coupling).