Having a debate in the office. This is kind of like when you know you've spelled a word correctly, but when you look at it, it looks wrong.
Say we have a 208V/120 3phase panelboard with a main breaker. Say we connect to it 6 heating loads, each 3kva and 208V single phase. Say these loads are ALL across phase A and B.
This is where the argument begins. What is the minimum size the MCB can be?
One person says you have your 6 loads. For each, 1500va is on phase A and phase B. Add this up. You have 18kva total, but because of the imbalance, you need to look at the current in legs A and B. Each Leg: 1500*6=9000VA. 9000/120 = 75A. Thus 75 is the minimum you can have.
The other says that because we are dealing with 208V 1ph loads, both phases will be experiencing the same current at the same time, because there is no neutral. So for EACH load, you have 3000/208=14.4A. Thus, on the panel, you have 14.4*6=86.5A on EACH leg, and thus 87A is the smallest it can be.
(Obviously this ignores breaker sizing issues like continuous load, etc)
Which is correct?
Say we have a 208V/120 3phase panelboard with a main breaker. Say we connect to it 6 heating loads, each 3kva and 208V single phase. Say these loads are ALL across phase A and B.
This is where the argument begins. What is the minimum size the MCB can be?
One person says you have your 6 loads. For each, 1500va is on phase A and phase B. Add this up. You have 18kva total, but because of the imbalance, you need to look at the current in legs A and B. Each Leg: 1500*6=9000VA. 9000/120 = 75A. Thus 75 is the minimum you can have.
The other says that because we are dealing with 208V 1ph loads, both phases will be experiencing the same current at the same time, because there is no neutral. So for EACH load, you have 3000/208=14.4A. Thus, on the panel, you have 14.4*6=86.5A on EACH leg, and thus 87A is the smallest it can be.
(Obviously this ignores breaker sizing issues like continuous load, etc)
Which is correct?