Subpanel Main breaker calculations.

[robert1234]

I have a project where a subpanel is needed to be added to the main panel 240/120 V single phase. The subpanel consists of 9 branch circuits, all 120volts line to neutral. The subpanel has 2 line conductors coming form the main panel(phase A&B).The total VA of connected load is 9250VA. When calculating the main breaker for the subpanel do I divide the 9250VA by 120V or by 240V?

 

[charlie b]

240.

Pretend, for a moment, that you have 10 circuits, all the same, each being 900 VA. You put half on Phase A - Neutral, the other half on Phase B - Neutral. Turn them all on. Half the time, the current will leave the upstream panel along its Phase A wire, pass through the Phase A circuits, then travel "backwards" through the Phase B circuits, and return to the upstream panel along its Phase B wire. The other half of the time, it will be reversed. So the upstream panel will see the load for all 10 circuits, a total of 9000 VA, travelling from its Phase A wire to its Phase B wire. The voltage across those two wires is 240. So the current seen by the upstream panel is 9000 VA / 240V, or 37.5 amps.

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