Main Panel 400 Amp - How to measure capacity -

[hardworker]

Main panel 400amp 120/240 single phase 3-wire service

Clamp meter shows 180 amps being drawn on phase A. Clamp meter shows 190 amps being drawn on phase B.

Will the main breaker of 400amp capacity trip when the total of both phases exceeds 400 amps?

OR

Will it trip only when either of these two phases exceeds 400amp?

 

[Smart $]

The latter.

 

[hardworker]

So am I correct to say, at this point the capacity being used is a bit less than half?

 

[Smart $]

Yes... but that depends on perspective. It is only true at the instance you measured. If you are attempting to evaluate whether loads can be added, your measured values will likely not indicate how much of the system is available. The NEC requires determination through 1) load calculation, 2) month-long monitoring for peak demand, or 3) POCO demand data.

 

[hardworker]

Yes, I understand that. Thanks for your information.

 

[Smart $]

You're welcome.