System Load 480 3 phase

Thread Starter


Joined Apr 18, 2016
Hello all, and first off thanks for the help in advance. I'm a maintenance engineer, but I'm basically illiterate when it comes to the electrical side of things. Hoping some of you folks here can help educate me, as this is a topic I'd like to learn more about.

We have a new machine that we're looking at bringing in within the next month or two. This is our system we're hoping to tie into.
Step 1) Utility 12470 coming in
Step 2) Transformer 12470 to 480
Step 3) Switchgear 480 3ph
Step 4) 400A Breaker
Step 5) Overhead bussbar

I've hooked up this to log what our amp draw is for each phase:

My question is this, with a 400A breaker what are the equations to calculate the load to trip the breaker for the following:
1) Single phase load........400A?
2) Total load.....................
3) Imbalance...................
I know part of this will be specific to the breaker, I just want to understand the whole picture. Due to arc flash requirements, we can't remove the panels, so I'm having to hook the fluke kit up where the wire lugs onto the bussbar, since they are level 0 because of the breaker. Also, the switchgear is about 30 yrs old.

I'm also overlooking what the load is on the switchgear itself, at least for now, under the assumption that the same equations will apply, and that we can't measure it due to arc flash restrictions. I'm assuming that we want it to be below 80% capacity, right?


Joined Nov 4, 2013
A 400 amp breaker is just that, a switch that opens itself if the load on any single line goes above 400 amps for a specific magnitude and period of time.

Also yes for continuous duty loads you typically size things to run at less than 80% of the services capacity which in your case would be 320 amps per phase.

Thread Starter


Joined Apr 18, 2016
The time and response is set by the dip switches on the breaker, correct, from low to high sensitivity?

So the only thing that will cause the breaker to trip is the single line amps exceeding 400, or whatever variance is set to that by the dip switches, which should be around 320-350 in my mind, since that's the recommended max load. So imbalance between phases shouldn't cause issues as long as they remain lower than the above number regardless of the combined amp load for all 3 phases?

Sorry for the noob questions, electrical is something I wish I'd taken more of in college. Realizing now that it's a gap I need to fill.