Finding my MCB size with an unbalanced load in a 3 Phase 480V Delta system

jakeparsons03

Member
Location
USA
Occupation
Engineer
I am having trouble making the final leap in my conceptual understanding of this problem.

I have a whole bunch of single phase 480V loads that I am feeding from a 3 Phase delta distribution panel. All of these single phase loads are different which causes the feed panel load to be wildly unbalanced. I moved things around and got the unbalance as low as I can go but now I am stuck. I need to size my panel and MCB to the size of the highest load (*1.25 of course) but I am having a hard time figuring out the calculations needed to get that number. So here is what I have

Phase A: 126.5kVA
Phase B: 127.75kVA
Phase C: 112.25kVA

Conceptually what I understand is that when phase b is peaking then that current is being returned on phase A and phase C. In a balanced system that wouldn't matter as Phase A and Phase C are at a combined point in their waveform which can even out the current from phase B. In this unbalanced system while phase B is peaking your other 2 waveforms are at a point where they are needing to return their current through phase b. So that being said your max peak current is Phase B peak + returned current from phases A and C. That total current is what I need to set my panel size and breaker value to.

My question for you is how do I go about calculating that absolute peak current for this system.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
You are making this more complicated than you need to. You don't need to worry about current imbalance when sizing feeders and protective devices.
Basically you have a 128kVA three phase load, why can't you use that to determine your current?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
You are making this more complicated than you need to. You don't need to worry about current imbalance when sizing feeders and protective devices.
Basically you have a 128kVA three phase load, why can't you use that to determine your current?
Effectively a loading of 128kVA per phase is equal to a three phase load of 378kVA.

This is about what topgone used in calculating his value.
 
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