3 phase vs 1 Phase 2

[2esingeek]

I have a 3 ph 75kva 13.8kV-208/120V XFMR connected to a 3 phase 208/120V panel. I have two 1 phase panels that I would like to connect to this system and no other loads. The two single phase panels are 100A each. I realize that I can take phase A and B to become 240V to panel A and Phase B and C to panel B but what will this do to my XFMR loading? I will have two phases of my three phase XFMR loaded. Will the neutral carry the panel fed from the XFMR's unbalanced load? Please help thanks in advance. Do I need to oversize the neutral? Thanks in advance.

 

[waross]

The common way to do this is to use three phase 120/208V panels throughout.
You may take two phases to feed a single phase panel but your voltages will still be 208?120V.
With equal line to neutral loads on the panels the neutral will carry about the same current as the hot lines (subject to power factor issues). The neutral conductor must equal be full ampacity in relation to the line conductors.
Be careful if you are going to load this installation to near capacity.
The current on "A" phase in the supply transformer will be the vector sum of:
The load current from "C" phase to "A" phase,
The load current from "A" phase to "B" phase,
and
The load current from "A" phase to neutral.
The simple case is balanced line to line three phase loads plus a line to neutral load, all at unity power factor.
In this case the three phase current may be added directly to the single phase current.
For other cases phase angles and vectors must be used to calculate the currents.
But, in practice, why do you want to know? If this is for homework, don't ask. We don't do homework.
If your superior has asked for an exact calculation, we have some calculation GURUs who will possibly be willing to assist you.
If you have a practical problem, consider the following:
You don't have to know all three phase currents, just the current on the heaviest loaded phase.
Note: Transformers are limited by KVA, not KW. Even though it may not be exactly correct to use KVA for loading, we will be concerned with KVA loading, not KW loading.
If your three phase line currents are 50A, 70A and 45A, just use 70A for all three phases. Add your line currents from the single phase (actually 2 of 3 phase) panels to the 70A for total currents.
Errors introduced by unbalanced loads and less than unity power factor will reduce the actual currents below these totals, so your installation will be safe.
If you have line to line loads on your 2 of 3 phase panels these loads may be considered as part of the three phase loads.
Three phase load currents and line currents.
If you have load currents of 100A line to line on all three phases, then your line currents will be 100A x 1.73, or 173A on each line. If your line to line load currents are not equal, use 1.73% of the highest current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[2esingeek]

Yes if this were a grass roots design then it would be better to use a three phase panel, but the two single phase panels are prewired to the trailer. Are you suggesting that I pull out the single phase panels and turn them into three phase panels?

 

[waross]

For trailers, leave the single phase panels and use two phases of the three phase transformer.
Best is to connect one trailer to "A" neural, "B" and the second trailer to "B" neutral "C". Yes, I know, you can't always do that either. Do it if you can. Otherwise, make it work. But, be aware of the transformer loading.
Sometimes more cable now is better than a new transformer and more cable later.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[2esingeek]

Thanks! Waross! Star for you!

 

[wareagle]

Quote "I realize that I can take phase A and B to become 240V to panel A and Phase B and C to panel B but what will this do to my XFMR loading?"
Ypu won't have 240 volts. It will be 208 volts. Suggest you install a 3 phase panel and split the single phase load between phases.