Unbalanced transformer loads

[kwired]

If you have two 320A, 208V (line to line), single phase loads fed from one three phase transformer, what should be the power rating of that transformer? 230kva?

thanks

Are you connecting both loads to same phases or are you going to connect to all three phases with one phase having two connections?

[LarryFine]

Looks like a maximum Load of 133.12 KVA, as the OP defined (2) L-L 208V Heaters, drawing 320 Amps each.

If both Loads are to be driven Coincidentally, Minimum 3 Phase Transformer Capacity would be 150 KVA.
If Loads are Non-Coincidental, the Minimum Capacity would be 75 KVA.

The problem with your thinking is that each individual load cannot be divided among the three phases. Each load must be supplied individually.

Each heater requires 66.56kva, or a 75kva 1ph transformer. Two such loads would each require a 75kva unit; three such loads, three units.

3 x 75 = 225.

Just because you eliminate one of the loads, does not lessen the required capacity each transformer secondary must be able to supply its L-L load.

A Delta transformer supplying a Delta load is basically three 1ph units supplying three 1ph loads, and they happen to share line conductors.

While the shared lines will see 1.732 times the load current, each transformer secondary will see only the individual load across its two lines.

So, each secondary must be sized for the load across it. The only way to get two 75kva secondaries in one case is a by using a 3-ph 225kva transformer.

[LarryFine]

Oops. See below.

[LarryFine]

Are you connecting both loads to same phases or are you going to connect to all three phases with one phase having two connections?

The latter, I hope. Otherwise, he'd need a 150kva transformer (and I'd have typed the above needlessly. )

[hurk27]

I come up with a much different Apparent Power Value than posted by others.




Looks like a maximum Load of 133.12 KVA, as the OP defined (2) L-L 208V Heaters, drawing 320 Amps each.

If both Loads are to be driven Coincidentally, Minimum 3 Phase Transformer Capacity would be 150 KVA.
If Loads are Non-Coincidental, the Minimum Capacity would be 75 KVA.

Connect Loads as follows:

Heater #1: Connect between Line "A" and Line "B"

Heater #2: Connect between Line "C" and Line "A"

Line "A" would be common to both Heaters and their corresponding 2 Wire Branch Circuits. The Heaters would be connected in an Open Delta fashion.

Scott

Would not Phase A then have twice the current on it?
A 150 kva transformer would only supply 416 amps per phase?

Sorry Larry didn't see the second page

[david luchini]

Would not Phase A then have twice the current on it?
A 150 kva transformer would only supply 416 amps per phase?

Not twice the current, but 1.732 times the current. Scott approach would certainly load phase A beyond its rating.

[kwired]

What are the 320 amp loads that apparently can not be balanced across the three phases? Being able to balance across all three phases will allow for a smaller transformer, feeders, switchgear, and so on.

Some have mentioned heaters, but the OP did not mention what the load actually is.

[LarryFine]

Being able to balance across all three phases will allow for a smaller transformer, feeders, switchgear, and so on.

For a given total load, yes. But, we're not talking about a bunch of small loads we can move around. This is two individual loads, each of which will be connected between two lines, i.e., across one phase.

The load current for each piece of equipment is 320a (even if intermittently.) Each source's phase must be able to supply that. That fixes the size of each phase's transformer to 75kva. Three of those is 225kva.

Some have mentioned heaters, but the OP did not mention what the load actually is.

It looks like scott said it first in post #8.

Ever play "Telephone?"

[LarryFine]

Scott approach would certainly load phase A beyond its rating.

Don't confuse line current and phase current; they're not the same in Delta (which I presumed the OP to intend to use, and which wouldn't change the power anyway.)

[kwired]

For a given total load, yes. But, we're not talking about a bunch of small loads we can move around. This is two individual loads, each of which will be connected between two lines, i.e., across one phase.

That is why I asked what the load is, A load that large is somewhat unusual to be limited to a single element or other component and if multiple components often are capable of being reconfigured for single or three phase, multiple voltages, etc.