I would go with a minimum of 225, and probably I would pick the 300 instead. I look at it as if I planned to install a third load of the same size and type, and connect it so as to balance the loads among the three phases. That is a total of 200 kVA. The 225 is the next higher size, and the 300 would give some room for additional loads. Keep in mind that this is a design process, not a code requirement.
I don't know of any such "allowance," but, it would be for the manufacturer to say.One more thing, I am actually thinking of going with 150kva; is there an IEEE guideline for overloading transformers for some time periods and duty cycles? the loads will be at peak for only a few hours a week.
I am missing something. Put three 320 amp loads on your tansformer. A-B 320 amps, B-C 320 amps and C-320 amps.
The KVA is KVA= Vl-l * I * 1.732
=320*208*1.732= 115.281 KVA
Your only using 2/3 of that but you can't go lower unless you configure the load differently.
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?
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
The problem with your thinking is that each individual load cannot be divided among the three phases. Each load must be supplied individually.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 latter, I hope. Otherwise, he'd need a 150kva transformer (and I'd have typed the above needlessly. )Are you connecting both loads to same phases or are you going to connect to all three phases with one phase having two connections?
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?
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.Being able to balance across all three phases will allow for a smaller transformer, feeders, switchgear, and so on.
It looks like scott said it first in post #8.Some have mentioned heaters, but the OP did not mention what the load actually is.
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.)Scott approach would certainly load phase A beyond its rating.
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.