If a 3-phase transformer is rated at 750kva, it has a capacity of about 900 amps. The question is, can I expect to support 900 amps on each phase, or 900 amps total of all three phases?

I believe it to be the latter, but need to be sure.

Thanks,

Bill



Moderator corrected
p.s. I see I can't edit the misspelling in the title bar. "Laoding" should be "Loading".

I don’t remember about the loading of an X-former. I did read over the topic and didn’t catch the misspelling. So don't feel bad some days I misspell my own name. Today is defiantly one of those days.

If I am Correct the KVA rating for the transformer is total load all three phases.

Volts x Amps x 1.73
---------------------------- = 3 Phase KVA
1000

The use of one phase on a 3 phases transformer isn't a good practice because the voltage to ground of that phase isn't a common voltage, however the kva of the primary must equal the kva of the secondary under the ideal transformer concept. If you need the use of a single phase you will need to use another transformer that take you to the correct voltage that you need, also this way you will maintain the correct balance in the phases. So you will have the total amps capacity (900) for all 3 phases; and don't forget to make the calculations needed to keep your kva at the correct rating. Hope this help.

The 750kVA and 900A would be consistent with 480V (you forgot to specify this little detail, tsk tsk).
But how can you even doubt this ?
There must be a nameplate on the trafo with all the relevant details.

Anyway, when 'looking' from one of the sides of the transformer, the simple formula is:
P=U*I*1.73
P is W
U is V across 2 phases. In 3 phase systems, when nothing else is mentioned voltage is assumed to be meant between phases.
I is A in one phase. In 3 phase systems, when nothing else is mentioned current is assumed to be meant inside one phase.
Notice that you can always measure U and I this way (U across, I inline) treating the trafo or motor as a 'black box'.
Also notice, this just shows what P goes in or out of the trafo. It does not account for losses. The simple formula only looks at one side of the trafo.

To answer you question: It is 900 A per phase @ 480 V between phases.

Thanks for the reply, but I never suggested using only one phase. The question is, if measuring the load on each phase, can I expect to support rated capacity on each leg (phase) or is rated capacity the total draw on all three legs.

Two other sources (Acme Transformers and Square-D) tell me that for my 750kVA 4160x480-volt transformers, 300 amps on each leg (total of 900 amps) is fully loaded.

Bill

Interesting take, thanks. I've got an electrical engineer en route today to size-up the situation for me. So far the two most direct answers I've gotten indicate that 300 amps per phase would be fully loaded.

Three Phase KVA =Volts x Amps x 1.73 /1000
480x900x1.73/1000
Answer = 747 KVA

Amps = 3 Phase KVA x 1000 /Volts x 1.73
Answer = 903 Amps Total

Electrical engineer was here. I have the answer, and some explanation/clarification of my original question.

The answer, in short, is 900 amps per phase. What I've done is install current transformers on each secondary phase, and add-up the resulting current draws. So, with approximately 300 amps being drawn on each phase, my Scada display of the 900 amp total is incorrect; that result must be divided by three.

Bottom line is that I am operating well within capacity and am therefore a happy camper once again. Thanks for the reply's!

Bill

That's a bad sign when the companies that make the xfmr. and an Engineer have two totally different solutions. I always thought the amp rating was for all 3 phases.

I agree, and I lost more than a little sleep over this issue. But the new load center that is now the major load on the transformer in question, has built-in current/voltage/harmonics monitoring. It, too, shows about 300 amps of load. That's what was bugging me; knowing the only significant load was in the 300-amp ballpark, where in hell was the other 600 amps going? This Plant is 35+ years old, I thought I really understood the power distribution, but could not assure myself of the situation/solution. I hate to change anything (in this case, re-distribute load) without thoroughly understanding what is going on and what will be the result of the change.

I'm really glad it worked out as it did!

Bill

I think I miss understood this phrase. If the load is only drawing 300amps thats all the amp meter is going to read. If you added more load, say 200amps, then the amp meter would read 500amps. Maybe you know this and miss quoted your thoughts. I do alot worse sometimes.

It is important to understand that AT THE TRANSFORMER (a few hundred feet from the load center with the ammeter), we installed current loops on the secondaries of each of two 750kva transformers. We were taking the total of the three current loops on EACH transformer and adding them to determine total load one each transformer. That method is incomplete; you add the three (phases) and then divide by three (because all three legs are not exactly evenly loaded). I repeat this only because it is NOT intuitive. And two reputable firms gave me wrong info.

Now, at the load center, the same 900-amp number we were seeing at the transformer was being properly displayed as 300 amps (give or take) because the built-in monitoring device was doing the math properly. But because of the massive amount of rewiring being done here during this project, it was not clear that I was looking at basically the same "juice" being calculated two different ways.

Make more sense now?

Bill

104 goodbuddy