Single phase kva

[stidwell]

I am trying to size a transformer for a 120/240Volt panel.

I have totaled up the loads on each phase and have 34A on phase A and 33.4A on phase B

I have totally confused myself and trying to get it straight in my head.

Do these loads give me 16.176KVA or 8.088KVA

The formula i am using is KVA=(A+B)*240/1000

is this correct or should i be using KVA=(A+B)*120/1000

The difference is between a 10kva and a 15kva transformer.

Please help and thanks in advance for any help

 

[Cold Fusion]

I'd likely use A*240/1000 = kva

cf

 

[augie47]

If you are going to add A+B then 240.

33.4 x 120 =4 kva Load 1
34 x 120 = 4kva Load 2

1+2 - 8 kw
or
34 x 240 = 8 kw

either way

 

[charlie b]

If you are going to add A+B then 240.

I think you said this backwards, but the rest of your math is OK. This sounds like you are suggesting taking 34 + 33.4, getting 67.4, and then multiplying that 67.4 times 240. I think you meant that if you add A+B you use 120, as you correctly explained thereafter.

To Stidwell: The two amp measurements you took are not measurements of two separate currents. The current leaving on one leg returns to the source on the other leg. It is the same current. Since the two numbers are not identical, you can infer that there is a slight imbalance in currents. The difference will be found as neutral current.

So as far as the 240 volt system is concerned, you have 34 amps of current. I would use the higher of the two readings, though you could get by with the average. So take 34 times 240.

 

[glene77is]

Charlie,

"The current leaving on one leg returns to the source on the other leg.
It is the same current."

Everybody forgets this one!
And then someday, they get into 3phase... :smile:

 

[Besoeker]

I am trying to size a transformer for a 120/240Volt panel.

I have totaled up the loads on each phase and have 34A on phase A and 33.4A on phase B

I'd go with 34*120 and double it.
Even if the 34A load was just on one of the two 120V windings and the other had nothing, you'd still have to do that.
The two windings are going to have the same kVA (and current) rating regardless of loading so you have to consider the more heavily loaded one to get total rating required.
Unequal loading just means you won't make best use of the transformer kVA rating.