Transformer Load 1

[cwhit03]

I am interning a a power company and a lineman turned in a sheet the other day on a transformer he was called out on. They replaced it with a 50 KVA and the readings were as follows: X1=182 Amps (Phase to Ground), X3=189 Amps (Phase To Ground). This transformer is a 120/240 single phase transformer. I was a little confused when determining the load. Do you take the voltage as 120 or do you take the voltage as 240, when you get it in the method of data he gave. Technically we had 21.84kW on X1 and 22.68kW on X3, which gives a total of 44.52kW for both legs. At this figure the transformer would only be loaded at 89% right? OR would it be (189 + 182) * 240 = 89.04 kW. Which one is the correct loading on this transformer.

 

[mls1]

Actually, 182 amps is flowing phase to phase and there is only 7 amps flowing though the neutral. So you can calculate it as 182*120+189*120 or 182*240 + 7 *120. Either way it's 44.52kW.

 

[waross]

I think the units shouild be Kilo-volt-Amps. You can not calculate the kW without knowing the power factor.
You have a 50 kVA transformer rated at 120/240.
That equates to a current of 50,000 VA / 240 volts.
That's 208.3 amps. That is the full load current rating of the transformer, and the number to use to determine loading as it relates to transformer heating.
208.3 Amps. Full load.
The pitfall of using KVA to determine loading is the day that someone reports a load of 215 amps and a measured voltage of voltage of 226 volts. The result is 48.59 KVA. It may look like a safe load on a 50 KVA transformer but the current rating of 208.3 amps is exceeded.
Use nameplate KVA and nameplate voltage to determine rated current. Then compare the field current reports to your calculated rated current.
respectfully

 

[stevenal]

Another pitfall: Each half winding is rated 25kVA. Comparing the total to the 50 may look fine while you are overloading the transformer by unbalance.

By the way, the lineman's clamp on ammeter can only sense the current in the enclosed conductor. It has no way of knowing where the current came from or where it's going.