Sizing a 120/240v transformer

KyleFowler

Member
Location
Maryville, TN
This is probably a dumb question but how do i properly size a single phase transformer with a 120/240v secondary that will feed a single phase panel with only 120v loads? Do I add up the wattage of each 120v circuit?

For example: I have one circuit on the A phase at 120 volts and pulls 16 amps which is 1920 watts. Then I have another 120 volt circuit on the B phase that pulls 4 amps which is 480 watts. So together this is 2400 watts. The next standard size transformer is 3000 watts. Is this the correct method?

I tried finding an example online but only found examples where the loads connected were the same voltage as the secondary of the transformer.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Each 120v secondary can supply half of the secondary's capacity, or 1500w. If wired like a typical 120/240v secondary, each half can supply 12.5a.

If wired in parallel, the same transformer can supply 25a at 120v. So, you need to do it this way to do what you want. The secondary has four wires, right?
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
This is probably a dumb question but how do i properly size a single phase transformer with a 120/240v secondary that will feed a single phase panel with only 120v loads? Do I add up the wattage of each 120v circuit?

For example: I have one circuit on the A phase at 120 volts and pulls 16 amps which is 1920 watts. Then I have another 120 volt circuit on the B phase that pulls 4 amps which is 480 watts. So together this is 2400 watts. The next standard size transformer is 3000 watts. Is this the correct method?

I tried finding an example online but only found examples where the loads connected were the same voltage as the secondary of the transformer.
Each of the two 120V secondary windings is rated at half of the transformer's nameplate KVA. So with the 1920 watt load on one phase you'll need a transformer of at least twice this or 3840 KVA. So a 4 KVA transformer would be the minimum size you could use.

The primary winding and core of a 3 KVA transformer would be adequate to support your total of 2400 watts. However, each secondary winding conductor can only support 3000 VA x 1/2 x 1/120V = 12.5 amps, which is 3.5 amps less than your 16A load.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
However, each secondary winding conductor can only support 3000 VA x 1/2 x 1/120V = 12.5 amps, which is 3.5 amps less than your 16A load.
If there are two separate secondaries that can be paralleled, the transformercan supply that load.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
If there are two separate secondaries that can be paralleled, the transformercan supply that load.
That's correct if the 120 loads can be placed on the same phase. I was going by the specifics in the OP's example that had one load on the A phase and the other on B phase.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
That's correct if the 120 loads can be placed on the same phase. I was going by the specifics in the OP's example that had one load on the A phase and the other on B phase.
You're right.

Kyle, you have to do it my way for the entire transformer to be able to supply the entire load. Half of it will not supply the 16a load.

Read post #3 again.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
This is probably a dumb question but how do i properly size a single phase transformer with a 120/240v secondary that will feed a single phase panel with only 120v loads? Do I add up the wattage of each 120v circuit?

For example: I have one circuit on the A phase at 120 volts and pulls 16 amps which is 1920 watts. Then I have another 120 volt circuit on the B phase that pulls 4 amps which is 480 watts. So together this is 2400 watts. The next standard size transformer is 3000 watts. Is this the correct method?

I tried finding an example online but only found examples where the loads connected were the same voltage as the secondary of the transformer.
It is all about the total VA. Add up all the loads and pick a transformer larger than the total load.

Personally, transformers being fairly inexpensive compared to revisiting it later, I'd be inclined to up size.
 
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