Transformer Sizing

When using a transformer sizing tool like this http://www.csgnetwork.com/xformercalc.html

when it ask if it's a single phase or 3 phase how does that apply to 480 to 120 transformers we use in control panels? Is it still single phase?

When trying to calculate amps you have to give it volts and KVA. Does it need primary or secondary voltage? which one do you to calculate?

If I do single phase and my KVA is 3 and I use primary voltage of 480 then amps is 6.25

If I do single phase and my KVA is 3 and I use Secondary voltage of 120 then amps is 25

Which one is right to calculate with primary voltage or secondary voltage?

Single Phase = KVA divided by Voltage

3 Phase = KVA divided by (Voltage x 1.73)

Both Calculations are Correct
your primary amps =6.25
your Secondary load amps = 25

For Single Phase Voltages

you need both for proper protection and wire size.


if your sizing you transformer you need to know total Load.

Yes but on a 480 x 120 control transformer do I use the Primary or secondary voltage?

a triphase TFO is when you have to connect all 3 phases IN to use all the 3 phases out.
Like converting a 3ph 600v to 480v to use a USA panel in Canada...

A Pannel TFO to give you a single 120v for control is 1 phase.

You dont really need to use a calculator to size a mono tfo, (P=VI)... your VA = 1 side voltage* amps regardless of which side it is. Then you divide by 0.80 to avoid loading your TFO above 80% (Missing from your calculator)
Their is some minimal heat lost from in to out but it shouldn't affect the sizing as you already calculate a 20% spare...

secondary voltage or the load voltage.
That is what the transformer will need to Handle.


if you know you need 25amps of 120v then use those two in your calculator. to get KVA

or you multiple 25a x 120v = 3000 va

When it ask if it's a single phase or 3 phase how does that apply to 480 to 120 transformers we use in control panels? Is it still single phase?

Click to expand...

I could say that the majority of control panels will have a single-phase 480/120 volt control-voltage transformer. But don't forget that there are always exceptions to the usual and expected, and that can get you into trouble.

You cannot know the number of phases simply by saying where the transformer is located. A control panel transformer could be either a 3-phase or single-phase transformer. The best method is to read the nameplate on the transformer (if you can't tell by looking at the transformer terminals).

I already have a transformer I am trying to use and see if it has enough amps for the circuit. It's 3 KVA so I was trying to calculate amps.

My question is that 3 KVA label on the 480/240 x 240/120 trans former based on the primary voltage or the secondary.

Primary is rated for 6.25 amps and secondary is 25 amps?

Which one applies to the label?

Which one applies to the label?

Click to expand...

Both! The amount of power is 3 kVA.

Secondary: 25 amps out of the secondary at 120 volts = 25 x 120 = 3000 volt-amps = 3 kVA. You have about 25 amps to use for your 120 volt control circuits. Probably you should figure on calculated loads of about 80% of 25 or 20 Amps load for the transformer, to allow a little margin for errors and future additions. To get 20 Amps through a fused circuit, you probably need to use a 25 Amp breaker or fuse (which could trip at 0.8 x 25, or 20 Amps.

Primary: 6.25 amps x 480 volts = 3000 volt-amps = 3 kV going into the primary.

A transformer can only transform or convert the voltage from one level to another. It cannot create, nor destroy power, to arrive at a different amount of power on each side (except for the heat losses and so on).

PS: It is 99.5 per cent probable that you have a single-phase transformer (based on the 480/240 x 240/120 wiring arrangement). However your secondary voltage from this transformer may only be 240 (instead of 240 and 120). Check to see if it has a center-tapped secondary. If not, to get 120 volts, you have to put in 240 on the primary, instead of 480.

usually a 480#240 x 240-120 will have a primary of 4 terminals and a secondary of 4 temrinals that you have to arrange each one to the voltage used. Serie for high voltage low current and in paralele for low voltage high current...

It would not make sense to have a transformer rated at 3KVA with only 2 terminals primary and 2 terminals secondary that would work at 2 different voltage on each side...Because the max current on both side is limited by wire size and metal core so if voltage change VA will change accordingly.
To keep the same VA when changing voltage, it is only reserved to multitap TFO

For exemple, if you use a 1000vA 240:24v TFO at 120v to obtain 12v, you will get only 500VA or a little less because the secondary is only able to make 42amps (not 84 because it is 12v)

Jeff23spl said:

For exemple, if you use a 1000vA 240:24v TFO at 120v to obtain 12v, you will get only 500VA or a little less because the secondary is only able to make 42amps (not 84 because it is 12v)

Click to expand...

YOU CAN'T CHEAT MOTHER NATURE. Remember the First Law of Thermodynamics - energy is neither created or destroyed.

For a given load on the secondary side of a single phase transformer VA = I x V

For the same load on the primary side the current decreases by the inverse of the ratio of the voltage change. VA = I x V

Secondary Side: 12V x 41.667A = 500 VA
Primary Side: 500 VA / 240 V = 2.083A

240 / 12 = 20 : 1
1 / 20 x 41.6666A = 2.083

The ratio of primary to secondary windings determines the voltage ratio. The load on the secondary side determines the currents (at least until you start letting smoke out!)

Tom, it may be my english but i think we speak of the same thing and you missed what i was trying to explain:

If you have the 1000va 240:24 TFO with only 1 ratio 10:1. The winding of this TFO is capable of +/-4.2 amps primary and +/- 42 amps secondary.
You can use it for a lower voltage on both side following the 10:1 ratio and regardless of the efficiency (too much impedance) The winding is still the size for 42 amps secondary and won't handle more without overheating making it a 500va TFO under a 120v-12v use

Then if you have another kind of 1000vA TFO (The kind i suspect the OP have) that it is multi taps with a 120-240 primary and a 12-24 secondary, it will keep the same 1000 VA regardless of the way you wire it because you will always keep the same voltage and maximum current thru each winding....You can either give it 2 separate 120v source with 4.2amps each or 1 common 240v 4.2 amps or a single 120v at 8.4 amps and either get a 42 amps 24v or 2x 12v 42amps or 1 single 12v 84amps...

So this is why i was responding to Lancie that a TFO cannot be marked for 2 voltage at the same VA unless it is a multi-tap TFO... And I never seen any TFO that would be marked to be used at a lower voltage than it's nominal for the above reason+efficiency with winding impedance...

Like wiring a 460v motor to a 230v source won't give the same horsepower without burning it because it would need more amps for the same power. But if this motor is a 460/230 (Like manny standard motor are) with wire arrangement that permit to split voltage, it would keep it's nominal power under 230v....

If the KVA rating of a given transformer's primary is different from that of said transformer's secondary, then what you have is a non-functional transformer.


480 x 6.25 = 3000

120 x 25 = 3000

Bit_Bucket_07 said:

If the KVA rating of a given transformer's primary is different from that of said transformer's secondary, then what you have is a non-functional transformer.


480 x 6.25 = 3000

120 x 25 = 3000

Click to expand...

So when fusing I would base my primary fusing on 6.25 which would be a 7-10 amp fuse

Secondary would be based on 25.0 which would be a 25-30 amp fuse?

disclamer: the following statement is from memory

If You Fuse the Secondary at 100% Then the Primary may be 125%. Check the NEC for proper sizing.
This will Also depends on Type of Fuse Used. Time Delay 125% Fast Acting 300% Again Check NEC.

Also you must consider a Transformer is a dumb animal it will produce no more current than is drawn from it. If you only need 20 amps. Then fuse your circuit accordingly. It will be your fuses that will limit the current produced. Just don't oversize protection to allow exceeding the transformer's Limit which is 3 KVA. You will let the smoke out.