How to size a 480 to 240/120V Transformer

[ActionDave]

Thank you ActionDave.

I need 3 phase transformer.

I spec out the wrong transformer because the transformer will be fed with 480V 3phase on primary side, and output will be 120/240V single phase.

So this means my current rating on the primary side will be 15000/ 480V x 1.732 = 18amps. So my OPCD is 20 amps.

Thanks for clarification.

You don't need a three phase transformer. You have a single phase 480V to 240V transformer. If you feed it with a two pole breaker from a 480V panel you will get your 240V. It does not matter that the 480V panel is three phase.

 

[Smart $]

Thank you ActionDave.

I need 3 phase transformer.

I spec out the wrong transformer because the transformer will be fed with 480V 3phase on primary side, and output will be 120/240V single phase.

So this means my current rating on the primary side will be 15000/ 480V x 1.732 = 18amps. So my OPCD is 20 amps.

Thanks for clarification.

I didn't look at the linked page but NO you do not need a 3 phase transformer just because the primary [source] is 3 phase. For single phase you just install a 2-pole breaker in your 480V 3P panel (AB, BC, or CA) and run two ungrounded conductors to the transformer primary terminals plus one EGC to grounding terminal [bus].

 

[GoldDigger]

And the primary current requirements will correspond to single phase numbers. You cannot use the three phase power to current formula since all of the power will be coming from one phase.

 

[goodoboy]

You don't need a three phase transformer. You have a single phase 480V to 240V transformer. If you feed it with a two pole breaker from a 480V panel you will get your 240V. It does not matter that the 480V panel is three phase.

Thank you ActionDave,

I am still learning, so still a bit challenging to see. Do you have a drawing or reference in the NEC book at can refer to see how to wire the transformer on primary and secondary using.

Thank you

 

[kwired]

Your transformer should have a connection diagram right on the unit, or if you don't have it yet, look at some of the data sheets on most any mfgr. site on a similar transformer.

Note that many of them available to you will have 240x480 primary or straight 480 with taps for plus or minus 10% of rated voltage on primary and 120x240 secondary.

 

[Smart $]

Thank you ActionDave,

I am still learning, so still a bit challenging to see. Do you have a drawing or reference in the NEC book at can refer to see how to wire the transformer on primary and secondary using.

Thank you

Your transformer should have a connection diagram right on the unit, or if you don't have it yet, look at some of the data sheets on most any mfgr. site on a similar transformer.

Note that many of them available to you will have 240x480 primary or straight 480 with taps for plus or minus 10% of rated voltage on primary and 120x240 secondary.

Here's the diagram for his transformer...

 

[goodoboy]

You don't need a three phase transformer. You have a single phase 480V to 240V transformer. If you feed it with a two pole breaker from a 480V panel you will get your 240V. It does not matter that the 480V panel is three phase.

Thanks,

I am confused. The transformer is being fed from a 40 amp Disconnect with 3 fuses, 480V 3 phase. I am not understanding where the 2 pole breaker comes in to play.

 

[kwired]

Thanks,

I am confused. The transformer is being fed from a 40 amp Disconnect with 3 fuses, 480V 3 phase. I am not understanding where the 2 pole breaker comes in to play.

There is no place to land the third supply conductor to the transformer - it is single phase and only has two input leads. You can supply it from a three phase source but will only use two of the three lines, whether it be via three pole breaker/fused switch or two pole. Could even be just one "hot" and the grounded conductor if the source were a corner grounded delta system using only one pole with overcurrent protection in it. All the transformer input cares about is seeing 480 volts between two input conductors and doesn't care which or if any of them are grounded or not.

 

[Smart $]

Thanks,

I am confused. The transformer is being fed from a 40 amp Disconnect with 3 fuses, 480V 3 phase. I am not understanding where the 2 pole breaker comes in to play.

If I recall correctly, you said this is being powered out of an MCC, correct? ...and you have a 480V 3-phase bucket available with (3) fuse holders. You put 40A fuses in two of the three. The transformer don't care which two, but it should be the two with the least amount of current on the MCC main.

From there, connect two 40A conductors to the load-side terminals of the fuse holders with fuses installed and run those along with an appropriate EGC to the transformer. Connect the two ungrounded (40A) conductors to H1 and H4. Interconnect 1—H2—H3—2. This is because there are two primary windings (H1 to 1 and H4 to 2), each rated 240V with a 216V tap at 3 & 4 respectively.

 

[templdl]

Oh boy, where do we go from here? Confused with he supply being 480v 3ph feed from an MCC feeding a 1ph transformer with a 480v primary.

 

[templdl]

Oh boy, where do we go from here after numerous posts? At yhis point being confused with the supply being 480v 3ph feed from an MCC feeding a 1ph transformer with a 480v primary.
One should be concerned with such a lach of familiarity that there may still be a misunderstanding which could result in a catastrophic failure, personal injury or loss of life. It may be best way this point to get some assistance locally from someone who is familiar with what y the OP is attempting to do. A mistake or misinterpretation could be disasterous.
We're trying to walk the OP through this and we are relying on the OP's description of what he has. We may be "assuming" one thing when in actually reality th OP may have something entirely different.

 

[kwired]

Oh boy, where do we go from here after numerous posts? At yhis point being confused with the supply being 480v 3ph feed from an MCC feeding a 1ph transformer with a 480v primary.
One should be concerned with such a lach of familiarity that there may still be a misunderstanding which could result in a catastrophic failure, personal injury or loss of life. It may be best way this point to get some assistance locally from someone who is familiar with what y the OP is attempting to do. A mistake or misinterpretation could be disasterous.
We're trying to walk the OP through this and we are relying on the OP's description of what he has. We may be "assuming" one thing when in actually reality th OP may have something entirely different.

I have to agree OP is in over his head, how much IDK.

 

[bhellmer]

Select transformer with KVA rating equal to or greater than load as calculated by Art 220. For single phase: KVA=Amps*Volts/1000. For three phase: KVA=Amps*Volts*sqrt(3)/1000.

For low-voltage transformers, primary and secondary over-current protection trip settings are limited by maximums in NEC Table 450.3(B).

Primary over-current protection trip settings should also be great enough to clear transformer in-rush current. Plotting in-rush current on fuse or circuit breaker trip curves from manufacturer. Multiply transformer rated primary current by 12 and plot at 100ms. Multiply by 25 and plot at 10ms. Select a trip setting whose curve is above and to the right of both in-rush points.

Size secondary conductors based upon secondary over-current protection trip setting and locate secondary protection based upon an appropriate transformer-secondary tap rule from NEC 240.21 (C).

Size primary conductors based upon primary over-current protection trip setting or based upon transformer-supply tap rule NEC 240.21 (B) (3).