Tranformer Main/Secondary Fuse Sizing.

[Digital]

Hello all,

I have a question regarding prm/sec fuse sizing. This is for an Extruder application. The primary feed will be 480 vac 3 phase and the secondary will be 240 vac single phase. The way it has been it set up is 2 legs from the 480 will feed 3 separate 7.5 kVA transformers. I can only assume the previous engineer used this setup for load balancing as he is not here anymore.

480 VAC
L1 & L2 ---> Primary of first transformer.
L2 & L3 ---> Primary of second transformer.
L3 & L1 ---> Primary of third transformer.

From the secondary side of the transformers all X2 legs will be tied to ground and a Neutral distribution block. All the X1 legs will be separated fused and terminated at a power distribution block.

The question of the hour is when sizing primary fusing should i size for three phase or single phase?

Thanks in advance.
Digital

 

[augie47]

you can look at it either way..
you have a fairly balance set up with 15 kva on any two legs or
22.5 kw at 3 phase.
Either way you are looking at about a 30 amp load.
Since you have secondary protection, you can primary protect anywhere from 30 to 40 amps depending on your wire size.

 

[Digital]

you can look at it either way..
you have a fairly balance set up with 15 kva on any two legs or
22.5 kw at 3 phase.
Either way you are looking at about a 30 amp load.
Since you have secondary protection, you can primary protect anywhere from 30 to 40 amps depending on your wire size.

Thanks for the speedy reply augie. Ive been coming up with the same numbers. Was not to sure and needed a second opinion.

Thanks again
Digital

 

[don_resqcapt19]

The code seems to say that you treat this installation as a 3 phase transformer.

450.3 Overcurrent Protection
Overcurrent protection of transformers shall comply with 450.3(A), (B), or (C). As used in this section, the word transformer shall mean a transformer or polyphase bank of two or more single-phase transformers operating as a unit.

You have polyphase bank of 3 transformers operating as a unit or single transformer.
Don

 

[LarryFine]

You have polyphase bank of 3 transformers operating as a unit or single transformer.

I had the exact same take as I was reading the OP, when he said "3ph primary and 1ph secondary".

He's actually going to end up with a 416Y/240 secondary system when he joins the X2's.

The neutral should be grounded, too, right? Or is this permitted, but not required?

 

[don_resqcapt19]

Larry,
Yes, 250.20(B)(2) will require this to be a grounded system.
Don

 

[kingpb]

Is the intent to have 240V L-L, or 240V L-N? Both could be considered single phase. If it is 240 L-N, then the connection as described is going to be consistent with what Larry said about 416Y/240V. With a 480V HV connection, I'd say this is a specialty transformer?????

 

[Digital]

Sorry about that. There are three separate single phase transformers (480 pri x 240 sec) with one leg on each of secondary 240 side grounded.