sizing a transformer

[Copperclad]

Hi all,
Im no engineer, just an electrician in need of assistance.
Im confused on how to size a step down transformer proper.
eg.. We have 480 volt 3ph. available for primary voltage, and would like to step it down to power misc. 240 volt 1ph equip.
Our exist. primary conductors consist of (3) #4cu. and (1)#8 cu.
My question is: Do I use the primary voltage or the secondary voltage to calculate the maximum kva transformer using the existing #4 conductors present.
We are looking to install the largest transformer possible, and still utilize exist. wires....thanks for ya'lls help

 

[yired29]

Hi all,
Im no engineer, just an electrician in need of assistance.
Im confused on how to size a step down transformer proper.
eg.. We have 480 volt 3ph. available for primary voltage, and would like to step it down to power misc. 240 volt 1ph equip.
Our exist. primary conductors consist of (3) #4cu. and (1)#8 cu.
My question is: Do I use the primary voltage or the secondary voltage to calculate the maximum kva transformer using the existing #4 conductors present.
We are looking to install the largest transformer possible, and still utilize exist. wires....thanks for ya'lls help

IMO you would have to use the primary voltage seeing that the #4's are on the primary side.

 

[Copperclad]

update

update

I think im making my question read a bit more complex than I intended.
Im trying to find out what is the largest transformer I can install using the exist #4's mentioned above as the primary ckt.?...thanks

 

[yired29]

I think im making my question read a bit more complex than I intended.
Im trying to find out what is the largest transformer I can install using the exist #4's mentioned above as the primary ckt.?...thanks

FLC of a 50 KVA transformer 480v primary would be 50x1000/(480x1.73)=60 Amps.

FLC of a 75 KVA transformer 480v primary would be 75x1000/(480x1.73)=90 Amps

#4 @ 75 C = 85 amps. Assuming 75 degree terms. I would stick with a 50 KVA transformer.

Someone correct me if I missed something.

This is all figured at 3 phase.

 

[charlie b]

A #4 copper wire is good for 85 amps. Multiply 85 times the primary voltage of 480, and multiply that by 1.732 (the square root of 3), and you get 70,666 VA. I would go with a 50 KVA transformer. It will have a rated primary current of 60.2 amps (50,000 divided by 480 and then divided by 1.732). You can protect that with a primary breaker of 1.25 times 60.2, or 75.2 amps. The next highest standard size overcurrent device is 80 amps.

I suppose you could go with the next higher rated transformer: 75 KVA. But you would not be able to use all of its capacity anyway. That is because its primary current rating is 90.2 amps, and your conductor is limited to 85 amps.

 

[yired29]

Hi all,
Im no engineer, just an electrician in need of assistance.
Im confused on how to size a step down transformer proper.
eg.. We have 480 volt 3ph. available for primary voltage, and would like to step it down to power misc. 240 volt 1ph equip.
Our exist. primary conductors consist of (3) #4cu. and (1)#8 cu.
My question is: Do I use the primary voltage or the secondary voltage to calculate the maximum kva transformer using the existing #4 conductors present.
We are looking to install the largest transformer possible, and still utilize exist. wires....thanks for ya'lls help

Are you installing more than one piece of equipment?
Are you installing a panelboard on the secondary?

 

[Copperclad]

Makes perfect sense to me now,,,thank you all!
Im not sure why that confused me............I was trying to make it harder than needed......thx again!

And yes, we are installing a load center feeding from the seconday side.....for multiple 240v ckts.

 

[winnie]

What type of conductor insulation do you have (THHN? XHHW? etc) and what sort of wiring method (conduit, cable, cable tray)?

How long is the run?

What are the temperature ratings of the termination on the supply side?

How important is it to squeeze every last drop of capacity from this feeder?

Can the single phase load be divided up, or are you looking at a single large 240V load?

Is the loading likely to be continuous, or are you looking for capacity which will only be used some of the time?

-Jon

 

[yired29]

Makes perfect sense to me now,,,thank you all!
Im not sure why that confused me............I was trying to make it harder than needed......thx again!

And yes, we are installing a load center feeding from the seconday side.....for multiple 240v ckts.

What is the rating of the load center. Watch your tap rules 240.21 C and 408.36

 

[winnie]

I'm with Charlie on the 'reasonable' response. Getting 'unreasonable':

You could go with a 75KVA transformer, supplied by a 80A '100% rated' breaker (are these available??) and have 66.5 KVA available to the load.

You could go further and interpose some 3AWG conductors between the terminations and your run, spliced using 90C rated splices. This would give you a conductor ampacity of 95A and get you up to 75KVA (using a 90A 100% rated breaker).

You could go still further, and add a 'buck-boost' transformer on the feed side, to get to 600V, and use a 600V to 240V transformer; this would kick you up over 90KVA.

At some point, being unreasonable is much more expensive than simply running a new feeder and using the reasonable approach

-Jon

 

[wasasparky]

And yes, we are installing a load center feeding from the seconday side.....for multiple 240v ckts.

240V 3-phase panel? (3 or 4wire?)
(240/120V 3ph-4wire center-tapped xfmr?)

 

[arnettr]

Transformers

Transformers

I recently found that someone had installed a Circuit breaker panel on the side of a step down transformer. Is this within the NEC code.