Wanted:One Xformer expert

Status
Not open for further replies.
S

shawn73

Member
Location
Napoleon, Ohio
Here my new project. I need to install a square D transformer (225 kva, 480 delta primary, 270 Primary side Amps, 208Y/120 Secondary, 625 Amp secondary) to power 3- 208/120, 200 Amp panels. I am planning on wiring the primary side normally. On the secondary side I plan to supply power to the top of 3 - 200 amp breakers (jumpering the tops with "full load" sized wire. The bottom side of each breaker will feed each individual 200 amp panel. I have several questions involving my plans.

1. Involving the top side of the 200 amp breakers: I have a potential of 625 amps on my secondary. I don't want to jumper 1250 kcmil wire across the tops of the 200A breakers. How do I calculate using 2 wires for each phase? For instance can I just use 2 runs of 300 kcmill (at 320 amps each for a total of 640 amps)

2. Is it acceptable to power three panels the way I am describing?

3. Is it legal to mount a breaker on the side of the transformer?

4. Finally, how do I calcuate the primary side breaker size, taking into account inrush current? Is there a % to full load especially for transformers?

Thanks a lot for your help. I just want to do everything correctly.
 
Re: Wanted:One Xformer expert

How do I calculate using 2 wires for each phase? For instance can I just use 2 runs of 300 kcmill
Click to expand...
Yes, that is how you calculate the ampacity of parallel runs (but you can only do this for larger wires - #1 or 1/0 and above I think is the minimum for parallel conductors without looking).

But you will need to protect the wires either at the transformer, or per the tap rules. So you may need a breaker at the transformer.

My guess is that you are going to have a very hard time getting 4 sets of 300's on a 200A breaker.

Steve
 
Re: Wanted:One Xformer expert

You can run one set of 200A conductors to each breaker from the transformer if you meet the tap rules in 240.21(C).
Don
 
Re: Wanted:One Xformer expert

Lots of ways to do the sec.

For pri. use an OCD that is at least 125% of the 270A rated PFLA. A 350A OCD with 350 kcmil copper should do the job.
 
Re: Wanted:One Xformer expert

In this situation the transformer primary OCPD must be NO LARGER than 125% of the primary FLA because no single secondary OCPD is provided.
 
Re: Wanted:One Xformer expert

In Aricle 240.21(c)(1)States that 1 phase and multi-phase (other than delta-delta, 3 wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protection device.

I have a 3 phase, wye configured secondary. So, am I corect that I will have to provide means of secondary overcurrent protection?

I have decided to install a 600A 3 phase 110/208V panel with 200A breakers to feed each of the 3 panels.
 
Re: Wanted:One Xformer expert

Transformer secondary conductors are considered to be protected if they are applied under the tap rules in 240.21(C) for transformer secondaries, as mentioned by Don.

Your conductors (some might say including the panelboard bussing) need to be not more than 25ft, and must be at least equal in size to the handle rating of the breakers installed in your panelboard and the output of the transformer.
 
Status
Not open for further replies.
Top