Transformer Tap rule

[steve66]

I would like to install a 75KVA, 480V to 208Y/120V transformer with primary only protection of 125A. On the secondary, I would like to use a 350KCM tap (rated at 310A, and less than 25' long) to feed two 150A circuit breakers (or maybe fused switches). Each circuit breaker would then feed a 1/0 wire to a 200A panel.

Do you all think this allowed by 240.21(C)(3)?

The reason I want to do this is so I can protect the secondary at more than 200A (so I can get the whole 75KVA without worrying about tripping a breaker or blowing fuse), but I don't want to run 4/0 or larger to both panels (each more than 25' away).

Steve

 

[cpal]

Re: Transformer Tap rule

The secondary current is rated for 208FLC, (disregarding any % of overload rating) If you ran 4/0 at 230A and fused at 225A Your secondary load current would still deliver full kVA to the connected load. If the sec OCPD tripped at that setting well, "it should".

The OCPD that your exampel appear to coomply with the provisions of 450.3(B) But I still don't see what you are trying to do! Are the 2-200A panels going to be loaded at or near capacity? If so the circuit capacity is undersized.


Where are you going with this

Charlie

 

[steve66]

Re: Transformer Tap rule

Hopefully, I'm not going anywhere

I'm just trying to get the most load capacity and flexibility while trying to keep the cost down.

Neither panel is loaded near capacity. The load calculation is about 40 KVA for one panel and 25 KVA for the other. But I would like as much spare capacity as possible for future flexibility.

 

[bob]

Re: Transformer Tap rule

Steve
You need to look at table 450.3B. The primary FLA of the transformer is 90.3 amps + 25% = 112.9 amps. The next size fuse 125 amps.
Your secondary installation appears to meet the requirements of 240.21(C)(6)1,2 &3. You did not say this was an industrial site.

[ February 18, 2005, 01:42 PM: Message edited by: bob ]

 

[steve66]

Re: Transformer Tap rule

Bob posted:

You did not say this was an industrial site.

Hmmmm....It is an industrial site, but it's a office & lab area in the front of the building. There is a definintion for "supervised industrial installation" (2002 code), but nothing for just industrial.

Steve

 

[bob]

Re: Transformer Tap rule

Steve
It looks like you meet both 240.21(C)(3)1,2 &3
and 240.21(C)(6)1,2 &3. I did not know if
240.21(C)(3)1,2 &3 was applicable.

 

[steve66]

Re: Transformer Tap rule

I just realized I can mount the transformer right above one of the panels. Now I'm thinking I would like to have double lugs put on the transformer (I assume that can be done). Then off one set of lugs, run a 3/0 tap less than 10' to a 200 A MCB panels. Off the other set of lugs, run a set of 1/0 to a 150A disconnect, and then to the other panel with the smaller load.

Steve