Transformer Secondary/Service Feeders

[KRG9729]

We are working on a project and are exploring different options for the customer. The facility currently has a 120/208, 3? 1200 amp service with a 300KW automatic generator between the transformer and main service disconnect. There current peak demand is 233 amps.

The customer is adding approx. 1200-1350 amps of new loads. The utility company?s transformer has built-in CT's with a meter mounted next to the transformer. The POCO stated the transformer and CT's can handle an additional 1200 amps.

The only way we have come up with using the existing transformer without changing the existing gear would be to install a new 2000 amp main service disconnect between the transformer and existing gear; then feed the existing service and a new 800 amp panel. Approx. 800 amps of the new load will be fed from the existing gear.

Is it possible to do it another way? It seems we are limited by the CT's being built into the transformer.


Please see attached one-line. View attachment One Line MU.pdf

 

[augie47]

I may be missing something, but if your existing 1200 amp service has less than six disconnects, why don't you just feed the 800 amp added service disconnect straight from the transformer ?

That said, how are you addressing 700.5 (or 701.5) regarding the load vs the generator capacity ?

 

[KRG9729]

There is only one meter because the CT's are in the transformer. We spoke to electrical inspectors and some other electricians. The inspector may allow it. The disconnects wouldn?t be group so a sign and map posted at each main will show where the other service disconnect is located.

On another note we will be running a 1200 amp service rather than an 800 so we don't exceed 80% of the generators rating. We are proposing to install 3 sets of 500kcmil. The calculated load is 1,059. Would this comply with the NEC?

 

[augie47]

There is only one meter because the CT's are in the transformer. We spoke to electrical inspectors and some other electricians. The inspector may allow it. The disconnects wouldn?t be group so a sign and map posted at each main will show where the other service disconnect is located.

If you can't group the two disconnects, you might stiull get AHJ approval as you say.

On another note we will be running a 1200 amp service rather than an 800 so we don't exceed 80% of the generators rating. We are proposing to install 3 sets of 500kcmil.

(3) 500 kcmil is not normally acceptable for a 1200 amp service with a main.

The calculated load is 1,059. Would this comply with the NEC?

Not the way I see it. The generator output would be less than the calculated load.
Some load shed devices might be in order.

 

[KRG9729]

If you can't group the two disconnects, you might stiull get AHJ approval as you say.


(3) 500 kcmil is not normally acceptable for a 1200 amp service with a main.

So 3 sets of 600 kcmil is what is generally accepted?


Not the way I see it. The generator output would be less than the calculated load.
Some load shed devices might be in order.

1059amps is the load calculation for the new 1200 amp service.
The generator is feeding the existing 1200 amp service and which is calculated just under 800 amps.

 

[KRG9729]

3 sets of 600 kcmil is typlical for a 1200 service. Why is this?

 

[Hv&Lv]

There is only one meter because the CT's are in the transformer. We spoke to electrical inspectors and some other electricians. The inspector may allow it. The disconnects wouldn’t be group so a sign and map posted at each main will show where the other service disconnect is located.

On another note we will be running a 1200 amp service rather than an 800 so we don't exceed 80% of the generators rating. We are proposing to install 3 sets of 500kcmil. The calculated load is 1,059. Would this comply with the NEC?

I still can't understand why you don't come out of the existing XF. CT's have nothing to do with it. I have several transformers that have 2 meters mounted on them. (one has three). The CT's aren't "built in", they are installed. If I have room on the side of the XF, I can meter as many different services as you can feed from the XF.

http://www05.abb.com/global/scot/scot235.nsf/veritydisplay/5007f1156407707d852576cf00523e2f/$file/1vap428201-db_cmv_cmv%20hi-temp.pdf

 

[augie47]

There is only one meter because the CT's are in the transformer. We spoke to electrical inspectors and some other electricians. The inspector may allow it. The disconnects wouldn?t be group so a sign and map posted at each main will show where the other service disconnect is located.

On another note we will be running a 1200 amp service rather than an 800 so we don't exceed 80% of the generators rating. We are proposing to install 3 sets of 500kcmil. The calculated load is 1,059. Would this comply with the NEC?

3 sets of 600 kcmil is typlical for a 1200 service. Why is this?

Your origianl post not3d (3) sets of 500. In most cases you are limited to 75? terminations ( 110.14).
(3) 500s = 3 x 380-= 1140
(3) 600s = 3 x 420 = 1260

 

[KRG9729]

The Poco has implied there could only be one meter on the transformer...

We are going to come out of the existing transformer.