Con Edison Elec. Installation Req.

[delkis]

I have a discussion we a collegue regarding Con Ed Ruling-The school service is 120/208,3 phase 4#500Kcmil,. The additional load to be added to the school was approximately (228kw/750amps).
Con ed ruling came that service reinforcement was needed and that con ed was to install 2-4" conduit and one set of 500kcmil service cable- my collegue think that this is wrong- that if come ed installed two sets of 5ookcmil, the customer will tap with max 2 sets of 500kcmil.-- I think that it is right. my interpretation of the ruling and con ed dwg. no. Eo-6208-c rev. 16 is that con ed will come in with one extra set of cables (remenber there is already one set), but the customer will tap to these cable with 3 sets of 500kcmil- i'm thinking that con ed will provide 2 new 4" conduit in additon to the existing one-
I have called the rep assigned to this project, but I have not heard from him.

thanks

 

[charlie b]

Re: Con Edison Elec. Installation Req.

You didn?t give us the existing load. The existing single set of 500?s is good for 380 amps, or about 137 KVA. But how much of that is the present load?

You are adding 750 amps, or 270 KVA. The total load could therefore be up to 407 KVA (or equivalently, up to 1130 amps. A set of three 500?s per phase can handle this, but a set of two 500?s per phase can not.

But the real issue comes back to load. When you say that you are adding 750 amps, how did you arrive at that figure? Did you perform a service calculation per NEC 220, taking into account the existing and the added load? If you did, and if your result was on the order of 400 KVA, it would not be unusual for a utility to offer a service connection that was only rated for 40% to 50% of that value. They could replace the existing service with a 250 KVA transformer, with a rated secondary current of 694 amps, and be well within their guidelines. A set of two 500?s per phase can handle this.

 

[charlie]

Re: Con Edison Elec. Installation Req.

Also, do not forget that the electric utility will normally size their cable for what they believe is the actually load. The transformer is also sized for the expected load. Just because Article 220 indicates that the load will require a 1200 ampere service at 208 does not mean the electric utility will put a larger transformer than required for the projected load that is done by the electric utility's engineer. In the above example, I would probably use a 300 kVA transformer it would depend on other information, not just the service size.

If the cable or transformer were to fail sometime in the future, the electric utility will be there to repair the facilities. Since we have effectively given the customer a guarantee that never expires, we are permitted to undersize our facilities.