Cable Sizing To A Transformer

[gaengineer]

I have an NEC question. The applicable code would be the 1999 edition.

I have a 10/12 MVA step-up transformer (4.16/34.5 KV) that is fed from a 4160 switchgear breaker. The switchgear is fed from a co-generation unit that has an output of 8.5 MVA. The cabling from the generator to the switchgear and from the switchgear to the transformer has a capacity of 10.5 MVA.

Per the 1999 NEC, does the cabling from the switchgear to the transformer need to be rated for the 12 MVA? I know it would be good engineering practive, but is it rerquired by code ?

 

[charlie b]

Re: Cable Sizing To A Transformer

This question relates to function and productivity and usefulness, not to code. The code doesn’t address such issues. The code only addresses the minimum requirements for safety. If you want to size the cables for the 8.5 MVA of the generator, that is fine, as long as you protect the cables by properly sizing the switchgear breaker. In the bargain, you will also be providing code-compliant protection for the transformer’s primary.

 

[gaengineer]

Re: Cable Sizing To A Transformer

Thanks for the answer, but what about the requirements set up in article 215 - Feeders ?

The 2002 edition has a new section 215.2(B)(2) that requires the ampacity of feeder conductors shall not be less than the sum of the nameplate ratings.....I would assume this means the higher (12 MVA) rating. The 1999 edition did not have this section, thus the reason for my question on how the 1999 edition addressed it.

 

[charlie b]

Re: Cable Sizing To A Transformer

It looks to me like your situation was not specifically addressed. The 99 code just said that the feeder must be big enough to handle the load as calculated in 220. Your original post said that you were powering a 10/12 MVA transformer, but did not say how that size was selected, nor what the ultimate load might be. I believe that you can size the feeder for the load that you intend to put through it, so long as you protect the feeder with an appropriately sized overcurrent device. And it looks like the maximum load you could put through the feeder is the 8.5 MVA of your generator.

 

[gaengineer]

Re: Cable Sizing To A Transformer

Thanks again for the reply. I agree with you, however, I have a customer who disagrees with me. He is increasing the output of the co-generation to 10.6 MVA. The cables actually are good for 1500 amps, or 10.8 MVA. He believes that the cables should have been sized for the maximum nameplate of the transformer, which is actually 12.5 MVA. I am trying to find a reference in the 1999 code, most likly in article 220, that confirms that if the maximum MVA that could be "pushed" thru it, 10.6 MVA, then this is the only requirement for the cabling. The protection is fine. There is a 4160 breaker with relaying protecting the cables.

Is there any resource you know of where I can get an "official" NEC ruling ?

 

[jim dungar]

Re: Cable Sizing To A Transformer

According to the NEC conductors are chosen based on the size of the load, not the size of the source.

Overcurrent protection is chosen based on the conductor that feeds the load, not on the source.

The source is then sized, at a minimum, to feed the load, not on the overcurrent protection chosen.

(There are exceptions for branch circuits feeding "general" outlets, and the short circuit of the source must be considered.)

 

[charlie b]

Re: Cable Sizing To A Transformer

I agree with Jim, that cables are sized for the load. But we still don't know what the load is here. I don't think we can say that the load is 12.5 MVA, for no other reason than that this is what the transformer is capable of handling. What does that transformer supply? How was that transformer’s load calculated? Lacking any other information, I am of the opinion that the “load” is no more than the generator is capable of supplying, and that cables sized for the generator are sufficient.

It should be obvious that the code committees have had other thoughts, given that the 2002 code has changed this requirement. You would have to look into the NFPA files of the code deliberations, to learn their reasoning. But for the immediate problem, the best resource for an “official” ruling is the AHJ.

 

[gaengineer]

Re: Cable Sizing To A Transformer

Charlie and Jim, I really appreciate your reply. Forgive my ignorance, but what is the "AHJ" ?

The transformer feeds the main plant 34.5 KV substation. This substation is connected to the utility. Also connected to substation is 3 step-down transformers that feed the plant. Since the 10/12.5 transformer is connected directly to the substation, there really is no way to calculate the "load", unless we consider the entire plant load. This would not be practical since they have to buy power from the utility (the overall plant load is much higher than the co-generation capabilities.

The transformer was chosen for the following reason. The nameplate of the generator is 8.5 MVA. This was seen as a maximum output at the time. The normal output was predicted to be around 7.5 MVA. Instead of buying a 7.5/9 MVA OA/FA transformer with the possibilities of having the fans on all the time, it was decided to go with a 10 MVA. It is an ABB transformer. ABB made a good deal on the transformer where the fans were included, thus the 10/12.5 OA/FA rating. Due to some upgrades on the generator, the output was pushed to 10.6 MVA. The existing cabling is capable of 10.8 MVA. Now the customer is attempting to upgrade the generator again and get 12.5 MVA out of it. He is going to upgrade the cables from the generator to the switchgear to handle the 12.5 MVA, but he is claiming that we were required to have installed cables that have the 12.5 MVA capacity from the switchgear to the transformer. If the project was installed per the 2002 edition, I would agree with him. That is pretty clear now, but I don't feel that this is true of the 1999 edition. If I am wrong, I don't mind admitting it, but I don't think I am and I am trying to find an "official" way to decide this. Thanks again for all your help.

 

[charlie b]

Re: Cable Sizing To A Transformer

“AHJ” means “Authority Having Jurisdiction.” It could mean the city’s Electrical Inspector or (in my area) Washington State’s “Department of Labor and Industries.” It is not uncommon for an electric utility to have jurisdiction over its own equipment. So if you were get a job to replace the cables, where would you go to apply for a new Electrical Permit? That is the person (or group) who can give you your “official” answer. Good luck.
Charlie B.