Building Service Maximum Amperage

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barryh

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I'm new to the forum, so if this has been asked before please accept my apologies, but I have a strict time requirement and don't have time to search the archives.

The question is: What is the maximum service amperage allowed by the NEC on a building with 480v/3ph? Shown below is an e-mail excerp I received concerning this. The writer states that the cose limits 480v services to 700 amps. Is this true? I will be looking though the code while awaiting a reply. I would like some other opinions to back me up. I jnow of quite a few manufacturers that are in violation if this is so.

Thanks
barryh


The drawing shows a 480V, 3-Phase, 60 Cycle, 4-line
> service. I can't meet code however, with 480 Volts concerning ampacities.
> We need to get about 1100 amps at 480 volts into the building to service
> 2-350 HP pumps (future size) running simultaneously, plus all the other
> loads. NEC won't allow ampacity over about 700 amps/480V with even the
> largest sized wires.
> If we go with a higher voltage, which seems to be the right thing to do,
we
> should try to meet a standard motor size for the pumps. The two standard
> motor sizes above 480 volts are 575 and 2300 volt motors. I could use
some
> help on what step-down transformer should be used to meet these standard
> motor sizes in voltage.
 
Re: Building Service Maximum Amperage

Thanks for the reply. Can anyone list the relevant code article or articles?
 
Re: Building Service Maximum Amperage

The code does not provide a maximum, only a minimum allowance. Therefore, one could theoretically go as big as they want.

See Section 240.6(A). There are at least 10 standard ampere ratings of devices over 700A. These would have no purpose if loads exceeding 700A were not allowed.
 
Re: Building Service Maximum Amperage

I don't think there is a reference for something that doesn't exist. :D

You might find 230.95 interesting though. ;)

Roger
 
Re: Building Service Maximum Amperage

Each electric utility will have a maximum size that they will permit the customer to have. This is based on the maximum size transformers they have in their inventory. If they special order a transformer for a customer, they will not special order a spare. This would not be a good situation if it failed. IPL has a maximum service size of 3000 amperes at 480Y/277 volts. :D
 
Re: Building Service Maximum Amperage

When you have a chance to look around the forum, you may notice that we frequently offer the following recommendation: If someone tells you that an installation (or even a design idea) does not meet code, then you should call upon that person to cite the source. In your case, if you go back to the person who sent you the email message, and ask for the code reference that was their basis for claiming the NEC does not allow over 700 amps on a 480 volt system, you will see that the person cannot cite a code article that backs up their statement.
 
Re: Building Service Maximum Amperage

Thanks Guys, I appreciate the responce and I was going to do exactly as charlie b suggested. I thought the 700A limit was a little strange. But when dealing with the NEC, I have found that the best way to go is to get lots of other opinions since the NEC is so convoluted.

Great forum, glad I found it and thanks again
 
Re: Building Service Maximum Amperage

It looks like your source may have looked at table 310.16 and quoted the ampacity for the largest wire (2000KCM). But you can put wires in parallel(i.e. 3 sets of 500 KCM's would be good for about 1140 amps). Maybe there is some reason they want one set of wires to each motor?

One other code reference, any single disconnect rated at 1000 amps or more, and at 480V or more, would require ground-fault-protection. That's something your source may want to avoid.

I would think standard motors would be available in any standard voltage that the utility may provide.

Finally, your utility may have some say in what voltage they are willing to provide.

Steve

[ July 07, 2004, 01:48 PM: Message edited by: steve66 ]
 
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