Demand loads and feeders

[jself]

This is going to be a decently sized question for my post here so lets see if I can get it and it make sense when I am done.

(refering to commercial applications)
We are looking at sizing feeders and trying to decide what load to use to size the feeders. I have read the rule in the NEC that says that the load is 100% of non-continuous load and 125% of continuous load. Does the non-continuous load in that statement allow me to size the wire for only the 65% of kitchen equipment load or do I have to use the full 100% of all the kitchen load on my feeder sizing. The same would be asked to the discounts given for receptacles. If the answer is is 100% of non-continuous and 125% of continuous no matter what then do my main breakers need to be sized that way to, and what is the point of the discounts when all is said and done, can I not just size the wire for the overcurrent protection protecting that wire?

 

[bphgravity]

Re: Demand loads and feeders

When performing a feeder or service calculation for any type of occupancy, you must consider 100% of non-continuous loads, and 125% of continuous loads. Some specific loads will be permitted to have a demand factor applied.

Check out Annex D of the NEC. There are several fine examples of feeder and service calculations you can review there.

 

[charlie b]

Re: Demand loads and feeders

Originally posted by jself:. . . when all is said and done, can I not just size the wire for the overcurrent protection protecting that wire?

No. That's the wrong sequence. First find the load. Next, pick a wire that can handle that load. Finally, pick an overcurrent device that will protect that wire.

Does the non-continuous load in that statement allow me to size the wire for only the 65% of kitchen equipment load or do I have to use the full 100% of all the kitchen load on my feeder sizing.

Unfortunately, the Annex D examples do not include a non-dwelling unit that has kitchen equipment. So here is my take on your situation: Kitchen equipment in non-dwelling units is counted at its nameplate value, and is not treated as a "continuous load." Once you add up all the nameplate VA, then the demand factor of Table 220.20 is applied.

The same would be asked to the discounts given for receptacles.

Duplex receptacles are 180 VA each. You do not ask whether the things being plugged in might be run for more than three hours at a time. You do not "biggie-size" the 180 VA for "continuous loads." You have two options for demand factors. See 220.13.

 

[jself]

Re: Demand loads and feeders

Thanks !

Checking out the Annex cleared things up... as it was what you guys said is exactly the interpretation I was trying to convey, so it worked out really well for me in our little interoffice squable.

[ July 29, 2005, 06:01 PM: Message edited by: jself ]

 

[james wuebker]

Re: Demand loads and feeders

Charlie b.
(No. That's the wrong sequence. First find the load. Next, pick a wire that can handle that load. Finally, pick an over current device that will protect that wire.) Charlie I agree with you to a point. It sounds that you are talking about only 1 load instead of several types of loads jself is trying to figure out. Your way is correct but in the form we talk about have more room down the road. So if he has 120 amps and installs and 150 amp main then there's no more room to add. I believe what he was trying to say is size the wire for a 200 main and then he will have room to grow some. That's how I like to do it but again your process is correct.
Thanks!
Jim

 

[charlie b]

Re: Demand loads and feeders

Part of the process of "finding the load" includes determining whether you need to (i.e., the owner wants you to) add in spare capacity. My main point is that the selection begins with finding the load, not with matching the cable to the breaker.