NEC Demand vs. Actual Utility Demand

[channing]

As a professional electrical engineering consultant working for an M&E Design firm, I work mostly in the commercial field doing building design, but occasionally my firm does some power plant design and utility study analysis.

I am bound by code to perform demand load calculations per NEC requirements, however, we all know that these are always much higher than the "actual" demand that a building sees. For example, the NEC demand load for a school may be 750 kW, however, the actual peak kW demand may be less than half that (around 300kW).

Does anyone have or know of any published references, charts, tables, design guidelines, etc.. that I can use to design my minimum generating capacity for the buildings with? My client wants some reassurance that we are using some recognized method of determining this and doesn't want us to use NEC demand because we would end up oversizing the generating capacity needed.

 

[robbie]

Re: NEC Demand vs. Actual Utility Demand

I would start out be contacting the local Utility Company to obtain loads for your application over a 1 year period. For example if you were designing a two-story office building, obtain loads for similar office buildings in the vicinity. The peak demand in the summer and winter months would give you a good guideline to start with.

 

[charlie b]

Re: NEC Demand vs. Actual Utility Demand

The NEC service calculation tells you the minimum size of the service equipment. For most facilities, there is no requirement to install any generators at all. Most often, the required egress lighting could be handled with battery backup lights, so a generator would not be needed for that purpose.

So I would venture the guess that your facility will gets its normal power from the utility, and that your client wants to have a generator for backup power. If this is correct, then the client is free to select any or all of the loads to be connected to the generator. It boils down to “What do the client want to be able to do, during the time that the utility source is unavailable?”

If you are dealing with a school, then does the client want to continue teaching all classes? Or just keep an area lighted and warmed, until parents can come by to pick up the students? Or keep the administrative offices going, even after the students are sent home? Get the answers, and then do a realistic calculation of the amount of power that would be needed. The NEC demand factors do not come into play here.

 

[channing]

Re: NEC Demand vs. Actual Utility Demand

The project is for the National Park Service out in rural Alaska where there is no utility company, NPS will be providing their own prime power. NPS is very concerned about the amount of energy used as they are trying to supply most of the load using a small hydro plant. We are trying to show them in a utility study whether they can realistically support all of the buildings in the community. According the NEC calculations that I have performed, they cannot do it with just the Hydro plant, additional diesel fired gensets will need to be added. However, I know for a fact that these numbers do not reflect reality, they are worst case numbers. What I'm trying to find is some case study information or some tables I can follow that help normal utility companies determine load sizes for different facilities.

Typically what happens on a normal design job is I will tell the local utility that I am adding a building that has an NEC demand load of 100 kW for example. The utility is not going to set a 125kW transformer to feed the facility, they typically install a transformer that is half that size, but I don't know where they get those calculations from. It might just be experience in the field or looking at the meter readings from a
similar building they have connected in the past. I was hoping that there might be a book that someone has published on this subject with some tables or guidelines I could follow.

 

[rb]

Re: NEC Demand vs. Actual Utility Demand

We have a publicly owned utility in Memphis. They use historical demand data accumulated over many years. This data is based upon the proposed use of the facility.

Who is the authority having jurisdiction for this project?