brother
Senior Member
Im just curious about the history of the NEC. Who and How did they come up with the 'Demand Factors' for dwellings and other buildings in the NEC. Does anyone have a code book that shows when it first appeared??
How they come up with DEMAND FACTORS
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e57
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Cant really answer your question - but I think they are far outdated...
Since no-one has answered yet - I'm gonna put my bets on the early sixties.
Just a quick run-through of the Golden Oldies collection, here's what I dredged up.....
From the 1928 NEC:
613. Demand Calculation for Feeder Sizes.
a. General and definitions.
Demand Factor. The demand factor of any system or part of a system, is the ratio of the maximum demand of the system, or part of a system, to the total connected load of the system, or of the part of the system under consideration.
American Electrician's Handbook, Terrell Croft, 1921, Pg 352:
331. To calculate the size of mains to supply more than one motor, the probable maximum current to be carried by the mains must be determined. If the demand factor of the load is not known, it may be assumed that: Probable Maximum Current=(The Sum of the Full-Load Currents of All of the Motors, except the Largest Motor, Which are to be supplied by the Mains) + (The Starting Current, Is, Required to Start the Largest Motor).
[I=amps, subscript s]
I hope this helps. If not, let me know and I'll see if I can dig anything earlier up. "Demand Factor" probably was initiated by using a simple plumbing analogy. You don't need to supply every water-consuming device in a building with the same size pipe that feeds the entire building..... you can use a smaller pipe to supply a bathroom, then use smaller pipes in that bathroom to supply each fixture.
From the 1928 NEC:
613. Demand Calculation for Feeder Sizes.
a. General and definitions.
Demand Factor. The demand factor of any system or part of a system, is the ratio of the maximum demand of the system, or part of a system, to the total connected load of the system, or of the part of the system under consideration.
American Electrician's Handbook, Terrell Croft, 1921, Pg 352:
331. To calculate the size of mains to supply more than one motor, the probable maximum current to be carried by the mains must be determined. If the demand factor of the load is not known, it may be assumed that: Probable Maximum Current=(The Sum of the Full-Load Currents of All of the Motors, except the Largest Motor, Which are to be supplied by the Mains) + (The Starting Current, Is, Required to Start the Largest Motor).
[I=amps, subscript s]
I hope this helps. If not, let me know and I'll see if I can dig anything earlier up. "Demand Factor" probably was initiated by using a simple plumbing analogy. You don't need to supply every water-consuming device in a building with the same size pipe that feeds the entire building..... you can use a smaller pipe to supply a bathroom, then use smaller pipes in that bathroom to supply each fixture.
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brother
Senior Member
Yes dig more.
give me more background.
e57
Senior Member
- Location
- San Francisco, CA
Yes and no.... If you go purley by sq' - and many people do - you could end up with either a seriously over-sized - or undersized service. I've done some retail showrooms that are seriously right there on the cusp of being to small for the actual load - and some 'every bell and whistle' residential as well. There are a few small holes in the resi, and commercial demands.... A while back I put in a 100a sub panel just for "AV equipment" - Tripped... Is there an 'AV demand factor' for people with too much money? Or the 'let's hire a team of people to spend $10K on X-mas lighting' factor? On the other side of the coin I've done work for some people I'd swear only live by candle-light, and few offices that still use paper and pens. Places where a calc would over-size actual use.
My point was that there are a few holes in the demand factors that need to be closed up. Most of them in the cord and plug side of things... Some of that stuff can really add up.
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