Do you derate when figuring number of lighting circuits?

[bjp_ne_elec]

Here's a question I would like clarification on:

If you have a 2500 sq. ft. house(dwelling unit), what is the minimum number of 15A, 120 vac general purpose lighting circuits?

What I'm not sure of is if you derate???

3VA x 2500 = 7500 VA

Now - do you divide by 1800 VA (120 * 15) or 1440 (120*15*0.80)?

Thanks

Brett

 

[infinity]

Divide by 1800 for a 15 amp circuit or 2400 for a 20 amp circuit.

 

[bjp_ne_elec]

Trevor - thanks for the reply, but can you help me just by siting the appropriate reference in the NEC.

I'm studying for my Master's License, and have a book - "Master's Electrician's Review" (published by Thomson) - that is a little confusing. It shows an example where they are asking a question about how many receptacles on a 120 vac, 20 amp circuit - and they derate. Now, they do specify the receptacles are for general use. I'm just trying to understand how these might impact how you calculate when specifically asked for number of lighting circuits.

Thanks

Brett

 

[gndrod]

Brett, The 80% 1440va breaker rate is for normal continuous operation 3 hours or more. Re: UL489 I agree with Trevor's 1800 for 15amp.
rbj, Seattle

 

[Mike03a3]

For the branch circuits, 210.11 says you are required to have enough to supply the load calculated IAW 220.3. In your case that's 2500 x 3 = 7500KVA. 7500 / 1800 (for a 15a circuit) is 4.17 circuits, so you'd need five. Use one 20a circuit and three 15a circuits and you could get by with four total, although that is probably not a good design idea.

Now, when you do your feeder or service load calculations you can derate the lighting load over 3000KVA. see table 220.11

All references are 2002NEC (current version here)

 

[bob]

Table 220.12 is for General Lighting Loads. Nothing said about receptacles.
220.14(J) states that no additional load caculations required for outlets
listed in J1,J2 and J3.

bjp_ne_elec

I'm studying for my Master's License, and have a book - "Master's Electrician's Review" (published by Thomson) - that is a little confusing. It shows an example where they are asking a question about how many receptacles on a 120 vac, 20 amp circuit - and they derate.

I am not sure what you mean by derating but the NEC does not specify the max number of receptacles on a circuit in a residence. You can put 2 or 100.

 

[haskindm]

The 80% load rule applies ONLY to continuous loads. There is nothing in the NEC that prohibits the loading of a circuit to 100% as long as it is not a contiuous load. Residential lighting is NOT a continuous load.