NEC Lighting Load Calculation for Services

[UFGatorEE]

When performing a SERVICE calculation, do you assume the lighting load to be at 125%?

I understand that branch lighting circuits are to be calculated at 125% of the connected load, but section 220-11 (and table 220-11) in the NEC say that the lighting load demand factors are at max 100% when calculating a service size.

Now, there's a good chance I'm intepreting it wrong, but I wanted to make sure because on service calculations for larger facilities, that extra 25% could be a couple hundred amps difference in main service feeder size. And with copper prices topping out, this can become a major issue.

 

[charlie b]

If you are accounting for lighting loads on the basis of watts per square foot, you do not need to multiply the appropriate value in Table 220.12 by 125%. I believe the tabulated values (e.g., 3 watts/sq.ft.) already take into account the fact that lights are likely to be on for more than 3 hours.

 

[haskindm]

I disagree. Example D3 and D3(a) shows that lighting in an Industrial/Store Building as a continuous load and figured at 125%. D3 specifically uses the VA per square foot method and adds 25%. My understanding is that the figures given in 220 do not assume a continuous load so they must be increased by 25%. This would include lighting, show window lighting, and the sign circuit.

 

[charlie b]

I disagree. Example D3 and D3(a) shows that . . . .

Thank you for sharing your disagreement. My four most compelling arguments in favor of my views are, in order of increasing importance,

1. The examples are entertaining, and perhaps educational, but they are not compulsory.

2. The text of the code article does not tell us to add 25% to the "3 watts per square foot."

3. The loads that are included in the "3 watts per square foot" will not all be on at the same time for periods of three hours or longer. In addition to "general lighting," the "3 watts per square foot" includes general purpose receptacle outlets, bathroom receptacle outlets, outdoor, basement, and garage receptacle outlets, and wall-switched lighting outlets. If you add 25% to the "3 watts per square foot," then you are also adding 25% to all of these outlets, and that is clearly overkill. Thus, if some lights (that are part of the "3 watts per square foot") are on continuously, that fact will be offset by (1) The fact that other lights are turned off in other rooms of the house, and (2) The fact that other circuits (that are also part of the "3 watts per square foot") are turned off. I conclude that "3 watts per square foot" is enough to account for all of these loads, regardless of how long any one of the loads will be on.

4. After you calculate the "3 watts per square foot," you get to add the small appliance loads and the laundry loads, then apply the demand factors of Table 220.42. It does not make sense to me that you would increase the amount you assign to a load, only to decrease it in the next step.

 

[haskindm]

Charles,
You are confusing dwelling calculations with commercial calculations. I think everyone would agree that the 3-watts per foot calculation in a dwelling is not a continuous load and not subject to the 125% calculation for all of the reasons that you state. These conditions and exceptions do not apply to a commercial occupancy. In a commercial occupancy the lighting load is seperate and the entire load is very likely to be on for 3-hours or more. You are not allowed to add the load from the circuits you describe to the lighting load. Both article 210 and 215 state that any load that is expected to last for three hours or more must be calculated at 125% of the actual load. In most commercial occupancies things such as lighting, show window lighting, and signs would be expected to be on for three hours or more and are subject to the 125% adjustment. Receptacles would not be as the total load would not be expected to last for three hours or more.

 

[charlie b]

Perhaps I missed something. But where in the original question is it stated that this is a commercial facility? I have been thinking large apartment building all along.

 

[haskindm]

Charles,
We may find ourselves in complete agreement...
I assumed from his question of 125% and his use of the word "facilities" that we were talking about a commercial installation. If this is a large "multifamily dwelling" then you are absolutely correct. The lighting load would NOT be calculated at 125%. I guess we need more information from the OP. Sorry for the confusion. I was somewhat concerned because I am usually in agreement with your statements....