NEC Demand Factors- Main panel vs Sub panels, Lighting Loads

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MEP246

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Location
Barbados
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MEP Engineer (Training)
There are many examples online of calculations to find the service size based on the NEC.
I am unsure, however, about how the demand factors should be applied to panel schedules, especially in the case of subpanels.

  1. How are the calculated demand factors applied to a panel schedule when there is a subpanel? Would the following example be correct? There are 4 Fastened in place appliances in a dwelling. A 75% reduction to the Fastened in place appliance load would be made to size the main breaker and wire. Two of these appliances are on a sub-panel. When sizing the breaker and cable for this subpanel, do we take this fastened in place load at 100%?
  2. In terms of the lighting loads, would you need to
    1. have a specific square footage associated with each subpanel
    2. calculate the minimum lighting load based on Table 220.12 and this fractional sqft of the whole building
    3. Find the demand from the calculated minimum lighting load (if the actual lighting load on the panel is less than the calculated value) and any other applicable loads
Thanks
 
Yes. When doing a subpanel calc, you are basically doing the calculation over again with only what is connected to the panel. If you don't have 4 fastened in place appliances, then you don't get to use the 75% discount. Lighting loads should be for the square footage that panel is covering. This can be a gray area, especially if two panels provide general lighting and appliance circuits to the same room as where exactly do you draw the line for square footage, especially if one panel does lights and the other does receptacles. You also get some weirdness in kitchens and laundries in that the receptacle circuits there are broken out into small appliance circuits and the 3VA per square foot for lighting/general is excessive for those rooms but helps make up for not enough in other rooms. Another anomaly is you can supply exterior only receptacles and bathroom receptacles only that have a load of zero in the calculation.

I wish the NEC would break out lighting from lighting and receptacles from the dwelling calculation. Pure lighting loads are minimal now, but there are many more things plugged that generally aren't lighting. And if I was NEC king, I'd add garage area to the square footage calcs and allow elimination of very intermittent loads like garbage disposals and garage door openers that have nameplate values of 12A or less as anything that runs for less than 30 seconds isn't going to affect your overall service or feeder load at all.
 
suemarkp said:
Yes. When doing a subpanel calc, you are basically doing the calculation over again with only what is connected to the panel. If you don't have 4 fastened in place appliances, then you don't get to use the 75% discount. Lighting loads should be for the square footage that panel is covering. This can be a gray area, especially if two panels provide general lighting and appliance circuits to the same room as where exactly do you draw the line for square footage, especially if one panel does lights and the other does receptacles. You also get some weirdness in kitchens and laundries in that the receptacle circuits there are broken out into small appliance circuits and the 3VA per square foot for lighting/general is excessive for those rooms but helps make up for not enough in other rooms. Another anomaly is you can supply exterior only receptacles and bathroom receptacles only that have a load of zero in the calculation.

I wish the NEC would break out lighting from lighting and receptacles from the dwelling calculation. Pure lighting loads are minimal now, but there are many more things plugged that generally aren't lighting. And if I was NEC king, I'd add garage area to the square footage calcs and allow elimination of very intermittent loads like garbage disposals and garage door openers that have nameplate values of 12A or less as anything that runs for less than 30 seconds isn't going to affect your overall service or feeder load at all.
Click to expand...
Thanks for your thorough response.

In the case of panels separated into lighting and receptacles, how would you handle that?
 
I would probably just overlap the 3VA per sq ft to each panel. That is if one panel did a 100 sq ft room lighting and another panel did the same 100 sq ft room as all its receptacles, I would assign each panel 300 VA of load as that is the more conservative answer.

You could probably go half each or (150 VA each). Dont think the code has guidance in this situation.
 
suemarkp said:
I would probably just overlap the 3VA per sq ft to each panel. That is if one panel did a 100 sq ft room lighting and another panel did the same 100 sq ft room as all its receptacles, I would assign each panel 300 VA of load as that is the more conservative answer.

You could probably go half each or (150 VA each). Dont think the code has guidance in this situation.
Click to expand...
Okay. Thanks for your feedback!
 
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