Sub-panel

F

frofro19

Senior Member
Location
VA.
Occupation
Master Electrician
I know this has been asked multiple times but I'm wondering why we can calculate the back-up heat at 65 percent if doing a load calculation for a house but if that same heat is coming from a sub-panel, it has to be calculated at 100 percent? I'm getting ready to install a sub-panel for an addition over a garage and wondering if everything has to be calculated at 100 percent?
It will have a dryer, water heater, washer, a 1.5 ton mini-split for the garage and I believe a 2 ton heatpump with 7.5 kw of back-up heat. If this same scenario was a "whole house" it would have some demand factors involved. I know you loose the diversity factor with a sub-panel and I know the total load calculation comes from the entire dwelling but just trying see what I need for the sub-panel feeder. I haven't checked the nameplate on the heatpump and mini-split yet, but will before choosing the feeder for the sub-panel.
 
augie47 said:
IMO, since 220.82 apples to feeder and service loads I would think it would be applicable to subpanels.
Click to expand...
Yes.

A common mistake I see done is that when doing the demand load for the main service, the demand load for the sub-panel cannot be used and the service demand must be calculated from the total connected loads.

Essentially, don't use a demand load inside of a demand load.
 
augie47 said:
IMO, since 220.82 apples to feeder and service loads I would think it would be applicable to subpanels.
Click to expand...
I find the wording quite ambiguous. Since the first sentence refers to "a dwelling unit having the total connected load served by a single
120/240-volt or 208Y/120-volt set of 3-wire service or feeder conductors", I have always assumed that every subsequent use of the words feeder and service in that paragraph refers to such a feeder or service, and does not include feeders not carrying the total connected load.

Cheers, Wayne
 
NEC 100 definition:
Feeder: All circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device.

Exhibit 100.6
1756500604887.png

Feeders are any conductors prior to the final ocpd, so therefore the calculation can be applied to the feeders between a main and a sub.
 
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