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Load calculation + subpanels

6659 Views 11 Replies 5 Participants Last post by  AllanJ
I'm working on a <1000 sq ft home w/detached garage and 100 amp service, I know just enough about load calculations to get confused when it comes to how subpanel loads fit in. Here's a list of questions:

Square footage: as I understand it the first step in any load calc is running a square footage formula to establish the general lighting/receptacle load. This accounts only for the area one wants to calculate the load for, right? So, in the case of a home with a main panel and two subs one would do the square footage calculation for 3 separate areas. I can elaborate on what's connected where if need be.

Safety factor: a few sources indicate that NEC calls for the gross electrical load for a subpanel to be multiplied by 1.25, can anybody confirm or even cite the NEC section that covers this?

Outdoor lighting/receptacles: some methods call for an accounting for all lighting/receptacle loads NOT in the main living area (outdoor/detached garage areas) in a separate calculation; this usually involves multiplying each receptacle by 180 (watts) and adding this to the sum total wattage of all fixtures. So in the case of the detached garage, does this calculation take the place of the square footage-based general load portion?

Watts to amps: most sources say that once I have my total wattage simply divide this by 240 (volts) to get my amperage load. At least two sources say to divide by 230 (???). Being that I'm in the US I think 240 makes more sense, where in the world is 230 coming from?

Maximum load vs service: finally, how close can my total load amperage get to my service amperage? With 100 amp service am I good with any number short of 100 or am I limited to 80% of that?

At this point I've looked up so many online sources that I'm dizzy. Seems crazy but almost every load calc method I find varies slightly from the previous. Any help I can get on figuring this out would be GREATLY appreciated.
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If the garage is bein used as living space (workshop, music studio, small manufacturing business included) you calculate its gross area and not as outdoor. If not living space you treat it as outdoor and not included in square footage. In both cases permanently installed items (freezer, band saw, electric car charger) are added in as individual loads. In both cases outdoor receptacles are treated as outdoor. Square footage is one cumulative or gross or total number regardless of the number of subpanels, I think it is 3 KW per thousand square feet. Two hundred thirty was an older standard voltage in the U.S. and is a standard voltage in some other countries.

Panels have amperage ratings specified by the manufacturer. The rating of any given subpanel does not affect the overall load analysis that in turn indicated whether the meter and service needs to be upgraded.
The load analysis for the subpanel is used to decide the size of the subpanel feed and the amperage rating for the subpanel. At first this load analysis has nothing to do with the whole house and the service.

You do a "main" or "whole house" load analysis, separately from the subpanel analysis, to determine whether the new load imposed by the new branch circuits in the subpanel together with the load prior to the subpanel project will necessitate an upgrade of the service.

If you will move some branch circuits from the main panel to the proposed subpanel then those circuits will contribute to that subpanel's load. Then when you do the "main" load analysis those moved circuits do not get counted twice because the main load analysis does not need to take into account how many subpanels there are or which circuits the subpanels contain.
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