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Load calculation + subpanels

6680 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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Here is an online calculator. I'm not sure what code cycle it is based on, but it has all the info you need.
You do not work backwards. The load calculations are the total load of the service coming into the house.

If you only have 12 circuits in the main panel, you need a bigger panel.

Unless you are going to make the detached garage a workshop, just add 1 circuit out there
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