Sub panel load calc

[maytrix1]

I have an inspector asking for load calcs on a sub panel in a home. I understand how to do a load calc on the house but he wants it on the sub panel. He suggested 3va per square ft, on all areas that this panel feeds but thos circuits are scattered through out the house. I believe technically we may be overloaded by 10 amps due to cook top and oven being moved to this panel by the homeowner doing the actual wiring. I would like to know how and be correct. I dont have a problem moving a circuit if needed.

we only have two inspectors in this county right now.

 

[charlie b]

The simplest thing to say is "make a list of all loads, and add them up." That may not be the simplest thing to do, I know. But if you could present a panel schedule that lists each breaker, states the name of each load, assigns a VA value to each load, and adds the numbers at the bottom of the page, then that is a "load calculation."

The problem, of course, is finding out what loads are on each breaker. For a single family residence, that is not too hard. I would suggest assigning that job to the homeowner (tell the HO it will save them some money). I did this once, long ago. I enlisted the help of my wife and two kids. I made a sketch of the whole house, showing every room and every electrical outlet. I plugged something into every receptacle in every room in the house, and turned on all lights and all other loads. I turned off one breaker, yelled "GO," and we collectively noted everything that just lost power, then I turned that breaker back on. I went through each breaker one by one. The whole exercise took only about 30 minutes.

The other approach is to do a 30 day current measurement on the sub-panel feeder.

 

[suemarkp]

Do the load calc just like one for a house, but cover only the circuits in this subpanel and the loads they serve. The problem here is that you may lose some of "discounts" you get in the full service calculation -- more than 4 fastened in place appliances, a different largest motor, etc). PLus the first X amount of VA is counted at 100%.

Some other considerations -

How large is the feeder? If the wires are rated at 100A or more, then you could use the optional calculation which usually comes in lower than the standard one.

If using the standard calculation, are you using the range circuit demand factor per 220.55 and not the nameplate amps (works for a cooktop and oven combo too)? You should be able to consider the load as 8 KW on a range that is 8.75KW to 12KW in size.

Are the general purpose circuits in part of a room, or just a duplex receptacle here and there? If a room or half of a room, try the 3VA/foot rule then divide by 2 if half the room is covered by this circuit (or by 4 if one wall is covered by this circuit). You could also try to see if he'll take the commercial value of 180VA per outlet if the receptacles are scattered about. For outlets that go to places that are never covered by the square foot rule (e.g. garages, unfinished basements, outside receptacles), their load would be 0. Kitchen counter and laundry circuits would be another 1500 VA small appliance circuit.