ECN Forum
Optional Method Calc - 04/22/05 08:04 PM
Hi,
What is the optional method for a load calc? Can this be used on a sub panel?

I used an online calculator and got the following:

General Light and Receptacle Load
1308sqft x 3VA = 3924VA
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Small Appliance Circuits Load
2 x 1500VA = 3000VA
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Laundry Branch Circuit(s) Load
1 x 1500VA = 1500VA
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Total General Load Less Demand
sum of above less T220.11 demand
sum of above = 8424VA
first 3000 at 100 % = 3000
next 117,000 at 35% = 1898
remainder at 25% = 0
sum less demand = 4898VA
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Fastened In Place Appliances Load
Disposal = 1176VA
Garage Door = 1000VA
Dish Washer = 1600VA
Attic Fan = 700VA
Microwave = 975VA
sum less 75% demand = 4088VA
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Total dryer circuit(s) load = 0VA
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Range 9.5kVA
T220.19 Column A = 8kVA
T220.19 Column B = 0kVA
T220.19 Column C = 0kVA
Total calc.d range load = 8kVA
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Heating or Air-Conditioning Load
Air Conditioner = 4240VA
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25% of largest motor = 1000VA
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22226 Volt-Amps
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Computed Amperage
22226 divided by 240 = 93Amps

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Is there anyway to get this down so I could connect a dryer to it?

Thanks

Re: Optional Method Calc - 04/22/05 10:10 PM
See NEC 220.30. This usually comes in smaller than the standard calculation. You also must have a service or feeder rated at 100A or more in order to use it (so I think you're OK with a 100A subpanel if its on a 100A feeder).

When I run your numbers with the optional calculation, I get 90 amps including a dryer. This was based on:
3.92 KVA living space
4.5 KVA small appliance circuits
5 KVA Dryer
9.5 KVA range
1.6 KVA Dish Washer
.98 KVA Microwave
1.18 KVA DIsposal
1 KVA Garage Door Motor
0.7 KVA Attic Fan

This comes to 28.38 KVA. The first 10 KVA are 100% and the remaining is 40%. So this reduces the load to 17.35 KVA. Then we add the Air Conditioner at 4.24 KVA giving a total of 21.6 KVA which is 90A.

About the only thing I can comment on your standard calculation you included is what air conditioner amp rating did you use? Was it MCA or FLA+RLA? If its MCA, that already has the 25% factor for the largest motor so you can remove that item from the calculation.