Load calculations for residential dwellings

S

Santa49

Member
Location
Pittsburg, ks
Occupation
retired
I have not posted anything for quite some time. Just recently I switched from my 15 or 16 year old desktop computer to a new laptop computer. For all those years I had a PDF Standard Load Calculation form for single family dwellings. I typed my individual loads in and it would automatically add them up. Made it really quick and easy. When I switched to the laptop I lost that calculation form. Now I can't find a new one that will do the same thing. Recently our City has passed an ordinance requiring a documented calculation for service size for new residential projects. Does anyone have any input for me as to what would be a good Service Calculator similar to what I had. Thank you, I appreciate any help.
 
Dennis Alwon said:
You can try this one from mike holt.

docs.google.com

Residental Load Calc MH 5.xlsx

docs.google.com docs.google.com
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Would a microwave be considered as cooking equipment and subject to the demand factors in T.220.55 since it's over 1 3/4 kW?
If not, if it is secured to the cabinet, shouldn't there be a 75% demand factor for four or more appliances fastened in place? 220.53
I've always been taught to make a final step of calculation to take my largest motor and increase 25% for inrush current on the service conductors? 430.24
 
Rick 0920 said:
Would a microwave be considered as cooking equipment and subject to the demand factors in T.220.55 since it's over 1 3/4 kW?
If not, if it is secured to the cabinet, shouldn't there be a 75% demand factor for four or more appliances fastened in place? 220.53
I've always been taught to make a final step of calculation to take my largest motor and increase 25% for inrush current on the service conductors? 430.24
Click to expand...
In regard to residential load calculations, when talking about a microwave, dishwasher and garbage disposal all rate at 15 amps or less, how do you apply the 25% inrush current for the largest motor? Which appliance do you choose? For the sake of learning, lets say the garbage disposal has the highest inrush current, how are you adding up the math on it?
 
Dennis Alwon said:
If you look at Annex D you don't see 125% of the largest motor being used in residential calcs.
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Annex D1(a) doesn't have any motor loads in the calculation. If you look at Annex D1(b), there are motor loads and the example shows a 25% increase of the largest motor and references Art. 430.24
WHayward said:
In regard to residential load calculations, when talking about a microwave, dishwasher and garbage disposal all rate at 15 amps or less, how do you apply the 25% inrush current for the largest motor? Which appliance do you choose? For the sake of learning, lets say the garbage disposal has the highest inrush current, how are you adding up the math on it?
Click to expand...
Your largest motor is not your biggest in terms of hp. Your largest motor in terms of the NEC is the motor drawing the most current. You would determine your largest motor based on T. 430.248 by FLC, then increase that by 25% for inrush current. Example: A 1hp 230v pool pump motor draws 8 amps and a 1/2 hp 115v garbage disposal draws 9.8 amps. If those were your only two motors in the calculation the garbage disposal would be your largest motor. 9.8a x 115v = 1127va 1127va x .25 = 281.75 You would then add 282va in the last step of calculation on both your line and neutral loads since it is 115v. If a 230v line to line motor was your largest motor, you would only increase the va on your line side as no neutral load needs to be calculated for this motor.
 
Rick 0920 said:
Dennis, would you count a 2000 watt microwave as a fixed in place appliance (220.53) or would you count it as cooking equipment and take the demand factor from Col. A in T.220.55?
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Rick, I don't think there is any question for me. I would use 75% on 4 or more appliances. Assuming it is fixed in place. I don't see a microwave as fitting in T.220.55
When I think of T. 220.55 I see ranges, single ovens, double ovens, and cooktops. Nowhere in there does it mention microwaves. Imo, if the nec wanted mw's in this table, they would have stated it.
 
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