Residential service load calculation

[nizak]

Could anyone take a minute and verify a residential service calculation for me.

Service is 120/240V 1phase. Load is as follows:
2800 sq ft
5- small appliance circuits (includes microwave)
2- laundry circuits
1-12A dishwasher
1-5A disposal
2-5A door openers
1-6KW wall mount oven
2-5KW clothes dryers
2-2 Ton (13SEER) AC compressors
2-furnaces 1/3 HP blower moters

I am calculating from the Michigan Residential Code, figuring from the NEC I would imagine should be fairly close.
I came up with 124A. I am trying to size a whole house generator and have been told by 2 different generator dealers that comparable size homes they have speced out in the past were not nearly this high on the load calc.
Am I miscalculating???
Thanks Much.

 

[Smart $]

I get 133A.

 

[Npstewart]

I actually come up with a load a little bit higher but that is probably because I used 8 kW for the range instead of 5, and a higher value for the garbage disposal. You also didnt specify the kW for the HVAC compressors, the tonnage doesent really indicate how much power it uses so I used 17.9 MCA for each.

In summary, I got about 132A @240 using the optional method for dwelling units.

132A @ 240 is about 32 kW. I would say that seems about right if you are sizing a whole house generator for 2800 ft^2.

 

[Npstewart]

I get 133A.

Funny, I didnt even see your reply and we were within 1 amp lol.

 

[nizak]

Thanks much everyone, didn't appear to be off by as much as I was being told by gen people.

 

[Horst]

Generator company

Generator company

We have traditionally installed undersized units on residential houses since the true power usage is traditionally much less than the calculated load. Makes for some interesting conversations with the "educated" AHJ. I assume that is what is happening to your salesman with his similar size house comment. I came up with the same as the others, you "should" base it on 130A (or 125 based on your load calc.)

PS: My "educated" comment is in no way a swipe on the AJH, I simply mean when one is versed on generator sizing.

 

[Daja7]

2 laundry circuits in a 2800 sq ft home? 5 small appliance circ?
how about water heater

I use the load calc sheet found on the mike holt free stuff. works great and all of the AHJ's accept it.

I come up w/ 124 using this calc form.
you can get away with a 20 KW if you use the load shed feature for a couple higher usage items. like dryer, oven or the A/C units.

 

[Npstewart]

Obviously the generator has to be sized at the calculated load but assuming you were to undersized it, you should be careful about doing so. Residential generators are not nearly as capable at handling high inertia loads when the loads are first starting. The extra fat in the generator will help a lot at starting any large motors should you have any. I have seen larger motors start on a residential generator where they begin to start the motor and then the motor simply stops. The only way to remedy this problem is to either replace the generator with a larger one, or use a commercial type generator that is designed to handel such loads.