How to do load calc?

[cartunes]

Hi Y'all,
Just completed my 40x30 detached and trying to determine if my houses's 200amp service can also handle the garage.
The house is all electric, with 2 heat pumps, 2 air handlers, electric stove, electric hot water, electric dryer, well pump, plus all the usual small stuff.
I had the electric company (Dominion VA Power) run a report of the house's usage for the last 18 months (I just moved here in May). The highest usage was during January with 85 KWH used.

That's my first problem, I don't understand how to deal with their KWH.

I calculated for the equipment I will have in the garage and it maxes out at about 89 concurrent amps = 18.4 KW.
The house service is 240v, 200a = 48KW. Correct?

So what else do I need?
Thanks, Ron

 

[rockwithjason]

there are a lot of little rules and nuances to load calcs but you can get a pretty good idea by adding up 100% of the continuous load such as lighting and things that are on most of the time and 150% of the non continuous load such as tools and things that you only use once in a while. most garage panels can use a 100a sub with no problems.

 

[Stuart in MN]

Remember that KWH = kilowatt-hours, which is what the power company charges you for, so you have to multiply your kilowatts by the number of hours.

Here is a useful worksheet for calculating electrical service size: http://www.finehomebuilding.com/PDF/Free/Service_Feeder_Calculations.pdf

 

[Schrodingers Cat]

85 KWH over what period? 1 hour, 1 day, 1 month?
without the time duration it's meaningless...except for billing
it's probably a 24 hr period...
so if you use 10 hrs as the base (work-sleeping = less load), and a 250% peaking factor
85 KW-hr / 10 hr x 2.5 ~ 21.25 KW or ~90A

one way to know for sure:
get an amp clamp and turn everything on...everything...
factor in the heat pump emergency heat...I'ld be surprised if you exceed 100A @ 240VAC

btw: 89 A x 240 V ~ 21.36 KVA or 21.36 KW at unity pf...at an average pf of 0.9 ~19.2 KW

 

[Gary S]

85 KWH over what period? 1 hour, 1 day, 1 month?
without the time duration it's meaningless...except for billing

Right. Trying to use KWH for determining load is the hard way. With KWH, you add time in as an element to compute and you don't want it.
Simplify your load calculations and use amperage draw.
Just add up the amperage of the devices you plan to use and compare the total to the size of your service.
Devices that need a continuous load like electric heat or motors that run 100% will use their rated amps directly off the data plate.
Other devices that run only occasionally like your electric range, dryer, and refrigerator are harder to calculate in because they aren't taking power all the time. You need to factor them in only when they run.
One more important factor to keep in mind is that 120v devices is that they take their amperage off only one side of the 240v service panel. That means that it takes two 20amp 120v devices on opposite legs of the 240v panel to use up 20 amps of its capability whereas it would take only a single 20 amp 240v device to draw the same current from the panel.

 

[cartunes]

Stuart in MN,
I filled out that load sheet for the house and it says 267 service amps needed for the house. Does that sound right? My 2 heat pumps and 2 air handlers (with emergency heat) yielded 32160 watts @ 100% per the sheet.

By my calculations for the garage, if I ran all these at the same time I'd use 69.8 amps:
Air Compressor 1 240 14
Automotive Lift 1 240 30
Ceiling Fans 3 120 1
Bench Grinder 1 120 3
Interior Lights 18 120 10
Exterior Lights 4 120 3.3
Heater (propane) 1 120 2.5
Electric Hand Tools 2 120 6
69.8

 

[Schrodingers Cat]

Stuart in MN,
I filled out that load sheet for the house and it says 267 service amps needed for the house. Does that sound right? My 2 heat pumps and 2 air handlers (with emergency heat) yielded 32160 watts @ 100% per the sheet.

By my calculations for the garage, if I ran all these at the same time I'd use 69.8 amps:
Air Compressor 1 240 14
Automotive Lift 1 240 30
Ceiling Fans 3 120 1
Bench Grinder 1 120 3
Interior Lights 18 120 10
Exterior Lights 4 120 3.3
Heater (propane) 1 120 2.5
Electric Hand Tools 2 120 6
69.8

I think you can deduct the heat pump compressors when the e-heat is on...
most systems lock them out...check on yours

32.2 KW at 240 VAC 1.0 pf ~ 135A...
a 200A service sounds about right

 

[Stuart in MN]

I would agree - the compressors are probably not running when the standby heat is on, but you should double check to make sure.

 

[cartunes]

So then if 200amp sounds right for the house, the garage is gonna need its own 100amp service to handle the additional load. Correct?
Thanks

 

[Gary S]

By my calculations for the garage, if I ran all these at the same time I'd use 69.8 amps:
Air Compressor 1 240 14
Automotive Lift 1 240 30
Ceiling Fans 3 120 1
Bench Grinder 1 120 3
Interior Lights 18 120 10
Exterior Lights 4 120 3.3
Heater (propane) 1 120 2.5
Electric Hand Tools 2 120 6
69.8

There's a small math flaw there. You added the current draw of the 110v devices as 220 draw. The 110 need to be divided by 2 as they draw only from one side of the service. Your total should be 56.9 which can be handled easily on a 100 amp service. I'm betting you will never have everything listed running at the same time, so you should never see 60amps of continuous draw.