Detached Garage Load Calcs

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Old 03-13-17, 02:16 PM
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Detached Garage Load Calcs

So in doing the whole house load calcs to figure if my panel can handle adding a detached garage, how exactly do I treat the garage? As it stands today, my calculation for my whole house is 124 amps, I'm on 150 amp service. Do I add the square footage to the total load? I don't plan to hard-wire anything in the garage. If so any additional amperage used by the garage would fall into the 40% group as 100% only applies to the first 10kW.
 
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Old 03-13-17, 02:27 PM
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Are you planning on running power to the garage is why you are doing load calcs? You don't need to get all technical with the calcs using square footage. Just estimate the highest total load on the main panel at one time and then estimate the total highest load you anticipate at one time in the garage.
 
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Old 03-13-17, 02:33 PM
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Based on another thread it seemed prudent. Realistically I'll never exceed 40 amps in the garage, but still want to wire for max capacity while I'm at it.
 
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Old 03-13-17, 02:50 PM
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Just guesstimate the most power you will ever use at any one time. Pick Thanksgiving, you have a mess of people over and are cooking the big dinner. Every burner on the range is running, a coffee pot perking away, maybe a toaster oven and the microwave along with the lighting. Now, with all these people in the way do you think you are going to go out in the shop and do some arc welding?
 
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Old 03-13-17, 02:53 PM
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Oh it's good to do some estimates of the loads but you don't need to get into the weeds doing it. If you have space in the main panel to add a DP breaker to feed the garage I'm confident you can add a 60A breaker to do so and will not have any issues. Now that is saying you don't have some uncommon high power usage in your home. Such as running a kiln or power lights to grow pot or on demand water heaters.
 
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Old 03-13-17, 02:59 PM
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Here is a great site if you are planning your garage. It covers everything from Electrical to Heat to Tools. The Garage Journal Board - Powered by vBulletin
 
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Old 03-13-17, 03:03 PM
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Now, with all these people in the way do you think you are going to go out in the shop and do some arc welding?
Haha, good point.

Oh it's good to do some estimates of the loads but you don't need to get into the weeds doing it.
Yeah that's what I figured, but it was informative doing the whole-house load calc. Just nice to have an idea of the capacity I'm working with.

I just ordered a Murray 50A breaker, that should do well with whatever I'll be doing in the shop (moderate woodworking/aircraft building). Thanks
 
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Old 03-13-17, 03:10 PM
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In case you aren't aware for power to the garage, you need a four wire feeder, isolated neutral in subpanel, and ground rods.
 
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Old 03-13-17, 03:20 PM
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In case you aren't aware for power to the garage, you need a four wire feeder, isolated neutral in subpanel, and ground rods.
Yup, though I did have a question on the ground rods? Apparently new structures need to have a UFER(sp), that is attached to the concrete foundation. My garage was built in 2001, I'm hoping 2 rods 6' apart will suffice.
 
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Old 03-13-17, 03:30 PM
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2 rods are ok if you don't have a ufer.
 
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Old 03-13-17, 03:35 PM
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Are you clear on what type of wire to use for the feeder. This is one thing that seems to bite a lot of people when they find out they purchased the wrong wire.
 
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Old 03-13-17, 04:20 PM
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Yeah I should be good there. 3 #8 THWN red-black-white and a #10 ground, copper, in 1" sch 80 pvc at the building entrances, sch 40 for the run, rated for 50 amps at 75C, 75C breakers at both ends. #6 bare copper ground, unspliced, to two 8' rods at least 6' apart buried below grade. That's the plan so far.
 
 

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