Detached Garage

[abc123]

I want to run power to a detached metal pole barn/garage from my friend's home panel. He has a 200 amp service to the house and wants 100 amp service to the garage. I know how to size the wire and do the work, but I don't know how to determine if the 200 amp panel in the house is sufficient to power a 100 amp sub in the garage. He also already has a 70 amp sub in the house. The local inspector said he won't require a load calc. since all he is gonna run in the garage are lights, gen. use receptacles and a small air compressor....but I don't want to overload the panel in the house. If I do a load calc. how do I figure it for the garage? Is there anything that says I can't put a 100 amp breaker in the house panel to feed the garage even if I am only going to use 50 amps? (besides obvious extra cost) ( He wants room for future expansion and has already had another electrician run the wire and install the panel in the garage.) If the house calcs. at say 150 amps, can we still use the 100 amp breaker to power the garage sub panel?
Any and all help is greatly appreciated!!

 

[hurk27]

I want to run power to a detached metal pole barn/garage from my friend's home panel. He has a 200 amp service to the house and wants 100 amp service to the garage. I know how to size the wire and do the work, but I don't know how to determine if the 200 amp panel in the house is sufficient to power a 100 amp sub in the garage. He also already has a 70 amp sub in the house. The local inspector said he won't require a load calc. since all he is gonna run in the garage are lights, gen. use receptacles and a small air compressor....but I don't want to overload the panel in the house. If I do a load calc. how do I figure it for the garage? Is there anything that says I can't put a 100 amp breaker in the house panel to feed the garage even if I am only going to use 50 amps? (besides obvious extra cost) ( He wants room for future expansion and has already had another electrician run the wire and install the panel in the garage.) If the house calcs. at say 150 amps, can we still use the 100 amp breaker to power the garage sub panel?
Any and all help is greatly appreciated!!

Use article 220, to determine a load calculation for the house, and let us know what you come up with, and we can help from there.

 

[LarryFine]

Michael, welcome to the forum!

I will respond to one thing right away:

Is there anything that says I can't put a 100 amp breaker in the house panel to feed the garage even if I am only going to use 50 amps?

Not as long as you remember that the feeder must be large enough that the 100a breaker protects it.

 

[abc123]

Thanks. I'll do the load calculation and get back with you guys.

 

[Power Tech]

Run a 1" or 3/4" and pull #6. He will never use that much power.

Set a sub panel and main breaker.

Sit quietly and ponder the universe with the time you have saved worrying.

 

[abc123]

200 amp main fed by #2 copper SE cable

200 amp main fed by #2 copper SE cable

Ok, I did the load calc. for the house and came up with 143 amps. 79 amps on the neutral. But in doing this I checked the current service a little closer and found that he is supplying the 200 amp main breaker in the panel with #2 copper SE cable...
Looks to me like we need to replace existing wire from weather head to house with 2/0 copper.... He has 1 and 1/2 inch riser. Here are my thoughts:
2 2/0 copper wires and one #4 copper for the neutral... #6 bare copper equipment grounding conductor to the ground rod.
Does this sound ok? If so, would it then be acceptable to install a 70 amp double pole breaker and feed the garage with #4 Copper USE-2 direct burial? Also, what's acceptable wire type from the weather head to the main breaker?

Thanks for your help.

 

[iMuse97]

Ok, I did the load calc. for the house and came up with 143 amps. 79 amps on the neutral. But in doing this I checked the current service a little closer and found that he is supplying the 200 amp main breaker in the panel with #2 copper SE cable...
Looks to me like we need to replace existing wire from weather head to house with 2/0 copper.... He has 1 and 1/2 inch riser. Here are my thoughts:
2 2/0 copper wires and one #4 copper for the neutral... #6 bare copper equipment grounding conductor to the ground rod.
Does this sound ok? If so, would it then be acceptable to install a 70 amp double pole breaker and feed the garage with #4 Copper USE-2 direct burial? Also, what's acceptable wire type from the weather head to the main breaker?

Thanks for your help.

1. FWIW, even if I saw and agreed with your max. unbalanced load calcs, I still would not install the #4 for for the main panel neutral, run a #1 or #2 minimum.

2. Depending on the Code you are under, you may have to run an equipment ground with your feed to the outbuilding--and if you have any metallic piping (gas or water) between the buildings you must run the EGC. Sizing looks fine, that is, #4 with 70A breaker.

 

[hurk27]

is this house all electric?

most houses, around here are mostly all gas heat, and appliances, which will rarely have a calculated load over 60 amps.

The #2 SE cable should be changed, as this looks like the house panel was upgraded without changing the service part. if your going to that trouble, you could think about setting a 320 meter that has double lugs, and feed this 100 amp panel off it, and by not entering the house, you don't need a disconnect at the house just the MB in the pole barn, since these would be service entrance conductors you only need 3 conductors, all the bonding require for a service will be required for the barn. this way you are not even adding the load to the house just to the meter and riser conductors.

but if you still want to connect the the house panel then you are going to be very limited to the amount of calculated load you can have in the pole barn.

We do the above 320 meter trick all the time, code only requires a disconnect if you enter a building, so with the double lugs you are not entering the house, just running up to the meter just like the utility would do if this was a underground service.

The size of conductors ahead of the 320 meter can go on the combination of the 200+the 100, so 250 copper, or 350 aluminum would do.

this would give you the full 100 amps in the garage and maintain the 200 amp service in the house.

 

[abc123]

Yep, it's all electric home except for the water heater. I will talk to the owner about the two options...although, I do like the 320 idea. Thanks for your help guys.