Do I need 400 amp service?

Mark Spohr · 10-24-2015, 11:12 AM

I'm in a bit of a quandary here and I was hoping to get some advice.
I'm building a new detached garage. The plans (approved by the building department and the power company) call for a sub-panel in the garage.
The house has 200 amp service with a 200 amp breaker panel. I figure my maximum electrical use if everything (hot tub, electric dryer, electric car charging plus other misc use) is turned on at once is 20kW (80 amps). (Hot water and heat are natural gas.)
Garage is to the point where we call in the electrician. He says no way he can put a sub-panel in the garage since I already have a 200 amp panel in the house. Says I have to have 400 amp service.
Consult the electric company about 400 amp service. They say I have to buy a new transformer ($7000) plus underground wiring to get 400 amp service. I should note that my current electric feed comes from a power pole mounted 25 kW transformer which also feeds 6 other houses plus an ATT DSLAM... seems a bit dodgy.

Do I need 400 amp service?
Can I put a sub-panel in the garage on my current 200 amp service?

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**Jim Port** · 10-24-2015, 03:31 PM

The correct method is to do a demand load calculation. Other than that it is a guess. Your electrician should be able to show you the figures.

**Jerry Peck** · 10-24-2015, 03:36 PM

You can put a 200 amp remote panel in the garage, even a 400 amp remote panel in the garage ... the only issue would be that the service supplying the house and garage is only 200 amp - so the most you could get through the service equipment to feed both the house and the garage is 200 amp combined.

The question is this: what are you going to be doing in the garage and how much is required for the use?

I can see installing a 100 amp ... maybe even a 125 amp remote panel in the garage fed from a 100 amp or 125 amp breaker in the garage.

You also need to install a remote panel which is listed as "Suitable for use as service equipment". I recommend installing a panel which has a main disconnect (this will be required any way if the panel has more than 6 circuits).

The potential problem would be when using everything in the house and the garage at the same time (which is highly unlikely) - in which case the 200 amp main would trip.

Mark Spohr · 10-24-2015, 04:37 PM

Originally Posted by Jerry Peck

You can put a 200 amp remote panel in the garage, even a 400 amp remote panel in the garage ... the only issue would be that the service supplying the house and garage is only 200 amp - so the most you could get through the service equipment to feed both the house and the garage is 200 amp combined.

The question is this: what are you going to be doing in the garage and how much is required for the use?

I can see installing a 100 amp ... maybe even a 125 amp remote panel in the garage fed from a 100 amp or 125 amp breaker in the garage.

You also need to install a remote panel which is listed as "Suitable for use as service equipment". I recommend installing a panel which has a main disconnect (this will be required any way if the panel has more than 6 circuits).

The potential problem would be when using everything in the house and the garage at the same time (which is highly unlikely) - in which case the 200 amp main would trip.

Thanks, that what I thought.
I went and did load calculations and I only need 100 amp service in each place. The garage has the electric car charger. The 200 amp service should be plenty.

**Mark Reinmiller** · 10-24-2015, 06:44 PM

It sounds like you need a new electrician.

**John Kogel** · 10-24-2015, 09:27 PM

Originally Posted by Mark Reinmiller

It sounds like you need a new electrician.

Maybe the electrician is confused by what you are asking for. You want the garage panel to be fed by a breaker in the main 200 amp panel. What JP says, 100 or 125 amps will be plenty. You do not want 200 amp service in your garage, you want 120/240 Volt power at whatever reasonable amperage, maybe 60 amps will be plenty for a few power tools, compressor, heat and lighting.