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jeffiebean 03-14-2011 10:25 PM

Detached garage subpanel options?
I have spent a few weeks now contemplating the best way to go about adding some 220V circuits to my garage, which has included many hours reading through many many threads on this site.

Overall question is: does what I am about to outline sound like the best solution to meet my needs?

I have a small detached 1 car garage in which the only electricity is fed thru 1 12/2 cable from the house, supplying a few lights and the garage door opener. The garage and current wiring are about 20 years old.

I have a few woodworking tools that require 220V, so I want to add a subpanel to the garage. I only need about 30A to support the electrical load of the dust collector (12A) and any other tool I have (tablesaw: 12A, miter saw: 15A, Router: 13A, etc). The only things ever running at the same time would be 1 tool plus dust collector plus lights, so a subpanel protected by a 40 amp breaker seems like plenty, taking into account the additional startup amp draw of the motors.

There is still capacity in the main 150A breaker panel (both physical and amperage).

Since it is a detached garage, (1) I need a main breaker in the subpanel as a main shutoff switch, and (2) would need to incorporate the current lights and outlets into the subpanel so I can get rid off the current feed jumped off the house circuit.

Unfortunately for me, the main panel in the house is all the way in the opposite corner, so the cable length from main panel to subpanel would be approximately 100'.

My plan:
  • install a 40A double pole breaker in the main box
  • Run 6/3 with ground cable thru basement
  • continue thru 1 1/2" PVC with (3) 6 awg THWN + (1) for ground, for the 30 feet from basement to garage
  • install 100A main breaker 20/20 subpanel in garage

of course, do all the appropriate install of ground rods local to garage, not bond ground and neutral until they hit the main panel, etc.

Is there a better way to go about this? I can't find a 40 or 60amp panel with a main breaker option. In some ways this all seems like overkill based on the load, but based on my calculations seems right.

anyone have a better way to go about it or does this sound good?


AllanJ 03-15-2011 12:09 AM

The project is going to be about the same whether you run a 30 amp (@ 240 volts) or a 100 amp line (size of the cable). Most homeowners will find a 60 amp line (6 gauge) out to the garage to be sufficient and also not detract from the resale value of the home (by being too small) when the time comes.

The existing 12-2 cable can no longer be used as an additiona power feed to the garage but it could be used to power a light back at the main house where the switch and also its power feed is at the garage.

It is possible to use a 60 amp side breaker pair as the "main" breaker for a subpanel, if you can't find a panel with a top breaker and of the desired size. This is called "backfeeding". It will have to be custom fastened in place so it can't snap out exposing its live underside.

Do It Right 03-15-2011 07:06 AM

If your going to pony up the money for #6 copper wire, there's no reason to limit it to 40 amps.
You can go with a 60 amp breaker.

jeffiebean 03-15-2011 10:21 AM

When supplying the subpanel 100 amp main break with #6 and a 60 amp breaker, do you have to mark the subpanel at all or blank out the 100A on the breaker so that a future owner doesn't think they have 100 amps in the garage?

jbfan 03-15-2011 12:29 PM


Originally Posted by jeffiebean (Post 609785)
When supplying the subpanel 100 amp main break with #6 and a 60 amp breaker, do you have to mark the subpanel at all or blank out the 100A on the breaker so that a future owner doesn't think they have 100 amps in the garage?

No. The breaker now becomes just a switched disconnect.

jeffiebean 03-15-2011 09:09 PM

One more question: where can I purchase 6 AWG THHN cable by the foot? I have to run 30' of wire outside to get from the house to the garage, and was going to run individual strands through conduit. Should I just start calling local electrical suppliers and ask if they sell it by the foot? The couple of big box stores I looked at quickly only appear to sell it by 250' spools.

Or, should I go with 6/3 UL cable and skip the conduit?


Jim Port 03-15-2011 09:20 PM

Try an electrical supply house.

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