1

I would like to run a sub panel out to my garage to run an 80 gal (4hp) compressor. -- I have a 200 amp shut-off panel on the outside of my house which runs to a 200 amp breaker panel in the house. I was told to think of the outside box as the "main" panel and the one in the house as a "sub-panel." So, I am essentially running a sub panel from a sub panel. - Any problems with that?

I'm thinking I'd do a 60 amp panel in the garage. The compressor would need a "dual 30A" breaker - correct? Can I still have another circuit in that panel for a 20A outlet and or a lighting circuit? Or would that require a larger sub-panel?

How many wires of what awg would I need to run from the house panel to the garage in each case?


In response to question - Total distance from indoor panel to garage panel is about 100 feet.

and, the plate on the motor says 230 volts -- 16.5 amps....

I also use a welder very intermittently that "works" on a 20a circuit.

As I continue to browse on this subject... I've learned that I can have four 6 awg wires in my 3/4" conduit... Am I correct that I will need 2 hots, a neutral and a ground running to my sub panel? At just under 100 feet, will 6 awg suffice for a 60 amp sub panel? Then the question becomes, what circuits will I be able to run out of that sub panel? My compressor, plus what?

  • Please revise to add circuit lengths. They can have a bearing on wire size. – isherwood Jun 8 '18 at 20:19
  • I'm a little confused on your labeling in particular the "200A shut off panel". Is it a 200A Panel, an Enclosed Breaker, a Fused Disconnect or an Unfused Disconnect? When you say it then "runs to a 200A breaker panel in the house". Is it being fed by a 200A breaker or fuse, or a lessor amperage? – Retired Master Electrician Jun 8 '18 at 21:30
  • Is the garage attached or detached? – Harper Jun 8 '18 at 22:05
  • What kind of conduit are you running? And yes, is the garage attached or detached? – ThreePhaseEel Jun 9 '18 at 3:36
1

You can run the sub for the barrage to the house sub panel. As far as the garage panel size I would go bigger, put at least a 100/125 amp panel in this way if later you need more power you only need to upsize the breaker in the house feeding the garrage and the wire gauge. Larger panels are not much more and I have never been asked to down size a panel but have upsized many over the years. For the breaker size on the compressor get the specs off the compressor. Compressor motors may not have "true power ratings" they use locked rotor current. Not FLA + for disconnect and wire size.(note your 16.5 amp size would work on a 20 amp circuit but the mfg instructions need to be followed). For example I have an 7 or 8 horse compressor that only requires a 30 amp breaker on 10awg wire but a true 5 horse compressor that draws more. The only way to know for sure is to check the mfg install instructions. At 100' usually the the wire size won't be needed to be upsized for compressors and welders because they are not continuous loads. With that 240v compressor 4 each 120v 20 amp circuits would be fine possibly more in a 60 amp panel but I would install a larger panel just in case of future needs.

1

Yes, you will want to come off of the panel inside the house. The one on the outside is likely just a shutoff. In which case it would be unsafe for you to open because you cannot dissconnect the power at the meter. So at least one side of the breaker would be hot. Flip the main cuffot outside so that you can work on the panel inside without worry.

For a 100' run I'd use service cable. 6/4 should be fine for 60 amps. You will need to run four wires. Two will carry the two phases, one will be a neutral, and the other will be a dedicated ground between the two panels. You may also want to add a ground at the new panel. Ground rod is best, or tie it to a copper cold water line (old school).

As for the appropriate breaker for the compressor, that will entirely depend on the compressor. If it runs on 240V then yes you will need a "double pole" breaker so that you grab 120V from each phase for a total of 240. But you will need to check the motor on the compressor to see if it requires 240v or 120V. It should also say how many amps it will pull.

Yes you could add a double 30 breaker and additional circuits for tools lights or whatever. Really you can have as many circuits as you like as long as the total used at any one time isn't more than 60 amps. So for example you could add a 30 amp circuit for a compressor and a 50 amp circuit for a welder and you will be fine as long as they don't run at the same time.

Edit: wanted to add that for the 60 amps panel you will need to use a 60 amps double pole breaker in the main panel to tie into.

  • 1
    8/3 is out of the question. 60A requires #6. – Harper Jun 8 '18 at 21:34
  • @Harper - Is this answer correct that it is permissable to run only a neutral out to the garage subpanel without a dedicated safety ground? – Michael Karas Jun 9 '18 at 4:17
  • 1
    @MichaelKaras No, it is not. You need to run a separate neutral and ground to subpanels, whether they are in the same building or an outbuilding, and outbuildings also need ground rods. There are some legacy installations that omitted the ground wire, but that's no different than NEMA 10 and NEMA 1 in pre-1970 homes. – Harper Jun 9 '18 at 7:39
  • @Harper - Thanks. That is what I thought. Hopefully PhillipBrantley comes back here and edits his answer to correct this detail. I originally thought I would edit the answer directly but then realized that it would be better for the answerer to learn this first. – Michael Karas Jun 9 '18 at 14:30
  • Thanks for the info. I updated my answer to include the correct AWG and advice to run a dedicated ground so it would be up to current code standards. Although, the equipment would work the same in either configuration. – Phillip Brantley Jun 11 '18 at 14:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.