tl;dr Want to know the easiest way to upgrade electrical in garage from 15A to 20A.

Actual Post

Garage is on a 15 amp circuit, doesn't work out too well for a 15A air compressor.

The main is in the house, ~20 yards away give or take. I can do the wiring myself, but I don't know how to go about finding the existing wiring's path, will standard fishing line work? I saw this related question, so if there are multiple 90s in my path then probably not.

What I know:

  1. I'll probably be adding a subpanel in there for convenience.
  2. I can calculate Amp/Gauge requirements for cabling and breakers.
  3. This is probably going to be a pain.

What I don't know:

  1. What codes will apply in this situation. I know there are certain requirements for buried cables though, just not what.
  2. How/Where the cable should enter the house.
  3. If I can or even if I should trace the existing wiring from the garage and use that path, or start from scratch.
  4. Any best practices or pitfalls to avoid for this type of job.

My question differs from this one in that I want to know what's involved with running new cable from the garage to the house. I know it can be done, I just want to know how it's done, essentially.

Info about the area:

There's a standard city backyard between the garage and house (a big tree, lawn, back porch w/ walkway), and this is MI in case there are any differences in code.

  • What's the gauge of the existing wire?
    – Edwin
    Commented Apr 5, 2014 at 17:17
  • don't know, it's old and thin, likely 14 or higher. I'll double check though.
    – MDMoore313
    Commented Apr 5, 2014 at 17:46
  • @Edwin yep, looks like 14/2.
    – MDMoore313
    Commented Apr 5, 2014 at 18:26
  • 2
    Check with your local building department, they might have documentation listing the requirements for properly installing electrical service in a garage.
    – Tester101
    Commented Apr 5, 2014 at 22:48

2 Answers 2


tl;dr - if you are going to all the work, and a subpanel, you presumably want a bit more than 20 amps (think it needs to be 30 amps minimum for code these days, and 60 amps is probably better.)

You'll have to dig a ditch. At that point, my opinionated opinion is that you should go ahead and put in conduit, and an additional conduit for any current or future possibility that you might want cable, phone, network, etc out there. Ditches are expensive and a lot of work - conduit is cheap, once you have the ditch...Often cheaper (with wire) than "direct burial" cable, and far more resistant to damage in the future - plus it does offer you the potential of pulling out the wire and pulling in new wire if there ever was a problem - but that's low odds of you making any use of it on he electrical side. Network, quite possible.

Before digging a ditch in a city (especially) backyard, call Dig-Safe and have all (Offically known about) services (gas, phone, electric, water sewer & things you may not know about) located. Turn off the circuit to the garage - I would not worry too much about where it is (unless it runs in a conduit that you might be able to re-use - which is not too likely), but odds are that you'll find it when digging, and it's less exciting if it's turned off when you do.

Any wire used must be rated for wet locations - not difficult, just be sure it is. Any exterior conduit is assumed by code to be wet (and that's generally true.)

If the portion of the backyard you are crossing is not travelled by cars and trucks (not crossing the driveway) depth is sufficient if the TOP of the conduit is 18" below finished grade. Be sure to lay "buried electric line below" tape in the top 6" of the trench fill. If you are not digging below frost line (4 feet or more where you are, probably) you definitely need to bring the ends of the conduit up vertically at buildings, and provide a slip (expansion) joint, as the conduit will move with frost. That's generally needed even if the conduit is buried below frost-line as well, unless it's going straight into a basement below frost-line, but its especially critical when the conduit is above frost line. If you don't find the cost phohibitive, a layer of XPS foam over the top of the conduit provides one more indication that there is something there (when someone else is digging, later) and can reduce frost movement a little bit (or a lot if it's wide.) Alternatively, 2" of concrete over the conduit provides some serious protection on top of the conduit, and reduces the required depth to 6" in Rigid or IMC metallic conduit (which may be well worth it in your situation to save on digging) or 12" in PVC conduit.

If you happen to want a walkway that would happen to run where the electric service would, a 4" thick concrete slab extending 6" beyond the conduit reduces the required burial depth to 4" (ie, right under the slab.)

Look for NEC table 300.5 for more detail.

  • ...bring the ends of the conduit up vertically at buildings.. Reading this answer, and looking at the house shot in this video, I don't have to enter the house underground, correct? I say that b/c the top of my basement is above ground, and it would be easiest to drop it in right above the circuit breaker, which would also happen to be right next to where the main comes in. Bringing it up vertically at both buildings wouldn't be a problem and looking at both it might be a straight shot, another plus.
    – MDMoore313
    Commented Apr 6, 2014 at 14:26
  • 2
    Correct - it's quite typical to bring the conduit up above grade (use a large-radius sweep if you are pulling the wire in rather than assembling the conduit around it - or in any case to make the possibility of pulling later reasonable.) It also reduces leaks around the penetrations into the basement, if you can make them above-grade. Just be very careful where you drill the hole - right next to the main means you don't want to shift your hole a little and hit the main!
    – Ecnerwal
    Commented Apr 6, 2014 at 14:43
  • Marking this as the official answer because it answers my original question, though it should be noted that the answer about the doghouse and moving the air compressor closer to the house is a valid solution to my actual problem, and something I hadn't even considered, and may be able to help someone else who comes across this question.
    – MDMoore313
    Commented Apr 21, 2014 at 19:48

Or: A completely different approach to the problem.

If the compressor is really the only thing driving you to want more power in the garage, and the electrical service in the garage, as it stands, would suit your needs adequately otherwise...move the compressor. Build it a "doghouse" in the yard where you can run a dedicated (and shorter) electrical line from the house (rewire the compressor for 220V if it can be wired for higher voltage and is presently 110V) and then run a compressed air line from the "doghouse" to the garage. That might be a lot less expensive than properly upgrading the garage electrical service...

  • Interesting approach, that's why i stated what the problem, I like out of the box solutions. Don't know if the hose would reach the garage with this approach, even upgrading to a 100 ft hose would be cheaper too if the compressor could handle it. What made you say rewire the compressor for 220, why not keep it at 110?? Something to do with it being outside??
    – MDMoore313
    Commented Apr 5, 2014 at 19:25
  • 1
    Anytime you can (easily) provide a tool with 220, and the motor is rated for operation both ways, feeding it 220 cuts the current required in half - so if you are not hauling the compressor around to places where you need to plug it into 110, (ie, it's going to stay put, or move between places where 220 is available) 220 ends up being a little bit more efficient, and also reduces the wire size required. In practice, they usually start a bit better on 220 (higher voltage, less current, less voltage drop, voltage drop as a percentage of supply voltage even less...)
    – Ecnerwal
    Commented Apr 5, 2014 at 19:32
  • 1
    As for the hose, probably getting a large-diameter airhose to run from the compressor to the garage, and then attaching your usual compressor hose or attachments to that would work, though I can easily over-engineer something more complicated in my head. ;-) Airhose diameter is somewhat like wire gauge - longer run wants larger size hose for reduced resistance and pressure loss.
    – Ecnerwal
    Commented Apr 5, 2014 at 19:41

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