Replacing underground wiring to garage

[Ashyukun]

So a few weeks back after a heavy rain, the breaker for the circuit that currently powers the outlets on our deck and my garage. The garage has a 220V outlet, so it's a dual breaker. After a lot of testing (by disconnecting the different legs at the junction box) and the fact that the breaker will not trip when everything after the junction box is disconnected but will trip when only the underground line to the garage is hooked up but the breaker (well, it's an old-style blade switch with fuses...) in the garage is disconnected. Which means the worst-case scenario appears to be in effect: there's something wrong with the wire underground from the junction box to the garage.

So, I need to figure out what to do. I can't not have power in the garage- I work on our cars there and need the power for lights and power tools, and at the moment I've only got power there because of a very long outdoor extension cord run back to an outlet in the house.

I've done a good bit of reading up on 220V wiring since I've not dealt with it before, and appear to have a few things to consider.

1. The current wiring running out to the garage is 3-conductor (2 hots and a neutral/ground). It's split off to both 120V and 220V circuits (though both are on the same subpanel fuses). From my reading, that's no longer up to code- it should be using 4-conductor wire with separate ground & neutral wires. I'm finding precious

2. The buried line is, well, buried- if it was buried properly (which I have my doubts about...) it will be down a fair bit and I don't relish the idea of having to dig it up and run a new wire.

3. Regardless of what ends up happening with wiring it up (if I stick with the 3-conductor or move to 4) I'm going to have to replace/upgrade the subpanel. It currently has a fuse for each of the 2 hot wires (both can be disconnected at the same time with an old-style blade switch) which I've been told is not safe since it's possible for only one side to blow and not the other and still be providing power to a shorted circuit.

So, I'm trying to figure out whether it seems like it would be reasonable to try and tackle this myself or if this is a case where I should suck it up and call in the professionals.

[Greg]

I would carefully double check your diagnosis before proceeding.  Not that it's impossible for the underground feeder to be bad, but unlikely.  I know electricians have special tools to test for line voltage leakage.

But, you could totally re-do this.  It will involve digging and the rest.

[unpolloloco]

Also, keep in mind that you may not have to actually dig up the old line (probably more pain than it's worth unless there's a high scrap value).  You could just run a new line and disconnect the old one on both sides.

[ncornilsen]

Per the letter of the code, you must demolish and remove all unused wiring. Period. So technically, you must remove it. Not to say it isn't left in place all the time.

Secondly, it may have been run in a conduit. that's how it was done at my house, which made the replacement very easy... make sure this isn't the case.

And as for the ground thing... I think that's a bit of a gray area. You may be able to drive ground rods outside your shop and attach the sub panel to that, unfortunately every code guy has a different opinion on this it seems like. At the end of the day, most wiring systems have the neutral (white) wire connected to the ground wire bar anyway where the main service enters the house.  The whole purpose of the ground wire is to provide a safe, low-impedance way of shorting the circuit directly to ground incase of a wire contacting a conduit or metal J-box (or other things), therefore ensuring an immediate breaker trip. A local ground in your garage should work, with the risk being a difference in potentials between the rods your main service is tied to, and the rods you drove by your garage, which would create a ground loop and maybe some noise on very sensitive electronics.

-Nick

[Spork]

I am not sure of the legality here (electricians please pipe in)...

If one of your wires is nicked and goes to ground...  You could theoretically repurpose the wiring.  I.e., identify the nicked wire and call that "ground"... then repurpose the other two for the two hots.  (This, of course, assumes your ground is an insulated wire.)

[Greg]

I am not sure of the legality here (electricians please pipe in)...

If one of your wires is nicked and goes to ground...  You could theoretically repurpose the wiring.  I.e., identify the nicked wire and call that "ground"... then repurpose the other two for the two hots.  (This, of course, assumes your ground is an insulated wire.)

Usually ground wires are uninsulated.  You would need at least one hot and one neutral.  A second hot for 220/240v service.  Best to redo it I think.

[Spork]

I am not sure of the legality here (electricians please pipe in)...

If one of your wires is nicked and goes to ground...  You could theoretically repurpose the wiring.  I.e., identify the nicked wire and call that "ground"... then repurpose the other two for the two hots.  (This, of course, assumes your ground is an insulated wire.)

Usually ground wires are uninsulated.  You would need at least one hot and one neutral.  A second hot for 220/240v service.  Best to redo it I think.

I knew that was true in normal romex... but is that also true in cable that is approved for underground?

I ask because:  I had an underground run that I found to be unsafe.  (I was doing other work and realized that whomever installed it did the bulldozer work after the cable was in... squashing all if it flat as a pancake.)  I worked with a friend that was a master electrician -- me doing the grunt work.  What he had me buy was a large length of the appropriate type of underground wire.  He cut it to lengths and just designated the ends as hot #1, hot #2, neutral.  They were all the exact same wire.

[ncornilsen]

Usually your conductors (L1, L2, N) will be the same size, but when you're wiring something that's more than 50 amps, the ground conductor is allowed to be smaller, even if it is insulated. In those cases, you couldn't re-purpose a wire.

If all the conductors and the ground are the same size, you could repurpose a ground wire for another purpose.

[Greg]

I am not sure of the legality here (electricians please pipe in)...

If one of your wires is nicked and goes to ground...  You could theoretically repurpose the wiring.  I.e., identify the nicked wire and call that "ground"... then repurpose the other two for the two hots.  (This, of course, assumes your ground is an insulated wire.)

Usually ground wires are uninsulated.  You would need at least one hot and one neutral.  A second hot for 220/240v service.  Best to redo it I think.

I knew that was true in normal romex... but is that also true in cable that is approved for underground?

It's true of copper wire, and used to be true of aluminum wire, but most long-run wire is aluminum now, which is now recommended to be insulated to prevent corrosion and loss of ground.