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My new house was built in 1980 (Virginia), and it has a detached garage. I went to install a 30A double pole breaker to power a 240V mini split, when I noticed the panel only had one powered 120V bus, the other was effectively dead. My garage subpanel only has 3 wires coming in via buried metal conduit about 30ft from the main panel, though there is a 100A safety switch between the two under the house (installed recently, probably the previous owner). The neutrals and grounds are bonded in the subpanel and the metal conduit is grounded as well. The metal conduit terminates at a metal junction box inside the garage right below the subpanel.

The faulty hot was left disconnected from the main 60A breaker, probably also the doing of the previous owner after they discovered the issue themselves. It looks like they had also installed a 220V dryer outlet (NEMA 10-30R) near the garage panel which is also not hooked up--again, probably because they ran into the issue I am now discovering.

Upon investigating the issue, I've determined the dead leg is a result of a broken wire between the safety switch and garage subpanel, somewhere in the buried metal conduit. I determined this by temporarily turning off the power and connecting the two hot wires in the garage junction box together and using a meter to measure resistance at the safety switch, which was infinite.

Main panel

Main Panel

Garage Subpanel

Garage Subpanel

Garage Junction Box

Garage Junction Box

Safety Switch

Safety Switch

So, now I have to decide.

  • Do I just remove the current wiring and pull 3 new wires, try to remove just the broken one and pull that, or do I throw it all out and pull 4 wires so that I can have 2 hots, a neutral, and a ground. Can I even use 3 wires or will I have to upgrade to 4 no matter what to meet code since I am "modifying" it by fixing the wire.
  • Can I replace the existing #1 (maybe #2?) aluminum wires with copper ones (say, #2 or #3 CU THHN). I'd also like to run something that could potentially support 100A in the future if I ever decided I needed more than 60A, though I doubt I ever will.
  • Lastly, do I need to upgrade my subpanel with one that has dedicated ground and neutral bars, and separate the connections (I am assuming yes if I upgrade to 4-wire).
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  • EV charging plug ?
    – Ruskes
    Aug 17 at 6:29
  • No idea what the previous owner was installing it for, but I only need 240V for a mini split, the dryer outlet isn't connected to anything.
    – Matt
    Aug 17 at 6:37
  • How is the JB below the subpanel connected to the subpanel? Aug 17 at 7:51
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    If you look in the JB pic, the two wires and bare braided cable leaving the top right of the box enter the center bottom of the subpanel (see garage panel pic). The JB is to the lower left of the subpanel. The "To Garage Panel" wire connects to the subpanel, and the "To Main Panel" wire goes into the conduit to the safety switch & main panel. They are disconnected in this picture but should be connected together.
    – Matt
    Aug 17 at 8:23
  • OK - the conduit is metal. That should be your grounding conductor. What size is it? I do wonder if you have an alien breaker situation there, though, since there are clearly at least two different types of breakers at the garage. Edit in a picture of the panel label on the right inside wall of the box?
    – Ecnerwal
    Aug 17 at 12:45

2 Answers 2

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99+% of the time a lost phase is lost at a bad connection, rather than the wire breaking, especially if the wire is in conduit and nobody has taken a backhoe or posthole digger to the conduit. If they have, you may not be able to pull anything out or in until the damaged area is dug up and repaired.

If you have 1 AWG aluminum in conduit, that's already good for 100A. If you have 2 AWG aluminum that's good for 90A (both assume 75 °C terminations and wire insulation, which are generally a good bet at this application. Aluminum at these sizes has never been an issue, and replacing it with copper is insanely expensive for no good reason, unless you really need more power and have no space in the conduit.)

If you actually have a bad wire rather than a bad connection, you can pull out 3 and pull in 4 (much easier than pulling an additional one or two without pulling out all of them, normally.) Or only pull in 3, see below. You can re-use wires that are not damaged.

You said buried metal conduit! If you have continuous steel conduit, you don't need a grounding wire, you can use the conduit as the ground conductor (safety ground) and have 3 wires in it for hot-hot-neutral, yet have a 4-wire subpanel. You will need to upgrade the connection from the garage junction box to the panel, but that's short. Looks like you will also need to upgrade the wiring from the main panel to the safety switch to a 4-wire connection.

Normally, all you need for a subpanel is either a retrofit ground bar (if the neutral provided is isolated from the case other than by a bonding screw) or a retrofit isolated neutral bar if the grounding bar provided is not isolated from the case.

Then again, a new panel is not terribly expensive, and you can get one with a main breaker to provide the required local (at the garage - one under the house does not meet the need) shutoff function and get plenty of spaces while you are at it. That can be a 100, 150 or 200A panel and main breaker, even if you are only feeding it 100A (the subpanel breaker need only be as large or larger than the breaker feeding it from the main. It does not need to "match.") You apparently had a "rule of 6" avoiding the shutoff at this location, but now you have 7 breakers and need a local shutoff. You will likely need to buy an accessory grounding bar kit for that panel.

Also, put that 10-30R in the trash or scrap where it belongs.

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  • Thanks, I attached a picture of the safety switch. I do feel a bit concerned that the metal conduit is only continuous up until the safety switch and does not extend to the main panel, wouldn't that mean the ground path ends at the safety switch? I tested the suspected broken wire by connecting it to the other hot in the junction box and then probing the exposed conductors where they terminate at the safety switch and got no reading (infinite). I'm assuming it broke when they were pulling it but maybe not. Guess I'll find out when I pull the wire. I will likely upgrade the panel after.
    – Matt
    Aug 17 at 14:31
  • Yes, you'll need to upgrade the wire to the safety switch as well, since it's "two hots and a bare" cable. Either run conduit and wires or run 4-wire cable to it, with the ground wire bonded to the box (and thus to the conduit.).
    – Ecnerwal
    Aug 17 at 14:34
  • Got it, I'll run 4-wire cable to the switch and then pull four new 1 AWG AL wires to the garage, and install a new subpanel with a main breaker and separated neutral/ground. If I can't physically pull 4 because of the conduit I guess I'll pull 3 and use the metal conduit as the ground path. Do I need to use 1-1-1-3 SER for the switch cable?
    – Matt
    Aug 17 at 14:55
  • 1-1-1-3 SER is fine for main-panel-to-switch and JB-to-subpanel Aug 17 at 20:31
  • If you measured the conduit as "about an inch and a half OD" (so it's probably 1-1/4" EMT) you'd want three 1 AWG and one 3 AWG if opting to pull a ground wire at all - four 1 AWG is just a hair too much for fill, and it will match the cables you're using.
    – Ecnerwal
    Aug 17 at 20:39
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Ecnerwal wrote the posting I would've written! Consider that an endorsement.

Don't rush off and buy new wire. I hate when people do that: Harper's Rule: buy the wire last. Normally, that's because you learn more stuff talking to people, but in this case it is to learn more stuff as you dismantle the installation.

Learning part A is to disassemble those gobbed up boogers of wire splices, to see what's actually under all that tape. Fair chance that's the problem: the tape makes them functionally uninspectable. Once exposed, you could try carefully insulating each exposed wire, then throw the power back on and see if hot is making it here. Wouldn't that be nice! Change those out to modern Polaris connectors. Split-bolt and gobs of tape was the state-of-the-art back then, but Polaris connectors are now readily available and they're inspectable!

If you're replacing all the wires, get enough wire to bypass (run through) this box. Avoid unnecessary splices. You'll know why when you price Polaris connectors.

Learning part B is you need to actually pull the wires out of the conduit to even know what the score is. That will tell us several things.

First: you want to do a careful physical inspection of the old wires. #1 if they're tip top, you can reuse them! #2 the wire condition is a Rosetta Stone for conduit conditions. If you find one particular zone where 2 wires have caked-on scale, that tells you exactly where the pipe is rusting out. Segue to:

Second: Is the conduit actually in good physical shape? The proof of the pudding is to pull the old wires out. If they can pull reasonably, that's a good indication the conduit is OK and you'll be able to pull new wire in. If not, it doesn't matter which new wires you buy, because you will not be able to pull them in without destroying them. Problems in the pipe will be revealed by either the wire being immovable, or the wire revealing damage either from the collapse or from being dragged past the collapse.

The cure is to, unfortunately, dig up the yard and replace pipes. Since FMC and thin-wall EMT is illegal for direct burial, most likely this is IMC or RMC which are entitled to being buried at 6" of cover. That means they may not be so bad to dig up - at that depth there's no need to call 1-800-DIG-SAFE or whatever your area calls it, and you can excavate with hand tools (a shovel hit will not faze RMC, but obviously a digger hit will destroy it.)

Wire size

If you want an honest 100A, you need #1 aluminum or #3 copper. If 90A is plenty, #2 aluminum is popular, meaning it's at a pricing and availability "sweet spot" - cheaper and available in more types. Keep an open mind while shopping.

Because of the way aluminum wire is priced, I wouldn't go any lower than 90A. The next size down is only 65A and barely cheaper.

90A is plenty for dual EV charging at 60A per car, due to "Share2" sharing technology, which allows the EVSE's to coordinate to keep their total ampacity down to a target figure such as 80A.

The Rule of Six

Your mini-split installation violated Code by adding a 7th breaker. Code requires that a subpanel in an outbuilding have either:

  • 6 or fewer hand throws to shut off all breakers, and all must be right next to each other. ("Rule of Six"). ...OR...
  • A main disconnect.

Once you have both poles working, you can position 120V breakers right next to each other and install an approved "Handle-Tie" between them. This makes them count as 1 hand throw. (it might also cause them to trip together, so choose wisely - not "table saw" and "lights" for instance.)

Otherwise you can retrofit a main disconnect. You do this by installing a main breaker of appropriate size, and a hold-down kit for said main breaker for that panel, and then connecting the supply wires into the now-main breaker instead of the main lugs. This will use up 2 of your 12 spaces.

Alien breakers must go!

This is a GE panel. I recognize the unique cruciform bus stabs (that support their "thin" breakers), but also the model number of the box.

Yet most of your breakers are a dog's breakfast of random other manufacturers. "Why not? They fit." Because the bus stabs are different shapes, and they don't make proper contact, and will arc and burn up a bus stab if not the garage. Thus, the only breakers permittable in this panel are:

  • General Electric THQL (full width)
  • General Electric THQP (half width, not handle-tie-able)
  • Eaton CL (which are UL-Classfied for this panel, and feature a distinctive circular UL-Classified logo).

The Eaton CL is hard to find. I suspect those Eaton breakers are Eaton Type BR/Type C, because many installers have not gotten the above memo (or deny inconvenient truths).

The GE half-width breakers will allow you to put more circuits in this panel; however they are not available in AFCI, GFCI or smart breakers.

Wait. What??? Smart breakers??? Yeah. They're in the pipeline, be here Real Soon Now. They will let you do things like let you power things that otherwise wouldn't fit on the service, e.g. interrupt the water heater while both A/C's are cycled on, or go after advantageous rate plans from the power company, or have an EVSE adjust EV charge rate on the fly to keep the house within limits.

Ground wire

If the old RMC or IMC proves out, there's no need for a ground wire for that section. The conduit covers the need. If you do need or want a ground wire, 100A requires #8 copper or #6 aluminum.

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  • Thanks. I have already swapped out the subpanel for a 100A Eaton main breaker box with more circuits and all Eaton BR breakers. I've also decided to just eliminate the junction box and extend the conduit to the subpanel, so I should have no more splices, but I have actually already ordered and received connectors similar to the Polaris connectors you are referring to, though they are Morris Product branded and not Polaris brand. Seems like the same design though. I am going to run #1 AL to the subpanel and 1-1-1-3 to the safety switch from the main panel since that isn't in conduit.
    – Matt
    Aug 18 at 15:02
  • And yeah I am worried maybe the conduit has some damage, guess I'll see when I pull the wire. I have a borescope that I might stick down there too. Guess we'll see. The conduit is under concrete so that'd be a right pain to fix. The concrete bordered a pool that has since been taken out so maybe someone somehow damaged it when that was going on.
    – Matt
    Aug 18 at 15:04

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