Updating electric in an attached shop. There are two sub panels, both fed 240 from the main panel in the house. The one has large aluminum conduit, with the conduit serving as ground. The smaller panel (screw in fuses!) just has a cable with red, black and white. I'd like to just run a separate ground for the smaller panel replacement to the conduit where they both enter the shop. It would be extremely difficult to get a new line to the main panel.

Shop is about 1000 sf.

Larger panel is on a 60A breaker. It might feed at most - a minisplit (MCA 15A) and a welder (under 20A) both legs, plus it might have 20a of misc 120 loads at one time. 6AWG in 2" conduit. The conduit is routed through the attic of the house, coming out in the ceiling of the shop.

Could get a line to the smaller from the larger, it might see 25A during vacuuming with a heater going etc though. Plug in vacuum and 120 heater intended for standard outlets, so 8-13.5A even if replaced with other models.

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    How many square feet of shop is there, and what other loads are on these panels? How many amps are the feeders breakered for? Can you get a new line from the larger panel to the smaller panel? May 30, 2021 at 12:03
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    How large is the conduit going to the larger subpanel? Can you give us more details about the heater connected to the smaller panel (watts/volts), and is that vacuum a plug-in unit or built-in? May 30, 2021 at 12:52
  • The amp rating you gave for the minisplit is its Minimum Circuit Ampacity, right? May 30, 2021 at 14:05
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    Are you sure it’s aluminum? I don’t know many sparks that use aluminum (imc)in ground because inspectors won’t allow it to be a ground, code states the grounding electrode conductor can not be in contact with earth if aluminum (within 6” or 18” I forget) as the conduit is a grounding conductor. Other than that issue I would connect to the conduit if emt or steel.
    – Ed Beal
    May 31, 2021 at 3:26

1 Answer 1


Sounds like rather old work, but the 2" conduit is nice, since you can upgrade to anything you want.

Metal conduit is a valid ground path. But underground, it tends to rust out. If the wires are still free-moving and pullable, I would pull in a #8 ground wire now, while it's still possible. Once the pipe rusts out, it ceases to be effective as a ground path.

#8 because that is good up to 100A (#10 is only good to 60A). Yes, you're at 60A now, but your #6 copper wires are probably good for 65A (breakered at 70A), assuming they are not NM, UF or TW types.

If you're thinking "upgrades" consider #1 aluminum for 100A. The stuff is like a quarter a foot x 3 wires. But you have plenty of headroom on your 65A right now.

One reason to put Big Amps in a garage is that home-buyers will pay thousands extra for it. Lots of home buyers have a "notion that they might get* an electric car someday, and they'll pay more today for a house that's ready for a 50A charger.

Yes, you can retrofit ground between the panels

Again run a #8 copper as it's good to 100A. It doesn't need to run with the other conductors.

It might also be a good time to replace the fuse box with a proper subpanel. And get a nice big one: "running out of spaces" is a problem everybody seems to have, and panel spaces are laughably cheap at panel purchase time.

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