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I plan to upgrade an older 40 amp 3 wire feeder to my shop to a safer 4 wire feeder. No issues at all running a #6 solid copper in existing conduit to an un-bonded ground bus-bar in the sub-panel. Problem I have is the Main load center for the house is mounted inside a room in the center of the house, making it not so easy to rout the ground wire from sub-panel into the main panel.

Question I have: would code allow me to connect the feeder ground wire from the detached shop sub-panel to the ufer plate for the Main Panel ? Seems this would be OK since the main panel has the bonded ground/neutral bus-bar. Thanks

  • Wait, are the 3 conductors in conduit the whole way? Is the conduit metal and non-flexible? Is it rigid/IMC (that is, threaded)? Or is it all indoors? – Harper - Reinstate Monica Mar 17 at 16:48
  • 3 wires, 35 feet run, under ground, 2.5" PVC type conduit, Jay box at shop and house, Sub-panel has disconnect and two ground rods already in place. – Spheresman Mar 17 at 16:59
  • Have you thought about pulling out the three wires, adding the fourth and repulling the whole bunch? You've probably got 3#8 so a #6 would be over kill (I mean over grounded). 4 wires in a 2.5" conduit for 35" totally doable in my humble opinion. – JACK Mar 17 at 17:27
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    Can’t use 75 because under 100 amp / #1 wire requires 60 deg table per 110.14.C.1.A – Ed Beal Mar 17 at 18:23
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    @EdBeal -- not if the terminations at both ends are 75degC rated (which panel and breaker terms have been for ages now) – ThreePhaseEel Mar 17 at 23:52
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Definitely separate out that neutral and ground wire.

Don't bother replacing the aluminum wires

Yeah I know there's some ooga-booga out there about aluminum, but that has nothing to do with your heavy feeder, which is fine stuff and you should keep it. The problem related to very small wires in 15/20A branch circuits, specifically use of outlets and switches whose lugs were rated copper-only, and under political pressure, UL hastily cross-rated them for aluminum without proper testing. Aluminum lugs are the universal donor, they cheerfully take Al or Cu wire. Hence many panels use Al lugs, correctly and without difficulty.

So all you need for the care and feeding of Al feeder is to keep it clean and use the anti-ox goop. If you're splicing that somewhere, use insulated Polaris lug splices just big enough for the larger wire; much nicer than using bare splices and wrapping them with globs of electrical tape. The Polaris bars are made of aluminum.

Your #4 Al wire is capable of being fed from a 60A supply breaker. That is also limited by other segments of cable/wire, as well as the subpanel's bus ampacity if it doesn't have a main breaker of its own.

An outbuilding requires a disconnect switch; 95% of the time you get that by choosing a panel with a main breaker. That breaker size does not matter for its role as a disconnect switch. You can never count on the convenience of the local "main" tripping first, however, with a 40A "local main" and a 60A feed, it might just work!

Anyway, I'd hold onto that nice ampacity of the aluminum. I know you've got your plans figured out, but honestly, the world is too full of surprises to ever say that for sure. Whoever guessed they'd work the bugs out of electric cars, for instance.

Retrofitting ground

So yeah. Just pull a #10 Cu ground wire through the conduit, add a separate ground bar to the panel, pull any N-G bonds, and you're all set.

It's a shame you didn't pull the ground wire in (or at least a pulling string) when you pulled the phone/intercom out. Of course the phone and intercom were completely illegal; you can't put signal wires in the same pipe as mains power.

I wouldn't grab the Ufer ground, but I'd grab the copper wire going right to it.

Generally you can take a retrofit ground back to any of these:

  • The panel that serves the load
  • Metal conduit that goes back to that panel, if the metal conduit is rated for carrying grounds (all non-flexible and some flexible conduit is).
  • Any junction box visited by a branch circuit coming out of that panel, which has a sufficiently large ground wire (any >=30A circuit must have a #10 ground wire, so that would suffice).
  • Anywhere along the Grounding Electrode System; i.e. the run from the service panel to the grounding rods/pipe/Ufer.
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  • Thank you for the info, very much appreciated. – Spheresman Mar 18 at 0:28
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I would think that would be OK. But to bring your sub-panel up to code you'll need 2 ground rods for your detached building as well.

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  • Yes the shop already has the two ground rods and a disconnect for conductors at sub-panel entry – Spheresman Mar 17 at 17:00

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