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I have a grandfathered subpanel that feeds my 5 ton package unit. 1988 install. Recently we added an AC system and they fed the condensing unit out of that sub panel and they tied the ground into the neutral because the grandfathered panel did not have a ground bus. It passed inspection so I guess the state inspector just missed this somehow. This sub panel is fed from my 200 amp main panel with a 100 amp breaker. I was hoping to feed my above ground pool pump by installing an additional sub panel fed by my grandfathered subpanel but it does not have a ground. Can I just run a ground back from the grandfathered panel to the main panel and then install a ground bus and then feed my pool sub panel or should I just go back to the main panel to feed my pool sub panel with the new circuit? I was thinking it might be a good time to bring my grandfathered panel up to date with a ground bus?

Being the creative guy I am I was also curious if I could just run a ground wire over to a nearby circuit that has a bare copper ground going back to the main panel even though it's only 12 gauge wire. I figured that would not suffice because I would need the earth grounded wire to be half the size of my conductors feeding my grandfathered subpanel to meet code. Is that correct?

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    1 - You may have an existing problem with the 100A breaker. That is a very large breaker for a typical subpanel, excluding of course "subpanels that feed half the house". Pictures of the subpanel and identification of the wire/cable feeding it would be very helpful. If it turns out the 100A breaker is in the subpanel and the feed breaker in the main panel for the subpanel circuit is smaller then it is more likely things are OK. 2 - Retrofit ground is, in general, fine. I'm not sure about how that works with subpanels though - e.g., whether you can retrofit a subpanel-fed circuit with a ground
    – manassehkatz-Moving 2 Codidact
    Commented May 16 at 15:06
  • that bypasses the subpanel and goes straight to the main panel. But in any case, the ground wire needs to be the size required for the particular circuit or larger in all segments. So piggybacking a 15A (14 AWG required) onto a 20A (12 AWG) ground works but not vice versa. 10 AWG works up to 60A, so often that means if you have a 30A circuit (e.g., water heater or clothes dryer) you can use that ground for 40A/50A/60A things.
    – manassehkatz-Moving 2 Codidact
    Commented May 16 at 15:09

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Whoa, just a minute there.

This original subpanel that did not have a 4-wire feeder. Do any loads in it actually use neutral for anything? HVAC equipment generally does not.

Because if they don't use neutral for anything, that may not be a neutral wire at all. It may be a ground wire. And that would be fine and current code. That is a 240V-only panel with no 120V support.

And if you daisy-chained off that to support a second group of A/Cs, then most likely that equipment is also no-neutral and you're fine again.

And if the pool pump also doesn't use neutral, you may be OK again.

Now, you say

because the grandfathered panel did not have a ground bus

Yeah, you generally have to pay extra for that, but we're talking like $8. The panel labeling should list model numbers of accessory ground bar that will fit pre-drilled sites in the panel. And on a 240V-no-neutral panel, the cheap thing is to just use the neutral bar as the ground bar.

Now for clarity's sake, this is not a Code requirement, on a 240V-only subpanel I recommend paying for the accessory ground bar, masking off the holes then painting it green to make it super obvious... and then isolate the neutral bar and attach absolutely nothing to it. That way, if some future person installs a 120V load without really paying attention, they'll attach neutral to the neutral bar and it just won't work.

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