Can I somehow use an existing 3 wire 220 volt circuit to power a 4 wire hot tub? Running a new 4 wire circuit back to the main panel will be long and difficult. I have nearby access to an old 3 wire sub panel once used for an air conditioner. This original circuit is missng the ground wire.

Dought the wire is big enough to work on the tub

You MUST have a dedicated ground wire AND a neutral for a 4-wire feed, and in most instances the ground MUST also be insulated.

You absolutely cannot use a 3-wire feeder for a 4-wire hot tub.
Also, Josh is right. What size is the old wire and what size is needed for the tub? It is doubtful that an old A/C line is that big.

The wires are big enough, 6 AWG at least, originally used for either a central A/C unit or heat pump. That shouldn't be a problem Could I run a separate ground wire either through the attic or crawl space?

Sorry, but no,

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Originally Posted by TennBrian

Could I run a seperate ground wire either through the attic or crawl space?

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A new 4 wire cable must be installed.

I apologize for pressing the point, but what difference can it make if the ground wire makes the trip to the sub panel a different way, as long as it's properly connected to both panels? I know it's not the most professional of installations, but I'm trying to avoid making a long run of expensive cable through and under concrete.

I suppose, ultimately, this conversation all boils down to following the code.

Exactly,

Quote:

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Originally Posted by TennBrian

I suppose, ultimately, this conversation all boils down to following the code.

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I hear what your saying, and I understand the concept, but I cannot suggest or recommend a practice that specifically is not allowed by Code.