[TennBrian]

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.

[Joshdta]

Dought the wire is big enough to work on the tub

[stanfortyman]

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.

[TennBrian]

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?

[tkrussell]

Sorry, but no,

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

A new 4 wire cable must be installed.

[TennBrian]

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.

[tkrussell]

Exactly,

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

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.