Can I use this multi-conductor wire for a 40 amp garage subpanel?

The main thing you need to look at isn't just the colors, but the actual printing on the jacket of that cable. If it's labeled as **#8 AWG** and has a rating like TC-ER (Tray Cable, Exposed Run), it might technically handle the 40-amp load you're talking about. Usually, #8 copper is good for 40 to 55 amps depending on the insulation temperture rating, but since you're feeding a subpanel, the identification of those conductors is where you'll run into trouble. Those red, blue, and yellow colors are typical for three-phase power or industrial control circuits, which is why they look out of place for a residential setup. For a standard 120/240V subpanel in a garage, you need four conductors: two hots, a neutral, and a ground. If that cable only has three wires inside, you're missing a conductor. You can't just skip the neutral or the ground, and you definately aren't allowed to bond them together at a separate structure like a garage anymore.

Another issue is the "control wire" designation. Even if the copper gauge is heavy enought, some control cables aren't listed for the same heat dissipation or ampacity as standard power cables when they're pulled through conduit. If the jacket says **Type TC**, it is often rated for 600V and can be used for power, but a picky inspector will see "control" and might fail you immediately because it's not being used for its specific listed purpose. If you've already gone through the effort of laying conduit, you're better off pulling the right stuff. Pulling individual strands of **THWN-2** in black, red, white, and green is the standard way to do this. It'll be much easier to pull through the pipe than a thick multi-conductor cable anyway. Free wire is a nice find, but if it doesn't have the fourth wire for the neutral-ground split, it's a non-starter for a safe, code-compliant subpanel.

That part about the insulation rating is interesting. I've noticed some industrial wires have that realy tough, plastic-like coating that's almost impossible to strip compared to the stuff from the big box stores.

If the OP ends up using that cable, woud the lugs on a standard residential breaker even be rated to hold it? I've run into issues before where the wire was technically the right gauge, but the strands were so big and stiff they wouldn't seat properly in the terminal. Seems like a recipe for a loose connection if the hardware isn't designed for that specific type of cabling.

If that cable is fine-stranded, you're going to have a nightmare at the terminations. Standard breakers and lugs are designed for Class B or C stranding—the stiff stuff. If this is high-strand count like machine tool wire, you need specialized crimp-on ferrules or lugs rated for that specific fine stranding. If you just jam it in there and tighten it down, the strands will migrate and you'll end up with a high-resistance connection that gets hot fast. Honestly, pull some THWN thru that conduit and sleep better at night.

One more thing to check is if that insulation is rated for wet locations. Any conduit run to a garage, expecially if it's underground, is eventually going to get water or condensation inside. If that cable isn't rated for 'W' (like THWN), the insulation will eventually fail and you'll be chasing ground faults forever. Also, keep in mind conduit fill. A jacketed cable takes up way more physical space than individual conductors. You might find it physically fits but still technically violates the 40% fill rule, which can lead to heat issues if you're running it near its limit.