Can I use a well pump cable to supply a sub-panel?
Assuming you are in North America somewhere based on your descriptions.
As a generalization without being able to see the cable, &quot;pump cable&quot; typically means a flat, rubber insulated type of multi-conductor submersible cable intended to drop from a control box at the top of a well to the pump that is submersed in the bottom. The extra conductors are usually because in those submersible single phase pumps, the starting capacitor is left at the top in that control box; it's not physically attached to the motor as it is on regular single phase motors, so the wires for the capacitor in the motor circuit have to travel in that cable. Submersible pump cable is considered &quot;conditional use&quot; cable and cannot be used for anything else, at least not per code. Running power to a garage is considered &quot;permanent&quot; wiring, so you have to follow all of the rules regarding that. If in conduit, the conduit must be buried at the correct depth for the type of conduit it is, then you have to make sure you do not over fill it (not likely in this case though).
What you could do is to use that existing cable to pull the correct cable back through that conduit. The rubber pump cable, if that's what it is, may not want to cooperate though. Good luck.