I have a pump cable (red, yellow, black, green) going from my house (in buried conduit) to my garage. It comes from a 200 amp service box on a 240v 30amp breaker. Previous owner was going to put in new pump, but city did so and wired it. Wire from house is just sitting there. I have a new Square D Homeline 100 amp 12 circuit box. Can I use the 240v pump wire to power the new box? If so, how? I only need 120v in garage. This is a flat cable with four individually insulated conductors, these conductors are also separated within the casing itself. wire is 10/3 with ground USA. THANKS IN ADVANCE.
Assuming you are in North America somewhere based on your descriptions.
As a generalization without being able to see the cable, "pump cable" 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 "conditional use" cable and cannot be used for anything else, at least not per code. Running power to a garage is considered "permanent" 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.
You already have the panel it is 240 and the breaker it is 240 with a 4 wire feed so yes it can be done , your neutral the white wire that has yellowed over time is the neutral and this will require an isolated buss if your panes has it great put that wire on the isolated buss the green goes to the grounded buss this buss is usually directly bonded to the box , some boxes both busses are isolated in this case you will need a jumper from the isolated grounding buss to the case The smallest bonding jumper allowed is #8 many small panels have a built in lug to the box for this reason. Red is one hot black is the other hot , the last thing is an additional grounding electrode you said you had conduit but plastic? Still requires a 8’ electrode connected by a #6 copper wire to the grounding buss, if metallic conduit you can use a grounding lock ring to connect the jumper to the buss. If you only need 120v just use single pole breakers put your lights on 1 leg and receptacles on the other if you ever trip the breaker you won’t be in the dark.
Remember all receptacles will require GFCI protection so GFCI breakers or the first receptacle in a string to be GFCI.