What are the disadvantages to running a pool pump at 120v v. 240v. Also would a 240v timer work with 120v pwr?

You probably have a dual voltage motor which will be able to run on 120v or 240v, But be careful. To switch from one voltage to another, it needs to be wired differently inside. Some of the newer models make it easier for you. There's a switch that you simply move from 240v to 120v and vice versa. If you don't do this, you will burn the motor. If you don't see the switch, then your going to have to rewire it for 120v which is considered "low voltage"in this case. There should be a schematic on the motor which will show you how to wire it for low voltage or high voltage. If you don't understand it, please take it to a shop to do it. As for the wires going to the motor, it should have been sized so that you can use either voltage and still be safe, but you still need to check. The lower voltage will pull twice as much amps as the higher voltage, which will cause the wires to get hotter. You need to give more info on the motor to answer that (Horse power, Amperage). I've seen a 1 HP, 14/7amp motor on a swimming pool pump that had a 20 amp breaker, and I believe it was 10awg wires, so check the nameplate of the motor and ask again about the wire size. As for the electric bill, it will make no difference. You are charged by the wattage or the VA volt-amps. To get that, you multiply the voltage being used and the amperage, which will come out to the same regardless which voltage you use, But I believe that the higher voltage is more efficient. It will make sense that with the higher voltage/ lower amperage, the motor will last longer too, because of less heat generated. If given the choice, go with the higher voltage. Are you sure the timer was a 240v? It should have been using 120v even before your rewiring. Just like a clock on an electric oven. The oven is using 240v, but the clock is using 120v. It's wired that way inside. You probably just didn't realize it. So have you tried it already? Does it even work? Do some more research on wire sizing for motors. It's not the same method as a regular lighting circuit. Good luck.