I have a 3 HP irrigation pump fed by a normal 240v service line from a relay activated by the irrigation controller. The pump uses 2 lines plus a ground. I have one extra conductor line running to the pump station.

Is there a code acceptable way to use that line, and one of the pump lines to add a small heater to keep the pump from freezing, without changing the pump/relay function?

Or, is my only option to plug in the heater or the pump but never both?

Preferably, the new wiring would contain a thermostat so that the heater would only run when needed.

What size is the circuit; wiring and breaker size?

What kind of heater are you looking at?

While Stan's questions need to be answered, if the circuit wiring is of ample size one to run both, issue I see is the heat will only work when the pump is running.

Another set of wires will need to be added for the heat, or the contactor will need to be relocated to the pump area, so the feeder can be converted to live constantly, and let the pressure switch control the pump contactor and the thermostat control the heat, again at the pump house/pit.

Is is possible to move the contactor?