The gauge of wire that is needed for a 220v heater is 10 AWG and a 30 amp breaker.
If the wire run is longer than 75 feet then you would need a 8 gauge or 8 AWG wire.
10 gauge wire can safely handle up to 30 amps at 220 volts.
For voltage above and up to 50 amps at 220 volts you would need 8 gauge wire to safely carry the amperage.
10 gauge wire can carry 30 amps as far as 75 feet and after 75 feet you would need to use 8 gauge wire for the 30 amps.
The gauge of wire you need for 30 amp electrical service at 220v is a minimum of 10 gauge wire.
For wires longer than 75 feet at 30 amps you would need to use a number 8 wire.
The smaller the number the larger the wire size when it comes to electrical wire.
A 10 gauge wire is required for 30 amps if the run is 75 feet or less.
A voltage drop of 3 volts is usually tolerated by most appliances or motors.
By installing an 8 gauge wire, the run could be approximately 125 feet without a voltage drop greater than 3 volts.
12 gauge wire is good for 20 amps, 10-gauge wire is good for 30 amps, 8-gauge is good for 40 amps, and 6-gauge is good for 55 amps.