Answer by e-lectrician for How do I install a GFCI receptacle with two hot wires and common neutral?

BMitch's post is partly correct and dangerously wrong. 3 wire circuits of (2) 120V circuits sharing one neutral should be connected to lines with a 2-pole breaker, and MUST be on opposite phase (even/odd circuit breaker positions, 240V across breakers). Shared neutral carries unbalanced load such that 15 amps on one circuit, 5 amps on the other, results in 10 amp neutral current. 15 amps on each opposite phase circuit balance, cancelling neutral current to zero (0) amps. Sharing neutral on 2 same phase circuits (even/even - odd/odd circuit positions, 120 V across breakers) should NOT be done as it results in additive neutral current where 15 amps on each circuit result in 30 amps on neutral, dangerously overloading conductor. Adjacent single pole breakers should not be used, and skipping panel spaces for even/opposite circuit positions, should NOT be done - for safety reasons where turning one breaker off to work on a circuit can result in dangerous condition if shared neutral splice is opened, being energized via the shared circuit. Both circuits must be OFF.