Originally Posted by
Terry Brogan
...
2. What's unsafe about taking the 220 off two separate breakers, i.e. two separate circuits? Let's assume they're both on the same side of the main bus.
First, if both breakers are on the same side of the bus (same leg of the incoming service) you won't get 240v at the outlet, you'll get 0 volts. At a given instant of time, one leg of the service is 120v above ground potential while the other leg is 120v below ground potential. That gives you a potential difference (voltage) of 240v between the legs. The potential difference between 120v above ground and 120v above ground, which is what you'd have if your breakers were both on the same leg, is zero (0) volts. The two breakers have to be on opposite legs.
CLARIFICATION: You'd have zero voltage between the "hot" wires of the circuit if both breakers were on the same leg. That does NOT mean there's no voltage on the wires. You would still have 120v between each hot wire and ground or neutral. So you could still get zapped by the hot wires, but you couldn't run a 240v load off the circuit.
Why is it dangerous to use two separate single pole breakers? Because if one leg developed a short to ground, the breaker on that leg would trip and break the circuit. However, with two separate breakers, the breaker on the other leg would not be tripped and that particular wire at the load would still be hot.
A 240v, 2 pole breaker is actually two separate breakers in the same body. The two breakers are interconnected so that if one breaker sees an overcurrent for whatever reason and trips, it forces the other breaker to trip also. That cuts power in both legs so that both legs are "cold" at the outlet/load/etc.
Last edited by Tom Veatch; 04-22-2009 at 7:03 PM.
Reason: clarification
Tom Veatch
Wichita, KS
USA