> "However if a 3 wire plus ground #12 cable is run then two separate 120
circuits can be used with a common neutral and the 240 volt circuit can also
be used. The two hots also need to be on a connected breaker (double pole)."
Gary's last sentence should be emphasized. If one were to use 4-conductor
cable (3-wire + Gnd) through a pair of single-pole 20A breakers on a common
neutral, then its possible to connect both breakers to poles on the same
distribution phase. If that happens, a 20A load on each 120V circuit can
result in 40A on a #12 neutral conductor. The worst part is there's no
overload protection from this wiring fault. 40A of current could continue
on that #12 conductor until the wring and the home burns to the ground
without either breaker opening.
When using a *connected* breaker as Gary points out, the two line conductors
are served from opposite phases. So, even if each line conductor draws 20A
on a connected breaker, the neutral conductor's current cancels to zero
since the neutral current is the vector sum of the two phase currents.
Also, check local codes on this. I have to believe where primary use is for
two 120VAC in a residential installation, the practice of splitting phases
on a common neutral is prohibited in some areas.
TowerTalk mailing list