mwbc tandems

[romex jockey101]

i went into a panel that i was about to run a home run from after looking through the panel i noticed a nuetral wire was burned arouned the insulation very badly. it was a mwbc terminated in a tandem breaker.the circuit was 15 amps and was feeding 2 general lighting circuits in a garage and office.. when i went into the office i noticed a 120 plug heater witch they were using it draws about 12 amps...then i went into the garage and the home owner had a table saw pluged in draws about 13 amps ...bc the tandem breaker only sits on phase a it does not cancel out the unbalanced load and the 14 awg could not handle the load ..so i made shure that one leg was on phase a and and i put the other leg on phase b....this was very dangerous and could of started a fire...mwbc on tandems are very dangerous

 

[M4gery]

They make Quad Breakers that will work in this situation.

 

[Chamuit]

It weren't the breaker. It were the installer person.

 

[romex jockey101]

u can not have mwbc on a tandem breaker that sits on one phase very bad when sharing a nuetral each leg has to be on a seperate phase to cancel out the unbalnced load....

 

[roger]

u can not have mwbc on a tandem breaker that sits on one phase very bad when sharing a nuetral each leg has to be on a seperate phase to cancel out the unbalnced load....

You can for some applications if the grounded conductor is sized accordingly, read 225.7(B) and it wouldn't be canceling the unbalanced load anyways, it would be canceling the balanced portion of the load

Roger

 

[kwired]

i went into a panel that i was about to run a home run from after looking through the panel i noticed a nuetral wire was burned arouned the insulation very badly. it was a mwbc terminated in a tandem breaker.the circuit was 15 amps and was feeding 2 general lighting circuits in a garage and office.. when i went into the office i noticed a 120 plug heater witch they were using it draws about 12 amps...then i went into the garage and the home owner had a table saw pluged in draws about 13 amps ...bc the tandem breaker only sits on phase a it does not cancel out the unbalanced load and the 14 awg could not handle the load ..so i made shure that one leg was on phase a and and i put the other leg on phase b....this was very dangerous and could of started a fire...mwbc on tandems are very dangerous

I'm not condoning the installation, but I don't feel the table saw was as big of a contribution to the problem as it was made to be.

How much actual run time does it see - especially at full load. This type of load only runs for a few minutes at a time when it is used and often does not run for days or weeks at times. I'm guessing the neutral termination was weak to begin with.

 

[Dennis Alwon]

In the 2011 NEC ,you can't oversize the grounded conductor to feed two branch circuits from the same phase except in certain instances-- Roger gave the only instance that I know about.

The op is correct in that the neutral would be carrying the load of 2- 20 amp circuits if they were on the same phase.

Ge makes a thin breaker that are side by side that can be installed on different phases.