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common trip bar on 110 breakers
- Thread starter xfrk
- Start date
wyliesdiesels
Well-known member
I have recently noticed that the 110 breakers on the garbage disposal and dishwasher in my apartment are set up like a 220 circuit, with a bar across the breakers that says 'common trip'. Why would this be done?
Circuits for garbage disposals and dishwashers are typically a multiwire branch circuit which consists of 2 hots 1 neutral and 1 ground.
Code requires all ungrounded conductors that feed MWBCs to be shut off simultaneously and this is accomplished with handle tied breakers.
75gmck25
Well-known member
I needed to run wiring up to my 2nd story, and using 12/3 and a MWBC with dual breaker meant I only had to pull one wire for two circuits. In my case I terminated it in a 4"x4" metal junction box and broke it out into two circuits in the attic.
Bruce
Bruce
Leaflessshadetree
Well-known member
Thanks for the response. Makes sense. Is there any chance that the coupling with the other breaker would inhibit the trip of the breakers if there was a short on one side?
No, the levers don't prevent the breakers from tripping.
Milton Shaw
Well-known member
- Joined
- Feb 11, 2011
- Messages
- 4,835
Safety problem, working on it thinking power was off because you threw the breaker but the other breaker is hot. Electrocution is what they call that....
wyliesdiesels
Well-known member
Thx again. Okay, so what could the consequences be if there were not a handle tie on the breakers? Sorry about the dumb questions, but I'm just curious and this is a new situation for me.
Biggest issue is getting bit by the neutral since its shared with 2 hots...just the reason why its required...
shaggyant
Well-known member
Multi wire branch circuits are strange.
Maybe I’m wrong but it always seemed to me that the neutral would be carrying twice the amps of each of the hot wires? Like if there was a 20 amp draw on each breaker the neutral would be returning 40 amps.
Maybe I’m wrong but it always seemed to me that the neutral would be carrying twice the amps of each of the hot wires? Like if there was a 20 amp draw on each breaker the neutral would be returning 40 amps.
wyliesdiesels
Well-known member
Multi wire branch circuits are strange.
Maybe I’m wrong but it always seemed to me that the neutral would be carrying twice the amps of each of the hot wires? Like if there was a 20 amp draw on each breaker the neutral would be returning 40 amps.
Because the 2 ungrounded conductors are 180* out of phase of each other, the current that is equal on both conductors cancels out. So the neutral only carries the imbalance between the conductors not the sum.
pattenp
Well-known member
Multi wire branch circuits are strange.
Maybe I’m wrong but it always seemed to me that the neutral would be carrying twice the amps of each of the hot wires? Like if there was a 20 amp draw on each breaker the neutral would be returning 40 amps.
That's why you don't use two single pole breakers on the same leg for a MWBC, you would have 40A on the neutral. The MWBC needs to be setup on opposite legs so there is no overloading of the neutral.
shaggyant
Well-known member
Because the 2 ungrounded conductors are 180* out of phase of each other, the current that is equal on both conductors cancels out. So the neutral only carries the imbalance between the conductors not the sum.
Ok, that makes sense. The neutral can only ever see 1/2 of the combined amperage due to the phasing.
wyliesdiesels
Well-known member
Ok, that makes sense. The neutral can only ever see 1/2 of the combined amperage due to the phasing.
not half.
It sees the imbalance.
Heres a better way to visualize it.
Phase A has 10a flowing on it feeding a 120v outlet.
Phase B has 7a flowing on it feeding a 120v outlet.
They share a neutral so the neutral sees 3a....