Single phase Multi wire branch circuit and neutral

[Abraham2015]

In a single phase system two ungrounded conductors can share a neutral as long as the hot wires are of different phases. If we use hot wires from circuit breakers one and three that would be ok. Let's say someone used one neutral wire with hot wires from circuit breakers 2, 4, and 6. That means two of those breakers are of the same phase. What happens and why? Thanks for you support.

 

[Dennis Alwon]

If you have 2 wires on the same phase then the load on the neutral is additive. So If circuit 2 has 12 amp load and Cir. 6 has a 10 amp load then the neutral will see 22 amps. I don't know what happens to the load from Cir 4. Normally the amp of the neutral is Phase A- Phase B. Same phase you add the loads. Not sure if this would be Circuit 2 - Circuit 4 + circuit 6 or not. I think that may be the case so you could still overload the neutral and it is not code compliant

 

[iwire]

That means two of those breakers are of the same phase. What happens and why? Thanks for you support.

Nothing may happen, the circuits may operate fine.

However the neutral has the potential to be overloaded.

For instance if the current on circuits 2 & 6 was 20 amps and there was 0 amps on circuit 4 the current on the neutral would be 40 amps.

 

[JFletcher]

Is using a three phase breaker (2,4,6) in a 2 leg panel allowed?

" Not sure if this would be Circuit 2 - Circuit 4 + circuit 6 or not". That's what I get. Dunno why this would ever be done in a residential setting, and sharing the neutral across 2 same leg breakers (2 and 6) could lead to massive overload if it were sized the same as the ungrounded conductors.

eta: what iwire wrote.

 

[Abraham2015]

Thank you

Thank you

Thank you very much for the informative responses. I really do appreciate it.

Perhaps you can answer another question that deals with this issue.

When using a multi-wire branch circuit the correct way you are supposed to use two circuit breakers with different potentials. In that case, the neutral wire would carry 0 amps since both phases "cancel" each other. Can someone explain this concept?

 

[roger]

Here's a simple illustration

Roger

 

[JFletcher]

Thank you very much for the informative responses. I really do appreciate it.

Perhaps you can answer another question that deals with this issue.

When using a multi-wire branch circuit the correct way you are supposed to use two circuit breakers with different potentials. In that case, the neutral wire would carry 0 amps since both phases "cancel" each other. Can someone explain this concept?

The reason they cancel is equal loads on each leg, which are 180* out of phase with each other. They have the same potential relevant to neutral (120V) but 240V L-L. If you look at an AC waveform for 120V, it rises and falls 60x/sec (Hz). 240V (split phase) has a second waveform, inverted, superimposed on top of the first; that is, as one waveform reaches its peak, the second is reaching its valley.

If you (wrongly) wired a 240V load onto the same leg (like to poles 2 and 6 of your 3ⱷ breaker in a single phase panel), you would get 120V L-N, and 0V L-L (continuity, not potential), instead of the 240V L-L you should be getting.

That cancellation works a bit differently on 3ⱷ power; you have to have equal loads on all 3 phases to get 0A on the neutral. 3ⱷ power is 120* out of phase line-line.

 

[Abraham2015]

I understand now. Thank you all for sharing your knowledge and experience.

 

[John120/240]

Overload a neutral & the insulation can turn a brown color and become brittle, crumbly.