Voltage Drop in Multi Wire Circuit?

[fastcar]

Someone asked me about running a 20 amp multi wire branch circuit where he would share the neutral between two 120 volt ungrounded conductors with a double pole breaker. The length of the circuit is 60 feet. My recommendation was to over size the wire to a 10 AWG.

I also suggested he should try to limit the voltage drop to a maximum of five percent--say two percent for the feeder and 3 percent for the branch circuit.

I know that with a 240 volt circuit I can carry the same amperage twice as far then a 120 volt circuit with the same voltage drop.

The question is, which voltage drop table would I refer to. The 120-V, voltage drop table or the 240-V, voltage drop table?

I'm leading towards the 120-V voltage drop table in this case. But I'm not certain.

Thank you in advance!

 

[LarryFine]

Welcome to the forum.

The length of the circuit is 60 feet. My recommendation was to over size the wire to a 10 AWG.

Not necessary for only a 60' run.

The question is, which voltage drop table would I refer to. The 120-V, voltage drop table or the 240-V, voltage drop table?

The 240v voltage drop applies.

 

[jaggedben]

The 240v voltage drop applies.

Only if the load is balanced.

 

[wwhitney]

Depends on the loads and how they are controlled.

If the only loading cases involve balanced loads on the two legs, e.g. pairs of lights, one on each leg, with each pair either on or off, then there would be no voltage drop/rise on the neutral, and the voltage drop would be based on 240V.

But you also need to consider the maximally unbalanced case, and then calculate the voltage drop based on 120V.

Cheers, Wayne

 

[winnie]

Voltage drop is dynamic, changing with the load. So you first need to answer why you are calculating voltage drop.

IMHO if you are concerned with things like flicker when large loads switch (for example what happens when a light shares a circuit with a vacuum cleaner) then you probably want to look at 120V drop.

For energy efficiency calculations where you are talking small loads randomly distributed across the two branches, I'd use the 240V drop.

Jon