Multiwire Voltage Drop

[brian ploskunak]

I have (2) 120 volt outdoor lighting circuits sharing a neutral with 12 amps on each phase. The run is 250'. Can this be calculated as a 240 volt circuit?

2x12.9x12x250/7.2

 

[kevinware]

I am assuming this is a 120/208 three phase panel? What size are the phase conductors now? What is the wattage of the lights? 250' is a long way to go to be using 12AWG and stay with-in the NEC recommendations.

 

[kevinware]

Just running through the math real quick I get this:

single phase: 2 x 12.9 x 12 x 250 / 6530 = 11.85 VD again what size wire are you using now??

three phase: 1.73 x 12.9 x 12 x 250 / 6530 = 10.26 VD WOW!!

210.19 (A) (1) FPN No.4 talks about the recommendations.

But if I am way off I hope to be corrected...

Edit to add:

Even 10AWG is not going to be with-in NEC recommendations. (3% for branch circuits)
2 x 12.9 x 12 x 250 / 10380 = 7.4 VD

Edit again to add:

210.4 (C)
"Multiwire branch circuits shall supply only line-to-neutral loads."
There are some exceptions........

 

[bphgravity]

Can this be calculated as a 240 volt circuit?

No. All your loads are line to neutral loads.

 

[kevinware]

Sorry Brian,

Didn't re-fresh my screen until I was done with my edit..... Did not see you had already put the info out there........


Edit again!! I am going to go home now.........Sorry again >>> Bryan <<<........ I can spell.

 

[infinity]

I am assuming this is a 120/208 three phase panel? What size are the phase conductors now? What is the wattage of the lights? 250' is a long way to go to be using 12AWG and stay with-in the NEC recommendations.

I'm curious as to why you would assume that this is fed from a 208 volt panel? This could be from a single phase 120/240 panel since the OP said 240 volts.

I would guess that since his two loads are the same size that the neutral current would be 0. That's why he's asking if he can compute this as a 240 volt circuit.

 

[bob]

I have (2) 120 volt outdoor lighting circuits sharing a neutral with 12 amps on each phase. The run is 250'. Can this be calculated as a 240 volt circuit?

2x12.9x12x250/7.2

You can if the load is balanced for the 250 ft. If so, I get about 9 volts drop,
3.8% using #6cu.

 

[winnie]

I always hate using the formulas such as the OP used; I much prefer to think in terms of ohms per foot of conductor and then multiply resistance by current to get the voltage drop, in two steps.

If this is a balanced 120/240V system, with two matching 12A loads on each of the two circuits sharing the neutral, and the two 'hot' conductors are on opposite 'legs' of the supply, then there will be no current flow in the neutral and thus no voltage drop in the neutral. In terms of voltage drop, this will be a 240V, 12A load.

In this case, I would expect a 4-5% voltage drop using 12ga conductors.

[12ga: 2.01 ohms per 1000 feet at 75C. Correct to 1.8 ohms per 1000 feet at 40C. 250 feet means 0.45 ohm. multiply by 12A to get 5.4V, or 4.5% of the 120V leg]

Note that the results are significantly different if the supply is a 120/208V three phase panel, because in this case there would be a full 12A flowing on the neutral, and thus significant voltage drop on the neutral.

-Jon
[edited to add calculation]

 

[don_resqcapt19]

Bryan,

No. All your loads are line to neutral loads.

While they are line to neutral loads, if both loads are at the end of the circuit, there is no current flow in the grounded conductor and the circuit can be treated as a 240 volt circuit for the purposes of voltage drop.
Don

 

[LarryFine]

While they are line to neutral loads, if both loads are at the end of the circuit, there is no current flow in the grounded conductor and the circuit can be treated as a 240 volt circuit for the purposes of voltage drop.

. . . which is why we say that a MWBC is more "efficient" than separate 2-wire circuits. The shared neutral effectively (and approximately) halves the voltage drop.

 

[brian ploskunak]

Thanks guys for all the great input. I guess I need to decide if I treat it as a 120V load and go with #6 or a 240V load and go with #8. That's not a big differance, but I have (10) circuits like this to do. BTW, the wire has not been run yet. Sorry, if I gave the impression that it was already fed with #12.

 

[Smart $]

Thanks guys for all the great input. I guess I need to decide if I treat it as a 120V load and go with #6 or a 240V load and go with #8. That's not a big differance, but I have (10) circuits like this to do. BTW, the wire has not been run yet. Sorry, if I gave the impression that it was already fed with #12.

In determining voltage drop you should always treat line to neutral loads as such even for mwbcs, unless it can reasonably be expected that all loads of the mwbc will always be concurrently on or off. This takes into account the worst case scenario where only one circuit is conducting.

 

[kevinware]

I'm curious as to why you would assume that this is fed from a 208 volt panel? This could be from a single phase 120/240 panel since the OP said 240 volts.

A classic example of what happens when you don't see what you read......

 

[brian ploskunak]

In determining voltage drop you should always treat line to neutral loads as such even for mwbcs, unless it can reasonably be expected that all loads of the mwbc will always be concurrently on or off. This takes into account the worst case scenario where only one circuit is conducting.

Yes, the circuits should probably be on 2-pole breakers. With that being said, I am just going to calculate them as 120v circuits and go with the bigger wire. Seems like the right way to go.