Voltage Drop & Lighting

[NathanF]

Hi All,

I am rather new to the Electrical field, but man do I love it. I had a hopefully quick theory/calculations question regarding voltage drop.

If I have a 9 amp lighting circuit and the furthest light from the panel is 350ft, can I still use a #12?

My answers

1. No you cannot run a #12 because NEC 210.19 (A) Max 3% VD - Correct? [I calculated a VD of around 12.5% (VD=2*I*R*L/1000) = 2*9*1.98*350/1000 = 12.5%]

2. (My theory question) if you did run a #12 would this just mean the current would basically increase by 12.5% and now be 10.125A? So the lighting would still work, it would just be less efficient? I get what voltage drop can do to motors, I guess in my head I don't find it to be a big deal when I think about lighting.

Honestly this is just a check on my theory of circuits. Hope I wasn't too confusing.

 

[augie47]

Other than a few specific areas (fire pumps, electronic equipment for example) the NEC does not mandate any restrictions as to voltage drop.
As a rule lighting is very forgiving on voltage drop.

 

[tank728]

Also as a practical matter, let's assume your lights are 1 amp per fixture. When you get to that last light at 350' away, it is only 1 amp of current traveling to the fixture. As an education exercise you would have a summation of 9 VD equations.

 

[NathanF]

Other than a few specific areas (fire pumps, electronic equipment for example) the NEC does not mandate any restrictions as to voltage drop.
As a rule lighting is very forgiving on voltage drop.

Yeah I am looking at NEC 210.19 Informational Note No.3 "Conductors for branch circuits, as defined in Article 100, sized to prevent a voltage drop exceeding 3# at the farthest outlet of power, heating and lighting loads..."

 

[NathanF]

Also as a practical matter, let's assume your lights are 1 amp per fixture. When you get to that last light at 350' away, it is only 1 amp of current traveling to the fixture. As an education exercise you would have a summation of 9 VD equations.

Right because the lights are in a parallel circuit

 

[augie47]

Correct and, as noted, that is an "Informational Note" and, as such, not a Code enforceable requirement.
VoltDrop is a design issue other than where specifically addressed by Code Sections such as 695.7

 

[NathanF]

Correct and, as noted, that is an "Informational Note" and, as such, not a Code enforceable requirement.
VoltDrop is a design issue other than where specifically addressed by Code Sections such as 695.7

Okay interesting. Thanks for the knowledge! So if you this were a job you were working on, you would just install a #12 and call it a day?

 

[augie47]

Okay interesting. Thanks for the knowledge! So if you this were a job you were working on, you would just install a #12 and call it a day?

Probably.
That said, a lot of job specs (engineered drawings) call for #10 on 20 amp circuits over a certain length.

 

[jaggedben]

2. (My theory question) if you did run a #12 would this just mean the current would basically increase by 12.5% and now be 10.125A? So the lighting would still work, it would just be less efficient? I get what voltage drop can do to motors, I guess in my head I don't find it to be a big deal when I think about lighting.

I noticed nobody answered this part.
The lights will consume less current because they will receive a lower voltage, but their impedance does not change. Look up Ohms law.
The lights will most likely be a little dimmer, although if we're talking 3% dimmer will anyone notice?

The lights will probably also consume slightly less energy at the lower voltage. However, a bit of extra heat will be generated along the additional wiring length, which will add to the power consumed. So the lights will be less efficient in the sense that you'll have less light (lumens) for proportionally more energy consumed.

 

[Knightryder12]

As others have said, the NEC does not say anything about VD. VD comes into play with the Energy Code. When you look at the energy code that is where the requirements are for VD. Florida's energy code says 2% VD for feeders and 3% max for branch circuits. I would not be OK with running #12 350' at all. There are plenty of free on-line VD calculators. I would suggest doing a VD calc. and see what wire size is really required.