0

I am running 5 - 8'lamp posts down my driveway a total of 400-500 linear feet. The lights will be 120v and will have LED bulbs vs. incandescent. Even though the total watts will be no more than 60 W, I know there will be voltage drop. Thank-you

2
  • 1
    Voltage drop will be microscopic. Low voltage lights might reduce your project cost a lot (500 feet of trench is expensive at adequate depths to cover line voltage conductors)
    – Ecnerwal
    Jun 10, 2020 at 14:16
  • Voltage loss on low voltage lighting is a bigger problem than 120v. 12v #10 would have 8% loss at 100 ft, and 13% at 200 feet. If you use 24v lights and #10 wire it probably would work (est. 7% loss at 500 ft.), but no chance to add Christmas lights or a battery charger. Jun 11, 2020 at 15:58

3 Answers 3

2

Plug your scenario into a voltage drop calculator.

If the entire 60W (0.5A at 120V) load is at the end of the run (which it isn't, but makes the calculation simple - your actual results will be better), 14 AWG copper wire fed by 120V has only 1% voltage drop. If you go up to 300W (2.5A at 120V) the drop would be 5.26%, which is still fine.

You could run 12 AWG copper instead if you want the future flexibility to run a much larger lighting load, or to do something like adding receptacles at the posts. The price difference should be relatively small compared to your full project budget.

0
2

There are voltage drop calculators online you can toy with if you would like.

The NEC advises 3% loss for branch circuits and 5% for total loss from service (which at a residential service can be considered one-in-the-same if all services on a transformer are similar lengths from the transformer.)

0.5 amps (calculated 60w/120v=0.5A) at the end of 500 feet with #14 will give you about 1.5% drop, and your current gets reduced in increments so it is likely the circuit will function just fine with wire as small as #14.

Would I use #14? Absolutely not! No way would I do all that digging then be unable to use the circuit for anything else. Some day I'm going to find myself wanting to add some Christmas lights, or a cement mixer. With conduit I might pull #12 because it could be replaced in sections, if UF I would use #10.

1
  • Thank-you. I always use #12 for 120V applications so this is good to know.
    – Bill Z
    Jun 20, 2020 at 12:49
1

Lights only

For a 15A circuit, you will need #14 wire, and your voltage drop will be less than 1.12%. (I assumed all 5 lights are at the far end).

For a 20A circuit, you will need #12 wire regardless because you can't put #14 on a 20A circuit.

Remember you must use outdoor rated wire NOT NM-B (Romex). If the circuit is GFCI protected, burial depth is 12" of cover. Otherwise it's 24" if direct buried or 18" if in conduit.

Lights and receptacle

With a receptacle at the far location, you will need 6 AWG copper wire resulting in a 4.60% voltage drop with a common 1500W appliance in use (NOT with the lights).

This is the point where you should be considering aluminum wire, and that would require #4 aluminum.

240V to reduce wire size

240V lights are readily available (your LED lights may already be). Is it worth evaluating?

Not just for the lights. But it changes everything for the receptacle. If you are willing to use a 240/120V stepdown transformer, perhaps a portable one that you take to the recep location you want to use that day. That would let you use #12 wire and have 4.52% voltage drop assuming a 1500W appliance in use at the far location. That's not so bad.

1
  • Thank-you for the detailed information. My thought is to now just run #12 for the lights with conduit & a GFCI protecting it. Great help.
    – Bill Z
    Jun 20, 2020 at 12:45

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.