Expert: Master Bob - 10/24/2006

Question
I am putting in lighting for my new driveway.  The distance is about 450'.  I am wishing to hook up lighting along the driveway with the farthest light always having power, but I want the ability to turn the other closer lights off with a switch.

Normally I would use a 12-3 wire to do this, but I do not know what voltage drop the 450 run will cause and what wire I should use to provide an acceptable voltage for the multi fixture small florescent lamps (4 lights per post).   Any advice for this tricky job.

Answer
Hi Steve,

  Based on your description...clearly you will have a substantial voltage drop issue at that distance (~450')....and #12 wire will simply leave you with an extremely unacceptable drop....far exceeding the 5% maximum drop.

Assuming these are 120 volt flourescent fixtures
Here's the calcs:

using #12 at 450' on a 120 volt 20 amp circuit

voltage drop 34.75
voltage at lights = 85.25
percent drop = 28.95%
(once you get past 85' - #12 wire rapidly goes over the 5% drop allowance)

#10 at 450'
voltage drop = 21.7
voltage at lights = 98.2
percent drop = 18.15 %
(#10 is good for up to 125' mark)

#8 at 450'

voltage drop = 13.75
voltage at lights = 106.25
percent drop = 11.46%
(#8 is good for up to 200' mark)


Things don't even start looking good until you get to a #6 copper wire run:
voltage drop = 8.84
voltage at lights = 111.16
percent drop = 7.37

(#6 is good for up to the 305' mark)

At 450 ' -  #4 copper wire is the FIRST acceptable voltage drop reading:
voltage drop = 5.54
votlage at lights = 114.46
percent drop = 4.62%

 At today's high prices for copper...a 500' spool of #4 copper will put a serious dent in your wallet...and then you'll quickly find that the #4 wire simply won't fit a 20 amp breaker's terminals.
 So - your next viable option is to use different fixtures and bump up the voltage to 240 volts and use a double pole 20 amp breaker...and now #8 copper can be used at 450'  and be acceptable...but the drawback now being you now need twice as much wire.
(we run into this problem frequently wih commerical parking lot lighting...but the voltages there are three phase and much higher - letting us get away with #10 and # 8 wire run to a transformer/ballast in the pole base or up at the fixture housing...usually all spec'd out for us on the blue-prints).

  There's no easy (or inexpensive) solution to providing power to lights at these kinds of distances...and you have to keep in mind that Ohm's Law dictates that when voltage goes down...amperage goes up...a 200 watt fixture will still want to draw 200 watts....making both wire and pipe size have to be bigger.(you didn't state whether you were running wires in PVC conduit or using UF cable...but at those distances...I'd imagine you'd certainly want to run in pipe..)

 Your looking at an expensive set up either way Steve...so you may want to go back to the drawing board and re-evaluate your lighting needs and do what you can to decrease the distances....you may have no other option but to go with higher voltage lighting just to keep it at an acceptable drop IF you can't decrease the power run distance.

 At those distances...calling this a "tricky job" would be an understatement....calls for some in-depth planning.

 Hope this helps Steve...if you need more details - or have new questions - please follow-up...

Best Wishes,

Bob Osgood
Master Electrician/ Owner

RKO ELECTRIC

www.rkoelectric.com

rkoelectric@verizon.net