voltage drop

[Rich Elec.]

I have a run of 5 light poles.
120 volts, 100watt fixtures, 500' long, 200' to first pole, 15 foot tall poles.
The spec is #10 AWG Cu., 20 amp circuit.
Is it an incorrect by code standards to run #10 to first and second light pole, and then #12 for the remaining 3 fixtures.
This would be at 3% voltage drop.
As the load decreases but the length increases is it incorrect to down size the wire.
Now this is spec. at #10, that is what will be installed, this is more a question in my mind and am I applying voltage drop with wire size correctly.
Also, it would be more trouble than what it is worth to set up different size wire for such a short length

 

[Dennis Alwon]

Not sure what you are asking but you could certainly run number 10 wire for part of the circuit as long as the overcurrent device matches that of the smallest wire. This sounds like what you are doing. I did not do the calculation but I will assume you are correct.

Remember the 3% is an FPN and not a code requirement.

 

[480sparky]

I have a run of 5 light poles.
120 volts, 100watt fixtures, 500' long, 200' to first pole, 15 foot tall poles.
The spec is #10 AWG Cu., 20 amp circuit.
Is it an incorrect by code standards to run #10 to first and second light pole, and then #12 for the remaining 3 fixtures.
This would be at 3% voltage drop.
As the load decreases but the length increases is it incorrect to down size the wire.
Now this is spec. at #10, that is what will be installed, this is more a question in my mind and am I applying voltage drop with wire size correctly.
Also, it would be more trouble than what it is worth to set up different size wire for such a short length

You can run any size wire you want at any portion of the circuit you want. The smallest wire on the circuit determines the breaker size.

 

[Rich Elec.]

Sounds good,
Thanks for the reply.
Rich

 

[weressl]

I have a run of 5 light poles.
120 volts, 100watt fixtures, 500' long, 200' to first pole, 15 foot tall poles.
The spec is #10 AWG Cu., 20 amp circuit.
Is it an incorrect by code standards to run #10 to first and second light pole, and then #12 for the remaining 3 fixtures.
This would be at 3% voltage drop.
As the load decreases but the length increases is it incorrect to down size the wire.
Now this is spec. at #10, that is what will be installed, this is more a question in my mind and am I applying voltage drop with wire size correctly.
Also, it would be more trouble than what it is worth to set up different size wire for such a short length

Commonly done what you describe.

 

[charlie b]

I did a quick check of the math. I had to make an assumption that the last four lights are spaced evenly along the last 300 feet of the run (75 feet between poles). To simplify things, I did not not account for the 15 foot height of each pole, but that would change things by no more than 0.05 volts at each light bulb location. I got the following results:
Source to Light 1: 2.07 volts
Light 1 to Light 2: 0.62 volts
Light 2 to Light 3: 0.74 volts
Light 3 to Light 4: 0.49 volts
Light 4 to Light 5: 0.25 volts
Total VD to last light: 4.17 volts (3.5%)

In order to reduce the VD to the last light to below 3%, you would have to run #10 all the way.

By the way, I agree with those who have said that (1) This is a common practice, and (2) The 3% VD value in the code is not a requirement.

By the way, the total VD value given above, in volts, is the same as the total current, in amps, as seen at the source: 4.17 amps. Go figure that one out on your own (homework assignment ).

 

[Rich Elec.]

Thanks guys!