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help with voltage drop

3K views 20 replies 11 participants last post by  Aegis 
#1 ·
Hey, I'm new here. I've got a bit of a complex lighting circuit I need help with.

It is a driveway with 8 stone pillars with a 100w fixture on each. There are 3 control devices, a photo cell in series with a single pole switch and motion sensor in parallel. The conductor would leave the panel and go 3' to the photo cell. One leg from there would go 25' to the switch then back about 60' to the first light. The second leg off the photo cell will go 300' to the motion sensor at the end of the driveway then back about 50' to the first light. The lights are in 3 sections totalling probably 375 to 400' between all of them. Every one is rated for 100w so about 10 amps total load. This is hard to explain, I wish I could draw a picture. Anyway, its all in sch40 3/4" pvc, two current carrying conductors max in each. I thought about just using 10s but I'd like to know for sure.
 
#7 ·
JoeLivesay said:
Hey, I'm new here. I've got a bit of a complex lighting circuit I need help with.

It is a driveway with 8 stone pillars with a 100w fixture on each. There are 3 control devices, a photo cell in series with a single pole switch and motion sensor in parallel. The conductor would leave the panel and go 3' to the photo cell. One leg from there would go 25' to the switch then back about 60' to the first light. The second leg off the photo cell will go 300' to the motion sensor at the end of the driveway then back about 50' to the first light. The lights are in 3 sections totalling probably 375 to 400' between all of them. Every one is rated for 100w so about 10 amps total load. This is hard to explain, I wish I could draw a picture. Anyway, its all in sch40 3/4" pvc, two current carrying conductors max in each. I thought about just using 10s but I'd like to know for sure.
Good luck with those 10's
The way you make it sound is way too confusing to even start to help you. If you don't know what your doing... You might want to get someone more qualified, that's a lot of money in wire to be wasted.
 
#8 ·
Thanks guys, I am a certified electrician, I can do voltage drop calcs and this is a code compliant installation. I guess I was putting too much faith in there actually being someone here that wanted to be helpful.

I got thrown off by the parallel switching and the fact that the load is distributed along the circuit instead of in one spot.

Nevermind, I'll figure it out, thanks.
 
#9 · (Edited)
You asked a basic question, we gave you the answer (or where to look for the answer) a simple to use calculator that makes all our lives easier and we are trying to simplify your work life HERE and in the FUTURE. Then you whine about we did not help.

Explain what you want or need with this very simple, very basic installation.

10 Amps with #8 AWG, 400 feet at 10 amps
ESTIMATED DROP 6.1 VAC
Load at End of Circuit 113.9
Voltage Drop 5.1%

10 Amps with #10 AWG, 400 feet at 10 amps
ESTIMATED DROP 9.6 VAC
Load at End of Circuit 110.4
Voltage Drop 8%

10 Amps with #12 AWG, 400 feet at 10 amps
ESTIMATED DROP 15.3 VAC
Load at End of Circuit 104.7
Voltage Drop 12.8%
 
#12 ·
I know Cali is a bit different but every time I hear certified I think of the "certified" electricians I see at places like home depot parking lots usually with signs saying electrical work.
Not to offend anyone just saying.
 
#13 ·
JoeLivesay said:
Thanks guys, I am a certified electrician, I can do voltage drop calcs and this is a code compliant installation. I guess I was putting too much faith in there actually being someone here that wanted to be helpful.

I got thrown off by the parallel switching and the fact that the load is distributed along the circuit instead of in one spot.

Nevermind, I'll figure it out, thanks.
I'll take a shot at it. There's a couple things to consider.
1) Is running through the photoeye and motion detector practical given the load and distance. Perhaps using these devices to operate a contactor may be better.
2) could you make a sketch and post it so that it would be easier to understand.
 
#14 ·
Mr.C said:
I'll take a shot at it. There's a couple things to consider.
1) Is running through the photoeye and motion detector practical given the load and distance. Perhaps using these devices to operate a contactor may be better.
2) could you make a sketch and post it so that it would be easier to understand.
Ok here is what I'm talking about. I am concerned with the leads on the devices, probably should've said that originally. That idea for using a contactor is great, I can't believe I didn't think about that. That would mean an extra circuit, which means more heat, tighter conduit. That's really what I need to know, how bad is that 14 or 16ga lead going to hurt me? Would it be worth it cost and performance wise to do it that way?

Drawing Text Sketch Line Artwork

I obviously omitted the neutral and ground but here it is.
 
#16 ·
ohiosparky99 said:
The photocell and motion detector won't even figure into the VD calc, and if your talking about the # 14 or 16 awg feeding the light fixtures, don't worry about those because they're normally to short to figure into the equation
Yeah I was talking about the device leads, not the fixture leads, they don't figure because the current doesn't run through them to the rest of the load. The devices (photo & motion) however would be like a inline resistor on the circuit, so how does that play? Also, because its in sections, can I taper the size down toward the end?
 
#17 ·
JoeLivesay said:
Yeah I was talking about the device leads, not the fixture leads, they don't figure because the current doesn't run through them to the rest of the load. The devices (photo & motion) however would be like a inline resistor on the circuit, so how does that play? Also, because its in sections, can I taper the size down toward the end?
Yes, you can taper down the size per each section, also since the wire going to the motion&photo doesn't have any current on them, the VD will be quite small
 
#18 ·
Ya the Contactor is a good idea. I would run 14AWG out to the motion sensor and back to the Contactor by the panel, assuming it draws less then an amp. All on the same breaker as the lights. I think you can get wireless motion sensors too but I've never installed one.

For the lights I'd run #10 from the Contactor to the 1st TWO lights and then #12 from the 2nd light to the 5th and 6th lights. #14 for light 7 and 8.

My 2 cents :)
 
#20 ·
Aegis said:
Ya the Contactor is a good idea. I would run 14AWG out to the motion sensor and back to the Contactor by the panel, assuming it draws less then an amp. All on the same breaker as the lights. I think you can get wireless motion sensors too but I've never installed one.

For the lights I'd run #10 from the Contactor to the 1st TWO lights and then #12 from the 2nd light to the 5th and 6th lights. #14 for light 7 and 8.

My 2 cents :)
I think I'm going to do that. What type of contactor do you recommend? I was thinking a dry contact relay.

I think to be safe I will run 10s and make the fixtures up as I go. I'll use my meter to read voltage and see if a) I can reduce and b) how the reduction affects it. I'll post what happens.
 
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