Electrical Question.

[Bill Lewis]

Up to what size conductors can be used with 1/2" PVC?electrical conduit. I need three conductors, hot, neutral, and ground.

I need to run an outlet/circuit for a gate opener about 350+ feet away. Circuit may also be used for seasonal lighting, and possibly a post light. The gate opener is low voltage, and according to the manufacturer, can be run up to 1000' on the LV line alone. I decided that if I was going to run the wire I might as well have an additional outlet available. I am going to run the conduit along the fence (4 board, wood) to avoid having to bury it. There are too many other lines, water, power, cable in close proximity. The gate is not so much for security, but to allow the dogs to do deer patrol in the front yard.

I am a little concerned about voltage drop over that distance. I'd like to keep it to no more than 12 gage wire for the circuit. I also want to use 1/2" conduit so it will be less noticeable. 12 ga. should be ok in 1/2", but not sure about anything larger.

[Robert Ducharme]

If the PVC will be above ground, you probably need to make certain it is Schedule 80 (electrical, not plumbing).

Note that for PVC, every 100' will expand 1" for a 25 degree temperature change and 2" for every 50 degree change. Make certain to account for that by putting in some bends. Also, for 1/2", support every 3 feet.

From what you said above, I believe you are running 2 conductor with ground. It looks like you are placing low voltage and high voltage wiring in the same conduit. If you are placing an outlet there, you really want 4 conductor plus ground. 2 for the low voltage (14-16 guage) and 2 for outlet (12 guage). Not certain that the code you have for your area allows that. I would talk to the state electrical board and ask that question.

In 1/2" conduit, you can have seven (7) 12 guage wires (good for 20 amps short range). At that distance, I would derate the amperage to 15 amps max). Assuming copper wires here. Even if using 12 guage wires, ground probably only needs to be 14 gauge at the most.

[Jim Becker]

Correct me if I'm wrong, but I don't believe you are supposed to install low voltage wire co-resident with line voltage wiring in the same conduit...

[Michael Perata]

Bill

You should consult a good electrician. You will be amazed at the wire size necessary to run 120v 350'.

[Chris Padilla]

As I understand the question, he is just looking to run 120 V 350' away from the source. That is all and that is only 3 total conductors. You can easily snake 3 #10 wires in there (use stranded) although your green wire (or bare one) can be smaller...like a #12 or #14 possibly.

For some understanding on the voltage drop you will experience, go to your local home supply store or hardware store and grab a 100' extension cord and read its little tag to see what it says. It should give you an idea of what size wire you might need to run.

Also, be sure to use some pull lube or pull the wire at the same time you are installing the pipe. Yes, be sure to use electrical conduit rated PVC. This is the grey pvc and readily available. You will need to use all water-proof fittings at the outlet as well and it will need to be GFCI protected...most likely.

I'm guessing #10 stranded will do the job and #12 green/bare but I only play an electrician on TV.... :)

[Bill Lewis]

As I understand the question, he is just looking to run 120 V 350' away from the source. That is all and that is only 3 total conductors. You can easily snake 3 #10 wires in there (use stranded) although your green wire (or bare one) can be smaller...like a #12 or #14 possibly.
Chris, You are correct :) , this is what I am doing.
To clarify, let me re-word my original question: What is the maximum allowable size conductor that can be run in 1/2" pvc conduit, for 3 conductors? Now that being said, I'd also like to keep the size of the conductors to a minimum to keep the cost down.


Note that for PVC, every 100' will expand 1" for a 25 degree temperature change and 2" for every 50 degree change. Make certain to account for that by putting in some bends. Also, for 1/2", support every 3 feet.
Good advise here. I think I'll add some junction boxes and use some water-tight flexible conduit to make expansion loops.


In 1/2" conduit, you can have seven (7) 12 guage wires (good for 20 amps short range). At that distance, I would derate the amperage to 15 amps max). Assuming copper wires here. Even if using 12 guage wires, ground probably only needs to be 14 gauge at the most.

So it looks like (3) #10 (gage) would fit fine in 1/2" if I have to go with it.

Robert, are you saying that 12 gage wire is acceptable for that length of wire run? The circuit will be derated to 15 amps and yes I'll be using copper (THHN). The outlet will also be GFCI protected. Obviously by now my preference is to use 12 ga. if it is acceptable.

[Chris Padilla]

Bill,

I haven't looked at voltage drops for this long run but I would encourage you to run your hot and neutral with #10. It is quite a distance you are running. Locally, Home Depot sells 500' spools of #10 stranded for $50/spool. Also, to be further safe, I would at least run the green/bare copper grounding wire in #14 (solid/stranded) but might look at #12 (solid/stranded).

Who knows, in the future you may wish to wire in another outlet or two along this run in any of the junction boxes you place. I did my whole garage rewire in #10 and it will fit GFCI outlets and the back-wired duplex outlets as well.

Good Luck!

[Bill Lewis]

Chris,

Now that you mention it. I think I know why I started down this path in the first place. I now remember seeing the $50 500' spools of #10 THHN in HD. I just didn't know if 3 #10 conductors is allowable in the 1/2" pvc conduit.

I considered down-grading the ground to #14 or #12, but they didn't have bare copper in those sizes on 500' spools (or any spools for that matter), so I firgured I'd just the same size for the ground.

[Ken Garlock]

Bill let me strongly suggest that you get a copy of the "Pocket Reference" by Tom Glover. It is a wealth of information, including voltage drop for common wire sizes.
From the reference, if you want to limit your voltage drop to just 2% in 450 ft, you need 1/0 cable. Now 2% may be rather restrictive for your application.

The formula for computing the voltage drop is:

Voltage drop = (K * 2 * wire length * current in amps)/wire area in circular mils.

K is the resistivity of the wire in question, available from tables. 11 for copper wire at 50% current load, 12 for up to 100% load. The 2 is because the wire runs out to the load and back to the source.

Wire area for 10 ga is 10400, 8 ga is 16500.

Let us look at your example 350 ft of 10 ga. wire, 20 amp load.

Drop = (12*2*350*20)/10400 = 16.15 volts. Not so good :eek:

Try 8 ga.

Drop = (12*2*350*20)/16500 = 10.18 volts. Not something I would be happy with. :mad:

Note that by halving the current to 10 amps, the new values are
10ga. - 7.4 volts. Low end of OK.
8 ga. - 4.66 volts. OK value IMO



So before you run out and buy a load of wire, think hard about the maximum current load you will have, and size yourself accordingly.

[Chris Padilla]

Ken,

Can you post the units for each value you are using in that equation? I'm not getting the units to work out correctly.

We have (ohm*ft*amp)/(mil) near as I can tell. We need the ohm*amp to get volts as the final answer but how do you get rid of the ft/mil without further unit conversions which multipy your answer by a huge amount (12,000). Clearly, I have something wrong.

Also, the fact that a circular mil is the diameter^2 of the wire does not compute to an area in my head. What happend to pi*r^2?


According to a table I have:

#8 has 0.62 ohm per 1000 feet
#10 has 1 ohm per 1000 feet

At 350 feet:

#8 has .22 ohm of resistance and #10 has 0.35 ohm.

So by my calculations, the voltage across 350 feet of #8 and #10 wire, at a max current of 20 A is:

4.4 V and 7 volts

Ah...but the resistance is in each wire...I need to double the resistance and therefore we can double the voltage:

8.8 Volts and 14 volts


Cool, similar numbers although mine are a bit lighter. I think step-up transformers might be in order here instead of larger wire or perhaps an AC to AC converter or something like that?

Incidentally, #6 at 350 feet and 0.4 ohm/1000 feet gives you a voltage drop of 2.8 V at 20 A...in each leg so a total of 5.6 V.

I'm not sure I've seen 6/2 Romex (I did buy some 6/3, however, at about $1.29/foot) but I do not think you could snake 6/3 through 1/2" PVC...3/4" would work I believe.

Thanks, Ken, for forcing me to waste some time at work to figure this out! :)

[Chris Padilla]

Another thought is to run only two hots and a ground 350 feet. Is there any chance that what you want to plug into at the end of the run can run at 240 V?

It doesn't change the current and resistance numbers nor voltage drop but now you have double the voltage to play with. Losing 14 V off of 240 V might work compared to losing it from 120 V. Again, remember we are calculating the voltage drop using 20 A. Good ole V=iR.

A device working at 120 V will use HALF the current working at 240 V. Some devices can work at both...you just move a switch.

[Ken Garlock]

Chris, the resistivity, K, in the reference manual is said to be measured in "ohm - circular mil/foot." I think what they tried to say is ohms resistance for a given circular mil cross section per foot.

Let's see if this works. Ref table says 1.02 ohms/1K ft. or .00102 ohms/ft. for 10 ga.

.00102 * 10400 cirmil = 10.608 The ref manual says use 11 for K, but up it to 12 for wire current above 50% of rated load. That is not "real close" except in nuclear war ;) They probably rounded it up as a fudge factor to make the drop look larger, and induce people to go with the next larger gauge.

Happy electrons.

[Bill Lewis]

Ken, Chris,

Thanks for all of your help. I actually do have a couple of "Pocket ref" books. They're just never in my pocket when I need them. :eek:

I think I am going to run #10 gage wire in the conduit. I'm only looking at a couple of amps load max. If I ever really need to up it to something larger, I can always replace it :o. After all it will be conduit.

The other day, I ran three high-quality 100' extension cords (#14) up to the gate to run my Circular saw (DW364). It ran ok, just a little slow to start. The saw pulls 15A, or that's what it is rated for on the DW site.

[Chris Padilla]

Bill,

Just place junction boxes at appropriate intervals...like just under length of your snake! :) Pull lube will be your friend...it works amazingly well.

[Bill Lewis]

Chris,

My snake has gotten way too short. I'd have junction boxes every 20 ft. :eek: Well maybe 40. I was thinkig about either blowing or sucking a pull line (with a mouse) through the conduit. I will also do it in sections. I'm going to add some flexible conduit to allow for thermal expansion/contraction of the pvc.

I know about the joys of pull-lube. I used to have some, but who knows where it ended up after the move.