Outdoor Electrical Wiring

[weldingisfun]

Can anyone tell me what would be the longest run I can do off of a 20 Amp breaker using 12-3 outdoor wire. It is for lights in a goat shelter located approximately 350 to 400 feet from the breaker box.

 

[kenstrac]

Go here and put in the numbers you have.

Voltage Drop Calculator

 

[mbohuntr]

You need to consider the resistance of the wire in calculating the current. I have been told that a rule of thumb is to increase the wire by one size for runs over 100'. So I'm thinking that 10-3 is in order. And I would also use UF type and either bury it in conduit, or deep enough to protect it. Whatever the NEC says.

 

[DennisArrow]

Hey.....I know you will hear about this from the doom and gloom crowd that will insist upon spending your money for you and surely not coming out to help you do the job. SO.....here goes.........

Am in almost the same exact situation. Goat barn/shed 300 feet from the house. Needed lights, fence charger, outlet for hair dryer for the kids, heat lamp, water warmer for low 20's, and maybe a charger for spotlight..............

300 feet from the breaker box. Rented a ride on trencher cause I had to go through MANY tree roots on the way. Ran 12/3 underground cable on a 20A breaker. NO problems. The breaker has NEVER popped.

Since running the circuit, have built a barn on the sight with the same circuit wired in.
This added a small dorm type fridge and 3 florescent fixtures to the mix. Yes, my wife, who's operation this really is, is judicious in using amperage. In other words she doesnt try running the hair dryer and the 3 water warmers at the same time; but everything else, lights, fridge, fence charger, and spot light charger WORK JUST FINE with no breaker blowing............

NOW........if I was to do it again............a 30A with 10-3 would be worth the extra money to be able to run everything........BUT......common sense does prevail.

At the same time, I ran to another barn where I park the Air Stream trailer. At first I used the 20A 12-3; but noticed when I ran the AC or heat pump that the wiring would be warm.....NOT HOT.........but a definate voltage drop.........Redug the trench, put in 10-3 on a 30A breaker.......NO VOLTAGE DROP and NO cable getting warm...........

So..........Again, if I had to rewire the goat barn/shed power feed, which I will one of these days when there is a problem.......I WOULD USE 10-3 on a 30A............God Bless.....Dennis

 

[mbohuntr]

Thought that's what I said??? Problem with voltage drop is not only that the wire gets hot, but that 'spensive appliance that wasn't in the origional plan,( but that someone added later cause there was power out there) and will burn itself out costing you more. (brown outs) Extra capacity never hurts, and hot wires start fires...

 

[EddieWalker]

Why 12-3? For lights, all you need is 12-2, unless you are wanting to have a three way switch at either end of the run.

The calculator says you will loose less then a quarter of your amps over the run, which leave you with over 15 amps to work with.

A 100 watt light bulb will pull 1 amp, give or take. It's an easy rule of thumb. How many lights do you plan to have and how many amps will they draw? If you have five 60 watt bulbs, you are only pulling 3 amps.

The breaker will trip when you pull more power then available. Don't go to the full 15 amps, or you will trip the breakers. Figure out your load in how much light you need and if you are under or around ten amps, you will never have a problem. The closer to 15 that you get, the more likely you will be to have issues.

For just lighting, you won't have any problems at all.

An example for me is that I have three 75 watt lights along my driveway and two cairrage lights at my gate on a 12 gauge wire. My total amp draw for those lights is 3 amps. My driveway is 700 feet long. That calculator said that I'm loosing 40 % of my 20 amps to the line, so I still have plenty to run those lights, which work great every night.

Eddie

 

[mbohuntr]

Thanks Eddie, your right. My concern was that they are using a breaker that is rated for the wire size, not for the application. Dennis mentioned that a heater and hair dryer etc were added after the fact, and a different user in your setup might not realize that it was not rated for a weedeater etc. If you didn't place outlets there, your good to go. I was only anticipating the likely uses down the road. I also missed the 12-2 part. that would save a lot of money!! good tips. My road lights were sized at 10-2 because the electrician I consulted asked me if I was placing outlets at the road end.

 

[EddieWalker]

As for the outlets, it really depends on what he's using and if it's at night when the lights are on. If it's during the daytime, he'll still have 15 amps and should be able to run most anything. It's a short enough run that his power drop is minimal. I'd put a couple outlets out there just because they are cheap and handy to have if needed. More then likely, he'll rarely use them, but you never know.

On mine, I mentioned that I have a 40% drop at my gate to power my lights. I put in an outlet there and have run my cement mixer all day long, and also my metal chop saw to cut cultured stone. I'm not sure what I'm pulling with those tools, but with the lights off, I'm able to get away with it. While I agree that I'm pushing things, the worse that can happen is that I trip the breaker and have to walk 700 feet each way to reset it.

Eddie

 

[dave1949]

With any cable, it would be a good idea to provide a grounding rod at the shed. Tie all the green connectors on outlets and fixtures together and run that to the grounding rod. This is really important if you are not carrying an earth ground from the subpanel to the shed.

Dave.

 

[20 20]

I've been there and done that, small wire works good for temporary. If you plan on using more power draining things or want better results BIGGER wire is better. Eddie also made a good point why not 12-2. If it were me I'd go with no less then 10-2 because of heaters and blow dryer. Just remember a buck saved now may cost more later???????

 

[eepete]

Can't argue with anything said so far. I've been through this process, so let me toss out how I'd think it through.

1) How much power (current) do you need? This is the hardest part of the problem because you have to be somewhere between clever and clairvoyant. You have to think about now and later. At any rate, you'll come up with some amount of current you need to use.

2) Google "wire resistance chart" and you can find a chart with ohms per 100 feet of wire (they may be at other units). You want to get ohms per foot, and it's going to be like the numbers I have here. A chart I use shows ohms per foot as:
#8 .000739 #10 .00118 #12 .00187 #14 .00297

3) Now pick a wire size that you think might work. Multiply the ohms per foot of that wire size times the length of the run in feet and you get the the resistance of a wire _one_way_. Barring an application where you are building an electric fence that can stop a moose, double this resistance because all the current that goes out has to come back. You now have the resistance of your circuit.

4) Now you can get the voltage drop as you pass your desired current through your circuit at the gauge you're trying out.
E(voltage) = I(current) x R(resistance)
so multiply the resistance times the current to get the voltage drop.

5) The next decision has a little "your milage may vary" going on, but I don't like to drag a 120 circuit down by more than 10 volts. So if you've got more than 10 volts of drop, go down a gauge in wire and see what drop it would have.

Now right about here you're going back to either step 3 if you need a lower voltage drop or step 1 if you're wallet is screaming, making you wonder if the arc welder requirement was such a good idea.

6) Now you have the correct size wire to use for your needs. Remember to put a circuit breaker on it based on your load not on the wire gauge. So if you used a #10 wire to get a circuit that will carry 16 amps maximum (because that was the load you figured back in step 1), then use a 20 amp breaker (even though #10 is usually used on 30 amp circuits). This will keep you from burning things out if you put too much load for the wire (based on voltage drop) but not enough to kick the breaker and the voltage drop gets to be too big. Light bulbs don't mind low voltage but motors do.

Here's three more thoughts:
a) I always put in conduit. I've done a hypot test (how much voltage can something take before breaking down) on the box store grey electrical conduit in the 3/4" size and it's happy at 7500 volts (the highest I can test). So I like the idea that a lightning strike has got to be just about on top of the buried wire to do anything. The UF buried wire (the grey stuff) pops at 1200 to 1500 volts. It's also handy if you every have to put something else in and cross a trench. You can gently dig by hand and find the conduit. Conduit also lets you recover from a mistake in step #1 without retrenching. If you're a mistake master (I know I am sometimes) go with 1" conduit. I recently check 3/4" conduit prices for a project I'm working on and the 3/4" sticks which are 10' long are one dollar. Unless you can re-dig a 400 foot trench for less than $45 (you have to buy the pipe cement too), this is a good deal.
b) Because I'm in conduit, I can use the individual wires (THHN) instead of the UF underground stuff. Consider running a white neutral if your budget allows, and then you get a 240 circuit and can break it out into two 120 circuits. This gets back to step #1 where you have to be clairvoyant.
c) Be sure you run ground. A ground rod at the other end is a good idea too. I use two. Around the house, I trenched in 1200 feet of ground wire (mix of #2 and #6) for the lightning roof ground, but some have accused me of overkill in the past. I made a ring around the house, and put a #6 wire in all trenches in the project (outside lighting, gutter drains, utility run to the road).

Note that the resistance charts are at room temperature, and that as copper heats up the resistance goes up a bit. You can ignore these effects for real world use. Also if you're just doing lighting, you can tolerate more drop, especially if it's incandescent. Goes back to step 1 again.

Sorry about the long post, hope this makes sense, and I don't claim this is the only way to approach this problem, it's just one that works for me. The key concept is to start by figuring out what you need, not buy picking a gauge and figuring out what it might do for you. Come to think of it, it's a lot like figuring out which tractor you want to get.

Pete

 

[heehaw]

20+ years ago, i put in two runs of 10-2wg, 250ft to my pump house: one ran the 3/4hp submersible pump an nothing else, the other ran a 1/2hp jet pump an lights: we were using a sand filter, so the water had to be pumped from it to the house: worked fine for the 20+ years, but when the jet pump came on, the lights went out: of course they came right back on, but at night it did get dark for a moment there. never had an issue with either pump motor during that time either. lots of things will work for a long time with no problems, would i do it again...you bet.
heehaw

 

[Lineman North Florida]

Eepete made a lot of good points most importantly about burying that wire in conduit, it is mighty cheap insurance if you ever want to upgrade your wire size and there is no telling what the future holds but if your like me you are contantly going to be upgrading ie adding load, very few people downgrade or are always happy with the status quo, hope this helps, good luck.

 

[RadarTech]

Guys,
I have one comment about conduit underground....
If you have never done pulled wire thru a conduit, and you don't put in a large enough conduit-- I'll keep you in my prayers for patience...

I watched a friend put in a 1 inch conduit cause someone said to...
He pulled a single 10/2 in it and hooked a rope to the leader of the conduit when he did...
well, later he decided he wanted another 10/2....
boy was that fun to watch...

So the moral of the story---

If you decide to use conduit--- make sure you pick the right size...

J

 

[eepete]

@ RadarTech: It keeps coming back to point #1- somehow knowing what you want. Conduit is interesting because you can hedge your bet. If 3/4" looks good for now, put in something bigger if you're not sure. I have a lot of 1 1/2" in the ground (currently $2.70 a stick). It's better to be flexible than be right. I put a spare 3" conduit out of the south side of the house during construction, and that's going to be very useful as I start on a tractor shed project in December. I also put in spare 3" PVC drain pipe, so I'll have water and low voltage stuff too.
Pulling the THHN also lets you get more wire in the pipe than pulling romex in conduit. Anyway, as with so many decisions you have to keep thrashing around on cost of conduit, wire, future plans, etc.

Finally, in the "learned the hard way" category, I got a 10-3 wG and a 12-2 wG in a 3/4" conduit (these were UF cables I had bought). It was a 100' run and it _only_ took me all day. I threaded the wire through as I put the conduit together. That wire will never come out of there. That's when the 1" to 1 1/2" started looking good.

One other conduit thought: For low voltage stuff, I've used 3/4 or 1" 160 PSI black PE pipe (like is used for water from wells). You can get it in long runs and it's easy to lay in the trench. Good if you might want phone, internet, security or fire detection at the remote building. It is greater than 7500 volt protection too.
At any rate, if you're putting wire in a trench, there is almost no excuse for not using at least the 3/4" conduit when you look a the trench cost/time and the wire cost. I agree that if there is any way you can swing it to go up a size or two. We'll leave for another day the how to get the wire in the conduit once it's in the ground, but consider treading something in there as you go along (if you're careful with the glue).
'nother long post...

 

[JoeL4330]

if you're going to run 3-wire (plus ground) why not run 220v and you can split the two legs at the distant end and have twice the capacity

 

[eepete]

Note sure what you mean here (and maybee you're not talking to me which is OK too). I ran the 10-3 so I could have a 240 or two 120 circuits. Used that to get saw power into the house during construction- got two 120 circuits inside the house. Never needed 240. It was a buried extension cord that didn't get crunched by all the traffic.

As for doubling up, you could parallel the red and black but it goes back one a single neutral (for 120) so it's not a double the capacity but a 50% improvement. Same thinking for 240 only load where you parallel up one of the leg wires with the now not needed neutral.

I did the 50% double-up during construction on another #10 run where I have 120/240 ability. Had to run a heat gun to soften up the black PE water pipe to get the splice fittings in. It was a 120 gun and drew 10 amps, so a little boost helped. Now it's a 800' run of #10 on a 10 amp breaker that gives me about 7 amps of usable power, either 240 or two 120's up by the road. That's a "in case I need it" hook for the future.

 

[20 20]

Guys,
If you decide to use conduit--- make sure you pick the right size...

J

Very good point, and when you think you've picked the right size go bigger I've used 4" because of live and learn experience.

 

[dave1949]

Very good point, and when you think you've picked the right size go bigger I've used 4" because of live and learn experience.

Double Ditto

A bottle of Yellow77 is good to have on hand too.

Lay your conduit in the direction that you are not pulling the cable against the lips of the inner pipe on the joints too. That means all your 'bell' ends would be closest to the end of the conduit you are feeding the cable/s into as you pull. I think that was going to be eepete's next post.

 

[eepete]

dave1949 is right on the money. You'l want some lubricant when it's time to pull. I think the only thing left is how to get the pull string into the conduit. Here are some things I've done, it's by no mean a complete list and I suspect other will have great ideas too.

If it's a short run and a fish tape will work, you're good to go. Otherwise, I've used a polypropylene string. Hay bailing cord works too (plastic, not twine). I've pulled the string in as I built it up, and I've blown it in after the fact. If you pull it it in/along as you go, just be careful you don't glue it in place. Right after you push/glue the sticks together, pull some more pull string out for the next stick. As for getting the string in later, I get some of those snacks in the aluminized mylar bags. Eat the snack. Doesn't hurt to eat a few. Then tie the string around the bag and use a shop vac to blow the string into the conduit. You might have to try a few sizes of pieces of the bag to get the right fit. Don't be afraid to eat another snack if you need a new bag. It's a funny kind of cup your hands, hold the shop vac hose, let the string go by thing. If you can have someone at the other end with a radio to tell you when the bag pops out, that's good too.
I've also used a cat toy, the round aluminum mylar toys, for big pulls on conduit larger that 1.5". If the toy came with a cat, make sure the cat is spayed or neutered. Could probably use the mylar bag from a full size bag of chips too if you don't like cats.

If you're pulling in really big wire, you'll probably want to use the string to pull in a rope, then use the rope to pull in the wire.

Remember when you pull in your wires to pull a new pull string with the wires so you can add stuff later. The snack bag / shop vac trick only works on MT conduit.

And lastly, when you cover the ditch think about putting a foot or so of dirt in, then putting in a plastic tape. Finish covering the ditch. It's a little extra work and time getting on and off the tractor but you've got to work off the snacks so it's OK. Like all such "little extra work to do it right" things, if you're ever digging in that area again you'll be glad you did.

I can't wait to hear others and their tips and tricks here. Hope this helps, another long post...

pete