eepete
Platinum Member
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
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
I've used 4" because of live and learn experience.