Post by capnjonny on Nov 14, 2012 11:28:22 GMT -5
I just found this very helpful guide to wire size over at Teardrops and Tiny Trailers
www.tnttt.com/viewtopic.php?f=30&t=11316
Here it is. Hopefully they won't mind me reprinting it here.
Thanks Bruce
Okay here's another technical topic stripped down to the essentials. You want to wire your trailer and don't have a clue how to pick wires and fuses. Lets limit our discussion to 12V for now. How do you pick sizes?
- Wire Sizes.
First we need a little explanation about wire. Wire is sized in the US using the American Wire Gauge standard. The wires we'll need to use for a trailer are 14G, 12G, 10g and 8g. Only even sizes are available. The bigger the gauge number, the smaller the diameter of the wire, so don't get confused because its backwards from what you think it should be. Each gauge has an upper limit to the amount of current it will carry before it gets too hot for the insulation to safely handle. It gets hot because there's some resistance in the wire and the more current there is the more heat that will be dissapated in the wire. If the insulation gets too hot it will melt and/or burn and expose the wire.
To pick a wire you need to know how much power you're going to expect the wire to carry. Wire is rated in amps so you need to figure out how many amps you'll need for each appliance on the circuit. Its just a little book keeping to figure that out. Look at all the things you want to circuit together, add up the amps and pick a wire that will carry that amount. But lets bump our wire size up a little for a safety factor and voltage drop. 25% is typical.
Here's how much current a typical wire will handle:
Wire Gauge / Maximum Ampacity
14g / 15a
12g / 20a
10g / 30a
8g / 45a
Wait a minute you say, I have this light and its a 24W bulb, it doesn't have amps on it. No problem, divide watts by 12 and you get amps. The 24 watt bulb uses 2 amps.
Clear as mud? Lets work an example:
You have two lights and a fan that you want to wire on one circuit. The lights are 24watts each and the fan is 2 amps.
Our total electrical load is 2+2+2=6 amps.
Then we add 25% for safety and voltage drop.
6+1.5=7.5 Amps
From the chart above we see that a 14g wire will easily carry the load. And you'll find that a 14g will work for most of the circuits in your trailer. Here's a couple of rules of thumb that will help also.
12V outlets- use a 12g wire because you don't know what you'll want to plug in and 20a is the maximum size for plug connected equipment you can buy. I like to use dedicated circuits for outlets so if Jean plugs in a 12v coffemaker she bought as a surprise there won't be any electrical surprises for her.
Wire from the Battery to the Fuse Block - Use a 10g if you don't have a lot of electrical stuff. Use an 8g if you're an electrical sheik and plan to use a bunch of stuff.
Wire from the Tow Vehicle - Use a 10g for charging the battery.
Don't know, don't care - Use 12g for all the branch circuits. Use 10g from the battery to the fuse block. Use 10g for the tow vehicle to the battery.
- Fuse Sizes
The hard part is picking the wire. The fuse is easy, just pick a fuse that is rated at the ampacity of the wire. Here's a chart to make it easy:
Wire Gauge / Maximum Ampacity/Fuse
14g / 15a / 15a
12g / 20a / 20a
10g / 30a / 30a
8g / 45a / 45a
Wait a minute, you say again. That could mean that the fuse is way bigger than the load. In the example you posted above I only have 7.5a on a 14g wire. Won't a 15a fuse be too big? Nope. We use fuses to protect the wire. If there's a short, we don't want the wire insulation to melt and burn our trailer up. The end devices are on their own. And, you can add another device in the future to that circuit if you want, assuming the wire will handle the additional current. But, there's nothing wrong with using a smaller fuse if it makes you feel safer. If it blows because its too small you can bump it up to the next size. Just don't put a fuse in thats bigger than what the wire can safely carry.
Okay, that seems simple enough you say. But I've read a bunch of stuff about inrush, fuse characteristics, time current curves and its all greek to me. Do I have to worry about that?
Nope. You *might* if you add a large invertor but in 99% of the cases a typical ATC/ATO fuses will work just fine. Even the older glass tube fuse are fine.
- Final notes
You could probably use 16g wire in a lot of locations but I didn't because its hard for me to make a good connection with that small of a wire.
I use stranded wire because its too hard to make a good connection with solid. Stranded is much easier to handle as well.
Hope this helps. Let me know if there's something in here thats confusing or worse, wrong.
Bruce
www.tnttt.com/viewtopic.php?f=30&t=11316
Here it is. Hopefully they won't mind me reprinting it here.
Thanks Bruce
Okay here's another technical topic stripped down to the essentials. You want to wire your trailer and don't have a clue how to pick wires and fuses. Lets limit our discussion to 12V for now. How do you pick sizes?
- Wire Sizes.
First we need a little explanation about wire. Wire is sized in the US using the American Wire Gauge standard. The wires we'll need to use for a trailer are 14G, 12G, 10g and 8g. Only even sizes are available. The bigger the gauge number, the smaller the diameter of the wire, so don't get confused because its backwards from what you think it should be. Each gauge has an upper limit to the amount of current it will carry before it gets too hot for the insulation to safely handle. It gets hot because there's some resistance in the wire and the more current there is the more heat that will be dissapated in the wire. If the insulation gets too hot it will melt and/or burn and expose the wire.
To pick a wire you need to know how much power you're going to expect the wire to carry. Wire is rated in amps so you need to figure out how many amps you'll need for each appliance on the circuit. Its just a little book keeping to figure that out. Look at all the things you want to circuit together, add up the amps and pick a wire that will carry that amount. But lets bump our wire size up a little for a safety factor and voltage drop. 25% is typical.
Here's how much current a typical wire will handle:
Wire Gauge / Maximum Ampacity
14g / 15a
12g / 20a
10g / 30a
8g / 45a
Wait a minute you say, I have this light and its a 24W bulb, it doesn't have amps on it. No problem, divide watts by 12 and you get amps. The 24 watt bulb uses 2 amps.
Clear as mud? Lets work an example:
You have two lights and a fan that you want to wire on one circuit. The lights are 24watts each and the fan is 2 amps.
Our total electrical load is 2+2+2=6 amps.
Then we add 25% for safety and voltage drop.
6+1.5=7.5 Amps
From the chart above we see that a 14g wire will easily carry the load. And you'll find that a 14g will work for most of the circuits in your trailer. Here's a couple of rules of thumb that will help also.
12V outlets- use a 12g wire because you don't know what you'll want to plug in and 20a is the maximum size for plug connected equipment you can buy. I like to use dedicated circuits for outlets so if Jean plugs in a 12v coffemaker she bought as a surprise there won't be any electrical surprises for her.
Wire from the Battery to the Fuse Block - Use a 10g if you don't have a lot of electrical stuff. Use an 8g if you're an electrical sheik and plan to use a bunch of stuff.
Wire from the Tow Vehicle - Use a 10g for charging the battery.
Don't know, don't care - Use 12g for all the branch circuits. Use 10g from the battery to the fuse block. Use 10g for the tow vehicle to the battery.
- Fuse Sizes
The hard part is picking the wire. The fuse is easy, just pick a fuse that is rated at the ampacity of the wire. Here's a chart to make it easy:
Wire Gauge / Maximum Ampacity/Fuse
14g / 15a / 15a
12g / 20a / 20a
10g / 30a / 30a
8g / 45a / 45a
Wait a minute, you say again. That could mean that the fuse is way bigger than the load. In the example you posted above I only have 7.5a on a 14g wire. Won't a 15a fuse be too big? Nope. We use fuses to protect the wire. If there's a short, we don't want the wire insulation to melt and burn our trailer up. The end devices are on their own. And, you can add another device in the future to that circuit if you want, assuming the wire will handle the additional current. But, there's nothing wrong with using a smaller fuse if it makes you feel safer. If it blows because its too small you can bump it up to the next size. Just don't put a fuse in thats bigger than what the wire can safely carry.
Okay, that seems simple enough you say. But I've read a bunch of stuff about inrush, fuse characteristics, time current curves and its all greek to me. Do I have to worry about that?
Nope. You *might* if you add a large invertor but in 99% of the cases a typical ATC/ATO fuses will work just fine. Even the older glass tube fuse are fine.
- Final notes
You could probably use 16g wire in a lot of locations but I didn't because its hard for me to make a good connection with that small of a wire.
I use stranded wire because its too hard to make a good connection with solid. Stranded is much easier to handle as well.
Hope this helps. Let me know if there's something in here thats confusing or worse, wrong.
Bruce
