What Size Copper Wire

BillyP · Oct 26, 2004

This is the question for today.

What size copper wire would I need from the main breaker panel to a sub panel. The wire will be ran 75' under ground. The total amp draw is about 70 but not continuous. I figure the most continuous amp draw, for several hours, would be around 45 to 50 amps.

slowrev · Oct 26, 2004

Number 6 copper stranded should work. Might want to check this link out, gives a good idea.

http://www.stanselectric.com/vdrop.html

Ben

lynxpilot · Oct 26, 2004

HERE is the NEC cut on it. Size based on maximum current. Says you need #4.

Deerlope · Oct 26, 2004

What size breaker are you going to feed it with? Also current use may increase a few years down the road. Do it right once or do it twice.

slowrev · Oct 26, 2004

Good info Lynxpilot,
Yes it would depend on the breaker size he is going to use. An interesting point on the NEC page you referenced...made no reference to length and voltage drop, just appears to be about safe heat dissipation capabilities. It shows that #6 will carry about 30% more current in open air than in an enclosed area, and that is ok, however it does not take into account voltage drop which can be a problem in longer runs. Which will cause problems in equipment on the downstream end of the run.

Ben

Inspector507 · Oct 26, 2004

One common flaw with all the responses so far. NEC® 110.14 says you need to use the 60°C chart for circuits under 100A or conductors smaller than #1AWG. #4 is good for maximum 70A based on the 60°C chart. So my feeling is a #3 conductor, minimum, fused at no more than 85A. That is unless all of the connections, terminals and wire are rated for 75°C then throw all of the above out....isn't the NEC® great?? /forums/images/graemlins/grin.gif

BTW a 75' run means very little except for REAL high current draw.

BillyP · Oct 26, 2004

slowrev
Thanks for the links. Looks like the voltage drop isn't an issue.

lynxpilot
Thanks for the link. I've looked high and low for that table.

Deerlope
The service is already in use. I'm just doing some upgrading to my poultry houses and was wondering if the existing wire would be good to go. I will be adding more fans but they are more efficient than the older ones (the older ones will be retired). The wire is #2 copper which by lynxpilot's provided chart is plenty big enough.

Thanks again guys.

BillyP · Oct 26, 2004

Jerry, I started to PM you with this question but really didn't want to be a bother. If I don't need to, I don't want to spend the extra cash to upgrade the wiring. This setup has worked for 14 years now and with all the new fangled gadgets that I'm installing, the continuous peak amp load isn't going to change. The total amperage will be higher but not everything will run at the same time.

If it would help you, I can list everything and their amp draws.

Inspector507 · Oct 26, 2004

If you currently have #2 Copper, then you have no problems at all. It is good for 115A and you are permitted to jump up to the next standard fuse size which is 125A, so there is always a little fudge room allowed too /forums/images/graemlins/grin.gif