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fw2007 03-26-2008 04:54 PM

Wire gauge VS length
I am going to run several circuits up to my attic, then back down walls upstairs to receptacles.
I don't think any run will be more than 50ft. I am trying to find a rating vs length chart.
This is standard NMB #12 cable that will be fed by 20A breakers.



micromind 03-26-2008 06:45 PM

Amp rating vs. length doesn't matter as much as voltage drop vs. length.

If you wanted to, you could use a 20 amp breaker on #12's 10,000 feet long. Of course, if you had any kind of load on it, there would be very little voltage left to feed the load. The resistance of #12 copper is 1.619 ohms per 1000 feet. It doesn't matter if its NM, THHN, an extension cord, or whatever.

Suppose you needed some light in a chicken coop a mile from the house. And you had a 5000 foot spool of #12/2UF. 5000 feet out and 5000 feet back (2 wires to complete the circuit) is 10,000 feet. There would be 16.19 ohms of resistance in this circuit. Suppose you used a 120 watt lamp. 120 watts at 120 volts = 1 amp. In this case you'd lose 16 volts in the wire itself, leaving 104 volts for the lamp. (Which would now be less than 120 watts, but lets not get too technical!) This would actually work, you likely wouldn't notice the dimming of the lamp.

Now, lets say you got more chickens, and you took a saw out there to add on the building. Lets sat the saw draws 10 amps. We won't consider starting current. At 10 amps we would lose (theoretically) 161 volts in the wire! See a problem?!?? The saw would barely turn, let alone actually cut anything.

Getting back to reality here, #12's are good for 100 to 300 feet, depending on the type of load. It wouldn't bother me a bit to run a heater 300' out, but if there were lights involved, I'd limit it to around 100' or so. You won't notice a heater 'dimming', but lights you certainly would.


nap 03-26-2008 08:06 PM

here is a jvascript voltage drop calculator. There is no code mandate for voltage drop but there is a recommendation to remain less than 3% in a branch circuit. Use that as a max voltage drop.

voltage drop calculator

fw2007 03-26-2008 09:14 PM

Thanks for the help guys;
The voltage drop calculator is really nice. Bookmarked it for future use.


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