Big enough wire?

OT. Electrical question · Jul 15, 2010

I am putting in power to a small office. I will have a computer, printer, small fan and a couple of light bulbs. The run is 100 feet. Will 12/2 with ground be big enough?
Thanks, Richard

Errin OH · Jul 15, 2010

No. If you figure about 15 amps at 100 feet 8 gage would do it (1.6% drop). At 20amp its close at 2.2% drop. There are some varibles though.
Voltage Drop Calculator

Bob · Jul 15, 2010

Run two wires and keep your lights separate from the equipment load.

You'll appreciate that someday when something trips a breaker and you still have lights.

DaveK(IN) · Jul 15, 2010

#12 will probably carry the load you describe. You could run #10 and have no worries. Voltage Drop is supposed to be 2-3% on the branch circuit. Since the labor is the same I would run a 12/3 and get a second circuit out there. That would let you run a small heater or other appliance. Since you don"t have a motor load the voltage drop is not so critical.

lucas boy · Jul 15, 2010

thats what i was thinking.# 8 wire for the 100ft run..

Richard G. · Jul 15, 2010

Thanks guys, I guess I will go with the 10 wire.
Richard

buickanddeere · Jul 15, 2010

Minimum but ok. However if you look into running 12/3 you will find little difference in the total cost of the job. That extra circuit will get used before you know it. Also allows a 240V load.
If you ever use AC or a heater on the 2nd circuit the lights will flicker less during startup.
Drive a ground rod out at the office and join the ground running from the supply. Don't let the neutral and ground bond at the office.

DaveK(IN) · Jul 15, 2010

Ground rod is always a good idea but is not required for a single branch circuit (including multiwire branch circuits). NEC 250.32(A) Exception

MarkB_MI · Jul 15, 2010

If you're going to be digging a 100 foot long trench, you might as well put in heavier wire than you need. I would go with 10/3 (meaning 2 hot lines, one cold and a ground) at a minimum. Use 20 amp breakers at the source. That gives you the ability to have your outlets on one phase and the lights on the other. You'll have less voltage drop this way, and if you blow your outlet breaker you won't lose your lights.

You might want to go ahead and put a subpanel in the outbuilding, if you think you'll need to expand in the future.

BlaineF · Jul 15, 2010

(quoted from post at 09:34:52 07/15/10) I am putting in power to a small office. I will have a computer, printer, small fan and a couple of light bulbs. The run is 100 feet. Will 12/2 with ground be big enough?
Thanks, Richard

I'd throw a extra 2 inch or 4 inch conduit in the ditch .Then you would have something to pull wires in for future use.The plastic stuffs not to spendy.Just my thoughts .BlaineF(WA)

Philip Doolittle · Jul 15, 2010

The Nat Elec. Code requires you to disconnect BOTH energized or hot wires if you run a 3 wire + Gnd circuit so as not to have a HOT wire/leg in the terminal box or panel if you have a overcurrent problem on one leg. This means you must use a TWO (2) pole breaker at the supply OR a 2 pole FUSED swtch. If you do use fuses you will gain the advantage (recommended) of being able to have light on one leg(ckt.) and motor on the other leg (ckt.),so if the motor has an electrical problem the lights will stay ON. #10 wire would be the size to use. Type UF/3 conductor with a ground is best for buried runs as the best buried conduits will eventually get water in them. I would also point out that a 2 pole Ground fault circuit breaker (GFCI) will probally be required at the "LOAD" end depending on the structure/use. "THATS FOR YOUR SAFETY " in case of a ground fault that could shock and electrocute YOU.Feel free to ask any questions.
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barn E · Jul 15, 2010

Call your inspector in and only have to do the job one time. State of Mich would be at least 3 circuits. Computer, plugs, lights if commercial endeavor. Still have my license.

paul · Jul 16, 2010

Why wouldn't anyone set up for at least 40 amps, and 240v setup, when you run wire that far? You'll end up using it, couple years down the road.

12-2 isn't good enough at any rate.

--->Paul

Big enough wire?

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