Bob Chansler/Boulder/IBM wrote:
>
> I installed lamp posts at the end of my driveway and I need to get power to
> it for the lights. The nearest connection point on the correct side of the
> driveway is 300 feet away. I was going to run 14 gauge wire out there, but
> wasnt sure if I need to bump up the gauge because of the distance.
- snip -
Well, that will be a long run, but this is one of those areas
where code is written to cover general conditions, and yours is a
specific situation. If you plan on no more than 200 watts of
load, then #8 clearly is overkill.
#8 can safely carry a 5 amp load (600 watts) 360 ft (one way
measurement) with a 2% voltage drop. This would be good for a
general run where you might want to plug in your corded Sawzall
in the receptacle at the end of the run.
Your forecasted 200 watt load is only 1.67 amps. I ran through
the formula for computing voltage drop for 14awg for you:
14 awg = 2.58 ohm per 1000ft = .00258 per ft.
your 300 foot run requires 600ft of wire (two way)
so
600ft x .00258 = 1.548 ohms
1.67 amps x 1.548 ohms = 2.59 volts (this is your drop)
2.59 vac drop / 120vac = .021543 or 2%
So instead of running #8, you can run 14 awg with a 2% drop if
you keep your load at 200 watts.
The 2% voltage drop is considered standard, and the length of the
runs dictated by code are in view of this voltage loss
parameter.
If you are running an electrical motor on the end of the run, or
large power consuming devices, then you should stick within the
specs for a 2% drop, as otherwise you are just wasting a good
deal of current due to resistance in the wires, and not doing the
work you want done.
In your case, even if you were to exceed the 2% drop figure, the
only damage is that your lights would burn a little dimmer.
However, a 10% drop would reduce incandescent lighting output by
30%, so it does become significant at higher levels.
This voltage drop is not really a safety factor, as 14 awg has an
ampacity of 15 amps -- it can carry 15 amps for as long a
distance as you wish without overheating, but at some point (the
2% or 4% point), it becomes less efficient and you should move up
to the next higher gauge.
If you wanted to be safe at higher loads, you could run 12 awg,
but at your current amperage requirements, 12 awg will not make
your lights any brighter than 14 awg, and 14 awg is perfectly
safe.
If you wanted to run some high output fluorescent, you could fall
well below the 2% drop level, but you should be fine with a 200
watt load and 14 awg and a 600 ft round trip run.
regards,
doug
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