Choosing the right wire size

If installing an outdoor receptacle that is 125 feet from the breaker box with a 20amp breaker, would 12 gauge wire be sufficient or would the voltage drop be to much?

Thx

Reply to
SteveP
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It depends on your expected load. At 20a you will be dropping 9.6v or about 8% which is regarded as too high.

10 ga will get you down to 5% with a 20a load. That is probably as big as you have to go since your real load should be less than 20. If this is multiple loads you can get pretty good bang for the buck by running a multiwire circuit with a shared neutral and 2 receptacles at the far end.
Reply to
Greg

What is the "real" load? Not enough information to calculate the VD.

Thx

Reply to
Brian

Guess I don't know the real load, This will be a receptale out next to a storage building so I don't have to keeping pulling out the drop cord when I need to do something. Will be running a flourescent light, and the occasional power tool a circular saw or drill. Possibly a small TV when something good is on while I'm working outside.

Reply to
SteveP

Two usual problems - the wire, and the protection.

First, size the wire - outdoor, 20 amp circuit.

Then upsize - how much? The old rule hidden somewhere in the NEC years back was that you upsized one level for every 60 feet after the first 60 feet. (Kept the house from burning down when the appliances had to pull extra amps thru the boxes to make up for low voltage and still deliver the power - and it all then gave off a fair bit more heat, given I^2 losses)

Then check to see if you can handle the voltage drop at that upsized level.

E.g., 125 feet, 20 amps - a value.

But is it undergound? - that amp value/required wire size is reduced in the table because of more loss to the soil and less voltage at the end when the appliance is running

Are you using an insulation in a conduit that breaks down at a lower temperature vs a higher one (e.g., MTW vs THHN) ? - a reduction in value because unless the running temp is lowered to a level compatible with the lesser insulation, the line loss thru the insulation is likely to increase from degradation due to heat as well as it may catastrophically fail and short out. Hotter appliances. So, here, 125 feet of total wire length is just past the cusp of the second upsize - if the deletrious conditions are present - undergound, damp environment or area, running heating elements or a lot of motor start-stops per hour, sitting for extended periods and then turning on, or like things that may affect integrity, then go with the second size up over the first.

When you connect to the GFI, it may not be able to fit that gauge - so you splice on a short length of the usual size wire for the outlet on the end of the long run wire - you are only upsizing to keep losses down, and a few inches in the box of the usual size for short runs is Ok. Don't skimp on box size, to have some room for the if-needed extra wires. Second - circuit protection - circuit protection is a science of its own, and the long runs tend to move one out of the under 50,000 amp let-thrus and into the larger numbers. A 30 amp breaker on a 500 foot line may not even trip with dead short at the other end. (I was a young engineer when i was amazed that we had a dead short on a 600 foot line feeding one phase of a 460 vac that was protected by a single phase

30 amp breaker - and the end welded and stuck and never took the breaker. A bit of checking, and a similar setup didn't take out the usual 30 amp fuses in dead short conditions, either. I retrofitted a fair number iof low energy let-thru - 4000 amp - fuses on those 30 amp circuits.)

GFI - The GFI will have to be at the outlet end to have any effectiveness. You may also need one at the box, if the local code says so, but after using these things on long lines for many years, I would not trust the one in the box to cover my ass at the outlet at 125 feet.

Fusing: part 1 - overload - (running 25 amps for a while on a 20 amp breaker and slowly heating wires ) A long line with too-small wires may trip the 20 amp breaker on running overloads. A 12 amp saw draws 12 amps, but it may take over 20 when running because of low voltage at the appliance which it m,akes up with by using more current -and it trips the breaker. Part 2 - short circuit - virtually no resistance to ground. lethal stuff. tthe impedance of the long runs limit the amount of current peaks - and thus can defeat short circuit protection while letting lethal currents and very hot arcs thru. You may have to use a low energy fuse rather than a common breaker, with that long a run. Check with a breaker mfg to see if their units can sense at that length and protect, or if you need a more sensitive breaker. The GFI at the outlet will help here, and maybe the GFI in the box may help mitigate fooling the short circuit sensors. Check with the breaker mfg.

Reply to
Hobdbcgv

The code load for a commercial duplex receptacle is 180 VA per outlet or 360 VA. At 120 volts this gives about 3 amperes. At 3 amperes the voltage drop is 1.5 volts or 1.3 percent. This is acceptable. However, if you plan to use considerable more power size up to No. 10. FYI As a rule of thumb the corps of engineers require one size up for every 70 feet for general purpose receptacle circuits in barracks.

Reply to
Mr. Smith

Voltage drop with #12 will be much too high, about 10 volts, (8.3%) at 20 amps.

Assuming that you allow the customary 3% drop for a branch circuit, it looks like #6 is required for 20 amps.

Regarding comments others have made about the actual load, take them with a grain of salt. While technically correct, they overlook the fact that good engineering practice is based on worst case values. Designing a 20 amp circuit so it works well only up to 10 or 12 amps is a false economy. If, however, you design a 20 amp circuit to work correctly at 20 amps, its performance will be satisfactory for all smaller loads.

Reply to
BFoelsch

There are people who will tell you to run wire as big as your wrist but if you have been getting along on an orange cord #12 will probably be plenty. If you are worried that you might actually be using a 20a load #10 will be fine. If this was a continuous load or something you knew was going to hurt you with I2R losses on your electric bill I would be a lot more concerned than you should be for an occasional use of a power tool and a TV.

Reply to
Greg

Thanks Greg, I'll probably go with #10

Reply to
SteveP

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