Underground UF Wiring Gauge Selection

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Chuck B

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I will be running a new 20-amp circuit to a currently unwired shed some 75' away from its departure from my cottage. Is it detrimental or advisable on the 20-amp circuit to use 10-2 gauge UF to combat resistance on a long run?

I also intend to run a 3-wire back to a spotlight 3-way switch inside the cottage. Using the same logic would that likewise be a 10-3 wire?

Would it then be acceptable to connect the 10-2 feeder wire once inside the shed for receptacles, light switches and the floodlight to 12-2 and 12-3 gauge wire? I am just a bit uncertain doing that but it appears safe to me. It would be easier to work with when wiring switches, (flood)lights and receptacles of course.

I would encase all UF wire in "grey plastic conduit" when entering the ground for burial at the cottage and shed. Not sure of correct depths since I do not wish to encase the UF throughout the trench. Is it 12, 18 or 24" depth until it reaches the plastic conduit if it is not encased? I will be coordinating UF wire burial into a newly trenched septic line trench to eliminate trenching on my own. Is there any problem sharing the space and locating the UF wire next to a septic line run? Could I plastic tie the two UF lines together before burial?

I will install a 20-amp GFCI breaker in the main panel to power it all.

Thanks.
 

Stuff

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If you are using a gfci breaker then you are good for 12" burial. Nothing should prevent you from sharing a trench. Plastic ties OK just don't cinch too tight.

10 gauge is fine but probably not needed even for lights. What kind of loads are you looking to support in the shed?

Note you could use a MWBC feeding the shed - double pole breaker and 12/3 cable. Then split at shed to use as two circuits.
 

WorthFlorida

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Your efforts may be better with a sub panel in the shed. You can run a 10-3 off of a 30 amp breaker at the cottage to the shed. Then at the shed run separate circuits for your needs. It will allow easy access to reset any breaker or easily switch a circuit off for maintenance. If you're only installing a few lights and outlets for convenience, you'll be OK. Use GFCI at the outlets since they can trip quite easily if your working on damp ground and be reset right there. As far as a three way switch, this remote switch may be easier to install since it will not require a three way switch and wiring between the cottage and house. It works with the remote it comes with and works with a your Chamberlian/Sears/Liftmaster garage door opener remotes. I use this to remotely turn on my pool light from inside the house when the light switch was on the outside. If you install a flood light at the shed, then with the remote you can turn it on from the cottage or from your car or truck remote. It can also be turned on and off at the receiver (switch) end.

http://www.midlandhardware.com/180793.html?gclid=CIO58IfSptICFUMdgQodbygDsQ

UF is designed for direct ground contact. Use Schedule 80 to protect the cable to 24-36 inches below grade and up to the entrance of the cottage. Same at the other end. You'll need to check your local codes since this will vary around the country. It may have to be below your frost line.
 

Chuck B

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If you are using a gfci breaker then you are good for 12" burial. Nothing should prevent you from sharing a trench. Plastic ties OK just don't cinch too tight.

10 gauge is fine but probably not needed even for lights. What kind of loads are you looking to support in the shed?

Note you could use a MWBC feeding the shed - double pole breaker and 12/3 cable. Then split at shed to use as two circuits.

Thanks. Load is small undercounter fridge, two led lights, a LED flood and several receptacles for power tools such as my Bosch portable table saw, drill, etc. Would only be using one power tool at a time.

The only reason I considered a 10 gauge wire is for current leakage over a 75 foot run. I'm now wondering whether the small fridge and a tablesaw might trip the 20 amp GFCI breaker.
 

Reach4

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Bigger wire will not help leakage.

It will help voltage drop. 10 AWG would be a good idea if you really will run 20 amps. The MWBC Stuff suggested will actually reduce the voltage drop usually.
 

WorthFlorida

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.... I'm now wondering whether the small fridge and a tablesaw might trip the 20 amp GFCI breaker.

GFCI will not trip due to load. The GFCI will “sense” the difference in the amount of electricity flowing into the circuit to that flowing out, even in amounts of current as small as 4 or 5 milliamps. For the table saw, if it is one of those light weight direct drive units a 15 amp circuit will work. If it is a heavy old fashion table saw with a drive belt, check the ratings plate. This one reason why I suggested a sub panel. If you're tripping a breaker it would be very easy to add another circuit.
 
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