Outdoor cable run for shed

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Old 09-23-15, 07:26 PM
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Outdoor cable run for shed

Hi,

I am installing irrigation this weekend and will have all of the poly pipe pulled on Friday. I will be putting in a shed next year about 80 feet behind the house and have done basic prep. for the pad it will sit on. I would like a 20 amp circuit for the shed and figured I should get the wire from the house to the shed now.

My thought was to use the 1" HDPE poly as an underground conduit for and run individual THWN conductors, and to use a shop vac with a mouse and string to suck the string through and just pull the conductors through this way. I would leave a small coil at each end and store in a waterproof box to figure out the splices later. Probably splice at a future outdoor outlet at each end (one on the house, one on the shed). The poly pipe I would pull 10-12 inches down in a loam/loamy soil, not too rocky. Per NEC code sec. 353, HDPE pipe is suitable for underground conduit, so I think I'm good here.

Just want to make sure I understand the temperature ratings and conductor fill tables correctly - per NEC sec. 110.14(C)(1)(a) for a 12 guage, 20 amp circuit, the temp. rating of the conductors must be at least 60 degrees celsius (140 fahrenheit). Per Chapter 9, Annex C, Table C.2 for electrical nonmetallic tubing, 25 THWN 12 guage conductors can be run in 1 inch conduit, so I think I'm all set there as well (I'm not sure the HDPE conduit is included within this classification but I think the point is that running 2 or 3 conductors appears fine). Any thoughts are much appreciated, thanks
 

Last edited by aerodan1; 09-23-15 at 08:30 PM.
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Old 09-23-15, 08:38 PM
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Plan sounds fine. Burial depth is 18" minimum. You will need a disconnect at the shed. A common 20 amp light switch can be used.
 
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Old 09-24-15, 08:04 AM
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Thank you. I was looking at column 4 of table 300.5 for the 12 inch depth requirement, but you're saying it's really column 3 (nonmetallic raceways listed for direct burial, etc.), which would be 18 inches?
 
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Old 09-24-15, 08:07 AM
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HDPE can be used as a lateral (horizontal) conduit underground, but you should switch to an electrical conduit for the sweeps and vertical risers. There are glue on and friction fit HDPE to PVC transition fittings to make the connection solidly.

The 12" burial depth only applies if you provide GFCI protection (usually via a breaker) prior to the wiring going underground, and you do not exceed 120V. This does limit the future potential of what you can power in the shed. BTW, the inspector measures to the top of the pipe, so the trench should be more like 14" deep for the GFCI version or 20" for the non-GFCI version.
 
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Old 09-29-15, 09:29 PM
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Thank you for the response. The PVC / poly connection sounds good to me, and yes it will be GFCI protected at the breaker.
 
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