Wiring a shed

Discussion in 'Electrical Forum discussion & Blog' started by rdj, Apr 14, 2010.

  1. rdj

    rdj New Member

    Messages:
    1
    Location:
    Tallahassee, FL
    I'm wiring a shed with a subpanel approx 150' from the main in the house. I'll be using 8-3 w/ grnd on 30amp breakers, 100amp main lug in the shed with 30amp as main disconnect back-feeding the panel. I'll be running a #6 ground to the ground rod and keeping the neutral isolated at the subpanel (bonded at the main service entrance).

    Note: the contractor put a 3/4" pvc stub in the slab when building the shed so that is what I have to work with.

    My first thought was to run 8-3 NM w/ grnd from the main panel overhead in the house out to the porch, go down the wall in a 3/4" pvc to a bell box, splice with #8 THHN underground in a 3/4" pvc into the ALREADY IN PLACE 3/4" stub that was put in the slab as mentioned before.

    My question is this: Would it be an option to just run 8-3 w/ grnd UF from the main service all the way out to the shed and up through the 3/4" stub and avoid the splice.
    a. Will the 8-3 w/ ground UF fit through the 1.5' 3/4" pvc stub?
    b. Does the code allow you to use UF from the main service in this way? or do I need a green insulated equipment grounding conductor from the main to the sub?

    Thanks for your help in advance and any other recommendations are welcomed. I was a commercial electrician for a few years but am not a residential expert by any means.

    DJ
  2. drick

    drick In the Trades

    Messages:
    392
    You can run 8/3 UF thru the house, out and to the subpanel. If it fits thru the 3/4 inch pvc or not I'm not sure. If it does fit it will be tight and I doubt you'll be able to pull it thru even a single 90. You need a ground, but it doesn't have to be a separate green THHN ground unless its a pool subpanel. You'll be digging more (24 inches) to get the UF properly buried as opposed to the PVC (18 inches). If your just worried about the quality of the splice use a terminal block instead of a nut or run the pvc indoors all the way to the panel.

    -rick
    Last edited: Apr 14, 2010
  3. cacher_chick

    cacher_chick Test, Don't Guess!

    Messages:
    3,248
    Location:
    Land of Cheese
    My 08 NEC guide reads 24" burial for UF & 18" for PVC. It also specs a max of 4 #8 THWN conductors in 3/4" PVC. I'm willing to bet that pulling UF through it might be a problem.
  4. Alectrician

    Alectrician DIY Senior Member

    Messages:
    689
    UF is a bad wiring method. Use pipe and wire.

    Keep bends to a minimum with no back to back 90's.

    Use a 6x6 jb at the NM/THWN transition (bond it)

    NEC says no NM in the sleeve outside (wet/damp location)

    NEC says backfed breaker in the sub must be secured to the enclosure (kit available)

    2 ground rods at the shed.
    Last edited by a moderator: Apr 14, 2010
  5. drick

    drick In the Trades

    Messages:
    392
    Oops, my 2008 NEC probably says that too. Edited my post to say 24 and 18 inches. Thanks.

    -rick
  6. loafer

    loafer Mechanical Engineer

    Messages:
    49
    Location:
    Maine
    Why two ground rods?
  7. Furd

    Furd Engineer

    Messages:
    446
    Location:
    Wet side of Washington State
    Two ground rods because the National Electrical Code requires a grounding "electrode" resistance of less than 25 ohms or to install two ground rods. Since the equipment required to measure the resistance of the electrode is rather expensive it is easier to just install two rods. The two rods must be a minimum of six apart and connected together and to the "grounding electrode conductor" (GEC). Usually a continuous piece of cable from the panel to the first rod and then on to the second rod. Splices are NOT allowed in the cable from the panel to the first rod nor between the first and second rod. The clamps must be of an approved type, usually "acorn" clamps that clamp the GEC directly to the rod.

    The above is the requirement for a "service", that is, for the first means of disconnect after the electric utility's meter. It is possible that one rod is sufficient for an outbuilding served from another building because this would NOT be a service but a feeder to the outbuilding, check your local Authority Having Jurisdiction (AHJ).
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