wiring a detached shed for AC power *and* generator backhaul
OK let's divide this into two main sections, with AC power coming and going...
I have a 15'x15' detached work shed located about 50' from my house (single family, 150A AC service). The work shed is weathertight, modern asphalt shingled roof, vinyl siding over CDX plywood, with a set of barn doors that open up almost the entire width of one side. The inside is unfinished with studs on 16" centers, and a poured concrete slab floor. There is currently no AC power in the shed, and it is unheated except for occasional use of a kerosene heater. In addition to needing AC power in the shed (which a typical 20A branch circuit will suffice for shop lights and perhaps a block heater for the tractor), I have another goal in mind as well while I have the trench open between the shed and the house. Read on...
1) AC power:
My plan is to run buried 1" dia Schedule 80 PVC conduit from the house out to the shed, using condulets/LB's at both ends to bring the conduit up out of the ground and through the cement basement wall and plywood shed wall. The conduit will be buried 18" the entire length. Through the conduit I would run three lengths of #12AWG THHN/THWN (black/white/green). In the basement, to get to the circuit breaker panel, I would wire-nut the THHN/THWN to 12-2 NM w/ground Romex in the condulet/LB. At the panel i will install a dedicated 20A GFCI breaker for this branch circuit.
At the shed I *believe* that for a detached structure I must use another small sub-panel, in this case a single or dual 20A breaker type (non-GFCI, as it's already protected upstream). I will need a pair of ground rods at the shed, AND I will need to separate N and ground at the small sub-panel. From the small sub-panel I can wire out a pair of outlets via 12-2 NM w/ground Romex and a switch for a cold-rated fluorescent fixture. For the most part I am copying elements of this diagram:
So far so good?
2) Generator backhaul:
I also want to be able to run my generator out in the shed (barn doors wide open of course, or with it out on the shed's cement apron if it is not raining/snowing) and not have to unroll a heavy, long cable back to the basement where the transfer switch will be located.
The generator produces 240Vac split-phase (4 wire output -- L/L/N, with ground) at a continuous 7.5KW with a momentary motor starting peak output of up to 10KW. The generator head has on it a pair of ganged 50A output breakers.
So, in ANOTHER, separate Schedule 80 1.25" PVC conduit in the trench, I would pull qty 3 lengths of #6AWG THHN/THWN(L/L/N), and qty 1 length of #10AWG THHN/THWN (ground). It is a little unclear to me from reading the NEC online info whether the ground needs to be the same gauge as the hots and neutral. From what I gather for this situation, for wiring protected by up to 60A breakers #10AWG ground wire is OK.
On the shed inside wall, after a short section of conduit I will install a box and the mating receptacle for the OEM-supplied generator cable (of course, the end of the cable is female, so there are no exposed energized contacts when the generator is running). For generator operation, I would connect up the heavy output cable to the aforementioned receptacle (which actually has male contacts), start the generator, flip the breakers on, and proceed to the basement.
At the basement end the heavy #6AWG THHN would be run inside conduit up to the transfer switch. The transfer switch would be wired into the main circuit breaker panel for selected circuits (fridge, some lights, well pump, etc) so that when the generator is running these circuits would be powered when the transfer switch is in the generator position.
extra credit) Finally, I would like to run some low-voltage wiring out to the shed as well. I believe I will have to use separate 3/4" PVC conduit for this as well. Primarily I would like to use the low voltage wiring to alarm the shed doors via my current house alarm system. So i would program a new wired loop zone into the system with magnetic door switches or some such out at the shed.
For the most part, the conduit (x 3) would look like this, copied from one of the tacked threads here:
Is there any reason I can not put the AC power out to the shed AND the generator backhaul wiring in the same (larger) conduit? Both runs would be THHN/THWN and appropriately sized. The low voltage wiring would be in a second, separate conduit.
Did I overlook something?
No sub panel required for the single circuit you are running. I would run an extra hot and have a multi wire circuit. This would give you 2 20 amp circuits for the shed. Still no sub panel needed. 1" conduit is way more than required for this.
Plan for generator sounds good .You could run all the wires(power to shed and genreator power) in the same conduit. You could fit them all in a 1" conduit.
Low voltage needs a separate conduit.
I would not protect the feed with a gfci, only the receptacles at the barn.
Everything else looks good.
If you do use a small subpanel and feed it with 240, you need 2 ground rods at the barn.
Joed and jbfan are certainly more well versed than I, but it sounds good to me too. Excellent planning and well thought out.
While I was expending money and effort I would consider to put 240 volts to the shed and have a small subpanel there, rather than 120 volts. While you are buying #6 you may find it economical to quantity buy enough to do the 240 volt power to the shed and the genny back to the house.
If no desire for 240 volts, I would add a wire to make it a multi-wire and have two 120 volts circuits as joed suggested.
My understanding is that you can't have more then 1 feed to a detached structure unless it is a MWBC
Best bet is to put the transfer switch out in the shed
Then run a sub-panel in the shed & feed that with the Gen & thus the house
I think I will probably do this with my Gen since I already have a sub-panel in the shed
Thanks for the replies and info above.
Question -- given that I can run everything in one conduit (of course, the AC stuff only, separate conduit for the low voltage) -- what size Schedule 80 PVC conduit do I need for the following:
qty 4 of #12 THHN/THWN (branch circuit, L1/L2/N/GND)
qty 3 of #6 THHN/THWN (generator backhaul, L1/L2/N)
qty 1 of #10 THHN/THWN (generator backhaul, GND)
The type of transfer switch I am talking about sits adjacent to your main breaker panel, and selected circuits from the panel (up to the ampacity or positions of the transfer switch) are brought out to the transfer switch. The position of the transfer switch determines whether these circuits are energized by either the utility power or generator power.
That said, in order to put this type of transfer switch out at the shed, LOTS of branch circuit wires would have to be extended out to the shed. I don't think this is a good idea at all, and I would never take this approach.
The problem is that you can only have one circuit to a detached stucture OR a MWBC
You aren't allowed to run multiple circuits between the house & shed
Actually the Interlock switch & transfer switch won't work either
I'm not sure if an emergency feed from shed to the house is considered a 2nd feed
I do think dave is right, though, about only being able to run one circuit (or one MWBC) to the shed.
It might be OK since it can be considered an emergency backup system
NEC section 700 (702) covers this
Reading thru it now.....
Anyone help me with closure on this?
One issue I see is you now have (6) current carrying conductors in the conduit
But 90c column is used to derate so you may be OK
80% derating required for 6 conductors
So #6 can still carry 60a, #12 wire is OK too
1" is the Min conduit size, I would up that to 1.5" for an easier pull
I pulled (3) #6 & 1 #8 in a 1" & that was a pain
I would run the Gen connection by your Inspector
He might ask for a 2nd conduit...better safe then sorry
I dont believe making a wirenut connection in a LB is acceptable. I was under the impression by my inspector that it was not to NEC.
I also would upsize the conduit to at least 1.5" and run the low voltage in separate SH 40 conduit separated by at least 6" of dirt in the trench.
Slightly different scenario.
I ran 100 amp service to my barn about 180' from the house.
I ran 3 #2 and 1 #6 wire for the main service.
I ran 4 or 5 - #12 in the same conduit for 2 - three way switches so that I could control some exterior lights on the barn from the house.
IIRC, I ran this all in 1.5" conduit.
The county inspector said this was fine and approved it. So technically, I have 3 circuits out there. (100 amp, and 2 - 15 amps)
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