Emergency Generator Wiring

[rodster]

I was thinking of running electricity to a shed about 70 feet from my house. While I'm digging a trench, I was wondering, could I add a wire for a connection plug for my portable generator to connect it to my house from my shed. That way I don't need to drag the generator closer to the house or take up space in my garage with the generator. Just open the shed door, wheel it outside and make the connections. The total distance from the connection point to my panel would be about 110 ft. I was going to use 8/4 wire.

Can anyone think of any issues?

Thanks

Rodster

[roger g]

I think that 8/4 will be more than enough for what you need. Depending how big your shed is and what you use it for I would think of burying a pipe so you could pull other items out to your shed.

roger

[Vector]

I'm going to assume you meant to say 8/3. 8/3 is 3 insulated conductors + ground.

Assuming you will be running no larger than a 30A generator, a 110' run of 8/3 will have a voltage drop of only about 5.2V, or about 2.2%. Anything less than 5% is OK, so 2.2% is great.

But! Because this is a generator, the voltage drop also has to take into account the longest load run that is on the transfer switch.

Except... Unless you have a really large generator (say larger than 7500 Watts), you're never going to be near a true 30A anyhow, so the voltage drop will be even lower.

Depending on the size of the generator and the length of the lines in the house, you might even be able to run 10/3. But that would be a bare minimum, and I'd not suggest that.

So, 8/3 is good unless you have a really big generator, or have very long circuit runs off the transfer switch.

I'm also assuming you're talking about using non-metallic sheathed wire rated for direct burial (UF-B). Don't just use NM-B. If so, then make sure you bury the cables at least 24" deep.

What are you running for power out to the shed? I'd pull at least 10/3 and run a "multiwire" circuit, giving you two 20A circuits. But multiwire circuits are a little tricky, so if you're going that route, make sure you know what needs to be done.

Me, if I'm going to dig a trench 70', I'm putting in a sub panel, at least 60A, with 6/3 wire. But not knowing the size of the shed, that might be ridiculous overkill :)

[rodster]

Thanks for the replies. I have a generartor that is 7550 with 13500 surge watts. Probably more than I need, but I got a good deal on it.

I will use the 8/3 rated for ground contact. Good suggestions on running power for things in the shed. A light or two and a couple of power tools will be great.

Just curious, why should it be buried 2 ft deep?

Thanks

[Vector]

That's a pretty hefty generator there. All my numbers were based on a 30A maximum from the generator, but you can go over that. What amperage is the outlet on the generator rated for? 40A and you're still OK.

24" deep because that's what code calls for for unprotected direct-burial cable. You could run Sch 80 PVC conduit with THWN wires and go only 18" down. If you used ridgid metal conduit you could go as shallow as 6".

Define "a couple of power tools" :)

That could be a 3/8" drill and a dremel tool, or it could be a lathe and a table saw... :twisted:

If you're just going to use smallish hand tools, I'd still go with the multiwire 2x20A setup I suggested earlier.

Other things to keep in mind.

- Outlets in the shed will have to have GFCI protection. This is a bit trickier with multiwire circuits, but not a real problem.

- You may need to drive a grounding electrode at the shed (if it was a sub-panel and not just a feeder circuit, I'd be 100% sure of this, but to be honest I'm not sure in the case of a feeder circuit. You'd want this for your generator hookup anyhow.

[rodster]

Thanks for the info. I was thinking about putting the wire in conduit and not going real deep 6-12 inches since I need to cross the wire from the power company.

I may have bought to much generator, but I want to be able to power my 1.5 hp well pump, refrigerator, deep freeze, and furnace blower if it is in the winter. It is a generac/briggs generator w/ electric start just in case I'm out of town, the wife can light it off. Also the oil filter for more durability. The largest circuit is 30amps. But I think I should be able to power a couple other things at the same time as well.

I think I will be consulting an electrician on the circuits. Not familiar with multiwire circuit. On tools, I'm talking a circular saw or a compound miter or maybe an air compressor.

[rodster]

So I'm finally ready to dig my trench. I've been reading other posts which refer to using UF-B and conduit for extra protection. If I use the 8/3 for my generator and 10/3 for a circuit to power the shed, can I run these in the same conduit? If so, what size. If they need to be in separate conduits, what size is required? Thanks

[rodster]

I think I need to add more information and another question. The total distance from the generator to the transfer switch will be about 140ft. Will I still be OK with 8/3 wire? Regarding the wire to power the shed, I found some information on another board 'It is against code to have more than one circuit installed between your home and your detached garage. If more than a single 20 amp circuit is needed then a subpanel must be installed in the garage. ' Does anyone know if this is true? If so, it would blow the idea of the multi-wire circuit, correct? And to do a 20amp circuit, would I need to go up to an 8 gauge wire?
My head hurts, I see why electricians are so expensive.
Thanks for the input

[Vector]

I'm surprised noone has responded yet, so I'll try.

I believe the multi-wire circuit falls under the "one circuit" limitation, and is legal, but I'm not 100% certain.

If you go shallow (6-12" like you mentioned above) you will need to run ridgid metal conduit.

For your multi-wire circuit, 10ga. wire should be just great. Especially since voltage drop on a multiwire circuit is treated like a 240v line. So at 140', the voltage drop would be 2.9% and therefore just fine (http://www.electrician.com/vd_calculator.html to run the numbers for yourself). In fact, you'd still be under 5% (barely) with 12ga, but I wouldn't do it.


For the 30A generator circuit, I'm going to assume your worst case is the length of the run from the generator and back to the detached garage, or 280' If that isn't the case, check it yourself witht he right numbers.

Assuming 280', you're looking at a 5.5% drop with 8ga. so you'd need to increase size to 6ga.

Things to keep in mind if you do this. You will need to bond the conduit to ground at one end, you do this by using a grounding bushing at one end. Like this: http://www.bptfittings.com/images/products/388-dc.gif

Don't pull sheathed NM (or UF) wire in the conduit. Instead buy individual THWN (it wil probably be THHN-2 and THWN-2 dual rated, but you want to make sure the "W" is in there). Technically you may be able to avoid running a ground wire, sing the conduit as your ground, but again, I wouldn't.

Your conduit sizing: You need to fill no more than 40% of the cross-sectional area of the conduit since you'll have more than two conductors. So:

THWN-2 in 6ga is 32.71mm^2 in area.
THWN-2 in 10ga is 13.61mm^2 in area.

That's 4*32.71+4*13.61=185.28 mm^2 total area. That would be less than 40% of the area in 1" Rigid (229 max allowable). You may wish to go larger in order to make pulling the wires easier.

There is a *ton* (perhaps too much) of information about wiring detached garages here: http://www.selfhelpandmore.com/homewiringusa/2002/accessory/detgarage/

Look Specifically at: "PICTORIAL EXAMPLE OF A TWO CIRCUIT 240 VOLT 15, 20 OR 30 AMP BRANCH CIRCUIT WITH UNDERGROUND CONDUCTORS WITH AN EQUIPMENT GROUNDING CONDUCTOR RAN WITH THE FEEDER SUPPLYING THE DETACHED STRUCTURE" (http://www.selfhelpandmore.com/homewiringusa/2002/accessory/detgarage/detgarageshow.htm#21)

Always remember, free advice is worth every penny you paid for it...

[rodster]

Great information. Thanks Vector

[rodster]

Thanks for the help with this project. Everything went as planned with the install of the wiring and generator panel. I decided to go with the multi-wire circuit for the feed to the shed. Although I haven't been able do the wiring yet. Wife says I need to finish the basement first. I an electrician friendcome by and do the panel hookups. He really liked the panel. It acts as both a sub panel and a generator transfer switch with generator usage guages. I've attached a couple of pics. 1 in the shed and another of the generator panel.