Since this will be a single 20-amp (or possibly 15-amp) circuit to the shed, you can run it from either the load side or the line side of the GFCI (i.e., with or without GFI protection). Furthermore, you shouldn't have to meet any additional grounding/bonding requirements normally associated with feeding an outbuilding, since it's a single small circuit. But I'd definitely check with your AHJ (inspection/permit department) to confirm this for certain!

As for making the 15' underground run, you have two decent options:

1. Run three THWN conductors (black-hot, white-neutral and green-ground) in conduit, which must be buried at least 18" below grade. 3/4" PVC conduit should make for a slightly easier time of it than 1/2" and the cost difference is very small. (Note that I'm assuming no obstacles along the run, such as a driveway.)

2. Bury a Type UF cable ("underground feeder") at least 24" inches below grade without conduit. (You would still need a couple of short sleeves of conduit, however, to protect the wire where it emerges from the ground at the house and the shed. That said, I'd opt for conduit all the way.)

BTW, what else will be on this circuit? It'd be cleaner, IMO, if you ran a separate independent circuit to the shed but YMMV. Also, I've heard (but can't confirm) that some local jurisdictions disapprove outbuilding feeds that tap off of existing branch circuits.

I'm assuming that you'll have maybe a light and a receptacle or two there. If you're planning for much more than that, I'd think twice about piggy-backing this on an existing circuit with other load demands.

TerribleTom,

Thanks for the detailed response. To answer your question, I will only have a receptacle and light in the shed, so there's not much draw. The circuit is 20 amp, there is only 1 inside receptacle on that circuit, with the outdoor GFI receptacle it would be 2 plus whatever is in the shed. Upon reflection, it would be very easy for me to run a dedicated 20 amp directly to the shed. I could dig a 18" ditch (only obstacles would be a couple of roots) and actually run that directly into the basement, below grade. That would be a straight run through the cinder block, I'd just have to make the hole in the wall to handle the 1/2" or 3/4" conduit.

Would that approach make sense or cause any issues that you know of?

Thanks again,
Bob

"Would that approach make sense or cause any issues that you know of?"

Sounds like goodness to me.

'Nother question. I've decided to separate the outdoor receptacle from the run to the shed, 2 different circuits. For the outdoor receptacle (mounted on a brick wall) - can I not use a GFI if the cable is coming from the load side of an internal GFI? I'm tapping into a receptacle in the laundry room which is GFI, seems as though the outdoor one wouldn't need on in this case.

Thanks,
Bob

"...can I not use a GFI if the cable is coming from the load side of an internal GFI?

So long as it's fed from the load side of another GFCI receptacle, you could use an outdoor rated non-GFCI receptacle and mark it with a label indicating "GFCI protected."

If it were my project, though, I'd opt to wire it from the line side of the existing indoor GFCI receptacle (i.e., without GFI protection from that receptacle) and install a new GFCI receptacle outdoors. That's not due to a safety consideration--it's to avoid confusion in the future as well as unrelated nuisance tripping. It's a personal choice because I hate it when an outlet goes dead and I have to go on a treasure hunt to find "which GFCI got tripped". YMMV.

In providing those new details, though, you've raised another issue that you may not want to hear. The National Electric Code (at NEC 220-4) specifies that at least one "20-ampere branch circuit shall be provided to supply the laundry receptacle outlet(s)" and the "circuit shall have no other outlets."

There are a few cases (including kitchen counter outlets, required bathroom receptacles and the required laundry circuit) where you're not supposed to tap off any other load.

Thanks again, Tom.

As for a separate 20 amp circuit for the laundry receptacle, I'm in luck. When we bought this house 10 years ago, I couldn't figure out why someone had wired 2 separate & dedicated 20 amp circuits to the laundry room. One was used for the washer & dryer, the other was unused. So I use the unused one for the outdoor receptacle.

Bob