GFCI have a circut board inside them to read how much power is going out on the hot and comming back on the neutral. If a differance greater than 4-6mA is detected they trp. The new GFCI are now easier for you to hook up. If you don't hook them up right, they won't let you push in the reset button. Black wire to gold screw, white wire to silver screw, bare wire to green screw. Hook up only to the line side of the GFCI. Leave the yellow sticker over the other set of screws( the load side).

That's presuming the receptacle only has one romex coming in. If the receptacle in mind is in line and has two romex connected to it and if you only want to protect that outlet, connect both pair of wires to the line side. Since we only wire GFCIs on 20 amp circuits I am not sure 15 amp models exist.

You can use a 20 A GFCI on a 15 A circuit.

Unless you have something that IS grounded nearby, even if the frame of the sewing machine became hot nothing would happen until you touched the machine and a grounded item.
If you install a GFCI you may have a lot of nuisance trips with older equipment from leakage.
While these 'leaks' are theoreticlly a hazard, making sure there are no grounds nearby will provide some additional protection.

The recept in question is at the end of a branch circuit so I won't need to connect to the load side of the GFCI.

For some reason my house has a couple of 120V circuits run with #12 wire on 15A breakers in the main panel along with about 2 dozen other 120V circuits on 20A breakers. But anyway the local Ace hardware does have 15A GFCIs in stock so I believe I'm all set to go. Thanks for the help guys.

I think what Brick meant was you can use 15A GFCI receptacles on a 20A circuit.

The sewing machine is in a basement with a concrete floor that is sometimes damp in winter. Not actually wet, but just damp enough that you can feel it. I don't know whether or not that would make a path to ground that could severely shock a person wearing shoes but I want to be on the safe side just in case.

There hasn't been any trouble with it for the last 9 years since we moved the machine down there but I would feel better about her using it if she's protected against shock, and installing a GFCI seems like a cheap and easy way to do that. I have already replaced receptacles for the washing machine and by the basement door with GFCIs, and I think I have read somewhere on this forum that all basement recptacles are now supposed to be GFCIs, is that right? If it is I will still have 3 more to replace after this one.

There is also a 240V dryer circuit on a double 30A breaker. It has red, black, and white wires plus a bare copper wire. The ground wire is connected to the receptacle and pigtailed to the metal box but it isn't protected any other way. Is there a 240V GFCI available that would protect that circuit?

If this is an unfinished basement (and most inspectors would say it is with a concrete floor), then GFCI protection is REQUIRED for all 120V receptacles that are accessible. You do not need to protect the dryer circuit.

Regarding the OP's first post, wouldn't it be in his best interest to replace the cord on the sewing machine?

I'll replace the recepts, but replacing the sewing machine cord would require a lot of disassembling of the machine which I'm not qualified to try on an antique which parts are no longer available for.

Most of the available GFCI receptacles are rated for 20 amps. You can use these on a 15 A circuit. They are only rated for 20 A to alow use on bathroom circuits.
Feeding them with a 15 A OCD will not result in a problem, and most do not have an actual 20 A receptacle pattern on them.
20 amp cord caps are actually pretty scarce on consumer equipment.

Yes, but they are listed as a 15A GFCI, rated for 20A feed through. You can not put a 20A listed GFCI (with the 20A pattern) on a 15A circuit.

I think rewiring the machine would be better and safer, you should still be able to get the modern equivalent of the cloth covered stuff, if you want.

I doubt it would be that complicated.

If the existing wiring isn't cracked or falling off there is no reason to replace it. Many old cords are still good and are safe to use. Obviously if the cord is frayed or is stiff and cracks when you bend it it needs to be replaced.

If the machine actually sits on a concrete floor my advice would be to get a couple of rug protectors, a large rubber mat, vinyl floor cover, or some floor tile to place both the machine and the operator on. If the cord is still in good shape there probably isn't isn't going to be a problem but with a floor covering of some kind you won't be tempting fate. As indicated above, if in an unfinished basement the receptacle should be a GFCI type.

Sewing machine motors usually have brushes inside. These are a carbon material that are used to make electrical contact with the rotating part of the motor. The carbon wears a bit in time and can deposit a fine dust that may eventually make contact with the frame of the motor. This may cause the GFCI to trip or could cause a shock. If either happens your best bet is to have the machine checked by a repair shop.

GFCI receptacles are all rated for a 20 AMP pass through, meaning they may be used on either a 15 or 20 AMP circuit, as can any listed receptacle, even though they say they are a 15 AMP device. A GFCI that is listed as a 20 AMP will have one slot shaped like a T and should only be used on a 20 AMP circuit.

UNK

Thanks for the info unclebill. Actually the 50+ year old cord appears to be OK. It's been coiled and hung up on the back of the machine so it doesn't get stepped on or dragged around on the floor. As for the concrete floor, I'm in the process of finishing out that corner of the basement by firring and drywalling the walls and installing a suspended ceiling, and it will soon have sheet vinyl flooring under the machine and the chair where she sits to sew. I would think that vinyl flooring would insulate her from the concrete floor pretty well in case the GFCI didn't do it's job for any reason.

I was tied up this morning with another new probelm involving wiring. Yesterday I opened the dryer receptacle box to make sure the box and metal conduit running down from the sill joist is grounded, and surprise, surprise it wasn't. Whoever wired my house must have been going by a 1910 vintage code book or something. Anyway, this morning I cut a piece of #10 bare wire and pigtailed the ground wire to the box. When I flipped the double 30A breaker back on I heard a sound like a muffled "whump" over at the dryer. I quickly turned off the breaker and rechecked everything in the box and found nothing wrong. So I closed up the box, plugged in the dryer and turned it on. The dryer works fine just as before and there is no smell, smoke, or anything to indicate an overheated wire or something like that. I ran the dryer for a while and nothing unusual happened. I have no clue what made that noise but I guess I'll just ignore it unless it happens again. There was quite a bit of concrete dust in the bottom of the box which came from the hole I drilled in the wall to fasten the box to the wall more securely with another 2-1/2" Tapcon screw. I don't see how that screw or the dust could have affected the wiring or the receptacle but it and the pigtail for the ground wire are the only things different from the way it was before I opened it up.