I've just moved house and my clothes dryer has a four prong plug but on the wall there's a three prong socket. Do adapters exist for this? Or do I need to get a new cord for my dryer? Or is there another solution?
You could replace the cord on the dryer, but you'd have to bond the chassis of the dryer to make that acceptable to current NEC codes (Article 250.140). This can be a safety hazard if done incorrectly, and it may or may not void the warranty on the dryer.
Since your dryer is set up to use a 4 prong receptacle, the optimal solution would be to update the receptacle to a 4 prong (NEMA 14-30R if I remember correctly). However, this will require you to run new cable to the receptacle, since you'll need a cable with a ground. Depending on the draw of the dryer and the length of the run, you'll either have to pull new 10-3 /w ground or 8-3 /w ground cable.
As @TheEvilGreebo pointed out, you may be able to simply swap out the cord on the dryer for a 3 prong version. Check the manufacturers documentation to verify the procedure, and to make sure your model supports this.
This schematic for an electric dryer might help you understand how the dryer could be wired (depending on make/model/manufacture date).
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EDITED for clarification:
The three prong plug has two hot legs for the two bus voltages to add up to 220 volts, plus a neutral wire. Prior to 1998 the neutral was also grounded to the dryer case. A four prong plug has the two bus connections, a ground, and a neutral return. Often these are used if the dryer circuit requires 120 volts at some point, the current would flow from one bus and return through the neutral. If you were to change from a four prong to a three prong and your dryer is designed to use 120 volts at some point, then that current will have to be returned on the ground leg. This is not acceptable by NEC code, nor is it safe. If the 120 Volt circuit were to short, the full 120 volt potential could be present on the dryer case! Imagine touching the dryer at that time, and maybe on a wet floor!
If you can determine that the dryer does not require 120 volts, and that no current is returned on a neutral leg, then I see no reason not to use the three prong (although why does it have a neutral leg at all?). I would ONLY do the conversion if it is recomended by the manufacture. With todays economy I don't think that a manufacture would go to the expense of a four prong plug if it was not required.
The conversion kits mentioned by others may be to convert from a three prong dryer to a four prong receptacle. That is okay, being that the neutral wire is not used.
Wiki tells it like it is. "Modern appliances require 4 conductor cordsets (separate conductors or wires for line 1, line 2, ground and neutral). Generally, this setup is safer, because the current-carrying neutral is not connected to the dryer case. However, it is useless if the wall receptacle does not have a separate ground slot. In that case, you'll have to do the very simple procedure of converting a 4-wire dryer to a 3-wire setup. Conversion of these appliances in the U.S. is prohibited by the NEC (National Electric Code). The receptacle and wiring should be modernized to a 4 conductor arrangement instead."
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