We just moved and had to change plugs on the dryer from 3 prongs to 4.
What is the fourth prong for? I know about positive, negative and ground, but I can't think of why a fourth would be needed.
Electrical – the dryer’s fourth prong for
dryerelectrical
Related Solutions
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.
EDIT:
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.
EDIT:
This schematic for an electric dryer might help you understand how the dryer could be wired (depending on make/model/manufacture date).
This is an obsolete socket type called a parallel and tandem socket, and is designed to accept either standard parallel plugs without a ground or older tandem style plugs (again without a ground):
None of the connections is a ground - it consists of 2 pairs of hot and neutrals, one oriented vertically, the other horizontally. If there is a ground wire to the box (which I doubt), this can be replaced with a standard 3 prong outlet. If not, you'd have to find a way to get a ground into the box.
Related Topic
- 4 prong Dryer cord not quite in all the way
- Electrical – Converting a Samsung Dryer from a 4-prong cord to a 3-prong cord
- Electrical – Dryer Outlet – 4 prong to 3 prong
- Electrical – Code requirements for moving 3 prong dryer outlet
- Wiring – Issues with 4 Prong to 3 Prong Conversion
- Electrical – I have 10/3 wire. Can I replace the 3 prong outlet with 4 prong for a dryer
- Electrical – Wiring a stove top infinite switch
Best Answer
Electrical Distribution
In the United States, most residential electrical service is what's known as a split-phase system. Which is a 3 wire, single phase system. The service entrance cable consists of two ungrounded (hot) conductors, and one grounded (neutral) conductor. Once at your house the grounded (neutral) conductor is bonded to earth, and a fourth conductor is added to the system. The fourth conductor is the equipment grounding; or simply grounding, conductor.
But why 4 wires?
A split phase system is aptly named, because the transformers secondary is center tapped (or split). This allows the system to provide different voltages, depending on how devices are connected.
120V
If a device only requires 120 volts it will be supplied using one grounded (neutral) conductor, either one of the ungrounded (hot) conductors, and one equipment grounding conductor.
240V
If a device requires 240 volts it will be supplied with two ungrounded (hot) conductors, and one equipment grounding conductor. In this case a grounded (neutral) conductor is not required, since the circuit is completed simply by connecting a load between the two ungrounded conductors.
120/240V
Some devices require both 120 volts, and 240 volts. These devices will be supplied with both ungrounded (hot) conductors, a grounded (neutral) conductor, and an equipment grounding conductor.
But wait... The old one only had 3 prongs?
In the olden days, we didn't care as much about safety (there were fewer lawyers around). Before houses were wired with the equipment grounding conductor, devices that required both 120 volts and 240 volts would be supplied with both ungrounded (hot) conductors and a grounded (neutral) conductor. Since there was no equipment grounding conductor, the circuit lacked one. This is why older homes only have 3 prong dryer (and stove) receptacles. The old three prong receptacles are NEMA 10 devices, and consist of two ungrounded (hot) conductors and one grounded (neutral) conductor (hot, hot, neutral).