In my apartment, I have a 4 wire 240V receptacle for a dryer where the neutral is not connected, only the 2 hots and ground. Is this configuration permitted for a dryer circuit? An electrician and maintenance person have both said the circuit is fine, but the electrician did point out that there is no box for the receptacle. How can the dryer get the necessary 120V to operate the panel and the other small parts in the dryer?
Here is a highlighted image of a random electric dryer schematic.
Notice that all the control circuits are 120V components, and that basically only the heater is 240V. Extending this image further, we can see how the dryer connects to a 120/240V split-phase system using a 3 wire cord.
Due to the nature of the 120/240V split-phase system, the grounded (neutral) and grounding conductors in a dedicated single appliance circuit are basically the same. The dryer will work just fine whether the
In the case of a three wire circuit, a NEMA 10-30R device should be installed.
When the wiring is connected to the proper device, the third wire in the cable becomes a grounded (neutral) conductor, and the code may be satisfied.
If you read point number three of the exception to section 250.140 of the National Electrical Code, you'll find that the neutral must be either insulated, or part of a Type SE cable. If this is not the case, your installation may still be a code violation.
An uninsulated, normally current carrying conductor running through your walls is typically a bad thing. Which is why this code exception is only valid, if the conductor is insulated.
Connecting a dryer in this way will work, but is a potentially dangerous code violation (according to the National Electrical Code).
If the receptacle is not of the self-contained variety, and is not in a box. That is defiantly a code violation (NEC 2011 406.5).
If a proper 4 wire cable existed, the schematic from above would look something like this...
If I've missed anything, or haven't explained something properly. Feel free to ask additional questions, or point out mistakes in the comments below.
Are you sure it's not two hots and a neutral, with the neutral also serving as the ground?
It is not allowed to connect the neutral to the casing of most appliances; however, the NEC specifically makes an exception for dryers:
If those four conditions are met, it is allowed to attach the neutral to the casing.
See this question for more information on grounding a dryer with three-prongs.
However, you specifically stated that it's the grounding conductor that's connected to both ground and neutral. If that's true, the above exception would not apply, since the outlet box (presumably?) would have both ground and neutral running to it.
I'll bet that the dryer's cord has the ground wire connected internally to neutral and also to ground. In fact, this is how 3 wire 120/240 appliances were wired up until rather recently (2002 if I recall correctly).
Unless the breaker is a GFCI, which is highly unlikely for a dryer, the non-separate ground and neutral is a minor issue. Technically, it is probably not to code. However, that could readily by fixed by replacing the receptacle and cord with a three wire and feigning the previous state of the art.
120 plus 120 = 240. Your panel has 120 AC on each side. AC stands for alternating current. A wave, not a line. Opposite sides of your panel have opposite wave signatures allowing the "hot" for one 120 to function as the neutral for the other.