I am an electrical student, actually having finished my course work now. I have worked with my electrician family for hands on experience, yet there are still some things I come across that throw me off a little. Today, I added a 4 prong 240v dryer receptacle for a friend. This is a task I have done many times. In fact, this is something I seem to need to do more than anything else. Well, I took off the old 3 prong receptacle, and when I did I noticed the cable was a three wire cable. I tested them to ensure their function. Here were the wires: one black, which was a hot 120v; one white, also a hot 120v; and one bare copper wire, (non current-carrying, no voltage) which was being used as a neutral on the old receptacle. Now, it's clear that the house was wired previous to the 1996 code changes, which now require the four wire, four prong setup on dryer circuits consisting of two line, one neutral, and one bare grounding wire. Anyway, I wired the new four prong receptacle as follows: the white and black hots into their corresponding slots, and then I wired the bare copper wire into the neutral slot, considering that it (the neutral) is sometimes needed to carry unbalanced currents. I, of course, left the grounding slot of the receptable open. Was this correct? My textbook says that it is not required that new cable be installed if one runs into this situation, only that the electrician use a "harness" to tie the neutral somehow to the ground, which would then tie them together to the frame of the dryer. Should I make a tie/bond between the neutral and gounding slot, as technically there is not grounding protection on the dryer scheme as of right now. Of course, the dryer works just fine, but I want to do this according to "the book." Please help. I very much appreciate it.
[This message has been edited by Chester (edited 04-24-2004).]
Bob, are you saying then that I should have just kept the old 3 wire (with no grounding slot!) receptacle on there, and then simply changed out the dryer cord from modern four prong, to outdated 3 prong? Would I not simply be dropping down in safety level by doing this? As I said, the code gives my permission to do this, if I adequately tie the frame via ground to the nuetral slot of the receptacle. Are you getting what I am saying? Please tell me where I'm going wrong. I can paste my textbook instruction I am basing my statements on, if you would like. Thanks....
Chester you have not raised the safety level at all as you sill have only three wires heading back to the panel.
If you want the safety level to increase run a four wire cable and install your new 4 wire outlet.
You have created a situation where the next person that comes along sees a 4 wire grounding type receptical and plugs in a dryer set up for that.
This new dryer will run great with no grounding.
250.140 Frames of Ranges and Clothes Dryers. This section shall apply to existing branch-circuit installations only. New branch-circuit installations shall comply with 250.134 and 250.138. Frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for these appliances shall be grounded in the manner specified by 250.134 or 250.138; or, except for mobile homes and recreational vehicles, shall be permitted to be grounded to the grounded circuit conductor if all the following conditions are met.
(1)The supply circuit is 120/240-volt, single-phase, 3-wire; or 208Y/120-volt derived from a 3-phase, 4-wire, wye-connected system.
(2)The grounded conductor is not smaller than 10 AWG copper or 8 AWG aluminum.
(3)The grounded conductor is insulated, or the grounded conductor is uninsulated and part of a Type SE service-entrance cable and the branch circuit originates at the service equipment.
(4)Grounding contacts of receptacles furnished as part of the equipment are bonded to the equipment.
Bob Badger Construction & Maintenance Electrician Massachusetts
From your description of the cable it sounds like it is type NM cable. If that is the case this installation was never code compliant. The bare wire in type NM cable is not to be used as a grounded conductor. I agree with Bob that you haven’t made this installation any safer and have made the problem worse. The frame of the dryer is not grounded at all now. I recommend getting a qualified electrician to replace the cable and install the 4-wire receptacle.
Thanks for the continued help here Bob. Please tell me if I am right - I have two choices at this point. 1) Put back on the 3 prong receptacle and swap out the current four-prong dryer cord to a 3 prong or 2) install a 4 wire cable with two hot, a nuetral, and a grounding conducter....correct? Thanks again Bob.
Re: 3 wire dryer circuit question...#37302 04/25/0403:46 AM04/25/0403:46 AM
The problem with the 3-wire set-up is that the current-carrying neutral is also used as the protective grounding conductor.
That means that in normal use, if there should be a break or bad connection on that neutral, then the frame of the dryer could become energized at 120V via the control circuits.
Swapping the 3-wire receptacle for a 4-wire one while leaving only a 3-wire feed doesn't really accomplish much. If you had linked the neutral wire to both the neutral and ground connections of the new 4-w receptacle (which I believe is against code anyway), then all you've done is make the system 4-wire from the receptacle to the dryer. A bad neutral on the feed would still result in the same situation as described above.
Leaving the grounding terminal open on the new receptacle means that there's no protective ground on the dryer at all now.
Thanks Paul, I gotcha there. See, I have already inherited a bad situation here. The cable being used was inadequate, it was meant for a 120v circuit and not a 240v. There are three wires in it, a black, a white, and a bare copper. The guy who did the install was using the black (hot) as a 120v hot in the receptacle, the white (supposed the be the neutral in 120v circuit) as the other 120v hot, and he was using the bare GROUNDING conducter as the neutral! Incredible! I was a bit thrown off when I saw this, as I have done this job numerous times for folks, and never saw this terrible scheme being used. Well, this shows me it was bad to begin with, with the wrong type of cable being used to begin with. As I see it, I only have one choice here: to install a new proper four wire cable with the new standard four slot receptacle. This is the only choice I see. Am I correct? What do you guys think? Thanks for sticking with me here.
[This message has been edited by Chester (edited 04-25-2004).]
[This message has been edited by Chester (edited 04-25-2004).]
Chester.. Thats the way I'd do it... 10/3w/ground from the 30A 2pole at your panel, Make up your 4 wire recept with the proper connections & you should be in the green A bare conductor in romex is not meant to be a current carrying conductor, therefore I'd certainly rerun this with the proper amount of conductors