4 prong dryer receptacle, 3 prong cord

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  #1  
Old 07-17-02, 11:45 AM
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Omega
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Question 4 prong dryer receptacle, 3 prong cord

We recently moved into a new house. The dryer receptacle has has four openings, and our old dryer has three prongs. Is it safe and legal to simply install a three prong receptacle or put a four prong cord on the dryer, or is this going to require new wiring from the box?
 
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  #2  
Old 07-17-02, 02:12 PM
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Matt Marsh
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Omega,

The national electrical code now requires a 4-wire circuit for clothes dryers and ranges. Replacing your present 3-wire cord with a 4-wire cord is the only code compliant choice that you have.

If you will be replacing the cord yourself, you will need to remove and discard the bonding jumper that runs between the neutral terminal where the cord hooks up, and the metal chassis of the dryer. The neutral wire in the new 4-wire cord will terminate to the neutral terminal as it did with the 3-wire cord, but the grounding conductor in the new 4-wire cord will terminate to the metal chassis where the bonding jumper use to attach. Matt
 
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Old 07-17-02, 03:52 PM
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Art. 250.140 allows a 3-prong cord-plug and the dryer frame Grounded to the Neutral for EXISTING Branch-circuits with (1) a #10 conductor-(2) an insulated Neutral (the White wire) and (3) if the Dryer circuit-breaker is in the same panel as the Service Dis-Connect.If you satisfy these stipulations you can re-place the 4-prong receptacle with one that matches dryer cord-plug.Standard electric dryer circuits use a #10 conductor.Good Luck!!!
 
  #4  
Old 07-17-02, 04:02 PM
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Matt Marsh
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PAT,

No, he cannot change the receptacle and still be compliant. You quote the correct article, but this very article says the opposite of how you've interpreted it.

Article 250.140 applies to "exsisting" 3-wire branch circuits. This circuit is not, it is an exsisting 4-wire branch circuit. If the "exsisting" branch circuit were 3-wire, the dryer cord can remain a 3-wire, but because this branch circuit is a 4-wire branch circuit, the dryer cord must be replaced with a 4-wire cord.
 
  #5  
Old 07-17-02, 05:21 PM
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Omega
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Matt,

Forgive my ignorance - in case you haven't guessed, I know next to nothing about this. When I remove the jumper that runs from the neutral terminal to the chassis, will it not disrupt the circuit? Or will the circuit flow through the terminal area?
Another way of asking the same question - how does the neutral wire (in the old setup) serve as both a ground and a conductor for the return flow? And why is it now necessary to run two wires for what was done by only one in the past?
 
  #6  
Old 07-17-02, 05:47 PM
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Matt Marsh
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Omega,

Clothes dryers and electric ranges use both 240 and 120 volts. The major portion however is the 240 volt loads. In the old days, copper was a very valuable comodity. Because the 120 volt loads are small, there use to be an exception in the NEC book to allow the equipment grounding conductor to carry this small neutral current. This exception did not apply to mobile homes, they have always been required to have 4-wire circuits for clothes dryers and ranges.

A few years back, the NEC realized that it was silly to continue this. On a 3-wire system, if for some reason the equipment grounding conductor that carries this neutral current were to accidently become lost or opened, there is a good chance that the only return path for this current is through a person touching the metal chassis of the dryer or range.

An equipment grounding conductor is not intended to carry current under normal operating conditions, only during a ground fault. In a 4-wire circuit, the two hots and the neutral carry the current for the loads being served, and the equipment grounding conductor is there only to provide a low impedance path for ground fault current back to the source. Its main function is to facilitate the opening of the breaker or fuse in the event a ground fault occurs. In the case of the jumper, it also only carries current in the event of a ground fault. Removing it will not interupt the circuit.

When the NEC made the change to require 4-wire circuits, this change would only effect new installations. It was not their intent to require us to rewire our exsisting 3-wire circuits that were compliant at the time they were installed (hence article 250.140). If your exsisting circuit is a 3-wire circuit, you do not have to re-wire your branch circuit just because you purchased a new dryer or range. Your case is the opposite however. Your exsisting circuit is a 4-wire circuit, it meets present day code requirements. Replacing the 4-wire receptacle with a 3-wire one would be a giant step backwards. Matt
 
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Old 07-17-02, 05:53 PM
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I agree with Matt. This is a new house wired to the NEC of 1996 or later. He has a four-wire service and he must stick with the four-hole receptacle he has. Besides, he wants to anyway because it will improve the safety of his dryer. And if he does it himself, it will take ten minutes and ten dollars.

Changing the cord is simple and inexpensive. Many places carry these cords, including any appliance store and even most stores like KMart and Walmart.

If you still have the installation instructions for your dryer, dig them out. Unless the dryer is pretty old, it will have instructions for both 3-wire and 4-wire installations. If you don't have the instructions, many dryer installation instructions are available on manufacturer web sites.

No, removing the jumper will not disrupt the current. This jumper was only in place in the event of a ground fault in your dryer. With your new 4-wire cord, the fourth wire will provide a path for that ground fault. This installation will improve the safety of your dryer. You should be grateful for the 4-hole plug.

If you don't care to do this yourself, I'm sure any appliance service center will do this conversion for their minimum charge, plus the cost of the new cord. You could probably even take the dryer down to the appliance store and safe the trip charge if you wanted.
 
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Old 07-17-02, 05:53 PM
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I also side with the replacement of the plug (not the receptacle). Follow the manufacturers instructions that you will get with the replacement plug/cord, which will review the seperation of the neutral and chassis connection. This will result in a safer installation, as well.
 
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Old 07-17-02, 06:46 PM
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Omega
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A couple more questions.

First, you say that the neutral wire in the three prong cord, which also served as a ground, carried little current, if any. If I understand the volumn of information on the Grounding Argument thread correctly, the current is approximately the same returning to the source from whence it came. Since it does not return through the neutral wire, it must return through the two hot wires. Is this correct?

Second, I know the dryer works when the neutral wire is not removed as was recommended above and remains attached from the terminal to the chassis. Is it then serving as a second ground instead of a conductor for the return flow? And is this creating a hazardous situation, or is it simply double protection?

Incidentally, the dryer is very old, and the instructions are unavailable with the dryer or on the manufacturers website, and the instructions that came with the cord were about two sentences long and basically said to consult the manual for the appliance itself.
 
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Old 07-17-02, 07:14 PM
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Since it does not return through the neutral wire, it must return through the two hot wires.
Yes, if you have 20 amps on each hot, you will have no current at all on that neutral. But if you have 20 amps on one hot and 22 amps on the other hot, you will have 2 amps on the neutral. I don't necessarily agree with the assertion that there is very little current on the neutral -- sometimes it might not be all that little. The drum motor on most dryers is 120-volt, and all the current it takes to run it will return on the neutral (not sure how much that is).

Second, I know the dryer works when the neutral wire is not removed as was recommended above and remains attached from the terminal to the chassis. Is it then serving as a second ground instead of a conductor for the return flow? And is this creating a hazardous situation, or is it simply double protection?
I assume you are talking about when using a 4-wire cord. In this case, you are using both the grounding wire and the neutral wire for returning neutral current (i.e., the grounding wire is serving as a second parallel neutral). It doesn't give you the advantages that you are due with a 4-wire cord, but it's no more dangerous than the 3-wire cord was. It is not double-protection, and it is not as safe as when done correctly as Matt described before. Finally, it's against code.
 
  #11  
Old 07-17-02, 07:19 PM
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Matt Marsh
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Omega,

First, in this case, the 3-wire cord does not have a neutral conductor, it is an equipment grounding conductor serving double duty as a neutral. For the purpose here, all circuits have required an equipment ground since very early in the 20th century, but all circuits do not require a neutral.

When you have a combination of 240 and 120 volt loads, the neutral carries only the imbalance of the loads on the two hots. In the case of a clothes dryer, the heating elements utilize 240 volts, the motor, timer, and other controls use 120 volts. This means that the loads on each hot are not the same, the neutral will carry the difference. For example, if the elements draw 15 amps on both hots, and the 120 volt loads draw another 5 amps on one of the hots for a total of 20 on one hot and 15 on the other, the neutral would only carry 5 amps. The only time the neutral would read zero is when there are identical sized loads on both hots (and of course when the circuit is off).

No, the neutral wire and the grounding wire are designed to do two entirely different things, they are not double protection. Again, the neutral wire is designed to carry circuit current under "normal" operating conditions, the equipment grounding conductor is designed to only carry current in the even a ground fault occurs. The neutral conductor is a circuit conductor just as the hot conductors are. The only difference is that it is bonded to the grounding system. The proper term for a neutral is actually "grounded circuit conductor". It originates from the windings of the transformer just as the hots do. The grounding system originates at the main service equipment.

When we speak of equipment grounding, we are refering to the equipment grounding conductors that run with all of the branch circuits, and also the bond that is made to the neutral conductor. Again, for all practical purposes, the main reason for the equipment grounding system is to facilitate the opening (tripping, blowing) of the fuse'(s) or circuit breakers. Matt
 

Last edited by Matt Marsh; 07-17-02 at 09:53 PM.
  #12  
Old 07-17-02, 08:07 PM
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Omega
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Outstanding! If I could only get this kind of response from my cell phone company I'd be a happy man. Thanks all for the assistance and education.

One project completed, 20000 more to go....
 
  #13  
Old 07-17-02, 09:30 PM
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FREDDYG_001
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Omega, Here are two examples with photos that will show the proper way of changing the cord for the two types of dryer connections today.


http://fixitnow.com/appliantology/dryercords.htm
http://forum.doityourself.com/showth...&theadid=96699


Fred
 
  #14  
Old 07-18-02, 06:30 AM
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Excellent references Fred. Your second link doesn't work as is, but here is a revised version that works.
 
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Old 07-18-02, 11:14 AM
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Thanks John!

Fred
 
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Old 07-18-02, 02:48 PM
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Omega
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Okay, everything's working perfectly.
Another question though, if you guys don't mind helping an electrical dummy learn something.
The reason I was confused at first was that Matt said, in his clear and simple explanation at the top, that I needed to remove the bonded jumper that ran from the neutral terminal to the chassis. When I looked at the wiring the bonded jumper wasn't attached to the neutral terminal. Now I know that the dryer has been in use for years (my brother had it) without incident. For my information (so I don't kill myself if I ever see this again), was the bonded jumper supposed to be attached? And if so, was my brother at grave risk all this time?
 
  #17  
Old 07-18-02, 03:05 PM
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Matt Marsh
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Omega,

Yes, if the dryer was equipped with a 3-wire cord, the jumper must be in place. Without it, the metal cabinet (chassis) would be ungrounded. This could have been a potentially deadly situation had a short to the cabinet occured. One probable scenerio could have been a person touching the "hot" cabinet with one hand, and the grounded metal cabinet of the washing machine sitting right next to it. Current would flow through one arm and out the other, right across the person's chest. If the jumper was in place, a short to the chassis would have caused the breaker to trip. Matt
 
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