Dryer/Range and 3vs4prong cords

[mhi]

I wanted to know if someone could explain/describe for me the reason that range and dryer appliance cords transitioned from a 3-prong for many years to a 4-prong type. I have been consulting my electrical inspection manual from my training and wanted to make sure I have it right. My perception is that the 4-prong circuit now required by code for new construction, provides an insulated ground and insulated neutral conductor. The enhanced safety features of this advancement are specifically what? Please correct me if I am wrong, but my understanding is that the 3-prong system required the appliance to be either grounded through an additional wire on the metal casing (bonding jumper to plumbing) or relying on the neutral terminal to be connected by a strap to the metal frame. This 3-prong scenario allows the current to return back to the service (through the neutral wire allowing the return portion of the circuit)as well as preventing an electrified frame?
Now the 4-prong circuit prevents the potential for danger by eliminating the need for the terminal strap which could very easily be disregarded in the installation process, and/or easily lose contact during the lifespan of the appliance? Please correct me if this is an incorrect understanding. Is it accurate to regard the most critical scenario involving the appliance becoming electrified thus a shock hazard to the user? Would this shock be the same potentially life threatening current that one might receive unplugging a major appliance and getting fingers in contact with the receptacle? I have some experience with this last scenario as an appliance installer for a few years. Hopefully you can help clarify my understanding of this process. Additionaly I was reading Mike Holt's saga regarding his Frigidaire dryer and he seemed to be touching on the subject with his questions. Unfortunately the story did not come to conclusion so I don't know what he found out ultimately

 

[charlie b]

Re: Dryer/Range and 3vs4prong cords

The purpose of a ground wire is to prevent the user from being shocked, in the event that a wire inside the device (i.e., the dryer, or the weed whacker, or the power tool, or the kitchen appliance) breaks away and comes into contact with the case. If that happens, the ground wire will provide a low-resistance path back to the source, and the resulting high current will force the circuit breaker to trip.

The neutral wires of every component in the entire facility and their respective ground wires are all connected together within the main service panel. This is usually done by landing all neutrals on one bar, landing all ground wires on another bar, and installing a bond wire between the two bars.

If you connect the neutral and the ground together anywhere else in the facility (for example, internal to an electric dryer), then the current flowing in that component's neutral (back to the source) will also flow in the ground wire (again, back to the source). That is not a good thing. It may mean that all "Grounded" surfaces may become energized (perhaps with a small, and not dangerous voltage, but energized nonetheless).

The older 3-prong dryer cords did not include a separate ground wire. So the case of the dryer is connected (by the wire you have described) to the neutral of the dryer. Not an ideal circumstance, and that is why newer homes have the 4-wire outlets. The house I am renting has an old electrical system, and the dryer outlet is 3-prong. I just bought a dryer. The installers connected a 3-prong cord, and I presume that they also connected the ground and neutral within the dryer. I didn't ask them that question.

But if I were to look into buying this house, and if a HI like yourself were hired to do an inspection, I would expect you to take note of the 3-prong dryer outlet. I would expect it to show up on your report as a statement of fact, but not as an item the must be corrected. It is not a violation of the NEC.

You seem to have a good handle on the situation. But did this help?

 

[mhi]

Re: Dryer/Range and 3vs4prong cords

I appreciate the reassurance that my understanding is accurate. Is there a source to consult regarding the statistics or rationale that motivated the NEC to change to 4-wire circuits only since 1996 I believe? Is there a documented number of injuries/deaths as a result of electrified appliances? The 3-prong cords are still widely used, I imagine it will be at least another generation before they become a relic of outdated households. To this day two prong outlets can be found in homes (lived in and used)built prior to the 60's many utilizing adapters that do not provide ground protection.

 

[iwire]

Re: Dryer/Range and 3vs4prong cords

Originally posted by mhi:
Is there a source to consult regarding the statistics or rationale that motivated the NEC to change to 4-wire circuits only since 1996 I believe?

I am not sure there is, it's my guess this change was as driven as much by safety as it was driven by the use of the grounded conductor for grounding on the load side of the service disconnect being a NEC anomaly.

Originally posted by mhi:
To this day two prong outlets can be found in homes (lived in and used)built prior to the 60's many utilizing adapters that do not provide ground protection.

In many rooms of the house those two wire outlets are all thats needed. You do not see many three wire cord caps in the living room, bedrooms or even bathrooms.

 

[mhi]

Re: Dryer/Range and 3vs4prong cords

The 2 prong outlets "work" but do not prevent shock hazards. Electrified light fixtures are not uncommon in these settings.

 

[iwire]

Re: Dryer/Range and 3vs4prong cords

Originally posted by mhi:
The 2 prong outlets "work" but do not prevent shock hazards. Electrified light fixtures are not uncommon in these settings.

Are we talking about wall and ceiling mounted light fixtures or are we still talking about 2 wire receptacles feeding cord and plug connected fixtures?

If we are talking cord and plug connected fixtures a three wire receptacle will not do anything.

I have never seen a residential lamp with a 3 wire cord.

 

[mhi]

Re: Dryer/Range and 3vs4prong cords

Granted the lamp cords are two prong, but plug the vacuum cleaner in using the adapter and you have another scenario. Come to think of it why aren't lamps equipped with 3-prong cords?

 

[rattus]

Re: Dryer/Range and 3vs4prong cords

In the case of a shared neutral and EGC, an open neutral would allow the cabinet of the dryer to be at 120V through any 120V loads--motor, timer, pilot lights, etc. The impedance of even a timer motor would be low enough to pass lethal currents through some unsuspecting user.

With the neutral and EGC separate, the 120V loads merely quit working if the neutral is opened.

That is the safety issue as I see it.

 

[rattus]

Re: Dryer/Range and 3vs4prong cords

Table lamps and floor lamps are rarely used where one could ground one's body, but if they are, they could be as lethal as the metal case power tools with no EGC.

The double insulated tools and appliances of today offer excellent protection unless one tries to use them underwater.

 

[mhi]

Re: Dryer/Range and 3vs4prong cords

Which brings us back to Mike Holt's search for answers from Frigidaire. How can one provide protection with a 3-wire cord?

 

[rattus]

Re: Dryer/Range and 3vs4prong cords

Originally posted by mhi:
Which brings us back to Mike Holt's search for answers from Frigidaire. How can one provide protection with a 3-wire cord?

Merely sharing EGC and neutral in a 3-wire system provides protection as long as the neutral stays intact, and with a home run and copper wire this is seldom a problem.

It is the occasional case where the neutral is opened that proection is lost; in fact what is meant to protect creates a hazard.

 

[charlie tuna]

Re: Dryer/Range and 3vs4prong cords

as the code changed from two wire receptacles to three wire grounding receptacles -- the clothes dryer was becoming popular. they used to have this thing they called "a clothes line" --- every house had them!!! most of the original dryers had 120 volt motors to turn the drum and feed the controls. since this appliance wasn't very "portable" and the fact that if the neutral conductor was lost the unit would not function due to the loss of control power -- the code allowed dryers and welders to use the neutral conductor as a ground. about this time wash machines were sold with a grounding kit since they still had two wire caps on their cords. the grounding kit had instructions--six feet of bare wire and a ground clamp. and the manufacturer also included a grounding plug adaptor for the new homes that had three wire receptacles for their washing machine. if the homeowner, with two wire receptacles didn't install the ground kit correctly and something grounded out to the washer's frame they would get shocked! as the next generation of wash machines came out it was standard to have a three wire cord on them.

i remember one of my first service calls was from a guy who worked for the airlines. his wife refused to wash clothes because she was getting shocked when she touched the washer and dryer!! he had gone to his credit union and got a loan for one thousand dollars to cover the costs of the service call. his house was in an older section of town -- two wire receptacles and the ground adaptor was not installed correctly... allowing the washing machine frame to become energized. she was being shocked by 120 volts between the washer's frame and the grounded (through the neutral) dryer's frame.

we all know it's a safer installation with a four wire cord -- with the code change we are going through another transition of matching the new with the old or the old with the new. after a new generation of houses --- the four wire will be standard --- but it's still code to have a three wire receptacle in an existing home. if the home inspector writes it up -- i think he is wrong! he should know the code and when this change took effect!

 

[iwire]

Re: Dryer/Range and 3vs4prong cords

Originally posted by charlie tuna:
but it's still code to have a three wire receptacle in an existing home. if the home inspector writes it up -- i think he is wrong! he should know the code and when this change took effect!

I disagree, the HI is not there as a code enforcement official.

I do not believe an HI should say that an existing 3 wire dryer outlet is a 'code violation' I do believe the HI should say something along the lines of "The current code" would require this outlet to be 4 wire"

An HI can make recommendations regarding safety well beyond the current NEC.

They could for instance point out that a kitchen now is required to have GFCI protection.

None of what the typical HI writes up is binding it can be ignored, used as a bargaining tool or the buyer may just walk away.

The HI is supposed to be the buyers representative, they are not there to make friends with the seller or the Realtor.

 

[stud696981]

Re: Dryer/Range and 3vs4prong cords

In regards to why there were only three wires for ranges and dryers, I was told (also read in one of my electrical books) that back around World War II there was a copper shortage. In order to save a little, the code allowed the three wire installation and the frame was bonded to the neutral. This was only allowed when the circuit originated from the main service panel, not a sub panel. Later after the copper shortage ended the code stayed this way and was not updated on this until around 1996.

 

[iwire]

Re: Dryer/Range and 3vs4prong cords

Originally posted by stud696981:
In regards to why there were only three wires for ranges and dryers, I was told (also read in one of my electrical books) that back around World War II there was a copper shortage.

Stud many of us would be grateful to get the real answer on this.

It may be that the war effort was the reason or it may be other reasons.

If the NEC had already required 4 wire outlets for ranges and dryers it seems unlikely in my opinion they would change the rule to save some wire.

Why not change the ampacity tables, or not require separate EGCs for panels? These actions would have also saved copper.

In my opinion there is more to this than the war needs.

Keep in mind that during the war the government simply rationed goods the the war needed, no reason to change the NEC.

 

[petersonra]

Re: Dryer/Range and 3vs4prong cords

one of my old code handbooks explained the reason that this was allowed was because it had been shown to be perfectly safe over time. That made some sense to me. why screw with something that is proven safe over a long period of time and saves money to boot.

 

[stud696981]

Re: Dryer/Range and 3vs4prong cords

Iwire,

I just looked up this issue in one of my residential books (Electrical Wiring Residential by Ray Mullins, 1996 edition). It states "By way of the Tentative Interim Amendment No.53, this special permission was put into effect in July 1942, and was supposedly an effort to conserve raw materials during World War 2."

Perhaps someone else can do a search on the web or something. The story appears legit, but I guess it could also be an urban legend so perhaps someone else could shed some light on this.

 

[rattus]

Re: Dryer/Range and 3vs4prong cords

Originally posted by petersonra:
one of my old code handbooks explained the reason that this was allowed was because it had been shown to be perfectly safe over time. That made some sense to me. why screw with something that is proven safe over a long period of time and saves money to boot.

Bob, fact is, this practice is not perfectly safe. It is safe 99.99% of the time, and the 0.01% can be lethal.

 

[iwire]

Re: Dryer/Range and 3vs4prong cords

Originally posted by stud696981:
Iwire,

I just looked up this issue in one of my residential books (Electrical Wiring Residential by Ray Mullins, 1996 edition). It states "By way of the Tentative Interim Amendment No.53, this special permission was put into effect in July 1942, and was supposedly an effort to conserve raw materials during World War 2."

Thanks, That is the closest thing I have seen to an answer.

 

[petersonra]

Re: Dryer/Range and 3vs4prong cords

Originally posted by rattus:

Originally posted by petersonra:
one of my old code handbooks explained the reason that this was allowed was because it had been shown to be perfectly safe over time. That made some sense to me. why screw with something that is proven safe over a long period of time and saves money to boot.

Bob, fact is, this practice is not perfectly safe. It is safe 99.99% of the time, and the 0.01% can be lethal.

Actually, the handbook does not say "perfect" its says "good".

The way you were required to wire it made it highly unlikely that a problem would ever develop that would create an unsafe condition.