2 dryers

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Jim W in Tampa

Jim W in Tampa

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Tampa Florida
If in a home i add a new dryer outlet it needs to be 4 wire.That part is easy.Question is what if the new dryer sits right next to and old dryer that is 3 wire .Will there be a hazard ?
 
It would be no more dangerous than it was before the new dryer was installed, unless of course there was no grounded surfaces with in touching distance before.

Roger
 
roger said:
It would be no more dangerous than it was before the new dryer was installed, unless of course there was no grounded surfaces with in touching distance before.

Roger
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They both are being fed from a subpanel.If they touch i am thinking i might cause a hazard.
 
If fed from a _subpanel_, then the old circuit _must_ be changed to a 4 wire circuit.

See 250.140 Exception Condition(3)...the branch circuit must originate in the service panel. (Note: there is some debate of the wording on this point, with some arguing that having a type SE cable with bare neutral forces the requirement of originating in the service panel.)

The safety issue is that the frame of the dryer will be energized, not to the voltage drop of its own neutral circuit, but to the voltage drop of the feeder to the subpanel.

I believe, but cannot confirm, that using a three wire circuit for dryers has _never_ been permitted from a subpanel, and that this use is not grandfathered.

-Jon
 
I think it is more of a hazard than the 3-wire dryer was by itself. I am not certain whether it would present a shock hazard severe enough to be barely noticeable, or whether it would be worse. But I would not do it. I would take the opportunity to replace the branch circuit to the old dryer.

I drew a picture, and used it to come up with that conclusion. But the difference is hard to explain in words. I?ll try.

Start with a 3-wire dryer. The problem starts with the fact that the dryer is likely to have some neutral current. The least resistance path for that neutral current to take (i.e., back to the source) is via the neutral wire. But the neutral point in the dryer is connected to the frame, so there may be parallel paths. One might be from the neutral point, to the frame, to your hand, through your body to your feet, to the floor, to the dirt below the floor, to the dirt surrounding the ground rod, up the GEC to the N-G bond point in the main panel, and now you are back at the source. Two things are worth noting. One is that planet Earth is part of the path. That?s a high resistance path, and it cuts down the amount of neutral current that can flow along the path. The other is that the neutral current is small to begin with. That is, it is not driven by the entire 120 volts or 240 volts that serves the dryer. It is the combination of these two reasons that causes the current to be so small as to be undetectable. That is why there is no shock during normal operation.

Now consider a 3-wire dryer and 4-wire dryer side by side. Put one hand on each dryer. It does not even matter if the 4-wire dryer is running. There can be a path for neutral current to flow, as follows: From the neutral point (of the 3-wire dryer) to the frame, to your right hand, through your chest to your left hand, to the frame of the 4-wire dryer, via the EGC to the N-G bond point, and now you are back at the source. Now please note that planet Earth is no longer part of the path. Instead, a low-resistance conductor (the EGC) takes the place of dirt in this scenario. The current will be higher; of that I am certain. But whether it will be enough to be a real danger, I cannot say without taking time (that I don?t have just now) to do some math. Perhaps someone else can chime in here.

If you are running new wire for the new dryer, then the simplest solution would be to place a small, 2-breaker panel near the dryers, run one circuit to it, and run separate circuits to two dryer receptacles.
 
If fed from a _subpanel_, then the old circuit _must_ be changed to a 4 wire circuit.
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I agree, also the cable must be a type SE cable, so a old installation with NM cable is not legal.


I believe, but cannot confirm, that using a three wire circuit for dryers has _never_ been permitted from a subpanel, and that this use is not grandfathered.
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The oldest code book I have is the 1978, and the requirement in that code, is that the SE cable originate at the service equipment.

Chris
 
Jim W in Tampa said:
They both are being fed from a subpanel.If they touch i am thinking i might cause a hazard.
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I agree with the others, the original installation is in violation being that it is fed from a sub-panel and needs to be changed to a four wire circuit as well.

Roger
 
If the existing receptacle is fed with 2-wire SE cable I agree with the other here. If it is fed with 10-3 NM with or without EGC I say the existing installation is code compliant. The commentary in the 93 Handbook also states that the circuit only needs to originate from the service panel if type SE cable with a bare neutral is used. I realize the commentary is not code but??..

From the 93 NEC:

250-60(c) The grounded conductor is insulated; or the grounded conductor is uninsulated and part of a type SE cable and the branch circuit originates at the service equipment.
 
curt swartz said:
If the existing receptacle is fed with 2-wire SE cable I agree with the other here. If it is fed with 10-3 NM with or without EGC I say the existing installation is code compliant. The commentary in the 93 Handbook also states that the circuit only needs to originate from the service panel if type SE cable with a bare neutral is used. I realize the commentary is not code but……..

From the 93 NEC:

250-60(c) The grounded conductor is insulated; or the grounded conductor is uninsulated and part of a type SE cable and the branch circuit originates at the service equipment.
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They both are 10-3 w/g.The fix is not hard.What i was thinking is along the lines of what Charlie is saying.What i have is an old install that is legal and a choice of breaking the code and installing the new one on 3 wire or changing the original.Its my own house and i do want it safe.Hardest part will be buying another cord and receptacle (ouch).

How would an inspector handle this ? The old is legal and so is the new.
 
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I bet that we could go back and forth all day on the specific requirements of 250.140. I previously posted on this topic:
http://www.mikeholt.com/code_forum/showpost.php?p=625762&postcount=13 and it is clear to me that there are 2 reasonable interpretations of this bit of code. I won't try to argue the code interpretation, but would extend Charlie's reasoning.

In the case of a three wire circuit feeding the dryer, the frame of the dryer will be energized at the level of the voltage drop in the branch circuit neutral, as Charlie discussed. Placing a nice chunk of ground bonded metal next to this dryer introduces a pretty significant potential parallel path, again as Charlie discussed. Now change the situation to a three wire circuit fed from a subpanel.

In this new analysis, the voltage on the frame of the dryer is not simply the voltage drop in the branch circuit neutral; additionally you have any voltage drop in the subpanel feeder neutral. If there were any metallic connection between the two dryers, you would have ground and neutral connected in a subpanel, a clear no-no.

IMHO an inspector would be in a position to argue strongly that the original installation was not code compliant, but it wouldn't be a slam dunk code citation, because of the alternate interpretations.

IMHO since you have the EGC available and so it is a simple matter of changing receptacle and dryer cord, then it is a no brainer: make the change; this is what I would do in my own house.

-Jon
 
Jim W in Tampa said:
Its my own house and i do want it safe.Hardest part will be buying another cord and receptacle (ouch).
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A simple and inexpensive way to address your concern would be to simply bond the two dryers together with a piece of #10 THHN. :wink:
 
raider1 said:
If the existing installation is code compliant and the new is code compliant then there is no problem.
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It?s not that simple, Chris. Article 80.9(B) is the NEC article that permits an existing installation that does not comply with the current code to remain in operation. It?s in the Annex now, so it is not even a mandatory article. But it does require that the AHJ be satisfied that the non-conforming condition not present an imminent danger to occupants.

In this situation, Jim has an existing condition that would not comply with current codes, if it were to have been installed today. By itself, that does not present an imminent danger to occupants. Now Jim plans to install another receptacle for another appliance, and plans to do it in accordance with current codes. Though I will quickly concede that I am not Jim?s AHJ, in my opinion that new installation, by virtue of its proximity to an existing installation, will present an imminent danger to occupants.

Therefore, if I were the AHJ, I would not permit this new installation, unless the existing installation were brought up to current codes at the same time. And if, by chance the local authorities have invoked Annex G as a part of their local requirements, I would be justified in citing 80.9(B) as the basis for a red tag.
 
charlie b said:

Now consider a 3-wire dryer and 4-wire dryer side by side. Put one hand on each dryer. It does not even matter if the 4-wire dryer is running. There can be a path for neutral current to flow, as follows: From the neutral point (of the 3-wire dryer) to the frame, to your right hand, through your chest to your left hand, to the frame of the 4-wire dryer, via the EGC to the N-G bond point, and now you are back at the source. Now please note that planet Earth is no longer part of the path. Instead, a low-resistance conductor (the EGC) takes the place of dirt in this scenario. The current will be higher; of that I am certain. But whether it will be enough to be a real danger, I cannot say without taking time (that I don?t have just now) to do some math. Perhaps someone else can chime in here.

Charlie...good point.
What is the difference between your installation and one having a grounded washing machine sitting beside a 3-wire dryer?
That's pretty common (my home included).
If the 3-wire dryer loses it's neutral and you touch it and the washing machine at the same time you're going to get shocked.
Requiring 4-wire dryer circuits will eventually eliminate this possible threat, but what about the thousands of existing circuits?
just a thought
steve
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charlie b said:
It?s not that simple, Chris. Article 80.9(B) is the NEC article that permits an existing installation that does not comply with the current code to remain in operation.
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I agree,
my comment was more directed at the statement "How would an inspector handle this ? The old is legal and so is the new." in regard to both the existing and the new installations being code compliant.

Chris
 
hillbilly said:
What is the difference between your installation and one having a grounded washing machine sitting beside a 3-wire dryer?
That's pretty common (my home included).
If the 3-wire dryer loses it's neutral and you touch it and the washing machine at the same time you're going to get shocked.
Requiring 4-wire dryer circuits will eventually eliminate this possible threat, but what about the thousands of existing circuits?
Click to expand...

Steve I like your quote. The only thing I see is, that entire situation was code compliant at the time of install. Now that the code is refining itself we have these interesting issues. I think the real question is, if you had to replace your 3 wire dryer would you take the time and money to replace it with a 4 wire installation? I would, regardless of the AHJ.
 
gary said:
A simple and inexpensive way to address your concern would be to simply bond the two dryers together with a piece of #10 THHN. :wink:
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Please rethink what that does.I know you mean well but that defeats the subpanels seperate ground and neutral.
 
hillbilly said:
charlie b said:

Now consider a 3-wire dryer and 4-wire dryer side by side. Put one hand on each dryer. It does not even matter if the 4-wire dryer is running. There can be a path for neutral current to flow, as follows: From the neutral point (of the 3-wire dryer) to the frame, to your right hand, through your chest to your left hand, to the frame of the 4-wire dryer, via the EGC to the N-G bond point, and now you are back at the source. Now please note that planet Earth is no longer part of the path. Instead, a low-resistance conductor (the EGC) takes the place of dirt in this scenario. The current will be higher; of that I am certain. But whether it will be enough to be a real danger, I cannot say without taking time (that I don?t have just now) to do some math. Perhaps someone else can chime in here.

Charlie...good point.
What is the difference between your installation and one having a grounded washing machine sitting beside a 3-wire dryer?
That's pretty common (my home included).
If the 3-wire dryer loses it's neutral and you touch it and the washing machine at the same time you're going to get shocked.
Requiring 4-wire dryer circuits will eventually eliminate this possible threat, but what about the thousands of existing circuits?
just a thought
steve
Click to expand...


I never gave that a thought.Its been that way for many years.Guess we were lucky.When i went to big orange for the cord my thinking was why not make it legal for the extra few bucks.But rest assured the old will be brought up to code.
Click to expand...
 
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