Why do some 240V circuits need only a 2-conductor cable with ground, while others require a 3-conductor cable with ground?
New knowledge is priceless.
Used knowledge is even more valuable.
Discussion Forum
Discussion Forum
Up Next
Video Shorts
Featured Story
Fine Homebuilding's editorial director has some fun news to share.
Featured Video
Highlights
"I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.
Replies
No expert here, but learning...
One reason is if appliance utilizes 120v as well as 240v. A range, for example. If my understanding is correct, the 120v portion requires the neutral (fourth wire)
Bill Hartman can shed more light, I'm sure.
Everything will be okay in the end. If it's not okay, it's not the end.
Two conductor circuits offer 240 volts between the two conductors. Three wire circuits offer both 240 between the hots and 120 between either hot and the neutral.
As stated above, for detail, theory, and massive info on implementation, see Sir Bill. ;)
Scott.
The other have the basic.
US residential power comes from a transformer with a center tap on the winding. So you get 120 from each leg (hot) to the neutral. But they are also out of phase. So that when one leg is going positive the other is going negative and together they add up to 240.
Now many loads that are pure motors or heaters run only on 240, so they don't need the neutral.
That includes things like AC, 3 HP motors for wood workign equipement, baseboard heaters, etc.
The only common equipment that needs both 120 adn 240 are stoves and dryers. And those mostly for historical reasons and limited real need.
Stoves used to have burners with 2 heating elements in them and they where connected one by one and in series and parallel across 120 to 240 with 7 position switches to get the different heat ranges.
Now days the only real thing that needs 120 is for the oven light. The controls can be made to work on either.
Likewise the dryer uses 120 for the controls and motor. But my guess is that the first ones used a commonly 120 motor that someone had. They could have as easily used a 240 motor.
William the Geezer, the sequel to Billy the Kid - Shoe
Thanks for clearing this up for me.New knowledge is priceless.
Used knowledge is even more valuable.
Something that was not mentioned: I believe that a lot of the older appliances such as stoves with a 120 convenience plug and dryers with a 120v timer tied the chasis and neutral in the classic two hots and ground/neutral (commonly called three wire vs the current (no pun) four wire design.
This can be stated more clearly I am sure.For those who have fought for it Freedom has a flavor the protected will never know.
Right. I learned about this setup (and about the reason for keeping ground and neutral separate) when I was maybe 12 and got shocked from our stove. The shock hazard is why the 3-prong scheme is no longer allowed in new installations.
The mark of the immature man is that he wants to die nobly for a cause, while the mark of a mature man is that he wants to live humbly for one. --Wilhelm Stekel
I've been shocked by older stoves too, but I never understood why. Are you saying that they used to bond the neutral and ground conductors inside the stove?That would be tantamount to installing a subpanel with the ground/neutral bond in place. A big no-no these days. Why did they do that?Scott.
Edited 12/22/2008 2:26 am by Scott
Yep.They didn't know any better, I guess. They should have changed the code in the 60s, but it took until the 90s, IIRC. Kind of odd how they're anal about other details but left this gaping hole for so long.
The mark of the immature man is that he wants to die nobly for a cause, while the mark of a mature man is that he wants to live humbly for one. --Wilhelm Stekel
"I believe that a lot of the older appliances such as stoves with a 120 convenience plug and dryers with a 120v timer tied the chasis and neutral in the classic two hots and ground/neutral (commonly called three wire vs the current (no pun) four wire design."I don't know how long the current system has been around.The NEC only changed in 96. And while it grandfathers in the 3 wire system it is only for ones connected directly to main panels. IE, apartments and mobile homes require a 4 wire connection. And I understand many local codes outlawed the 3 wire systems earlier.I bought a USED dyer in 79 when I moved into my house. And it was adaptable to either, just like current models.So I am guessing that this started happening when the grounded circuits started becoming common in the 60's..
William the Geezer, the sequel to Billy the Kid - Shoe
Bill or anyone else knowledgable about electrical stuff: I have a follow up question.
I just completed a new house and the dryer receptical had a four prong set up and I had to buy another cord for my older dryer.
No problem with that but now with winter here I'm focused on getting a portable generator. The house has the seperate panel and the inlet box outside on the front porch all ready for the cord from the generator.
All the generator's I've looked at have the 4 prong female 240V connnection while the inlet box (made by Reliance) has a 3 prong male connection. All the inlet boxes I've looked at online have a 4 prong configuration.
Should I have a 4 prong male connection in the inlet box or will the 3 prong work? I had it all installed by a licensed electrician (same guy that did the whole house) and he knew all about having to modify the older dryer.
Thanks,
Runnerguy
Generators are a confusing area.And I am not sure that I have it all right.The confusion is over whether the generator is equivalent to a service entrance or not. Now when used standalone, it is. And I think that I have heard that OHSA says that they should have a ground rod and the neutral and ground bonded at the generator.Some transfers switches switch the neutral. But I don't think that is common.And reading a similar discussion in another forum people where saying that many of the portable generators now come with a clearly marked bonding strap that can be removed when used for something like powering a house.Hopefully he ran the right cable or it easily accessed.What inlet did he install. Here is a place that has lots of different versions.http://www.stayonline.com/reference-nema-straight-blade.aspxhttp://www.stayonline.com/reference-nema-locking.aspxIn the straight blade they only go upto 20 amp..
William the Geezer, the sequel to Billy the Kid - Shoe
Thanks Bill. I'm going to give the electrician a call. There's going to be no problem with him as he's been accomodating during the construction, I was just trying to educated myself a little beforehand.
I guess I got a little confused because the inlet box is a Reliable PB50 http://www.steadypower.com/catalog/product_info.php?products_id=1525 and their photo shows a 4 pronged receptacle.
The one I have has 3 prongs looking mostly like this: http://www.stayonline.com/detail.aspx?ID=4433 except I've got two curved prongs with the little leg like their "G" prong and one prong without the little leg, just smooth. It says "50AMPS 125/250 so it looks like it's rated for the power I'll need.
All of the generators have 4 prong female connections for the cord.
In the generator product lit I've read it talks about grounding the thing when powering a whole house. I'm all set up inside with the transfer panel with an assortment of outlets, lights, the well pump, microwave, etc. on it. I just need to be sure I get the generator connection correct.
Runnerguy
I think that they are using a generic picture.http://www.steadypower.com/catalog/default.php?cPath=257_260Click on the PB15, 20, 30, 31, 50 and they are all identical although some are 3 wire (120 volts) and some are 4 wire (120/240)I think I figured what you got, but not 100% sure.If you look the L14-50 has 3 connections in the in the circle and a 4 round ground pin in the center. While the L14-30 has all 4 connectors in the circle.http://www.nooutage.com/nema_configurations.htmThis site does not list the L14-50, but a CS6364. That has a center pin, but I think that it is only for indexing. The ground is on the outer shell. And reverse mechanical polarity.http://www.islandgenerators.com/browseproducts/LL550C-CS6364-50-amp-Power-Cord-Connector.HTMLThe receptacle, female power terminals, has the mail part of the ground terminal.And that one has the legs on 2 of the terminals.Here is the whole line.http://www.passandseymour.com/pdf/I33.pdfThey are a non-nema California Standard.I have heard these mentioned in another forum. Not sure, but something about California Ohsa requiring locking connectors on commerical construstion site and these where developed before NEMA standards.Now Island Generators are based in FL and not CA. But there 50 amp cordsets go from non-locking 14-50 to the CA standard.http://www.islandgenerators.com/index.asp?PageAction=VIEWPROD&ProdID=414And I see that Gen-tran also has one
http://www.gen-tran.com/eshop/10Browse.asp?Category=50AAnd the remainder have CA standard plugs and receptacles on both ends..
William the Geezer, the sequel to Billy the Kid - Shoe
Yes. That's what I have. There's the copper ground connection on the rim of the receptical. Before closer inspection just now I thought it was a tension clip to help keep the plug in place. Mine's exactly like the CS6364/65 configuration.
I plan to get the generator during the next week or so, then I'll have to get a cord that matches both recepticals.
Thanks again Bill. You've been a big help.
Runnerguy
bill,i just had the same problem, had to retire a gas dryer and was given an electric. so i installed the electric 240 dryer with a 3 prong plug (30 amp circuit). i was a bit confused about the "2 hot and the ground" instruction. why no neutral? i admit to having a simplistic idea of how electricity works, but none of my home depot purchased electrical manuals really made it clear :)))i always thought of it as a circuit, i.e. hot goes out, neutral comes back. is that where the "phases" you mentioned come in?thanks,john
"i was a bit confused about the "2 hot and the ground" instruction. why no neutral?"Actually it is 2 hots and a neutral.At the dryer the case is allowed to bonded to the neutral so that the neutral can also be used for the ground.But technically it is a neutral and if poperly wired should be wired with a neutral wire. That will be an insulated wire wire unless SE (service cable) is used. SE has a bare neutral wires wrapped spirally around the 2 hots and then covered by a sheather. But typically that might be used for a stove with larger current requirements.Hopefully this explains the 120/240 better.http://en.wikipedia.org/wiki/Split_phasehttp://www.allaboutcircuits.com/vol_2/chpt_10/1.html
.
William the Geezer, the sequel to Billy the Kid - Shoe
Edited 12/22/2008 4:52 pm by BillHartmann
perfect, thanks.....that's gonna take some reading :)my electrician said he used a 4wire, left the neutral (white) for "future if necessary" and put the ground in the grounding bar on the panel and the red and black to the breaker? so i have a "white" if i need it.you said "poperly wired should be wired with a neutral wire"
yikes! should i be worried the "ground" is uninsulated?thanks for all the info
Well if he ran the 4 wires you really should use the 4 wire receptacle and 4 wire cordset on the dryer.http://www.stayonline.com/reference-nema-straight-blade.aspxThe 3 wire uses the 10-30 receptacle. And the W terminal is for the neutral.The other 3 wire receptacle is the 6-30. That has 2 hots and a ground and is for 240 volt only.The 4 wire receptacle for dryers is the 14-30. It has both a neutral W adn a ground G terminal..
William the Geezer, the sequel to Billy the Kid - Shoe
the plug is attached to the dryer,.....came that way from sears :) so i got the receptacle based on the plug. electrician got the 4 wire cause it was cheaper at 50' than the 100' of 3 wire. some HD sale i guess.it's the 10-30.so if i read you right, i should disconnect the ground and use the white wire at both ends, leaving the un-insulated ground out.right?REALLY appreciate the help,john
"the plug is attached to the dryer,."Not really. They come without any plug. When it is installed the appropriate cordset is installed and the bonding jumper is installed or removed. The electrician should have installed the 4 wire receptacle and changed out the dryer cord set.But I find that many people, including pro's, are often confused by this.I would around with McDesign about this. In building inspector insisted on a 3 wire installation although it was a new installation.And while I will not mention any names I know someone very close to me that has a 3 wire dryer connection using a ground and not a neutral connection. Now it was done in the early 90's when the 3 wire was legal. But not to use the ground wire."so if i read you right, i should disconnect the ground and use the white wire at both ends, leaving the un-insulated ground out."Yes, but to be honest I would not bother with it unless you where also going to replace the receptacle and dryer cordset with the 4 wire version..
William the Geezer, the sequel to Billy the Kid - Shoe
thanks,seriously, sears delivered it with the plug attatched. no kiddin' i swear. is that a "no-no"just curious, what's the downside? do 4 wires "balance" the load better? or maybe "dissipate"?article....."Addition of neutral conductor allows loads to be individually driven"i read what you sent about phasing, though i admit much of it goes right over the old noggin'.does the insulation on the neutral make a difference? over the ground in this case?
The whole concept internal electrical wiring is that the ground is use for a reference so that all grounded equipment is at the same voltage. And that ground can be a return to clear a fault. That is if the hot shorts to a metal case, then and only then, current will flow in the ground wire and trip the breakers.To accomplish this the neutral is bonded to the ground ONLY at one place the service entrance which is usually the main panel. Any sub-panels the neutral is isolated from the ground wire.The 3 wire dryer connect violates this whole concept as the case is connected to the neutral connection rather than a separate ground wire..
William the Geezer, the sequel to Billy the Kid - Shoe
Bill,
Long time lurker, particularly electrical question/answer threads (the ones with posters saying "Bill will be along shortly to correct me...").... again I thought I understood the basic 120/240 stove/dryer, 3 wire/4wire but after following the above, I now have a much better understanding.
Your replies are always well composed and even if brief, succinct and understandable even for a rookie. Kudos for your insight and guidance.
Best of the season
TN
PS Just curious, what is your trade/training/electrical background...?
Thank. Hope that you have happy holidays.My training is in electrical engineering.But my work was all in designing control systems using microprocessors and then software to do the controlling.Most of my electrical knowledge comes from my DIY work and later handyman work.But the knowledge from the engineering and understanding technical specs does help..
William the Geezer, the sequel to Billy the Kid - Shoe
That is if the hot shorts to a metal case, then and only then, current will flow in the ground wire and trip the breakers
Better explain that one a little better Bill.
Current on the ground wire does not cause the breaker to trip. The breaker is an over current protection device. Current in a wire that is not connected to the device will not trip the device.
The ground wire will provide a safe path for the current to ground only. The fault current on the hot wire(s) must excceed the fault rating of the protective device for the breaker to trip or the fuss to blow.
Just because that ground wire is in place doesn't mean one is completely protected from a fault to ground (equipment case). If the current to ground does not exceed the breaker/fuss rating the device will not function.
What normally happens on a fault to ground is the supply side quickly exceeds the fault rating and the time delay of the device and it trips or blows. All is good then. Both the ground and the device did what they were suppose to do and the equipment/lives/ and property are saved.
What you should do is: Get a 4 prong cord for your dryer and hook it up properly to the dryer. Most new dryers have the wiring block very clearly labeled. You hook up the black, red, white wires to the appropriate terminals on the dryer and ground the green wire to the frame.
Then hook up the white wire on both ends. One end to the neutral bar in the panel and the other end to the appropriate terminal in a 4 prong receptacle.
The way you have it, the bare grounding wire is conducting current, which is illegal. (unless the dryer operates on 240volt only)
The chances of you having a new dryer that operates on 240volts only, is pretty slim. I have never seen one, not to say they don't exist.
If you actually have a 10-30 receptacle and plug, then you have a dryer that operates on 120/240 volts and it is wired incorrectly.
Edited 12/23/2008 12:21 am ET by BoJangles