Question about Outlets and terminals/push-in(real question, not a troll)

Discussion in 'Electrical Forum discussion & Blog' started by cswilson, Nov 4, 2010.

  1. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
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    Location:
    North Carolina
    Jim and thatguy look straight into the end of a piece of size 14 AWG copper conductor. It will look like a copper circle. That is all the area that carries electrons down the wire. How can having half inch or better of this conductor under a screw be any better than the area of the wire?

    Take the end of that conductor and establish a connection no larger than the end of the #14 and the device will carry as much current as the wire will carry. The connection will allow as much current to pass as the conductor carries. It is evident that the area of coverage need not be any larger than the area of the end of the conductor.

    Unless you are going to say that a #14 that is 1000 feet long will carry the same or more current than #14 10 feet long. Would there not be more area of copper in the 1000 foot long piece than the 10 foot long piece?
    Or we could say that a piece of 14 that is as long as it is wide would carry the same current as one that is 10 feet long. Well then why would we need more area in contact with the screw than the width of the wire? The one thing that we can’t argue with is the laws of physics and the thought that having ½ to ¾ of an inch covered by a screw will carry more current than a stab-loc should refrain from things such as wago connectors and terminal blocks. By the way these are approved all the way up to and including 30 amps.

    The one thing that we all can be certain of is; if this stab-loc connection was as unsafe as some are trying to make it sound like, then why would the practice have been allowed for over 40 years now??
    Why wouldn’t people like the Fire People be doing something about it? We all are aware how quick the Fire Marshall can close the doors to something unsafe. Wait a minute ain’t it the National Fire Protection Association that does the NEC? Yep it is those good fire boys that says this push-in connection is safe but then again they ain’t members of a Plumber’s Discussions Board, hee…hee…

    You can argue till the cows comes home but until you get all these Nationally Recognized Testing Labs such as UL and Met along with all those great minds that govern the NEC to say that the screw is safer than the stab-loc you can rest assured that something that someone post on one of these discussion boards will not change my mind on the safety of the stab-loc connection.
    I would hope that the general reader would also put their faith in those who are charged with our safety over the wives tales that spread throughout all the trades of how much better some installation is over another listed installation.
  2. Thatguy

    Thatguy Homeowner

    Messages:
    1,460
    Location:
    MD
    Three problems, at least:
    You don't understand contact resistance.
    You don't know that you don't understand contact resistance.
    You have cherished beliefs that are probably false.

    Along with "contact resistance", you could search

    asperities
    "Kelvin clips"
    pressure
    area
    deform
    "current density"

    The good news is that you can measure contact resistance with a 10 A load like a toaster, and almost any DVM.
    I got 0.05 ohms for a badly installed wirenut. The good ones didn't register at all.

    And if you spring for a micro-ohmmeter it will open a whole new world for you.

    As for the cherished beliefs - they only make themselves known when somebody steps on them. Like now.
    Last edited: Nov 8, 2010
  3. Furd

    Furd Engineer

    Messages:
    446
    Location:
    Wet side of Washington State
    Thank you for the response, JW. If I am correctly reading your response the NEC does indeed limit most branch circuits to 80% of the maximum ampacity regardless of whether or not the load is continuous. That the inclusion in the text of a limitation to 80% of maximum ampacity for certain loads is only a re-iteration of the general limitation.

    As a practical matter then it would seem to me, (I do not claim to be a code expert by any means) that the only circuits that can in fact be operated at 100% of rated load would be fixed lighting circuits (but not where harmonic currents are present) that could be reasonably expected to not be energized to the maximum in excess of three continuous hours and then only when there were no provisions (receptacles) for any plug-and-cord connected equipment.
  4. Furd

    Furd Engineer

    Messages:
    446
    Location:
    Wet side of Washington State
    JW wrote:
    I don't entirely agree. While the analogy between electricity and water is often used that analogy does break down eventually. Water DOES travel through the center "nothingness" (for want of a better term) of a pipe or hose but electron flow is through the mass of the conductor and the shape of that conductor plays a small role, if any role at all, in how much of a flow occurs. If you take that #14 AWG copper conductor and flatten it to the thickness of one electron you will have a huge increase in surface area compared to the original conductor whether you measure the outer circumference or the cross-sectional area. Since the electron flow is through the copper at a sub-atomic level the real-world shape of the conductor is meaningless.

    HOWEVER,

    Thatguy mentions contact resistance and THAT IS a real-world concern. Higher resistance through the contact of one conductor to another is a measurable phenomena and all other things being equal, or at least within the bounds of real-world situations, the contact with the lower resistance will pass a higher current with less heating than will a contact with a lower resistance. Contact resistance is directly correlated with both the pressure and the area of contact in addition to a few other things that are less of a real-world factor.

    I have a device for testing GFCI receptacles and circuit breakers. It has a provision for applying a specific load (I think 12 amperes but it could be 15) to a circuit for a specific time. It may be a pulsating load, I don't remember but the purpose is to see if a specific circuit is capable of sustaining a "proper" load without excessive voltage drop, I think 3%. When I have used this device on backstabbed receptacles in my own home I am regularly greeted with a "FAIL" message but the act of removing the ultra cheap backstab only receptacles and replacing them with relatively inexpensive Leviton Decora receptacles with screw connections (but not "side wired" using pressure plates under the screws) now allows these same branch circuits to pass the test every time.

    I'm NOT stating that backstab connections are unsafe or that they do not meet the minimum requirements when properly installed according to the manufacturer's instruction. What I AM stating is that more often than not the use of the simple screw terminal or the better screw with pressure plate does indeed provide a SUPERIOR connection than does the average back stab connection. Maybe it has something to do with the methods used in the field to make the backstab connections that is at fault rather than the backstab itself, I don;t know. What I DO know is that there are far fewer problems of all kinds with screw termination than with backstab connections.
  5. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,529
    Location:
    North Carolina
    Furd,

    We are in agreement and are saying the same thing just saying it in different ways with the exception of heating of a back stabbed device.

    In your opening statement you said that to take the #14 and flatten it out it would still carry the same amount of current as when it is round. To simplify this statement you are saying the mass of copper would be the same round or flat. You are also saying that the mass of copper is what is capable of conducting the maximum amount of current for a #14 copper conductor.

    Am I correct so far?

    This is like the two college students arguing over Ice. On student says his ice is prefect at 10 degrees Fahrenheit. The other student says his ice is better simply because his is at minus 10 degrees Fahrenheit. Which is correct?

    Then all that would be needed to carry that current would be the same amount of mass with the same resistance of the conductor. So if the contact of the conductor to the frame of the receptacle with a spring steel that has the proper foot pounds covers that amount of area or mass with a designed resistance equal to or lower than mandated ft.lbs. of the screw pray tell me how the screw can be a bit better.

    As to your Sure Shot the 12 amp load is simulated. Not knowing much about this resistance stuff I think that Ohm’s Law will tell you that it is not a real 12 amp load.
    Ohm’s Law states that the wattage (heat) is equal to the amps times the volts. This equals 1440 watts of heat in your little meter. Unless that meter is producing as much heat as 14 hundred light bulbs would be producing then it is not a continuous 12 amp load.

    If your meter is showing no reading at all through receptacles that have a voltage on them I would say is a pretty good indication that you need to have your meter looked at.
    I use one myself when called out by one of the three realtors I work for here in my area when there is a dispute between them and a Home Inspector concerning the electrical system. Strange but I don’t find these same problems as you mentioned concerning stab-loc devices. I would say I average at least two or three homes a month.

    In all honesty I must say that through out my 40 plus years of doing this type of work I have found just as many stripped or loose screws as I have bad stab-loc. I am sure that I have seem many times where the conductor was not properly installed around the screw which one can’t do with the back stab. I can remember a few where the insulation was never removed and the device installed. Just as with any method the installation is no better than the person doing the installation.

    For all those that just can’t seem to understand this push-in connection think about it for just a few minutes. Every receptacle in your house is a push-in pressure connection. The breakers in your panel is a push-in pressure connection and low and behold the very cash register (meter) for the power company is a push-in pressure connection. The disconnect for your air conditioner is a push-in connection. And to top it all off you push a handle on your water closet in order to flush.


    The NEC does not limit the branch circuit to 80% it only limits the load for a receptacle to 80% of its rating. I can put the other 20% on another receptacle on the same circuit and now have the circuit loaded to 100%.
    Last edited: Nov 9, 2010
  6. TWEAK

    TWEAK New Member

    Messages:
    86
    Location:
    Bay Area CA
    Interesting thread. I have some comments.

    1. I don't buy the contact area argument. Test results say stab-loc works, and the area is adequate. Obviously there are lower resistance connections possible. But it doesn't matter. The spring loaded backstab works at 15A, there is not excessive voltage drop or the related joule heating at the contact. Personal preferences are perfectly fine -- that's the individual's choice. But IMO the test results have to hold more sway than opinion. BTW I've suffered many burnt plug/receptacle blades over the years, and *no* problems with back stabbed spring connections on receptacles.

    2. One thing to consider is that a spring loaded contact is more compliant than a screw contact. As the wire heats up and cools down, wire expands and contracts. The spring is better suited to take this up. Screws can get loose under conditions of temp cycling. This is especially true of SHORT screws... such as we see in receptacles and switches. Those with some engineering background in fasteners will surely get this point. Those that have ever had the header bolts on their hotrod loosen may have wondered why that happens... the reason is header bolts are very short and exhaust systems go through temp cycling. The car engineers (as opposed to the aftermarket header people) generally use longer bolts on their manifolds, and that is one reason why they do so (there are others, too).

    3. I have never seen anyone wire a receptacle or a switch that used a torque wrench to tighten the screws. I have, on the other hand, found some that weren't tight. An advantage of the spring contact is that the contact pressure is achieved by design - not by a human tightening the screw.

    4. A poorly formed loop around a screw and inconsistent tightening of the screw seems to me like it would be just as much of a problem if not more than a spring contact. Many loops are far less than perfect. Many straight strips, used with backwire screw connections, are also not straight or have large burrs. The spring is more tolerant.

    5. Push in multi-wire connectors are commonly used these days instead of wire nuts, to save time and also to save space. Same technology... certified by UL, ok with NEC, and they work fine and reliably.

    Regarding 12 amp limits.... I disagree with jw on that one. I appreciate your code citations, but the fact sis thatt here are several plug-in devices are rated for more than 12 amps and come with UL 15 amp cords/plugs. Your sawzall may be a 13 amp. Your wife's blow dryer is in all likelihood a 1500W unit (12.5 amp). There are plenty of other examples. Do we really think that these won't be plugged into branches that carry two or more receptacles? It's not realistic. But it's ok. A 15A receptacle IS rated for 15A, not 12. Having said that, back stabbed spring style connections are fine at 15A.

    I imagine if someone manhandles the connection or just jams everything into the box they could damage the connection. Like anything else.

    BUT.... I most often use the screw terminals because I don't like finding something small to stick in the release hole when it's time to take it apart. That's the only reason. Even so, I use the back-stab hole when I'm tight on space in the box. To me, that's what it's there for and I think one is making the job harder than it needs to be not to use it. But that's just my opinion.
    Last edited: Nov 9, 2010
  7. Thatguy

    Thatguy Homeowner

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    Location:
    MD
    That's so they don't need a big heat sink or forced air cooling for the load resistor. You can do the same test with a steady 10 A load like a hair dryer, and a DVM.
    Ideal makes a tester, maybe #65-165, that likely uses the same principle. If you need to dissipate one kw in the load resistor and your duty cycle is 1% then you can use a 10 W resistor.


    This thread is one reason why this book
    http://www.amazon.com/unSpun-Findin...=sr_1_1?s=books&ie=UTF8&qid=1289325803&sr=1-1
    was written.

    Along with companies who paid for patents on improved push-in connectors, here are some people who put their money where their mouth is regarding this connection method.
    http://www.google.com/search?client...receptacle&gs_rfai=&pbx=1&fp=6e8733203d1b4e27
    Last edited: Nov 9, 2010
  8. TWEAK

    TWEAK New Member

    Messages:
    86
    Location:
    Bay Area CA
    Your second citation may be the best reason for your first.

    Your link points to lots of problems with Federal Pacific panels. No news there. Federal Pacific panels are well-known to be garbage. They company went under. Unless you have a specific article in mind... sorry I can't guess about that, and can't spend the time looking at every hit from a google search to pick out something that might be relevant. Federal Pacific had some technical issues that have precious little to do with the viability of spring loaded pressure contacts, such as are used in receptacles and switched, in general.

    Every panelboard out there uses spring loaded contacts, including Square D, Cutler, etc. They are fine. Badly designed products happen, and they prove only that the product was badly designed. Nothing more. Generalizing a specific failed design to an entire class of products with no qualifying data is a great way to generate disinformation.

    I don't see that there is any demonstrated problem with the technology. Federal Pacific lawsuites certainly mean nothing. And in general, lawsuits don't prove that there is a problem with the technology, anymore than they disprove it. Lord help us when we rely on lawyers arguing in front of juries to make technical decisions for us.
    Last edited: Nov 9, 2010
  9. TWEAK

    TWEAK New Member

    Messages:
    86
    Location:
    Bay Area CA
    At least it's a fairly cordial discussion. I agree with you, jw, that the spring loaded contacts are perfectly safe and acceptable as long as they are executed to a reasonable level of workmanship, and within their ratings, etc. Which is the case with ANY connection. As you point out, it's a technology that's well established, has some significant advantages, and has not been demonstrated to be a problem.

    As you also significantly point out, the NFPA holds the reigns on the NEC, and they think it's fine. Being familiar with NEC should tell anyone that that they are very fire-safety conscious, as well as shock-hazard conscious. From an engineering viewpoint, if anything, the NEC tends to include generous margins on most of what they approve. Which is as it should be.

    But people can and will do whatever they like. As I mentiond in an earlier post I prefer to use other than the stab-locs unless I am tight on space, but that preference is not due to workmanship or safety reasons. The issue in this case is if they promote some disinformation that says that using these teminations is somehow the hallmark of inferior workmanship. I think that is wrong.
    Last edited: Nov 9, 2010
  10. ballvalve

    ballvalve General Engineering Contractor

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    Location:
    northfork, california
    The average homeowner doesnt know an amp from a watt and even where his panel is located. Dont test the water with friction terminals.
  11. ballvalve

    ballvalve General Engineering Contractor

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    3,261
    Location:
    northfork, california
    To the guy that says heat will loosen the screw clamp connections, I would ask hiim to have a look at his main panel before making such comments. All those wires must be wiggling around!

    When we see PUSH IN connections on breakers and neutral bars, I'll be the first to use them.

    comparing a STAB, clamp, double sided connection to a bus bar or a meter terminal to chinese push in outlets is absurd.

    The ONLY advantage one might mention is if the electrician is mildly forgetful and neglects to turn the clamp screw at all. If you are a dopey electrician, by all means STAB dont SCREW!
  12. Thatguy

    Thatguy Homeowner

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    Location:
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    Temperature changes will probably loosen machine screws unless precautions are taken, like using a close fit.
    http://www.sizes.com/tools/thread_screw.htm
    Last edited: Nov 14, 2010
  13. ballvalve

    ballvalve General Engineering Contractor

    Messages:
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    Location:
    northfork, california
    It seems that main panels and breakers use screws with threads a bit "tight" or a slight interference fit. Back stab screw terminals have a bit of angular offset which gives flex, as well as [typically] serrations on the tab.

    Always the issue is that copper and aluminum are soft and re-torquing should be done.
  14. alternety

    alternety Like an engineer

    Messages:
    650
    Location:
    Washington
    Just an extra 2 cents as I have had problems with push-in units. Perhaps there is a bit more sensitivity to manufacturing tolerances/errors/materials variance in the push-ins.
  15. jetlag

    jetlag New Member

    Messages:
    84
    Location:
    Ga.
    You are already over the box allowance , the 3 14/2 are 6 conductors but you must count one for all the bare grounds that makes 7 , then you must count 2 for the recep or switch , that makes 9 times 2.0 cu in per each #14 , that equals 18 cu in box . The conductor allowance stamped in the box must say 9 or more #14 to use it . As far as the attach the way you said it is allowed but very hard to push in the box , best to use pigtails and you dont have to count the wire nuts for box fill .
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