Electrical connector wire-contact area differences on electrical receptacle and switch wiring connectors:
Push-in type back-wired electrical receptacles & switches have significantly less electrical contact area than other connectors on these devices. Pressure or torque forces vary as well.
This article describes differences in electrical-wire to device connector surfaces between push-in type backwire connectors, compression plate connectors, and binding head screw connectors, all of which are found on many electrical receptacles and switches.
These area differences can be significant, up to ten times variation in surface area. Small surface area and possibly other properties of push-in back-wired electrical receptacles and switches may explain some field failures reported on those devices.
This article series explains receptacle types, receptacle grounding, connecting wires to the right receptacle terminal screws, electrical wire size, electrical wire color codes, and special receptacles for un-grounded circuits.
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At BACKWIRED RECEPTACLE FAILURE PHOTOS where we discuss the key factors in the decreasing reliability and and increasing risk of push-in backwire electrical receptacle wire connections we emphasize the inherent weakness of the spring-clip connector.
But differences in actual contact area between the wire and the connector may be factors that increase the risk of a connector failure, overheating, or fire.
In this article we illustrate the typical connector used in some receptacles and switches that accept a simple push-in connection usually found on the rear of the device.
The rectangular opening is used to release an installed wire. Simple screw terminals are also visible in the lower left of the photo.
[Click to enlarge any image]
This article compares the approximate size of contact areas between an electrical wire and the connector surfaces in an electrical receptacle across three different connector designs:
This article concludes that
On an electrical receptacle or switch the binding head screw wire contact area offers about four times the contact surface as a perfectly-made push-in backwired receptacle or switch connection, and if the wire is bent in a push-in connection, the binding head screw offers nearly ten times the contact area as the push-in device.
Why do we care about the size of the contact area between an electrical wire and the connectors on the device such as a receptacle or "outlet" or a light switch ?
Because it is both true in theory and appears true in field reports that a smaller or more fragile electrical contact between the wire and the device can increase the risk of an electrical failure, overheating, and possibly building fires.
In concept, if we keep voltage and current flow (120V and up to 15A in the electrical receptacle connections examined here) then a smaller contact between connections may be expected to generate more heat. The circuit breaker or fuse protecting a circuit is intended to avoid un-safe overheating but an electrical connector that uses minimal contact area may be at increased risk of field failures.
See DEFINITIONS of ELECTRICAL TERMS - or go directly to DEFINITION of ELECTRICAL RESISTANCE, OHM's LAW in that article for details. There you'll read that Ohm's law is basically a measure of resistance which in turn is a measurement of the heat that will be generated by an electrical wire (or contact) that is carrying a given current (amps or load).
At above left we illustrate a #14 solid copper wire being pushed into the backwire opening of a common electrical receptacle. The wire has not been pushed fully into the connection as I wanted to show the diameter of the wire entering the push-in connector opening.
Our page top illustration of a spring-clip type back-wired push-in receptacle connection comparing that to a pressure plate or screw connection is discussed in detail separately at BACKWIRED DEVICE SPRING CLIP DETAILS.
Below measurements indicate the approximate length of contact area between an electrical wire and the surfaces in a push-in backwire connector used in a typical electrical receptacle. Keep in mind this is only the length of contact area.
The actual contact area expressed in square mm will be slightly greater than the length depending on the width of actual physical contact between the rounded wire surface and the relatively flat area of the contact plate or the contacting edges of the leaf spring.
Besides the actual contact area in mm2, the force of the spring against the wire is a critical factor in the quality and reliability of the wire-to-connector contact, and it certainly will be a factor in the depth of notch cut into the wire by the end of the leaf spring during and after wire insertion.
See WIRE-TO-CONNECTOR FORCE COMPARISONS for a comparison of the connection force of the spring clip connector with the force exerted by a binding head screw connector.
Above we show that the approximate width of the end of the leaf-spring that pushes against a back-wired push-in wire is 3mm. The actual contact width of the end of the spring with the wire will depend on how precisely the wire radius matches the spring notch, the depth of the cut that the spring causes into the wire, and perhaps something else I've not considered.
Above you see that the width of the contact plate against which the push-in backwire receptacle connector forces the wire is about 9mm, possibly reduced a bit by the notch cut out of that surface. As we discuss in this article, the actual wire contact with the contact plate may be significantly reduced if the wire itself is not very straight.
Above is a close-up photo of a #14 copper wire against a millimeter scale. The actual wire diameter of #14 copper wire can vary among manufacturers and wire types. At SE CABLE SIZES vs AMPS we give the typical diameter of #14 copper wire as 2.05 to 2.32 mm or about 0.081 - 0.092 inches.
Using the measurements above, ignoring the effects of spring force against the wire, just considering the length of wire-to-connector contact here are some rough contact area estimates & calculations for a back-wired push-in electrical connector such as the receptacle connector shown in the photograph just above:
Note that this 7mm estimate is about half the length of bare wire indicated by the strip gauge on the back of the receptacle case. The "strip gauge" molded into the rear of the receptacle and shown below indicates a 14-15mm strip length (about 3/8") for the wire to be used with this device.
But it is the size of the contact plate (shown above) that determines the maximum possible wire contact along its length when pushed into the back-wired connector. That length (in this receptacle) was between 8 and 9 mm, and observation when I inserted a wire showed that by no means did the wire actually contact all of the available connector surface.
CASpring = contact area of wire to the end of the leaf spring
CAPlate = contact are of wire to the contact plate
CAT = Total contact area for a relatively-straight copper wire in the connector
Note: these are estimated contact surface areas. An actual physical measurement of contact area may be possible using other methods.
Discussion: these are estimated wire-to-connector contact surface areas.
An actual physical measurement of contact area may be possible using other methods. Additional important effects of the condition of the wire and connector surfaces such as due to corrosion, prior arcing, oxidation, have not been considered here.
A similar measurement and contact area estimates can be performed for the contact areas beneath a binding-head screw (photo below) that is an alternative connector for push-in backwired devices.
You'll note that the binding head screw outer diameter is between 8 and 9 mm for the product shown, an Eaton 2427AD 15A 125V rated electrical receptacle that was purchased at a Home Depot store for $0.69 U.S.D. in October of 2015.
Here are some rough calculations, again ignoring the effects of differences in contact force between that imposed by the leaf spring of a push-in backwired connector and the force imposed by a binding head screw or screw-operated compression plate connection of electrical wires in a receptacle or switch.
[Click to enlarge any image]
Using a duplicate of the same electrical receptacle model as was employed above for measuring the areas beneath the head of the side connection "binding head screw" we found:
Since the wire must be under the screw to be secure we assume that it is under the screw but as close to the screw outer edge as will leave the outermost edge of the wire "under" the screw head, as illustrated above. For maximum possible contact area (and to simplify calculations) we assume that the wire makes a full circle (which it won't). The diameter of the full circle of electrical wire will be calculated as follows:
Screw-head diameter minus one wire thickness
(As the wire contact point will be at half of the wire thickness under the head at opposing sides of any diameter measurement point).
The total contact area of wire under the screw head as well as on the contact plate below the screw head will each be calculated as the circumference of the wire circle x the estimated contact surface width at the point of contact between the screw head and the wire or the contact plate and the (round) wire surface.
Convert wire circle diameter to circumference: Circumference = 2 π r
Total electrical wire contact surface area below the binding head screw is then contact length (circumference) x contact width for both the screw underside and the surface of the contact plate below the wire.
Electrical wire contact area in a binding head screw connection including both screw head and pressure plate is then calculated as
A similar measurement and contact area estimates can be performed for compression plate back-wired electrical receptacles - a (probably) better-performing design alternative to push-in backwired devices. At above left you can see a typical pressure-plate screw-clamp type electrical wire connector on an electrical receptacle.
Above is a #12 copper wire against the manufacturer's wire-strip gauge on the back of this device, and below we show a measurement of the length of the strip gauge as 9 mm.
The wire strip gauge indicator found on the back of a typical electrical outlet or switch provides for stripping about 15mm of insulation from the wire. Estimating that about 13 mm of wire is ultimately pinched between two flat pressure plates that are squeezed over the wire, we can calculate the approximate electrical wire to connector contact area as follows:
CATotal Compression Plate Connector mm2 = 2 x wire contact length x wire contact width
CATotal Compression Plate Connector mm2 = 2 x 9mm x 0.1 mm
CATotal Compression Plate Connector mm2 = 1.8 mm2
This makes sense in that the total wire length of a loop of wire under a binding head screw will surely be longer than the straight length of stripped wire pushed into a back-wired device.
Summarizing the estimates of contact area between the electrical wire and these three different types of electrical receptacle wire connection devices
Summary of Approximate Electrical Wire Contact Surface Area in Electrical Receptacle & Switch Connectors
|Connector Type||Estimated Contact Area in mm2||Comments1|
|Push-in Backwired Connector||CATotal Push-In Connector = 0.8875 mm2||Assuming the inserted wire is straight|
|Push-in Backwired Connector||CATotal Push-In Connector = 0.3875 mm2||For an electrical wire that is bent or curved|
|Insert-in Backwired Pressure-Plate / Clamp-type Screw Connector||CATotal Compression Plate Connector mm2 = 1.8 mm2||Assuming the inserted wire is straight|
|Wire loop under binding head screw||CATotal Binding Head Screw mm2 = 3.68 mm2||Assuming a perfect circle of wire|
1. The effects of contact force on electrical resistance between the wire and its contact on an electrical switch or receptacle are not included here
2. Because my assumption that the wire under a binding head screw is a perfect circle may slightly over-state the wire contact area, I used 8mm rather than 9mm as the screw head diameter.
3. On an electrical receptacle or switch the binding head screw wire contact area offers about four times the contact surface as a perfectly-made push-in backwired receptacle or switch connection, and if the wire is bent in a push-in connection, the binding head screw offers nearly ten times the contact area as the push-in device.
The significance of these differences in wire-to-connector contact area is significantly affected as well by differences in force exerted against the wire among these connectors and may be affected by differences in the condition of the wire with which the connection is being made.
These estimates are based on calculations given in the article above and assume a wire contact area width along the wire against a plate or screw of 0.1mm using #14 copper wire. The extent of flattening of the surface of the wire due to compression by a connecting screw will affect this estimate as may other wire conditions such as re-using old, previously-bent or twisted electrical wiring in a re-wire or device replacement repair operation. As we discuss just below, the true contact area and thus electrical resistance of the connection is not so simple.
Contact Resistance is a function of (force x micro-contact points x other factors such as oxide resistance)
Watch out: as electrical engineering experts point out in numerous research articles (some cited here), the effectiveness of an electrical contact derives from the combination of the size or area of the contacting conducting surfaces and the force applied to their contacting surfaces. The table above describes only contact area size for different electrical connectors on electrical receptacles (and some switches). Below at left is the spring-clip connector of a push-in backwire electrical receptacle. Below at right is a binding-head screw on the side of an electrical receptacle.
[Click to enlarge any image]
Research (cited below) points out that the actual physical contact and thus electrical current flow between an electrical wire and the electrical connector surface actually occurs through numerous microscopic points of contact. (Greenwood 1966 et als).
Those authors make a clear distinction between the "constriction resistance" and the "real area of contact" in electrical devices and cite the Holm radius and Holm's equation that provides "... a method of obtaining a practical estimate of the interaction term ..." to describe clusters of contact points.
Below is a screw-clamp or pressure-plate type connector on an electrical receptacle.
This connection combines back-wiring and stronger connector force between wire and connector than seems likely from the spring type connector shown at above left.
Greenwood (1966) points out that "Whenever we have a reasonably large number of not-to-small contacts, the self-resistance term becomes small and the constriction resistance is close to that observed if the entire area of the cluster were in electrical contact". [Emphasis ours].
The author continues to note that in addition to contact area (and force) as factors in electrical resistance, oxide films or coatings must be considered, and makes this interesting remark in section 4.3 of the article I cite:
The mechanical area of contact [between a wire and an electrical connector] is often very much less than that deductible from bulk properties. - Op. Cit.
Separately at WIRE-TO-CONNECTOR FORCE COMPARISONS we discuss obvious differences in the wire-to-connector contact force available in these three connector types, of which it appears that push-in back-wired spring-type connectors exert considerably less force than a properly-tightened binding head screw or clamp and pressure-type screw connector. Also see Tismit (1998) and Park (2002) cited below.
At AlumiConn TORQUE TESTS we quote expert advice from King Innovation electrical engineers who said that when using that company's AlumiConn™ aluminum wire repair connectors useful for pigtailing aluminum wire ends to copper:
Torque Recommendation: 15 inch-pounds when connecting ... All Solid or Stranded Copper conductors
I did some further crude experiments using the same S.K. torque screwdriver provided by King Innovations. This tool has an adjustable torque setting range of 4 to 22 inch-pounds of torque.
[Click to enlarge any image]
I tightened a #14 solid copper wire in the pressure clamp of a modern back-wired receptacle, observing just how physically difficult it was to turn the screw to 22 inch-pounds. I started with the tool set at 15 inch-pounds of torque, tightened the screw, then re-set the tool to its maximum torque - 22 inch-pounds and tightened the clamp screw again.
I selected a flat-bladed screwdriver bit that was the biggest that would fit into the screw slot - thus minimizing slippage. (Slippage occurred anyway.) When installers use power tools to driver up screws on electrical devices they usually use a Phillips-driver bit and apply down-pressure onto the screw while turning it.
For these tests I mounted the electrical receptacle in a wooden vise (above), providing the best feasible position for applying considerable hand force to tighten its screw. In normal electrical wiring in the field nobody tightens wires onto a receptacle that is held in a vise.
And nobody - at least nobody normal - even owns an SK torque-measuring screwdriver. You won't find this tool at most electrical supply stores. My opinion is that I was able to apply more torque with this method than would normally be applied in the field. Modest damage to the flat-blade screwdriver slot of the screw is shown below.
It was physically difficult to turn the binding head screw on the receptacle to 22 inch-pounds of torque, and even 15 pounds was a bit of a challenge. My opinion is that
most by-hand field-tightened electrical screws used on receptacles and switches are going to be at 15 pounds of torque or less.
power-tool-tightened electrical screws on electrical receptacles and switches is likely to be at 15 pounds of torque or more and is limited in part by the position of the device being tightened and the ability of the installer to hold it in place.
Researchers determining the nature of actual true electrical contact between surfaces and the concomitant resistance and connection quality probably have to take more accurate pressure measurements. Patents that I surveyed referred to "exceptionally high torques" available when tightening screws using power tools but did not include specific measurements. (Jasch 2001).
I did not test the torque necessary to un-screw the electrical receptacle screw in the pressure clamp connector as the S.K. device I used provides torque only in the tightening direction. However we know that considerably more torque would be used to loosen a tightened fitting than to turn it tight in the first place. (Simonin 1992).
After tightening the heck out of the pressure-plate electrical connector on the receptacle shown above (Heck = 22 inch-pounds of torque), it was suggestive to consider these questions:
Power dissipated or heat produced is proportional to the square of the current. Because of the following formula, even a small resistance can cause a lot of heat when a large amount of current is being drawn.
Your issue is that the contacts should not reflect any resistance. Small contact area means more resistance. Sorry but I do not know the details of ohms per contact area. (My guess is that there is more to it than just contact area such as contact pressure or surface oxidation of the wire.)
Power (heat) = current x voltage
since voltage = current x resistance
Power (heat) = current x current x resistance or P=I2r
Any small amount of resistance can produce significant heat when used in an environment where there is heavy current demand. e.g. vacuum cleaners, toasters, microwaves, etc. The increased heat at the contact point is probably what causes the deterioration of the connection over time.
Are those little springy things (photo at left) really copper? I would not have thought that copper would have the needed strength. This especially if there is any heat being produced. - 11/8/2015 Paul Galow, E.E.
Mr. Galow is a frequent contributor to InspectApedia.com and a personal friend of the publisher/editor. His contact information is at TECHNICAL REVIEW COMMITTEE MEMBERS
Thanks Paul. The push-in backwire spring assembly looks like copper but I suspect it's an alloy. While I have not measured it accurately, just the subjective experience of the force necessary to push a #14 copper wire into the connector argues that there is a palpably-strong spring force involved. Just how that spring force will be maintained if a wire is later removed and re-inserted is not known.
Aronstein as well as researchers I cited above emphasize that while contact area is important in understanding electrical connectors, the force with which the two surfaces are pressed together is critical as well.
The article above discusses only the comparative areas of different electrical contacts used in receptacles and some switches and finds that even before considering force, the areas vary considerably.
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