Back wired electrical receptacles:
Using the back-wire or push-in type connection points on an electrical receptacle or switch may be just fine, or it may not be reliable nor safe, depending on the age and type of back-wire connector provided. Here we describe the types of backwire connectors used on electrical receptacles and light switches and we explain the safety and performance questions that may arise.
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 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.
This article illustrates and explains possible electrical failures and fire risk when using the push-in rear connectors on some back-wired electrical devices such as receptacles and switches.
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.
A lower price ($0.69 U.S.D.) push-in type back-wired receptacle purchased at a Home Depot store in 2015 is illustrated above. Above the photo gives view of the neutral wire side screw connectors and a green grounding conductor connector screw.
[Click to enlarge any image]
Just above you can see on the back of this device the round opening that accepts a push-in connection of a #14 solid copper wire (white arrow). The small rectangular opening permits insertion of a probe to open the spring clip to release wires that have been installed through the round opening.
We disassembled the push-in backwire receptacle shown above to permit a view of the wire connections inside this device
. In the photo at below left, and viewed from the front with the electrical receptacle's plastic front cover removed, the white and black arrows indicate the side screw connectors while the white and black circles show the interior location of the push-in spring-clip connectors.
However wires are pushed in from the other side - the "back" of this device as I show below.
Below at left is a close-up of the spring clip connector. Click to enlarge this image and you'll see that the end of the spring clip has a rounded notch (red arrow) that I think is intended to increase the contact area and security of spring grip on the wire when it's pushed through this opening.
Below you can see a high magnification (stereo microscope) photo of the areas of contact between a relatively-straight copper wire and the spring clip connector surfaces.
There are two points of electrical contact with the wire: the notched end of the spring blade (red arrows in the photographs above) and the side contact plate against which the wire is pushed (green arrow). Details about this connection and a discussion of wire-to-device contact area and contact force are given below
at BACKWIRED DEVICE SPRING CLIP DETAILS
[Click to enlarge any image]
Modern back-wired electrical devices like receptacles and light switches use a clamp-type screw and plate that permits very good contact between the electrical connector on the device and the wire - as shown in our photograph at above-left. With wires stripped to the proper length, not nicked, and with the screw tightened, these are, in our OPINION, good electrical connections. Shown at above left is a FS Spec Grade 20-Amp 120V electrical receptacle listed by UL, CSA, and other agencies.
This device is marked as suitable for copper wire only.
At above right is a top view of a #12 copper wire inserted into one of the clamp type electrical receptacle openings. At right in the same photo you can see the silver edge of the clamp pushed against the copper plate beneath the screw when the device is fully closed.
Watch out: to make proper use of this electrical connector on a receptacle you must be sure to insert the electrical wire between the silver coloured clamp and the inner face of the copper screw plate. Otherwise when you tighten the connector screw it will not clamp onto the wire. A close-up view of the wire inserted properly into the screw-clamp connector is shown at below right.
Definition of "FS" and "FD" electrical boxes & devices: The label "FS" on an electrical device indicates (F) it is suitable for mounting on a metallic ("ferrous") electrical box and (S) indicates it will fit in a shallow box. Other devices may be labeled "FD" indicating "Ferrous" and "Deep" electrical boxes. An "FS" type electrical box has a minimum depth of 1 3/4" and 13.5 cubic inches and can accept up to 6 #14 or 6 #12 electrical wires. An "FD" type electrical box has a minimum internal depth of 2 3/8", 18.0 cu. in., and can accept up to 9 #14 wires or 8 #12 gauge wires. - NEC Table 314-16 (A)
Older electrical receptacles and light switches and some lower-cost modern products use a spring-clip type contact that presses the mere edge of a metal spring or clip inside the device against the edge of a wire that is pushed into the device through a hole on the device back. These devices offer two different methods of connecting wires: through use of a binding head screw usually found on the device side, or through a simple push-in connector on the device back.
Notice that the electrical receptacle shown above sports only push-in type backwire connections. No side screws and no screw clamp options are provided.
Typically these electrical switches or receptacles are referred to as "backwired" if the push-in connectors on the device back surface are used to make electrical wiring connections. An example of a back-wired electrical receptacle is shown at above-right. Most-likely this receptacle was a split-wired unit - note the red and black wires on the receptacle's left or "hot" side and the common neutral wire in and out of the receptacle on its neutral side. This image was contributed by reader J.K.
These devices provide less electrical contact area between the device electrical connector and the wire surface than the clamp type connector, but are code-permitted and may work acceptably provided that the devices are not re-used. That is, in our OPINION, if you remove and reinsert wires in devices that rely on a spring clip rather than a screw and plate clamp connector, the spring may be weakened and the connection less reliable.
Still older receptacles and switches (shown at left, no longer sold in the U.S. or Canada) used a hole diameter on the device back that would accept either No. 12 or No. 14 copper wire to be connected by a push-in back-wired connection method (red arrows).
Not only did these devices sport a limited electrical contact between the wire and the connector (just the edge of a flat spring in contact with the edge of a round wire), but worse, if a device was back-wired using No. 12 wire and later re-used with smaller diameter No. 14 wire, the contact spring, having been bent open by the No. 12 wire, performed poorly against a No. 14 wire later inserted into the same opening.
In our OPINION these older devices were less reliable, less safe, and should not be used by backwiring. The devices would perform acceptably, however, if the screw connectors on the device side were used to connect the wires.
4. Binding Head Screw Connectors on Electrical Receptacles & Switches
Above is a typical connection of a #14 copper wire to the binding head screw of a 15A electrical receptacle. Most electrical receptacle models that offer a back-wiring option also offer a screw or screw-clamp option.
At WIRE-TO-CONNECTOR PERFORMANCE SUMMARY we suggest that binding head screw connections such as shown here are likely to be significantly-better performers over the life of the electrical system in a building, adding that screw-clamp connectors also perform well.
Above is the same electrical device and binding head screw connection with a variation: we closed the hook or loop around the screw either by pushing the open end of the loop against a plastic or metal protrusion on the right side of the screw or by using pliers. This detail provides a still more-secure electrical wire connection and a bit more electrical contact area between the screw and the wire.
This discussion has moved to its own page at WIRE-TO-CONNECTOR PERFORMANCE SUMMARY
For electrical receptacles and switches that offer a choice of using a push-in type spring-clip backwire connector and using a binding head screw connector to connect a No. 14 copper wire to the device, the overall quality and thus long term reliability of the electrical connection made by the combined effects of contact area (wire to connector) and contact force (wire to connector) under a binding head screw is between 300x (straight wire) and 750x (wire not straight) better than that obtained using the push-in spring-clip connector.
For electrical receptacles and switches that offer a choice of using a push-in type spring-clip backwire connector and using a pressure-plate screw connector, other constraints kept as in the case above, the pressure plate screw connection is between 150x (straight wire) and 200x (wire not straight) likely to be more reliable in long term use.
Overall, the binding-head screw offers the best electrical wire connection quality and long term reliability, all other features being equal, because it offers the greatest bulk surface contact area and significantly-higher contact pressure. The pressure-plate connector is also likely to perform significantly better than the spring-clip connector.
(June 3, 2014) William said:
I am changing a wall plug with a double plug but the wires are too thick to go through the hole of the new device. HOW can I resolve this?
Use the screw terminals not the backward device.
Watch out. What are those large diameter wires? If aluminum you have a bigger safety hazard to address.
See ALUMINUM WIRING HAZARDS & REPAIRS - home
Is it safe to plug in an ac unit that runs 10 amps , into outlet that is backed wired, i had read that you don't like this method, the outlet is on third floor and is on a 15 amp breaker - Johnny B 5/2/w
Johnny, that's an interesting question and one I'm scared to answer - by online posting one cannot assure the electrical safety of your building.
Here we will illustrate three different types of electrical receptacles that can be wired from their back-side.
Our photo (left) illustrates a spec-grade 20-Amp, 125V rated electrical receptacle that looks as if it is "back-wired" - in fact while a wire can be wrapped around the terminal screws on this device, the screw is intended to be used to tighten a rectangular brass plate against a square metal nut (silver in color) that makes a very strong and positive connection over a good area of wire surface. This receptacle is marked on its back surface as CU Wire Only - copper only. [Click images to see enlarged details.]
That said, I agree that older, spring-type back-wired electrical connections (shown at below left) are not as reliable as connections made under a screw or clamp, as the total contact area between the back-wire spring edge and the wire surface is minimal.
Nevertheless, on a 15-A circuit using 15-A devices such as receptacles, the circuit and its devices are rated and intended to be able to support the 10-amp load you describe, so long as the sum of all of the items plugged into that electrical circuit don't overload it.
Thank you for responding, my town home was built in 1999, not sure if that is considered newer or older, lights do dim though when i use 10 amp vacuum . - Johnny B.
Backwiring electrical receptacles is a permitted installation and might be found in a 1999 home - but as we show above, there are two different approaches, the second of which is a better quality installation and is in our opinion more reliable. See the details just below.
Contractor-grade 15-A spring-type-connector back-wired electrical receptacles (below left) provide a single opening at each of the four terminals (two neutral wires - yellow arrows, and two hot wires - red arrows) on the back of the receptacle.
The white arrows point to the smaller rectangular opening giving access to a press-to-release spring that will allow removal of the wire, but we prefer not to re-use this type of back-wired receptacle. Tightening the screw at the main wire terminal (blue arrow) has nothing to do with the spring-clamp that is securing the back-wired terminal wire.
You can see that this receptacle also includes two binding head screw connectors on each side - silver screws for neutral wire (right side in the photo) and brass-coloured screws for hot or black wire (left side in the photo). I consider these screws a more secure electrical connection than the push-in backwire connectors though of course a bit more labour is involved as well.
Some newer heavy-duty 15-A back-wired electrical receptacles (below-left) do not rely on a simple spring-edge to contact the electrical wire, as we illustrate in our photo below. Rather, when the wire is inserted into any of 4 receiving holes on the back of the receptacle (red arrows) on the line side (black wire) and another 4 receiving openings on the load or neutral side (white wire) of the device.. When the terminal screw is tightened that actually snugs up a clamp that contacts a much larger surface area of the back-wired wire. That's a more secure connection mechanically. On this receptacle, instead on a single back terminal accepting a single wire, there are a pair of back terminal openings at each of the four terminal screws.
The 20-A rated electrical receptacle shown below is a variation on the 15-A pressure-clamp connector discussed just above. This receptacle dispenses with the round holes in the plastic receptacle back and exposes the wire clamps for a more clear view of what's happening. The electrical wire (copper-only according to markings on these device) is inserted between a copper face plate (red arrow) and a thicker silver colored base plate (green arrow). Turning the screw (blue arrow) at any of these four connectors (each of which will accept two wires) pinches the stripped wire-end between these two plates.
A comparison of crude contact areas between the copper wire and the different types of wire connectors is at RECEPTACLE WIRE-TO-CONNECTOR CONTACT AREA SIZES.
Below is a side-view of the tightened connector.
10/24/2015 Markus said:
Is there a picture somewhere that shows the flimsy spring clip contact against the inserted wire? I have a hard time convincing friends that the large and secure contact area of the clamp style backwire outlet is superior to the spring clamp.
Great suggestion, Markus. It's of course difficult to photograph the poor spring-clip-to-wire contact without disassembling the receptacle as the pushed-in wire covers the opening through which the spring is visible.
Shown at left is a close-up photograph showing the connection between the edge of a push-in type "backwire" electrical receptacle contact spring and the edge of a copper electrical wire. The photo at left was provided by Dr. Jess Aronstein,
You can see that the contact area on the spring-side of the connector is quite small - just the point at which the edge of the spring touches the rounded surface of the copper wire. In some of the spring clips that I've examined and not visible in the photo at left, we may find that the end of the spring clip that contacts the wire is cut out in a round profile or "notch" to increase the contact area against the rounded surface of the wire, but still the total contact area will be very small as only the edge of the angled clip touches the wire.
You can also infer that all of the spring force against the wire has to come from the arc of the copper spring.
[Click to enlarge any image]
Dr. Aronstein points out that in the connector shown at left there are two contact areas: the edge of the spring clip (red arrow #1) and possibly the larger surface area of the contact plate (red arrow #2). Certainly there will probably be at least some contact between the wire and surface #2 or the spring clip may not hold the wire in place at all.
I have added a sketch below to illustrate the types of contact between the electrical wire and the connector within or on the device such as an electrical switch or receptacle.
The sketch (below) compares the wire-to-receptacle (or switch) contact area sizes of an older type flat spring with the contact area of a compression plate or screw type connector pushing against the same wire size.
In the sketch below we depict the labeled items as follows
[Click to enlarge any image]
The large and small gray-colored arrows depict differences in force between the contacting device (spring or plate or screw) and the surface of the wire. The force exerted using a screw type connector will be significantly greater than the force exerted by the thin metal spring clip in older back wired electrical receptacles or switches.
Watch out: the older thin-spring type push-in back-wired electrical receptacle or switch hazard arises not just the very small surface area contact at the spring. After all, both types of wire-to-device connectors include a contact plate against which the spring or screw force the wire. But in a connector-spring-type "push-in" electrical wire connector, anything that weakens the spring - itself a thin flat copper component - such as age, re-use of the electrical device, removal and re-insertion of the wire, or changing wire sizes from a #12 to a smaller $14 wire gauge or possibly even bending forces exerted by the pushed-in wires as the device is pushed back into the electrical box are likely to weaken the contact force as well.
In a binding-head screw connection, Aronstein also points out, there are again two current paths: one at the interface between the wire and the contact plate (#2 in the right-portion of the illustration above) and a second through the screw to the connector plate (#4 in the illustration above) or through the under-side of the screw head to the wire directly depending on the connector type.
In my opinion the effects and thus the benefits of the greater contact area and the greater contact force of the newer screw-type connections are additive and are features not available using the simpler spring-clip push-in type back-wired electrical connectors shown here.
Watch out: Dr. Aronstein warns (Aronstein to DF 2015) that while the screw-clamp type connections are an improvement on electrical switches and receptacles that permit back-wiring, they are not fail-safe, warning that "... they sometimes fail due to the wire moving when the receptacle is pushed back into the [electrical] box, loosening the connection."
Below is a magnified view of the connection between the spring clip and the copper wire after it has been pushed in through the back-wire opening.
You can see the rounded notch in the end of the spring clip that we mentioned earlier (red arrow). In this connection the two current paths are through the wire to the notched end of the spring clip (red arrow) and through the wire to the contact plate (green arrow).
Our lab photographs below illustrate conditions that may reduce size of the contact area between the electrical wire and the electrical receptacle or switch when a push-in, back-wire type connector is in use. The factors described here are opinion and have not been peer-reviewed by a professional forensic electrical engineer or other industry experts. Use our page bottom CONTACT link or the comments box to offer suggestions or critique.
The photograph above illustrates the two intended contact areas between the push-in backwire electrical connector and a #14 copper wire: the contact spring (A in the photo) and the contact plate against which the spring pushes the wire (B in the photo).
At above left is a typical, relatively-straight #14 new copper wire pushed into the spring-connector of a push-in back-wire electrical receptacle connector. You'll notice that the wire at above-left is not perfectly straight. Some contact between wire and contact plate may be lost at arrow A. Even when trying to keep the wire straight some bending may occur from handling, during insertion into the electrical device, or near the end when the wire was cut to the proper insertion length - arrow B in the photo above.
Above is a curved #14 copper wire illustrating the reduction in contact between the wire and the contact plate against which the push-in wire connector spring is exerting force. Click to enlarge the photo and you'll see that the wire is in contact with the connector plate over about the wire length indicated by the red lines and double-headed arrow.
Curved wire ends may be more likely to occur in old-work, re-wiring, in electrical outlet or switch replacement and similar situations, particularly if the wire being inserted was previously connected by having been bent or curved around the shaft of the screw of an electrical receptacle switch that is being replaced.
The width of the contact plate surface is about 11mm (see the red scale). I estimate that the wire is in contact with the contact plate (ignoring any effects of the cut-out opening) over about 2mm. of that distance.
Additional wire-to-connector effects from notches, abrasion or damage to an older wire are not depicted here: I was using new #14 wire.
Above at left the photo shows a typical contact point between the notched end of the spring and the copper wire in a push-in backwire connector in an electrical receptacle.
The rounded notch appears to be designed both to increase the wire-to-spring contact area and perhaps to improve the cut or notch formed in the wire by the spring pressure - an effect that would probably improve the resistance against wire withdrawal as well as the wire-to-connector contact. The notch cut by the spring in the copper wire seems to be increased if the wire is rotated at all during installation or when the receptacle is pushed back into the electrical box.
Above at right is a close-up of the wire notch I'm discussing. You can see that notch increases the actual contact between spring and wire includes more than the mere sharp edge of the spring as it permits contact between the back side of the spring (red arrow) as well as the face or end of the spring (green arrow). Incidentally we note that the cut-out area of the contact plate (blue arrow) might reduce the contact surface between the wire and the contact plate against which the spring pushes the wire.
The article cited just below 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.
Please see RECEPTACLE WIRE-TO-CONNECTOR CONTACT AREA SIZES - separate article - for details.
This discussion has moved to WIRE-TO-CONNECTOR FORCE COMPARISONS
There we compare estimates of the connection force exerted by a spring clip wire connector and a binding head screw wire connector. Stronger contact surface force means a more reliable contact and better connector performance.
This topic has moved to BACKWIRED RECEPTACLE FAILURE PHOTOS
In that article we quote:
Low contact force between the wire and the receptacle connector where spring-type push-in backwired terminals are used on receptacles is probably the most significant reason that these connections can be expected to deteriorate over many years.
Continue reading at RECEPTACLE WIRE-TO-CONNECTOR CONTACT AREA SIZES for a discussion of the areas of wire-to-receptacle contacts on electrical receptacles., or select a topic from the More Reading links or topic ARTICLE INDEX shown below.
Or see ELECTRICAL OUTLET, HOW TO ADD & WIRE - home - for general wiring procedures, connections & advice for connecting electrical receptacles.
Or see ELECTRICAL RECEPTACLE CONNECTION DETAILS - where & how to connect black, white, red, green, ground wires.
Or see WIRE-TO-CONNECTOR PERFORMANCE SUMMARY for overall conclusions on this topic
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