Plastic electrical box (C) D Friedman Electrical Box Types & Sizes for Receptacles
How to choose the proper type of electrical box when wiring electrical receptacles (wall plugs or "outlets")
     


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Guide to types of electrical receptacles (wall "outlets" or "wall plugs"):

How to choose the right type of electrical receptacle when adding or replacing a wall outlet in a building. Here we describe matching 15-Amp receptacles to 15-Amp circuits, 20-Amp receptacles to 20-Amp circuits, two-wire receptacles where no ground is present, GFCI and AFCI electrical receptacles, and the proper electrical box to hold and mount these devices.

This article series describes how to choose, locate, and wire an electrical receptacle in a home. Electrical receptacles (also called electrical outlets or "plugs" or "sockets") are simple devices that are easy to install, but there are details to get right if you want to be safe.

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Choose the Proper Electrical Junction Box When Adding a Receptacle

Arcing in an under-sized electrical box (C) Carson Dunlop Associates
  • The proper sized and type of junction box must be used to house the electrical receptacle, must be properly secured in the wall, and must be located at the proper height from the floor.
  • The National Electrical Code Article 314 contains complete details and tables of electrical box sizes in dimensions and cubic inches and should be consulted for complete accuracy because the actual size of the box required, in cubic inches, depends on the number of wires that will be within that enclosure. We give some basic electrical box size examples just below.
  • If the junction box is too small you face several problems such as crowding which forces wires to be jammed and bent into the box, increasing the chance of a loose connection or damaged wiring, and worse, arcing.
  • If the clearance between the metal box and other live electrical parts is too small, arcs and short circuits could occur, as we depict in this sketch courtesy of Carson Dunlop Associates.
NEC Table 314.16 (A) Metal Electrical Boxes for Devices
Box Dimensions (Inches)
& Trade Name
Minimum Cu.In. Max # Conductors AWG 14 Max # Conductors AWG 12
3 x 2 x 1 1/2" Device Box 7.5 cu. in. 3 3
3 x 2 x 2 " Device Box 10 5 4
3 x 2 x 2 1/2" Device Box 12.5 6 5
3 x 2 x 3 1/2" Device Box 18 9 8
4 x 2 1/2 x 1 1/2" Device Box 10.3 5 4
4 x 2 1/2 x 1 7/8" Device Box 13 6 5
4 x 2 1/2 x 2 1/8" Device Box 18 7 6
4 x 1 1/4" square 18 9 8
4 x 2 1/4" square 21 10 9
4 x 2 1/4" square 30.3 15 13
Notes: The National Electrical Code Article 314 contains complete details and tables of electrical box sizes in dimensions and cubic inches and should be consulted for complete accuracy because the actual size of the box required, in cubic inches, depends on the number of wires that will be within that enclosure. This is an excerpt pertinent to wiring electrical receptacles and switches; other wire sized and applications are given in the source NEC.
NEC 314.16(B) Electrical Box Volume Required Per Conductor (wire)
Conductor size (wire size) Free space within the box for each conductor
No. 14 2 cubic inches
No. 12 2.25 cubic inches
No. 10 2.5 cubic inches
Notes: This is an excerpt pertinent to wiring electrical receptacles and switches; other wire sized and applications are given in the source NEC.

Typical Electrical Boxes for Single Receptacles

Typical electrical junction boxes used for receptacle installation to accommodates a single receptacle are 2" x 4" metal or plastic boxes of varying depth. Choose a deeper box if you have more than the minimum number of wires entering & leaving the box or it will be over-crowded and may violate building electrical codes.

It's likely to be permitted and safe to install a larger sized junction box than you need for an outlet or switch or wire splice, but you may need to purchase a special adapter-cover for the box in order to properly secure an electrical outlet therein. Installing a junction box that is too small is illegal and unsafe.

Typical 4-inch Electrical Boxes used for Single (or more often) Duplex Electrical Receptacles

Below-left we show a typical 4-inch metal junction box, and below-right a common and deeper plastic junction box.

Metal electrical box (C) D Friedman Plastic electrical box (C) D Friedman

Electrical junction boxes are required for all wall plugs ?

Burned up electrical receptacle (C) Daniel FriedmanReader Question: Do I really need an electrical box to put a new wall plug in ? - Thomas

Reply:

Yes, Thomas, electrical devices such as switches and receptacles (wall plugs) need to be mounted in a code-approved plastic or metal receptacle (box) for fire safety as well as to assure that the device is mechanically secure.

In fact when you purchase a "wall plug" you'll see that its metal mounting ears and screws are spaced and designed to connect to an electrical box.

Watch out: while it's physically possible to install a wall receptacle or "plug" without using an enclosure, doing so is dangerous, risking fire and shock, and of course, it's also illegal in virtually every building code jurisdiction.

Take a look at our photograph at left - a fire was contained within this electrical junction box. Had the box been omitted there is a good chance the fire would have spread to the building itself.

When we hear a question like this it makes me very afraid for you and for future building occupants - as amateur electrical wiring is dangerous.

Details about how to wire up an electrical receptacle are at CONNECTION DETAILS - where to connect black, white, red, green, ground wires .

Reader Question: When adding an electrical outlet in a garage, what's code: metal or plastic junction box?

I am putting outlet in garage wall that has kitchen on the other side. What is code, plastic or metal? I would think in a garage fire that a plastic box would melt and fire would go through the wall faster? - Steve Smith

Reply:

Steve both plastic and metal receptacle boxes are code-approved and neither, properly installed and wired, should violate the fire-rating of the wall.

Reader Question: how do I increase the projection of outlets into a room so I can add a kitchen backsplash

Gang Box Extender Electrical Box Extension from Arlington IndustriesI am unable to find instructions on how to increase the projection into the room of existing electrical outlets so that I can tile the kitchen backsplash and have the outlets be at the appropriate depth for use and safety. Do I move forward the box to which the outlet is screwed and if so how? - Anne 3/22/12

Reply: use electrical box extenders - shop for an "electrical box extension" of the proper thickness

Anne,

Building suppliers like Home Depot and also your electrical supply house sell "box extenders" in varying thicknesses, made of plastic, code approved, for the purpose you describe. The electrical box extender is sized and shaped to match the electrical receptacle box to which it is to fit.

By removing the electrical receptacle from its mount on the existing box, the box extender is fit as a sort of large rectangular plastic washer, mounting between the existing box edge or surface and the mounting ears of the receptacle or switch.

Electrical box extensions are sold in plastic and steel and in thicknesses from about 1/8" up to an inch or even more. The plastic electrical "gang box extension" shown at above left is produced by Arlington Industries but there are several manufacturers. Just choose an electrical box extender that brings your receptacles far enough forward to suit the thickness of the kitchen backsplash or tile.

Watch out: don't try a makeshift substitute using washers or junk - that's an improper and unsafe repair, leaving a gap around the electrical box sides.

Reader Comments:

Anne,
I'm in the midst of a remodel that posed the same 'problem'. Work box extender rings are available at Home Depot and Lowes in the electrical department. They are plastic, color-coded frames that fit between the front edge of the box and the outlet/switch. The screws that secure the outlet/switch to the box also secure the frame in place. The frames are available in multiple thicknesses. I suggest you take a tile sample with you so that you can get the correct thickness for your project. Depending on the thickness of the tile, you may need to combine two frames of different thicknesses.

While I was changing a failed plug I noticed that the box was too deep. I looked into extenders, and plastic ones (Arlington BE1) are less expensive. Are CSA approved plastic box extenders code compliant for homes? - Gary 7/19/12

Question: how many electrical receptacles are allowed on a 20-amp circuit? How many receptacles on a 15-amp circuit?

How many receptacles can be wired To one 20 amp circuit No. 12. Wire - John K.

Reply:

20 Amp electrical outlet (C) D FriedmanJohn K:

Our photo (left) shows a 20-Amp electrical receptacle - you can recognize it by that horizontal opening that makes the left-hand slot look like the letter "T" on its side.

In general, the Electrical Code [NEC] allows

  • 10 electrical receptacles to be wired on a 15-Amp (#14 copper) wire circuit, and the Electrical Code [NEC] allows
  • 13 receptacles on a 20-amp (#12 copper) wire circuit.

    Watch out: When purchasing the receptacles to use on a 20A circuit, be sure to also buy receptacles that are themselves rated for 20Amp use.

    You'll see that those least-costly receptacles found in a big pile at building supply stores are more likely intended only for 15-Amp use.

Our photo (left) illustrates an electrical receptacle intended for use on a 20-Amp circuit.

Notice that extra horizontal slot? You won't see that on a 15-Amp electrical receptacle.

Reader Question: what is the minimum height that indoor house wiring must be above the ground or floor level?

When running wire for a basement, is there a min height the wires must be off the ground? Not the outlet box, but the wire running through the joists. Justin Sheppard

Reply:

No, Justin. But if there is the slightest danger that wires will be nicked by someone driving a nail into a stud though which the wires are run be sure to use steel plates to protect the wire where it passes through the studs. Simple nail plates are available at any building supplier.

See HEIGHT above FLOOR for OUTLETS for details.

Question: are electrical junction boxes required for wall plugs?

Burned up electrical receptacle (C) Daniel FriedmanDo I really need an electrical box to put a new wall plug in ? - Thomas

Reply:

Yes, Thomas, electrical devices such as switches and receptacles (wall plugs) need to be mounted in a code-approved plastic or metal receptacle (box) for fire safety as well as to assure that the device is mechanically secure. In fact when you purchase a "wall plug" you'll see that its metal mounting ears and screws are spaced and designed to connect to an electrical box.

Watch out: while it's physically possible to install a wall receptacle or "plug" without using an enclosure, doing so is dangerous, risking fire and shock, and of course, it's also illegal in virtually every building code jurisdiction.

Take a look at our photograph at left - a fire was contained within this electrical junction box. Had the box been omitted there is a good chance the fire would have spread to the building itself.

When we hear a question like this it makes me very afraid for you and for future building occupants - as amateur electrical wiring is dangerous.

 

Question: Which end of electrical outlets go "up"? The ground hole should be up, down, or sideways?

Electrical outlet with ground connector down (C) Daniel Friedman Electrical outlet with ground connector down (C) Daniel Friedman

Can the outlet be installed any way? For example ground hole facing up, down, or sideways? thanks, - Anon

Reply:

Anon, the position of installation of an electrical outlet won't affect its operation and should not normally affect its approval by the electrical inspector.

In some areas I see the outlet installed with the ground connector always "up" as in our photo at left, though to me that's less attractive than the position shown in our electrical outlet photo at far left.

I've also seen arguments expressing the OPINION that the position of the grounding pin connector might help resist the tendency of a plug to fall out of its connection. That's nonsense. If a plug is falling out of a receptacle, one of the two objects is worn or damaged and should be replaced to assure a safe, mechanically secure connection.

Question: When adding an electrical outlet in a garage, what's code: metal or plastic junction box?

I am putting outlet in garage wall that has kitchen on the other side. What is code, plastic or metal? I would think in a garage fire that a plastic box would melt and fire would go through the wall faster? - Steve Smith

Reply:

Steve both plastic and metal receptacle boxes are code-approved and neither, properly installed and wired, should violate the fire-rating of the wall.

Question: can I connect a pigtail from multiple hot, neutral, or ground wires over to a receptacle

I have 2 receptacles that are both side and back wired, 3 hot and 3 neutral wires. I eliminated one receptacle (capping the 3 wires together) but want to keep the other. Is it safe to just run a pigtail from the 3 wires to the receptacle? - Greg

When wiring multiple boxes in series, how do you connect both incoming and outgoing ground wires to the back of the receptacle? With 12 ga. wire, only one wire will fit under the green screw (and not very tightly, at that - there's no washer or clamp.) - Bob M.

Reply:

Yes, Greg, that's a common practice. Be sure that your junction box is big enough to contain all of the wires and twist-on connectors.

Bob, similar to Greg's question, I see two approaches to hooking up the ground wire in junction boxes and at electrical receptacles.

  1. If the incoming ground wire from the feed circuit was left long enough, it can be run continuously, connected to a grounding screw that connects the wire to the metal junction box (skip this step if plastic junction boxes are in use), on to the ground screw terminal at each electrical receptacle, and ending with a ground clamp crimp connector that ties the incoming ground to the ground wire of the outgoing wire that continues to the next junction box.
  2. If the incoming ground wire is not long enough to run as above, then an additional length of ground wire is pigtailed to the incoming ground and makes the other connections I've described above.

Electrical receptacle mounting strap and screw are not a ground (C) D FriedmanIn sum, all of the grounds are tied together in the box: the incoming ground, outgoing ground, and ground wires to each of the electrical receptacles.

Watch out: while the electrical receptacle ground may also be electrically connected to the metal strap that mounts the receptacle to the junction box (photo at left), and while the junction box may be metal, do not rely on the receptacle mounting screws and receptacle strap-to-box contact to serve as the grounding connection.

It's easy for the receptacle mounting screws to be deliberately left loose or to work loose - making that ground connection unreliable. Use a ground wire.

Question: how many electrical receptacles are allowed on a 20-amp circuit? How many receptacles on a 15-amp circuit?

How many receptacles can be wired To one 20 amp circuit No. 12. Wire - John K.

Reply:

20 Amp electrical outlet (C) D FriedmanJohn K:

Our photo (left) shows a 20-Amp electrical receptacle - you can recognize it by that horizontal opening that makes the left-hand slot look like the letter "T" on its side.

In general, the Electrical Code [NEC] allows

  • 10 electrical receptacles to be wired on a 15-Amp (#14 copper) wire circuit, and the Electrical Code [NEC] allows
  • 13 receptacles on a 20-amp (#12 copper) wire circuit.

    Watch out: When purchasing the receptacles to use on a 20A circuit, be sure to also buy receptacles that are themselves rated for 20Amp use.

    You'll see that those least-costly receptacles found in a big pile at building supply stores are more likely intended only for 15-Amp use.

Our photo (left) illustrates an electrical receptacle intended for use on a 20-Amp circuit.

Notice that extra horizontal slot? You won't see that on a 15-Amp electrical receptacle.

Question: how to fix loose electrical receptacles in a ceramic tile or glass mirror wall wall

Is there a way to repair electrical outlets on finished (glass and ceramic tile) walls that were not installed properly without damaging the tile? The outlets and the covers pull away from the wall when the electrical cord plug in removed?
Also, what does it mean when an electrical switch with multiple switches which control recessed lights, the ceiling fan and light on the ceiling fan gets hot; what is happening? Is this a fire hazard? - Mrs. Spencer

Reply:

Mrs. Spencer:

It sounds as if you need a licensed electrician to check and secure your loose electrical outlets - I agree that a loose electrical receptacle is unsafe. But an inspection is needed to understand the underlying problem. It could be simply tightening screws, or it could be that the electrical box itself is not adequately secured in the wall. Luckily there are retrofit parts that can be used and inserted along the box to make it secure, usually without disturbing the surrounding ceramic tile.

Some dimmer switches use a resistor to dim the light and it is common for them to get warm. Very hot - a subjective judgment for homeowners - may indeed be a fire hazard and should be investigated.

Question: how do I increase the projection of outlets into a room so I can add a kitchen backsplash

Gang Box Extender Electrical Box Extension from Arlington IndustriesI am unable to find instructions on how to increase the projection into the room of existing electrical outlets so that I can tile the kitchen backsplash and have the outlets be at the appropriate depth for use and safety. Do I move forward the box to which the outlet is screwed and if so how? - Anne 3/22/12

Reply: use electrical box extenders - shop for an "electrical box extension" of the proper thickness

Anne,

Building suppliers like Home Depot and also your electrical supply house sell "box extenders" in varying thicknesses, made of plastic, code approved, for the purpose you describe. The electrical box extender is sized and shaped to match the electrical receptacle box to which it is to fit. By removing the electrical receptacle from its mount on the existing box, the box extender is fit as a sort of large rectangular plastic washer, mounting between the existing box edge or surface and the mounting ears of the receptacle or switch.

Electrical box extensions are sold in plastic and steel and in thicknesses from about 1/8" up to an inch or even more. The plastic electrical "gang box extension" shown at above left is produced by Arlington Industries but there are several manufacturers. Just choose an electrical box extender that brings your receptacles far enough forward to suit the thickness of the kitchen backsplash or tile.

Watch out: don't try a makeshift substitute using washers or junk - that's an improper and unsafe repair, leaving a gap around the electrical box sides.

Reader Comments:

Anne,
I'm in the midst of a remodel that posed the same 'problem'. Work box extender rings are available at Home Depot and Lowes in the electrical department. They are plastic, color-coded frames that fit between the front edge of the box and the outlet/switch. The screws that secure the outlet/switch to the box also secure the frame in place. The frames are available in multiple thicknesses. I suggest you take a tile sample with you so that you can get the correct thickness for your project. Depending on the thickness of the tile, you may need to combine two frames of different thicknesses.

While I was changing a failed plug I noticed that the box was too deep. I looked into extenders, and plastic ones (Arlington BE1) are less expensive. Are CSA approved plastic box extenders code compliant for homes? - Gary 7/19/12

Question: electrical outlet height requirements

I was looking at some height requirements on electrical outlets this is a very informational site.

thanks Jerm 4/19/12

Reply:

Jerm, in the article above at HEIGHT above FLOOR for OUTLETS we give the data you want. Let me know if anything is unclear.

Question:

I have one line with power coming into a box that will have one two way switch, one three way switch and one power line exiting. Is it proper to splice the incoming black wire to make 3 black wires by pig tailing with wire connectors and doing the same for the white wire and ground? - is it ok? 12/13/12

Reply:

I'm sorry but I'm confused by the question. I think it's safe to say that in general it's common practice to use a twist-on connector to splice pigtails or individual wires at an individual hot or neutral wire where more connections are needed than fit with the original wire.

Just watch out to avoid violating the space or number of connectors permitted in a junction box of the particular size you're working on.

 

 

 

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