Ground Wiring Details when Installing an Electrical Receptacle
The illustration at left shows the typical wiring of an electrical outlet or "receptacle", courtesy of Carson Dunlop Associates.
You can see the incoming ground wire aimed towards its connection point - the green screw on the electrical receptacle. But additional grounding connections are often required as well - as we will detail here.
Watch out: Electrical components in a building can easily cause an electrical shock, burn, or even death.
Even when a hot line switch is off, one terminal on the switch is still connected to the power source.
The bare ground wire - this wire, visible in our photo as the un-insulated copper wire seen between the white (top) and black (bottom) wires, connects to the green ground screw usually found on the bottom of the electrical receptacle (photo at left).
What if there is no ground wire in the circuit?
If you are connecting an electrical receptacle to an older circuit that just provides hot and neutral wires, that is, no ground, and assuming you're not going to re-wire the circuit to provide a proper ground, there is no ground wire to hook-up. In this case you must use a type of electrical receptacle that does not include the third opening for the grounding prong on the wall plug.
Proper grounding connections for an electrical receptacle
The electrical receptacle must be properly connected to the building grounding system - not shown in our sketch.
That connection is made from the ground screw on the receptacle to the grounding conductor (usually bare copper) in the wire leading back to the electrical panel where in that location it is connected to a grounding bus and from that bus to the building grounding system, one or more earth-driven electrodes or their equivalent.
The incoming ground wire is connected to the ground terminal on the electrical receptacle (usually a green screw such as shown in our photo at left).
If the junction box is metal (not plastic) the ground wire is also connected to the metal junction box itself, usually by a special green screw that connects to a tapped threaded hole on the junction box back side, or by a grounding clip that secures the ground wire to the edge of the metal box.
If there is more than one feeder wire entering the electrical junction box then all of the grounds are connected together as well as being connected to the ground screw on the receptacle itself.
If the junction box is plastic, you're done.
If the junction box is metal (photo above right) then a ground wire is also connected to the metal box using an approved grounding screw or clamp device to tie the wire end to the box. Most metal electrical boxes have a threaded hole intended for use as a connection point for the ground wire connecting screw, as illustrated in our photograph (left).
In 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 and if it's metal, to the junction box itself.
Watch out: Don't rely on the connection between the electrical outlet's steel mounting strap and the steel screw openings of the junction box to provide the ground connection. That's not a legal ground and it's unreliable
Use the a ground wire and ground screw on the receptacle itself to be sure that this important safety feature is correctly installed.
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, as the connection through the receptacle mount screws is simply not reliable.
Watch out: mis-wired electrical receptacles are dangerous. This article series describes how to choose, locate, and wire an electrical receptacle in a home.
Reader Question: how is an electrical outlet wired to the electrical panel?
how do wire the outlet plug to the electrical panel - Anon
Anon, the electrical circuit that powers an "outlet plug" or receptacle is connected, usually through building walls, ceilings, or floors, from the first receptacle in the particular series back to a fuse or circuit breaker connection in the electrical panel. The fuse or circuit breaker, by its connecting mount in the electrical panel, receives electrical power from the income electrical service.
Ultimately in the electrical box where the electrical receptacle ("wall plug" or "wall outlet" in common speech) is mounted,
The incoming black or hot wire in the electrical box is connected to the bronze or brass colored screw on the receptacle; the other end of the hot wire is connected to the circuit breaker or fuse connector screw in the electrical panel.
The incoming white or neutral wire in the electrical box is connected to the white or silver colored screw on the receptacle and the other end of the neutral wire connects to the neutral bus in the electrical panel.
The incoming ground wire or grounding conductor in the electrical box connects to the green ground screw on the receptacle and also, by extension or pigtailing, to the junction box if the electrical box is metal not plastic. In the electrical panel the other end of the grounding conductor connects to a ground bus bar that in turn is connected to earth, usually by grounding electrodes at the building exterior or in some jurisdictions to a buried metal water pipe.
Reader Question: what do I do with the screws to which no wire is connected on a conventional "plug" (wall receptacle)?
At the end of a circuit, I'm only using 2 of the 4 screws on a conventional plug. What should I do with the 2 unused screws? Should they be screwed all the way in? Or left partially unscrewed? Or does it matter? - Chris Rasko 7/8/12
regarding the un-used screw terminals on an electrical receptacle, you should simply screw them all the way in and leave them alone. Don't remove the screws - it's not necessary, they are deliberately hard to remove completely, and they could be needed in some future wiring change.
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When wiring an electrical receptacle, what do I do with the red wire? Can I ground it?
I have a 3 wire (Black, White, Red and ground) feeding a outlet and I want to add another receptacle to run further down the line. The line out is 14/2. What do I do with the Hot Red wire? Can i attach it to the ground. - Rick
NEVER connect a hot (red or black) wire to ground (nor to the white neutral wire) - doing so would form a dead short, should trip a breaker, or if not, could cause a fire or could cause a dangerous shock.
If there is a hot wire that is not used in a junction box, SOP would be to cap it off with a twist-on connector.
It sounds as if you'd be best served by hiring a licensed electrician.
After capping off the red wire, can I extend the line to the next plug by following the diagram above and adding the black and white wires to the respective second screw connections?
You see, the wiring has already been installed by the builders and they left the boxes without receptacles so all I have to do is connect them to the. I don't know why the extra red wire is there. It was done over a year ago. I want to finish the connections. It runs 14/3 and then 14/2. That's why I have the extra red.
Reply: how three-wire circuits or multiwire branch circuits with a common neutral (and ground) are used and wired
Rick often electricians run a 3-wire system into a building area using two hot wires and a shared neutral, to permit providing two circuits in an area while having to pull just one wire to the area. But to sort out how your wires were connected and are being used requires some expertise, visual inspection, and testing using a VOM.
Take a look at multi-wire branch circuit wiring information and hook-up details at MULTI-WIRE CIRCUITS.
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 The 2008 NEC National Electrical Code (ISBN 978-0877657903) Online Access LINK (you'll need to sign in as a professional or as a visitor)
 Special thanks to our reader Steve who pointed out prior errors in our illustrations.
 Simpson Strong-Tie, "Code Compliant Repair and Protection Guide for the Installation of Utilities in Wood Frame Construction", web search 5/21/12, original source strongtie.com/ftp/fliers/F-REPRPROTECT09.pdf, [copy on file as /Structures/Framing/Simpson_Framing_Protectors.pdf ]. "The information in this guide is a summary of requirements
from the 2003, 2006 and 2009 International Residential Code
(IRC), International Building Code (IBC), International Plumbing
Code (IPC), International Mechanical Code (IMC), 2006 Uniform
Plumbing Code (UPC) and the 2005 National Electrical Code."
"Electrical System Inspection Basics," Richard C. Wolcott, ASHI 8th Annual Education Conference, Boston 1985.
"Simplified Electrical Wiring," Sears, Roebuck and Co., 15705 (F5428) Rev. 4-77 1977 [Lots of sketches of older-type service panels.]
"How to plan and install electric wiring for homes, farms, garages, shops," Montgomery Ward Co., 83-850.
"Simplified Electrical Wiring," Sears, Roebuck and Co., 15705 (F5428) Rev. 4-77 1977 [Lots of sketches of older-type service panels.]
"Home Wiring Inspection," Roswell W. Ard, Rodale's New Shelter, July/August, 1985 p. 35-40.
"Evaluating Wiring in Older Minnesota Homes," Agricultural Extension Service, University of Minnesota, St. Paul, Minnesota 55108.
"Electrical Systems," A Training Manual for Home Inspectors, Alfred L. Alk, American Society of Home Inspectors (ASHI), 1987, available from ASHI. [DF NOTE: I do NOT recommend this obsolete publication, though it was cited in the original Journal article as it contains unsafe inaccuracies]
"Basic Housing Inspection," US DHEW, S352.75 U48, p.144, out of print, but is available in most state libraries.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
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