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What size electrical wire should you use when hooking up an electrical receptacle (wall plug or electrical outlet)?
Here we explain the choice of No. 14 or No. 12 copper wire for 15A and 20A electrical circuits where receptacles are being wired.
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.
What Size Electrical Wire Should I Use When Wiring an Electrical Outlet (Receptacle)?
The electrical wire used for the receptacle circuit must be the proper type in size (thickness or gauge) and number of conductors for the ampacity of the electrical circuit
The electrical circuit must be properly protected by the right fuse or circuit breaker
For a 15-Amp circuit use #14 copper wire (or #12 copper-clad aluminum wire). The fuse or circuit breaker feeding this circuit is rated for 15 amps.
For a 20-Amp circuit use #12 copper wire (or #10 copper-clad aluminum wire).The fuse or circuit breaker feeding this circuit is rated for 20 amps.
For a 30-Amp circuit use #10 copper wire and a fuse or circuit breaker rated for 30 Amps.
Note1: it's safe and OK to use a smaller (lower ampacity) fuse or circuit breaker, such as a 15-amp fuse protecting a circuit wired with #12 copper wire.
Note2: it's generally safe and OK to use a larger size electrical wire, and a larger wire size may in fact be required for longer wiring runs. Generally you want a 3% or less voltage drop across the wire from source to point of use. For the U.S., electrical wire sizes vs. circuit ampacity are given in National Electric Code Table 310-16.
Watch out: in complex circuits that have many connections within a single junction box you could get into trouble: the number of connections that are allowed within an individual junction box depends on the wire size and the size of the box itself.
So increasing just the wire size could require that you use a larger electrical box. Sketch courtesy of Carson Dunlop Associates.
Electrical Conductors: 2-Wire? 3-Wire? 2-Wire with Ground?
The electrical wire must have the proper number of conductors. In modern electrical circuits used to wire receptacles (electrical outlets).
Typically an electrical receptacle is wired with two insulated wires and a bare ground wire, all three of which are encased in a plastic (NMC) or metal (BX) jacket.
You'll see this wire labeled as 14/2 Type NM B with ground (photo at left) or 14/2 Type NM C with ground.
These wires are color coded black, white, and bare (photo below right). Sketch at left showing the number of conductors in types of electrical wire is provided by of Carson Dunlop Associates.
Our photo at page top is not an example of a proper electrical outlet installation.
This website provides information about a variety of electrical hazards in buildings, with articles focused on the inspection, detection, and reporting of electrical hazards and on proper electrical repair methods for unsafe
electrical conditions. Critique and content suggestions are invited. Credit is given to content editors and contributors.
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.
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: Wire size required for a 30A Wall Plug for an RV Power Drop? What about voltage drop on the circuit?
2016/09/11 Scott Davidson said:
I would like to install a 30 amp 120 volt outlet to plug in my rv camper For the length of the run it says i should use number 8 /2 wire it is about 42 feet from panel to where plug would be installed is this correct Thanks Scott
Reply: when to use a larger size electrical wire for long wiring runs - keep voltage drop under 3%
Wire sizes vs amps vs run lengths are given in the National Electric Code Table 310-16. ( Allowable ampacitys of conductors ). [Consult The Electrical Code for further details.]
Watch out: you do NOT want to provide 30 Amps to a typical electrical receptacle ("outlet" or "wall plug") as devices connected to those wall receptacles expect to be protected against overcurrent at either 15A or 20A; you'll see that a 20A receptacle looks a bit different, having an extra slot. Above I illustrate a typical RV temporary outlet box for taking power to an RV. This is a GE unit sold at various electrical suppliers and building supply stores.
Another sort of 30A electrical receptacle and the wall plug that plugs into it uses a different design: you'll see that the openings in the recptacle are curved and the plug's "prongs" are also curved or in other designs the receptacle and mating plug use an angled opening and prong. DO NOT just run 30A to a typical wall receptacle.
Typically a #10 wire can handle 30A 120V loads and lengths such as what you describe. Tell me where you got the requirement for #8.
You're right that it's safer and sometimes required to go to a heavier wire gauge (#8) for longer runs. If your installation permist a 3% voltage drop, then a typical wire calculator or voltage drop calculator (there is a plethora of them online) will say you could have used a 42 foot length of #10, so #8 sounds safe to me. I used both www.elec-toolbox.com/calculators/voltdrop.htm and Southwire; Southwire also has a nice voltage calculator and as a wire manufacturer they of course have both an interest in proper wire use and engineers on staff.
Watch out: when going to a larger wire size, depending on the number of wires and connections in the electrical boxes on the circuit you may need to go to a larger electrical box size to meet the wiring space requirements. See ELECTRICAL BOX SIZING TABLE
Watch out also: ordinary electrical wire is not intended for use as an "extension cord" and is not intended for use being repeatedly plugged in and out nor left exposed and unsupported. .
Here's what Southwire's voltage drop calculator told us when we assumed the wire was 120V, 30A, 42 feet long, "underground / direct" though in conduit or overhead didn't change the result by much: all instances gave less than the typically-allowed maximum of 3% voltage drop.
1 conductors per phase utilizing a #10 Copper conductor will limit the voltage drop to 2.21% or less when supplying 30.0 amps for 42 feet on a 120 volt system.
For Engineering Information Only:
30.0 Amps Rated ampacity of selected conductor
1.1417 Ohms Resistance (Ohms per 1000 feet)
0.05 Ohms Reactance (Ohms per 1000 feet)
3.5999999999999996 volts maximum allowable voltage drop at 3%
2.644. Actual voltage drop loss at 2.21% for the circuit
0.9 Power Factor
**Note to User: All ampacity values are taken from the Section of 310-15 of the NEC. The conductor characteristics are taken from Table 9 of the NEC. The calculations used to determine the recommended conductor sizes for branch circuits are based on 60°C ampacity ratings for circuits rated 100 amps or less or marked for use with #14 AWG - #1 AWG. Circuits rated over 100 amps or marked for conductors larger than #1 AWG are determined using 75°C ampacity ratings. Calculations to determine service and feeder conductor sizes are based on overcurrent device ratings rather than actual expected loads which are conservative and may yield oversized conductors. No calculations take into account temperature correction factors or conductor de-rating.
This voltage drop calculator is applicable only to NEC applications. It does not optimize conductor sizes for several different loads at various points in a circuit. The total combined load and length of the circuit must be used. Consult with an engineer if your application requires more complex engineering calculations.- www.southwire.com/support/voltage-drop-calculator.htm
Watch out: finally, I note that you plan on running 8/2 wire? Surely you mean 8/2 with ground, right? Check with your electrical department about required permits and safety inspections.
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Timothy Hemm has provided photographs of various electrical defects used at the InspectAPedia TM Website. Mr. Hemm is a professional electrical inspector in Yucala, CA.
Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Mr. Cramer serves on the ASHI Home Inspection Standards. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com
John Cranor is an ASHI member and a home inspector (The House Whisperer) is located in Glen Allen, VA 23060. He is also a contributor to InspectApedia.com in several technical areas such as plumbing and appliances (dryer vents). Contact Mr. Cranor at 804-747-7747 or by Email: firstname.lastname@example.org
 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
Recommended books on electrical inspection, electrical wiring, electrical problem diagnosis, and electrical repair can be found in the Electrical Books section of the InspectAPedia Bookstore. (courtesy of Amazon.com)
The ILLUSTRATED HOME illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
The HOME REFERENCE BOOK - the ENCYCLOPEDIA of HOMES, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
The HORIZON SOFTWARE SYSTEM manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones