14_2_NMB_electrical wire (C) D FriedmanElectrical Wire Size Required for Receptacles
How to choose the proper wire size for an electrical plug outlet or wall plug

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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.

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What Size Electrical Wire Should I Use When Wiring an Electrical Outlet (Receptacle)?

Common house electrical wiring sizes (C) Carson Dunlop AssociatesThe 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

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.

14_2_NMB_electrical wire (C) D Friedman

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.

Details are at NUMBER of WIRE CONDUCTORS needed


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?

20 Amp electrical outlet (C) D FriedmanHow many receptacles can be wired To one 20 amp circuit No. 12. Wire - John K.


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

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?

RV Power Outlet (C)

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.

Curved locking prong electrical plug rated for 30A (C)

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 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. -

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.


Continue reading at WIRE SIZE FAQs or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see COPPER-CLAD ALUMINUM WIRE for information about copper clad aluminum wiring

Or see SE CABLE SIZES vs AMPS for more wire size guidelines


Or see UNDERGROUND SERVICE LATERALS for more wire sizing guidelines

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SIZE of WIRE REQUIRED at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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