Electrical wire sizes & gauges:

These Tables of Electrical Service Entry Cable Sizes, Electrical Circuit Wire Diameters, Circuit Ampacity, Allowable Voltage Drop, & Wire Size Increase based on Run Length assist in determining the electrical service size or other required electrical wire sizes at buildings.

We include tables of aluminum or copper wire sizes for long runs of service entry cables.

This article series gives photos and tables of electrical service entry cable sizes, electrical branch circuit wire sizes, bell wire, telephone wire, thermostat wire, and ampacity or fuse/circuit breaker ratings.

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Here we discuss and present a table of electrical wire sizes, ampacity, and fuse or circuit breaker sizes for common residential electrical wiring circuits.

What is the diameter of service entry electrical cabling? What are the common diameters of household copper or aluminum electrical wiring?

What is the diameter of thermostat wire, telephone wire, bell wire?

How to determine the size, capacity, or ampacity of electrical service at a building. Illustration of common electrical wire sizes for both service entry cables and in-building electrical circuits: wire size versus circuit ampacity and fusing requirement.

[Click to enlarge any image]

"How do I determine the service amperage?" Start by taking a look at the service entry cables outside and at their entry into the electrical panel.

A quick look can tell us if the property is served by 240V or only a 120V service, even before measuring the gauge or wire thicknesses that we discuss below.

In the photo above we see a three wire mast-head: 3 wires means that there are two 120V lines + a power company neutral. Two 120V lines usually give us a 240V service. But we did not like the position of that weather-head, and we considered that water may be entering the SEC.

This article series explains how to estimate the electrical service size (or "electrical power" or "service amps") at a building by visual examination of the service entry cables, electric meter and meter base, electrical service panel, main switch, and other details.

The tables above are for electrical circuits up to 50 Amps.

For larger wire sizes (higher Amps) (above 50A)

see SE CABLE & WIRE SIZES FOR LONG RUNS

The amperage that can be supported by any electrical wire is a function of its materials and diameter and the run length.

Often the actual cable type and size is printed right on the cable insulation. Otherwise some rough measurements of cable diameter are in order.

Carson Dunlop Associates' sketch shows common electrical wire sizes for both service entry conductors and in-building branch circuit wiring.

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We use to use a plastic vernier caliper or other non metallic instrument to measure external (insulated) width of the whole cable as a reasonable guess at wire ampacity. Some inspection tool companies sell, and others give-away a plastic "wire gauge" with notches labeled to tell electrical inspectors the cable size for aluminum and copper SECs. Or you can make your own.

For a simplified table matching service entry cable size to ampacity

see SERVICE ENTRY WIRING & AMPACITY - a separate article.

Branch circuit electrical wire sizes such as the #14 copper electrical wire shown just below are included in the tables on this page where we give wire sizes for circuits from 15A to 50A.

Above is a close-up photo of a #14 copper wire against a millimeter scale. The divisions on the scale of my metal ruler each represent 1 mm. You'll see that this wire looks as if it's about 1.5 mm in diameter. [Thanks to reader Max for careful reading 2018/06/14]

The actual wire diameter of #14 copper wire can vary among manufacturers and wire types.

The wire size table below gives the typical diameter of #14 copper wire as 0.073- .064 / 1.84-1.63 inches / mm. [Thanks to Anonymous for careful editing]

The tables and notes below expand and provide detail about the ampacity and temperature ratings of various wire sizes and gives a brief history of U.S. NEC wire sizing table changes. For copper wiring the following wire sizes and ampacity ratings or fuse/circuit breaker sizes are common on 120V residential electrical circuits.

AMPACITY Rating

COPPER

Wire Conductors

Recommend Max Fuse Rating

AMPACITY Rating

ALUMINUM

Wire Conductors Also for Copper-Clad Aluminum Wire

Recommend Max Fuse Rating

Electrical Wire

Overall Diameter

(approximate)

inches / mm

20 / 20 / 25

15A in common house wiring applications ca 1970's

115 / 135 / 150[5]

1. See Article 100 (maximum amperage a conductor can carry under conditions of use without exceeding its temperature rating) and Article 310 of the U.S. National Electrical Code (NEC). The U.S. NEC can be purchased from electrical suppliers and online from nfpa.org.

Since some readers request historical data on electrical code provisions, we note that the data in this article has been excerpted / adapted from the U.S. National Electrical Code. The locations of wiring size vs ampacity ratings changed in the 2011 NEC and again in the 2014 NEC.

Prior to the 2011 NEC wire size vs ampacity table data was provided in NEC Table 310.16. Many sources continue to quote this table.

Beginning in 2011, the pertinent wire sizing tables were found in NEC Table 310.15(B)(2)(a) (Ambient Temperature of 30°C) and NEC Table 310.15(B)(2)(b) (Ambient Temperature of 40°C). Ampacities of not more than three current-carrying conductors in raceway, cable, or earth, were typically based on an ambient temperature of 86°F / 30°C.

This excerpt from the 2014 NEC explains:

310.15(B)(7) For one-family dwellings and the individual dwelling units of two-family and multifamily dwellings, service and feeder conductors supplied by a single-phase, 120/240-volt system shall be permitted be sized in accordance with 310.15(B)(7)(a) through (d).

(a) For a service rated 100 through 400 amperes, the service conductors supplying the entire load associated with a one-family dwelling or the service conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling shall be permitted to have an ampacity not less than 83% of the service rating.

(b) For a feeder rated 100 through 400 amperes, the feeder conductors supplying the entire load associated with a one-family dwelling or the feeder conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling shall be permitted to have an ampacity not less than 83% of the feeder rating.

(c) In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that of its 310.15(B)(7)(a) or (b) conductors.

(d) Grounded conductors shall be permitted to be sized smaller than the ungrounded conductors provided the requirements of 220.61 and 230.42 for service conductors or the requirements of 215.2 and 220.61 for feeder conductors are met.

Informational Note No. 1: It is possible that the conductor ampacity will require other correction or adjustment factors applicable to the conductor installation.

Informational Note No. 2: See example in Annex D.

2. Assuming the electrical current in these wires is AC or DC at 60Hz or less.

3. The U.S. National Electrical Code NEC 240-3 requires that the electrical branch circuit, feeder wire, and electrical service conductors shall be protected against overcurrent in accordance with their ampacity ratings as specified in NEC Table 310-16.

4. Additional NEC rules that pertain are in NEC Sections 210-20 (a), 215-3, and 384-16(d). Also NEC Section 240-6(a) gives a list of standard wire sizes and overcurrent protection (fuse or circuit breaker) required.

5. For dwelling units, conductors, as listed below, shall be permitted as 120/240 volt, 3 wire, single phase service-entrance conductors, service lateral conductors and feeder conductors that serve as the main power feeder to a dwelling unit and are installed in raceway or cable with or without an equipment grounding conductor.

For application of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and appliance branch-circuit panel board(s) and the feeder conductors to a dwelling unit shall not be required to be larger than their service entrance conductors. The grounded conductor shall be permitted to be smaller than the ungrounded conductors provided the requirements of Sections 215.2, 220.22 and 230.42 are met.

6. For information about copper clad aluminum wiring

see COPPER-CLAD ALUMINUM WIRE