Table of Electrical Service Entry Cable Sizes, Electrical Wire Diameters & Ampacity assists home inspectors in determining the electrical service size at buildings
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 requirements
Questions & answers about electrical wire sizes, diameters, and ampacity ratings
Electrical wire sizes: this article 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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In answering the question "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.
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. Readers of this article should also be sure to review Safety Hazards and Safe Electrical Inspection
Procedures for examining Residential Electrical Panels.
SE CABLE SIZES vs AMPS - How to Inspect Electrical Service entrance cables for Ampacity, Voltage, Condition
It's not as difficult as one may think to get a reasonable handle on the electrical service capacity at a building without sophisticated analysis. But there are some pitfalls that can make for big mistakes in your guess at the service ampacity for a property, and the process itself is dangerous.
At left we can see a three wire mast-head, suggesting that the building has a 240V service. But we did not like the position of that weather-head, and we considered that water may be entering the SEC.
The amperage provided by the electrical service entrance cable is a function of its materials
and diameter. Often the actual cable type and size is printed right on the cable insulation. Otherwise some rough measurements of cable
diameter are in order.
If, inside the panel, the inspector could see the ends of the entrance cable [Figure at left], measure metal wire diameter,
and if the inspector knew the manufacturer of the cable and its specifications, a certain identification of the cable's ampacity could be made.
However
a safer, faster and common practice is to examine the exterior of the cable at a point outside of the electric panel.
Many SECs include printing right on the external jacket of the cable that will tell you the number of conductors, the metal (AL or CU), and the wire size.
Watch out: Make sure you
look at both outside (mast head down to meter) and inside (from meter into service panel).
They may be different! Inside the panel, stripped
of ground and insulation, you may see only wires as in Figure 3. Don't confuse guides for external measurements of the whole
cable with in-panel measurements of the wires themselves.
Carson Dunlop Associates' sketch (left) shows common electrical wire sizes for both service entry conductors (SECs) and in-building branch circuit wiring.
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.
But this method is not precise. Plastic insulated cable,
for example, is often thinner in total thickness than older fabric-covered cable. Look at the table of wire dimensions in the accompanying article.
Fortunately for building inspectors, even though wire thickness varies among manufacturers, you can generally
find the measurement closest to one of the standard sizes and you're likely to be correct.
Table of Common Electrical Wire Sizes and Amps or Fuse Ratings - Residential
For copper wiring the following wire sizes and ampacity ratings or fuse/circuit breaker sizes are common on 120V residential electrical circuits. :
Electrical Conductor Wire Size or Gauge - AWG
Ampacity Rating Copper Wire
Recommend Max Fuse Rating
Ampacity Rating Aluminum Wire
Recommended Max Fuse Rating
Electrical Wire Overall Diameter (approximate)
inches / mm
Comments
"Bell wire"
/ 0.5mm
Typical solid conductor telephone or bell wire size, thermostat wire, or 12-24V wire for controls
Usually used for thermostat and HVAC control wiring
16
0.058-0.051 / 1.46-1.29
OK for thermostat and HVAC control wiring
14
15
n/a
0.073- .064 / 1.84-1.63
Smallest size normally used for household 120V wiring (copper 15A)
12
20
15
0.092-0.081 / 2.32-2.05
Smallest size normally used for household 120V wiring (aluminum 15A)
10
30
20
.116-.102 / 2.93-2.59
8
50
30
.146-.128 / 3.7-3.26
6
80
50
.184-.162 / 4.66-4.11
4
125
80
.232 / 5.88
2
200
125
.292 / 7.42
1
250
200
.332 / 9.43
1/0 or 0
325
.373 / 9.46
2/0 or 00
400
.419 / 10.6
3/0 or 000
.471-.470 / 12.0-11.9
4/0 or 0000
.528 / 13.4
About the largest wire size likely to be found at residential electrical SECs
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.
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.
The articles from which much of this online material originated
appeared first in the ASHI Technical Journal, Vol. 2. No. 1, January 1992, "Determining Service Ampacity," Dan Friedman and Alan Carson,
and the ASHI Technical Journal, Vol. 3. No. 1, Spring, 1993, "Determining Service Ampacity - Another Consideration," Robert L. Klewitz, P.E.,
with subsequent updates and additions to the original text ongoing to 2/19/2006. Reprints of the originals and reprints of the Journal are available from ASHI, the American Society of Home Inspectors www.ashi.com
Questions & Answers regarding this article
Questions & answers about electrical wire sizes, diameters, and ampacity ratings.
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[1] "Thermostat Wire", Southwire Corporation, OEM Division, One Southwire Drive, Carrollton, Georgia 30119, Website: southwire.com/
Applicable Standards for thermostat wiring: 150 Voltage rating per NEC® Article 725 ETL listed to UL 13 Meets UL 1581 Vertical Tray Flame Test (FT-1) Type CL2
Web search 12/14/2011, original source: http://www.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheetOEM112
The Original Authors: Alan Carson is an ASHI Member, national home inspection educator, author and building failures researcher in Toronto, Ontario.
Daniel Friedman, an original author of this article and the editor and producer of InspectAPedia where this article now appears is an ASHI Member, first ASHI Technical Committee chairman, editor and publisher of the ASHI Technical Journal, licensed home inspector, educator, and building failures researcher in Poughkeepsie, NY.
Robert Klewitz is a licensed professional engineer, a professional home inspector, an ASHI Member, and has served on the ASHI Technical Committee as well as in other ASHI activities. His practice is in Issaquah, WA.
Douglas Hansen, Robert Stead. Mark Cramer. Photographs: Daniel Friedman.
N. Srinivasan, MSEE, is a senior member of IEEE with 30 years experience in the electrical industry. Mr. Srinivasan is in Vienna VA.
Louis P. Babin generously contributed technical editing about the effects of doubling ampacity in an electrical circuit (September 2007)
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book 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. InspectAPedia.com ® editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
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)
Aluminum Wiring Information Website Aluminum Electrical Wiring Hazards and Repairs: in-depth authoritative info, photos, documents including selection of proper vs. ineffective repair methods. E.g.: Ideal 65 "Twister" purple connector fails in field and lab testing with aluminum wire.
Circuit Breaker, a bad one fails to trip failure at aluminum bus-to-circuit breaker connection - field report and photographs
Electrical Panels, How to Inspect in buildings, safety for electrical inspectors, electrical panel, fusing, wiring defects, defective products. Inspection Class Presentation
Federal Pacific Electric (FPE) Stab-Lok Circuit Breaker Panel Hazards Website - Latent fire hazards, in-depth authoritative research, documents, advice on Stab-Lok electric panel and circuit breaker failures and what to do when this equipment is found in buildings.
"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.
"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.
"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.
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