Unsafe service entry wiring (C) Daniel Friedman Electrical Service Entry Cable Sizes, Branch Circuit Electrical Wire Sizes & Ampacity Tables

  • SE CABLE SIZES vs AMPS - CONTENTS: 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
  • POST a QUESTION or READ FAQs about electrical wire sizes, diameters, and ampacity ratings
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Electrical wire sizes & gauges: 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.

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.

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SE CABLE SIZES vs AMPS - How to Inspect Electrical Service entrance cables for Ampacity, Voltage, Condition

Electrical service entry masthead (C) Daniel Friedman

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
for examining Residential Electrical Panels.

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.

[Click to enlarge any image]

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.

Common electrical wire sizes (C) Carson Dunlop Associates

Carson Dunlop Associates' sketch (above) 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.

For a simplified table matching service entry cable SEC size to ampacity

Branch circuit electrical wire sizes such as athe #14 copper electrical wire shown just below are also included in the tables in this article.

Close-up measurement of the diameter of a #14 copper electrical wire (C) Daniel Friedman

Above is a close-up photo of a #14 copper wire against a millimeter scale. The actual wire diameter of #14 copper wire can vary among manufacturers and wire types.

In the wire size table below give the typical diameter of #14 copper wire as 2.05 to 2.32 mm or about 0.081 - 0.092 inches.

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.

Table of Common Electrical Wire Sizes and Amps or Fuse Ratings - Residential

SEC Cable & Other Wire Ampacities for 1-3 Current-Carrying Conductors in Cable, Earth, or Raceway For 120/240-volt systems sized between 100 and 400 amperes

Electrical Conductor Wire Size or Gauge - AWG or kcmil

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

Conductor Rating Temperature of 60 / 75 / 90 °C
Conductor Rating Temperature of 60 / 75 / 90 °C
"Bell wire"     / 0.5mm Typical solid conductor telephone or bell wire size, thermostat wire, or 12-24V wire for controls
20     0.036 / 0.91 Also used for thermostat wiring [1]
- / - / 14
  0.046-0.040 / 1.16-1.02 Usually used for thermostat and HVAC control wiring
- / - / 18
  0.058-0.051 / 1.46-1.29 OK for thermostat and HVAC control wiring
15 / 20 / 25
Not Used
0.073- .064 / 1.84-1.63 Smallest size normally used for household 120V wiring (copper 15A)
20 / 25 / 30

20 / 20 / 25

15A in common house wiring applications
ca 1970's

0.092-0.081 / 2.32-2.05 Smallest size normally used for household 120V wiring (aluminum 15A)
30 / 35 / 50
25 / 30 / 35
.116-.102 / 2.93-2.59  
40 / 50 / 55
30 / 40 / 45
.146-.128 / 3.7-3.26  
55 / 65 / 75
40 / 50 / 60
.184-.162 / 4.66-4.11  
70 / 80 / 95
55 / 65 / 75
.232 / 5.88  
95 / 115 / 150
75 / 90 / 100[5]
.292 / 7.42  
110 / 130 / 145
85 / 100 / 115[5]
.332 / 9.43  
1/0 or 0
125 / 150 / 170
100 / 120 / 135[5]
.373 / 9.46  
2/0 or 00
145 / 175 / 195

115 / 135 / 150[5]

.419 / 10.6  
3/0 or 000
165 / 200 / 225
130 / 155 / 175[5]
.471-.470 / 12.0-11.9  
4/0 or 0000
195 / 230 / 260
150 / 180 / 205[5]
.528 / 13.4 About the largest wire size likely to be found at residential electrical SECs


1. See Articles 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

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

The three different ampacity ratings shown for most of the wire types above reflect the wire types and temperature ranges as shown in the table below.

Table of Wire Types vs. Temperature Ratings Reflected in the Wire Ampacity Chart Above

Temperature Rating Wire Types
60°C TW, UF


1. Excerpted / adapted from the U.S. National Electrical Code for 2002 for ampacities of not more than three current-carrying conductors in raceway, cable, or earth, based on an ambient temperature of 86°F / 30°C

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

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

Increase in Electrical Wire Size for Long Distance Wire Runs

Question: at what circuit length do I need to increase the wire size to allow for voltage drop?

(Oct 23, 2015) Anonymous said:
What size wire should run from my meter to the main fuse box and does distance affect this?

Apr 28, 2015) Grant said:
What is the maximum distance for 10 gage wire, 30 amp circuit ,Direct burial 10-3 UF ? 1 hp motor

(Aug 1, 2015) Anonymous said:
what size wire would i have to use to run from a 200 amp 240 v disconnect to a structure that is 1300 feet away to avoid unacceptable voltage drop? prefer to use aluminum.

2016/02/09 JD said:
What size wire do i need to run from a 200 amp pole service to a house 450 feet away?


The voltage drop in a wire run has to remain within code limits, that's how we get to larger wire sizes for long runs. There are online voltage drop calculators for which of course you need to plug in wire size, type, length, and the electrical load or current. However many electricians simply jump up one wire size for each 100 feet of run length.

A more engineering approach is cited at the Mike Holt's electrical forum where a reader says as an engineer he calculates the voltage drop (and thus the need for a larger wire size) when the run length of the circuit exceeds voltage rating of the circuit. If we try this approach, for a 15A copper wire circuit using #14 copper wire in a typical household 120V circuit, if the circuit length exceeds 120 ft. the voltage drop must be calculated (or "considered"). This gives a result close to the first rule of thumb that argued just increase the wire size one step for each 100 ft. of run.

For a costly SEC run it would be smart to have your engineer calculate the actual voltage drop for the actual length and choose the larger wire size accordingly.

Voltage drop is not discussed at length in the U.S. NEC, except in Article 647 that addresses "Sensitive Electronic Equipment". There, the U.S. National Electrical Code PP 674.4. Wiring Methods, Paragraph (D) Voltage Drop states:

(D) Voltage Drop. The voltage drop on any branch circuit shall not exceed 1.5 percent. The combined voltage drop of feeder and branch-circuit conductors shall not exceed 2.5 percent. - 2002 NEC Article 647.4(D)

Watch out: "rule of thumb" approaches to electrical wiring may produce unsafe results, particularly where higher voltages or service entry wiring questions are involved. And as we suggest above, using a rule of thumb on SEC wiring may also produce unnecessary costs. In general, voltage drop is a particular concern in low voltage wiring systems (such as 12V or 24V solar systems) while maximum current carrying capacity is a greater concern in higher voltage systems (such as a 120V residential circuit).

Voltage Drop Index - VDI

The voltage drop index or VDI is a reference number that is based on the electrical resistance of a wire and is calculated as

VDI = (Amps x Feet of run) / (% Voltage Drop allowed x Voltage)

Typical allowable voltage drop is 2% or 4% depending on the application. The calcuated VDI for a specific circuit is then compared with VDI's given in wiring charts. The type of electrical load is important as well as the circuit type (AC vs DC). For typical residential electrical circuits with resistive loads (such as lighting, not motor loads) using a power factor of 1 and assumign a negligible line reactance, here is an example table (see warning and citations below).

Maximum Wire Length vs Wire Size & Circuit Amps

Wire Size AWG AMPS Rating Voltage Drop Percentage
10% (this may be too high)
14 15 5 16 48 160
12 20 5 18 54 180
10 30 6 20 60 200
8 55 5 17 52 175
6 75 6 19 58 192
4 95 8 25 76 253
2 130 9 29 86 286
0 170 10 35 104 346
00 195 11 38 114 382
000 225 12 42 125 416
0000 260 14 46 137 457


Typically the maximum allowed voltage drop in residential wiring is 3%. For motor circuits and other applications less voltage drop may be acceptable, and you may need to consider both the starting voltage drop and the running voltage drop allowed.

Watch out: the engineering work and derivation of the table above is uncertain. Check with your engineer. See the citations below.

Adapted from "Wire, Cable & Harness", provided by the California Department of Transportation, retrieved 2016/02/09, original source: also "Universal Wire Sizing Chart, a 2-Step Process", [PDF] found at

Siemens offers an XLS spreadsheet that can be used to calculate voltage drop as do other sources, and there are numerous online voltage drop calcuators, but be sure to compare the calculator's result with what the applicable electrical code will permit.

According to the zetatalk version,

This chart works for any voltage or voltage drop, American (AWG) or metric (mm2) sizing. It applies to typical DC circuits and to some simple AC circuits (single-phase AC with resistive loads, not motor loads, power factor = 1.0, line reactance negligible).

Articles on Determination of Ampacity & Voltage at Building Electrical Services


Continue reading at AMPS VOLTS DETERMINATION or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see SIZE of WIRE REQUIRED for receptacle circuits and similar wiring situations

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