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.

Service Entry & Electrical Cable or Wire Sizes vs Amps vs Run Length

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.

"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.

Article Contents

• INTRODUCTION to WIRE SIZES - size matters
• WIRE SIZE TABLE - Electrical Wire Sizes and Amps or Fuse Ratings - Residential - Bell wire up to 400 Amp 240V Circuits
• WIRE SIZE INCREASE for LONGER RUNS - 120V - 15A - 50A Branch Circuit Wire Size vs Run Length: 120V Circuits
• WIRE SIZE INCREASE for LONGER RUNS - 240V - 15A - 50A Branch Circuit Wire Size vs Run Length: 240V Circuits
• MAXIMUM WIRE RUN LENGTH for each WIRE SIZE - how long can the wire be without overheating?
• WIRE SIZE INCREASE using VOLTAGE DROP CALCULATORS - what to do if your wire size & length aren't in the table
• VOLTAGE DROP INDEX - allowable voltage drop in circuits

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

For larger wire sizes (higher Amps) (above 50A) see this separate article

at SE CABLE & WIRE SIZES FOR LONG RUNS

Introduction to Choosing Electrical Wire Size

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.

[Click to enlarge any image]

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.

Electrical Wire Sizes and Amps or Fuse Ratings - Residential

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.

Notes to the Table Above

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

 

Branch Circuit Wire Size vs Run Length: 120V Circuits, <=5% Voltage Drop

Notes to the table above

• Branch circuit wire sizes for long runs: The long-run wire sizes given in the table above are for typical residential branch circuits up to 50A.
• Service entry cable wire sizes for long runs: Detailed tables of copper or aluminum Service Entry Wire SEC cable sizes for long runs at higher ampacities entering the building's main electrical panel, and where we give sizes for both single phase and three phase power are at
  
  SE CABLE & WIRE SIZES FOR LONG RUNS - separate page
• Sources include Cerrowire, "240 Volt, Conductor Size (AWG or kcmil) Single Phase, Max 3% Voltage Drop" retrieved 2021/12/03 original source: Cerrowire, https://www.cerrowire.com/products/resources/tables-calculators/voltage-drop-tables/
  Notes included by Cerrowire:
  
  *The tables assume steel conduit, a power factor of 0.9, an ambient temperature of 86°F, and no more than 3 current-carrying conductors. We have taken reasonable care to establish the accuracy of these values; however, this tool is intended only as a guide, consult a professional engineer to determine suitability for your application.
  
  **Based on 60°C per NEC 110.14(c)(1)(a).
  
  ***Based on 75°C per NEC 110.14(c)(1)(b).
• VOLTAGE DROP TABLE, [PDF], retrieved 2017/09/26, Cerrowire LLC, 1099 Thompson Road SE, Hartselle AL 35640, USA, Tel: 256-773-2522, original source: http://www.cerrowire.com/files/file/voltagedrop.pdf

  300 ft and 400 ft cable length data: Cable size recommended based on voltage drop calculations. Calculated by Online voltage drop calculator: 2019/07/23, Southwire, Tel: 1-800-444-1700 Website: www.southwire.com Email: CableTechSupport@southwire.com Web page: https://www.southwire.com/calculator-vdrop

• Watch out: the engineering work and derivation of the table above is derived from the sources listed below including manufacturer's tables and online voltage drop calculators but may not be correct for your specific installation. Check with your engineer or electrician.

1. These circuits are 120VAC (single phase) single conductor. Three phase power typically reduces the wire size to the next smaller.
2. Watch out: These aluminum solid conductor wires should NOT be used in new residential branch circuit wiring such as for lighting or electrical receptacles but may be found in older homes built in the 1970s.
   
   However multi-strand aluminum wiring is used in special or single purpose circuits in new work such as for high-amperage devices like an air conditioner compressor. Proper wiring and connection details are required for such circuits to perform reliably and safely.
   
   See ALUMINUM WIRING HAZARDS & REPAIRS - home
3. Unlike the service entry cables, branch circuits may permit up to a 5% voltage drop but
   
   Watch out: a limit of 3% voltage drop is the most-common recommended limitation on wire size vs run length for residential properties
4. Cable size recommended based on voltage drop calculations. Calculated by Online voltage drop calculator: 2019/07/23, Southwire, Tel: 1-800-444-1700 Website: www.southwire.com Email: CableTechSupport@southwire.com Web page: https://www.southwire.com/calculator-vdrop
5. Watch out: these are example wire or cable sizes vs. run or length for the specific parameters named here and entered in the wire size and length calculator.
   
   Your installation may differ. For safety and for code compliant electrical wiring, be sure to check with your local electrical code inspector and electrical code provisions.
6. For electrical wire runs longer than 400ft. you will probably need to use a voltage drop calculator like those we describe
   
   at WIRE SIZE INCREASE using VOLTAGE DROP CALCULATORS
7. Also see the Table Maximum Wire Length Permitted vs. wire size and amps given just below.

Branch Circuit Wire Size vs Run Length: 240V Circuits, <=3% Voltage Drop

Notes to the table above

• This table respects the requirement to limit voltage drop to <= 3%
• Sources include Cerrowire, "240 Volt, Conductor Size (AWG or kcmil) Single Phase, Max 3% Voltage Drop" retrieved 2021/12/03 original source: Cerrowire, https://www.cerrowire.com/products/resources/tables-calculators/voltage-drop-tables/
  Notes included by Cerrowire:
  
  *The tables assume steel conduit, a power factor of 0.9, an ambient temperature of 86°F, and no more than 3 current-carrying conductors. We have taken reasonable care to establish the accuracy of these values; however, this tool is intended only as a guide, consult a professional engineer to determine suitability for your application.
  
  **Based on 60°C per NEC 110.14(c)(1)(a).
  
  ***Based on 75°C per NEC 110.14(c)(1)(b).
• Watch out: the engineering work and derivation of the table above is derived from the sources listed below including manufacturer's tables and online voltage drop calculators but may not be correct for your specific installation. Check with your engineer or electrician.

Table of Maximum Electrical Conductor Run Length: Wire Size vs Amps vs Length

Notes to the Table Above

• 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 derived from the sources listed below including manufacturer's tables and online voltage drop calculators but may not be correct for your specific installation. Check with your engineer or electrician.

1. Maximum wire run distance changes by required temperature rating:
   
   The Amperage rating and distances shown are for 90°C (194°F) THWN-2, THHN, XHHW-2, USE-2 - copper wire;
   
   Watch out: Note that dropping to 60°C (140°F) NM-B, UF-B copper wire, the maximum run lengths will be substantially shorter. For example #4 copper wire can be run just 70 ft. instead of 95 ft. as shown in the table above.
2. See this ELECTRIC WIRE AMPACITY CHART [PDF] from CerroWire, retrieved 2021/11/25 original source: https://www.cerrowire.com/wp-content/uploads/2021/04/Cerrowire_Ampacity_Chart_210405.pdf
   
   For example, if your circuit permits a voltage drop of 5% (3% is more-common) you could run a 20A circuit using #12 wire run up to 50 ft.
3. Using a typical online voltage drop calculator, specifying source voltage 120VAC, Single Phase, 70Amps, 70 ft run, 3% voltage drop, for copper wire
4. The National Electrical Code limits voltage drop to a maximum of 5% of nominal. Most experts recommend limiting the voltage drop to 3% of nominal.
5. For longer wire runs & larger wire sizes in the kcmil range, and a table of allowable current (amps) & run lengths

   see SE CABLE & WIRE SIZES FOR LONG RUNS

Adapted from "Wire, Cable & Harness", provided by the California Department of Transportation, retrieved 2016/02/09, original source: http://www.dot.ca.gov/hq/eqsc/QualityStandards/Electric/Electric-01.htm

VOLTAGE DROP TABLE, [PDF], retrieved 2017/09/26, Cerrowire LLC, 1099 Thompson Road SE, Hartselle AL 35640, USA, Tel: 256-773-2522, original source: http://www.cerrowire.com/files/file/voltagedrop.pdf

Excerpt: To avoid excessive voltage drop, select a size wire that will minimize voltage drop, You need to know the length of the wire run and the amp load or current that will be on the circuit.

To determine amps, add up the wattage of all electrical devices that will be on the circuit and divide this total by the voltage of the circuit, 110 or 220.

WIRE SIZES AND MAXIMUM LENGTH DETERMINATION, [PDF] (2007) [no author, no authority cited] retrieved ca 2007, retrieved anew 2017/09/26, original source: http://www.zetatalk.com/energy/tengy10s.pdf [Beware this is a pretty weird website]

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).

WIRE SIZE GUIDE, 6,12, 24V [PDF], Thomas & Betts, ABB Group, Emergi-Lite, retrieved retrieved 2017/09/26, original source: http://www.emergi-lite.com/usa/modules/Files/el_92c_EL_US_Wire-guide_13-6-2016.pdf

LOW VOLTAGE ELECTRICAL WIRE SIZE TABLE [PDF] Emergi-Lite emergency lighting company, original source: http://www.emergi-lite.com/usa/modules/Files/el_251c_el-us-wire-size-guide_13-3-2019.pdf

Credits:

The articles from which some 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 2091/07/236.

 

How to Use a Voltage Drop Calculator to Determine Required Service Entry Wire Size

Here we give examples and compare the results of using two example wire size and voltage drop calculators.

You will see that Southwire and Paige calculators give similar results.

1. Southwire's Voltage Drop Calculator example calculating SEC wire size

Southwire's wire size calculator at http://www.southwire.com/support/voltage-drop-calculator.htm is easy to use and clear.

The user specifies the number of phases (1 or 3), conductor (copper or aluminum), installation (direct buried, conduit, or overhead) and the input voltage, maximum allowed voltage drop (I use 3%), the length of the cable run, and the desired current (amps) at the end of the cable run. The calculator gives an appropriate wire selection and its parameters, such as this:

Example: Single phase, aluminum conductor, direct buried, 120VAC, 3% maximum voltage drop, 250 ft. cable run length, 100A current at end of cable

Result: 1 conductors per phase utilizing a #400 Aluminum conductor will limit the voltage drop to 2.94% or less when supplying 100.0 amps for 250 feet on a 120 volt system.

Changing the example above to 240 VAC, keeping other parameters the same gives this result:

Result: 1 conductors per phase utilizing a #3/0 Aluminum conductor will limit the voltage drop to 2.82% or less when supplying 100.0 amps for 250 feet on a 240 volt system.

2. Paige Wire Size Calculator example calculating SEC wire size

Paige Wire's voltage wire size calculator at http://www.paigewire.com/pumpWireCalc.aspx is also easy to use and clear and is an alternative to the Southwire wire size, length, and voltage drop calculator we described above. Testing with the following parameters:

Example: 240 VAC Single Phase, 100A, 250 ft. run, 3% voltage drop allowed,

Result: 1 AWG Copper or 2/0 Aluminum

Changing the example above to 120VAC, keeping other parameters the same gives this result:

Result: 3/0 AWG Copper or 250 MCM Aluminum

3. Siemens offers an XLS spreadsheet [this address leaves InspectApedia.com] that can be used to calculate voltage drop as do other sources, and there are numerous online voltage drop calculators - I like the Southwire calculator given above. Be sure to compare the calculator's result with what the applicable electrical code will permit.

Voltage Drop Index - VDI - when to increase wire size for long runs

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.

3% is a typically-accepted voltage drop target.

The calculated 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 assuming a negligible line reactance, above on this page is an example table (see warning and citations throughout this article and atReferences or Citations ).

Wire Ampacity Ratings - Temperature Ratings

Three different ampacity ratings shown for most of the wire types above reflect the wire types and temperature ranges as shown in the tables now given in detail

at ELECTRICAL WIRE TYPES CODES USES

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

What is the watt capacity of No. 4 SEC Wire?

How many watts is the capacity of number 4 service wire.? - 2021-09-06 by Nestor -

Reply by inspectapedia.com.moderator - watt capacity of number 4 service wire?

@Nestor,

Let's start with the wire size table above on this page where look up the wire size in your question.

A No. 4 COPPER wire is rated to carry 70 Amps.

Using the on-page search box for InspectApedia and searching for

"Convert Amps to Watts" or searching for "Convert Watts to Amps"

finds several articles describing how to do that; if we just try the very first article in the list

https://inspectapedia.com/electric/Electric_Motor_Horsepower.php ELECTRIC MOTOR HORSEPOWER & CIRCUIT WIRE SIZE

we find this really simple formula

AMPS = Watts / Volts OR Watts = Amps x Volts OR Volts = Watts / Amps

So if your #4 copper service wire is 120Volts (as is the case in North America where we are guessing you're located)

And if we pretend your wiring is legal so is correctly sized to carry 70 Amps of current,

We convert 70 Amps to Watts as follows:

AMPS = Watts / Volts

70 = Watts / Volts

where we know Volts is 120

using simple algebra we write

70 x120 = Watts

or

8,400 = Watts

so your No. 4 copper wire can support 70 A of current or 8,400 Watts of electrical power.

Please take a look at DEFINITIONS of ELECTRICAL TERMS https://inspectapedia.com/electric/Electrical_Definitions.php

to find definitions of each of these terms.

And don't hesitate to ask follow-up questions if you need.

Minimum Wire Size for Street Lighting Controller

I have a street lighting controller that is rated at 100 Amps and has four 30 amp breakers (one for each circuit). Each circuit is 240 VAC. I am trying to determine the minimum wire size for electrical service from the streetlighting controller to the power pole.

The streetlighting controller has this note: "Field installed conductors shall be 60 degrees C, or 90 degrees C, sized to 60 degrees C rating for 110 amps or less; and 75 degrees C or 90 degrees C for 125 amps and above."

Does this control more than the voltage drop? What size service wire would I need for a 1 phase, 3 wire, 240 volt system and a 100 Amp service? -On 2021-03-18 by Bruce

Reply by Inspectapedia Com Moderator - what wire size for a 100A street light panel?

@Bruce,

IF the main switch and panel are 100A that tells you the wire size needed - as you'll find easily in the tables above on this page.

When you set out to buy the SEC cable, you'll see temperature range rating among its data provided stamped right onto the cable jacket.

#1 copper would provide 100A BUT you'll need to go to a larger size like 1/0 copper for a longer wire run length - data you've not provided.

Measure your wire run length and then check the SEC WIRE SIZE tables given above.

An example of #1 copper and aluminum wire ampacity ratings in the 100A range, showing different temperature ranges and the codes telling you the temperature ranges is given below, courtesy of cerrowire.com - you can see the full chart at

CERROWIRE AMPACITY CHART (image file)

From your question:

I have a street lighting controller that is rated at 100 Amps

Field installed conductors shall be
60 degrees C, or
90 degrees C,
sized to 60 degrees C rating for 110 amps or less;
and 75 degrees C or 90 degrees C for 125 amps and above."

I agree that whoever wrote those instructions lives in a mental fog and was unable to write clear, intelligible, unambiguous instructions.

I THINK what they're trying to say is

your wires can have a temperature rating of 60C or 90C

your wire can have a temperature rating of 60C if your control panel is 100A or less (yours is 100A)

your wire should have a rating of 75C OR 90C if your panel is 125A or higher (this is not your case)

The voltage drop for the wire bringing power to your control panel is figured by the run length of your SEC wire to to the panel

Separately, you will want to look at the length of your run from panel to each street light and to size those wires properly as well.

You can see that to meet the 60C requirement for a 100A panel you'd start (before increasing wire size for a longer run)
that

There are NO Cerrowire copper wire products rated for 60C in the 100A current range - the most you'd find is that #1 NM-B UF-B that can deliver up to 95A - too small

Instead

You'd have to use #3 - 75C Copper THW THWN SE USE or XHHW wire to get 100A

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

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?

Reply: crude rule of thumb: up one size for each 100 ft. of run

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).

SEC Electric Service Entry Cable Specifications & Instructions for THHN type SEC Cabling

I'm helping a friend out in a attic and noticed that he had thhn #2 awg feeder cables to a 100 amp subpanel that ran across his attic.

The cables are not in conduit. I know generally, THHN should always be in conduit. Does the same rules apply to feeder cables of that size?

My friend thinks it should be okay since the home was built in 2011 so it must have passed inspections. - SH asked: On 2018-10-14

Reply: THHN electrical wire is intended to be run in conduit

NEC 300.3(A) prohibits THHN (or any other type of single wire save for a few exceptions such as single conductor MI and SE/USE, overhead wires, or an EGC) from being run outside of conduit or approved protection.

"A) Single Conductors. Single conductors specified in Table 310.104(A) shall only be installed where part of a recognized wiring method of Chapter 3."

Here are Southwire's specifications and general instructions from that company's notes at
www.southwire.com/support/THHNGuideSpecifications.htm (dead link in August 2024 - Ed.)

Annex A Type THHN Conductor Specification 26 05 19 WIRE AND CABLE TYPE THHN

PART 1 – GENERAL

1.1 – SPECIFICATION INCLUDES

1.1.1 Cable Type: Type THHN/THWN for use as services, feeders and branch circuits

1.1.2 General Applications: Type THHN/THWN cable may be used in the following general applications per the National Electrical Code®

1.1.2.1 In Conduit

1.1.2.2 In Cable Tray

1.1.2.3 For Services

1.1.2.4 For Feeders

1.1.2.5 For Branch Circuits

1.1.2.6 Wet or Dry Locations

1.2 – SUBMITTALS

1.2.1 Product Data: Submit manufacturer’s product data confirming that materials comply with specified requirements and are suitable for the intended application.

1.2.2 Installation Instructions: Manufacturer’s installation instructions shall be included in submittal. Industry guides may supplement the manufacturer’s instructions.

1.3 REQUIREMENTS

1.3.1 Underwriters Laboratories: Type THHN cable shall meet the following Underwriters Laboratories (UL) standards and listings.

1.3.1.1 UL 83 Thermoplastic-Insulated Wires and Cables

1.3.1.2 UL 1063 UL Standard for Safety Machine-Tool Wires and Cables

1.3.1.3 UL listed sunlight resistant in black sizes 2 AWG and larger.

1.3.1.4 Sizes 1/0 AWG and larger listed for CT USE

1.3.1.5 Sizes 14 through 1 AWG shall be rated VW-1

1.3.1.6 Sizes 8 AWG and larger shall be rated THWN-2

1.3.2 ASTM Standards: Type THHN cable shall meet all applicable ASTM standards.

1.3.3 Federal Specifications: Type THHN cable shall meet Federal Specification A-A-59544

PART 2 - PRODUCTS

2.1 MANUFACTURER

2.1.1 Southwire Company, One Southwire Drive, Carrollton, Georgia, 30119. Website: http://www.southwire.com/

2.1.2 Senator Wire and Cable, 102 City Hall Ave., Carrollton, Georgia, 30117 Website: http://www.southwire.com/

2.2 CABLE CONSTRUCTION

2.2.1 Conductor: The conductor shall be soft annealed copper

2.2.2 Insulation: The insulation shall be high-heat and moisture resistant PVC.

2.2.3 Jacket: The jacket shall be abrasion, moisture, gasoline and oil resistant nylon or listed equivalent with

2.2.4 Self-Lubricating Jacket: Jackets on conductor sizes 2 and larger shall be SIMpull™ or equivalent having integrated self-lubrication such that the cable coefficient of friction is less than or equal to 0.17.

PART 3 - INSTALLATION

3.1 INSTALLATION

3.1.1 Manufacturer’s Instructions: Type THHN cable shall be installed per the manufacturer’s published installation instructions. Industry guides may supplement the manufacturer’s instructions.

3.1.2 Field Support: Manufacturer shall provide, when requested, field engineering support for Type THHN cable installation.

3.1.3 Manufacturer: Type THHN cable for circuits, feeders and services shall be supplied from a single manufacturer.

3.1.4 Minimum Bend Radius: Bends in Type THHN shall be made so that the cable will not be damaged.

3.2 SPECIFIC USES

3.2.1 Type THHN cable may be used in conduit, raceways and cable trays for services, feeders and branch circuits as specified in the applicable section of the NEC®.

3.3 USES NOT PERMITTED

3.3.1 Type THHN cable shall not be used in direct burial applications.

3.4 AMPACITY

3.4.1 The ampacity of Type THHN cable shall be determined in accordance with Article 310.15 of the National Electrical Code®. The installation should not exceed the temperature ratings of the terminations and equipment.

...

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