Copper Wire Resistance and Voltage Drop

AWG stands for "American Wire Gauge" and is a standardized wire gauge system  used in the US since 1857 for diameters of round, nonferrous, electrically conducting wire.  The cross-sectional area of a wire determines it's resistance and current-carrying capacity.  The large the wire diameter, the less resistance it has to the flow of electrons, and the more current it can carry without overheating.  The table below list copper wire resistance for various gauges of copper wire.  These should be used as a rule of thumb as there are other factors that play into the current ratings of wire including ambient temperature, insulation temperature limit, air convection, etc.  You should consult the National Electrical Code (NEC) for specific guidelines.

AWG Wire Sizes and Resistance

  AWG gauge Conductor Diameter Inches Conductor Diameter mm Ohms per 1000 ft.  
0000 0.46 11.684 0.049
000 0.4096 10.40384 0.0618
00 0.3648 9.26592 0.0779
0 0.3249 8.25246 0.0983
1 0.2893 7.34822 0.1239
2 0.2576 6.54304 0.1563
3 0.2294 5.82676 0.197
4 0.2043 5.18922 0.2485
5 0.1819 4.62026 0.3133
6 0.162 4.1148 0.3951
7 0.1443 3.66522 0.4982
8 0.1285 3.2639 0.6282
9 0.1144 2.90576 0.7921
10 0.1019 2.58826 0.9989
11 0.0907 2.30378 1.26
12 0.0808 2.05232 1.588
13 0.072 1.8288 2.003
14 0.0641 1.62814 2.525
15 0.0571 1.45034 3.184
16 0.0508 1.29032 4.016
17 0.0453 1.15062 5.064
18 0.0403 1.02362 6.385
19 0.0359 0.91186 8.051
20 0.032 0.8128 10.15
21 0.0285 0.7239 12.8
22 0.0254 0.64516 16.14
23 0.0226 0.57404 20.36
24 0.0201 0.51054 25.67
25 0.0179 0.45466 32.37
26 0.0159 0.40386 40.81
27 0.0142 0.36068 51.47
28 0.0126 0.32004 64.9
29 0.0113 0.28702 81.83
30 0.01 0.254 103.2
31 0.0089 0.22606 130.1
32 0.008 0.2032 164.1


The diagram below shows many of the common copper wire sizes used when wiring a home.  General ampacity ratings are listed as well but you should consult the tables below for more accurate ampacity ratings.  This illustration is meant to show relative sizes of the common gauges of wire.

Common Copper Wire Sizes

common-copper-cable-sizes   

This table provides the amperage rating for common insulated conductors including Romex.  Insulated conductors should have the temp rating and type (i.e. THWN 75ºC) printed on the outside of the cable.  You can then follow the chart below to see how much current you are allowed to put through the conductor.  This table assumes not more than three conductors in a raceway or cable or earth (directly buried) and is based on ambient temperature of 30ºC (86ºF).

Ampacities of Insulated Conductors

Size Temperature Rating of Conductor Size
AWG 60ºC 75ºC 90ºC 60ºC 75ºC 90ºC AWG
(140ºF)  (167ºF)  (194ºF)  (140ºF)  (167ºF) (194ºF)
Types Types Types Types Types Types
T
TW
UF
THW
THWN
XHHW
USE
RHH
THHN
XHHW 
T
TW
UF
THW
THWN
XHHW
USE
RHH
THHN
XHHW 
0 Copper Aluminum  
14 20 20 25 ---- ---- ---- ----
12 25 25 30 20 20 25 12
10 30 35 40 25 30 35 10
8 40 50 55 30 40 45 8
6 55 65 75 40 50 60 6
4 70 85 95 55 65 75 4
3 85 100 110 65 75 85 3
2 95 115 130 75 90 100 2
1 110 130 150 85 100 115 1
0 125 150 170 100 120 135 0
00 145 175 195 115 135 150 00
000 165 200 225 130 155 175 000
0000 195 230 260 150 180 205 0000
250 215 255 290 170 205 230 250
300 240 285 320 190 230 255 300
350 260 310 350 210 250 280 350
400 280 335 380 225 270 305 400
500 320 380 430 260 310 350 500

The table below provides the maximum number of THNN conductors that you can put in a given size conduit.  Correction factors should be used if putting more than 3 conductors in a raceway.

Maximum Number of THNN Conductors in Conduit

Conduit Size (inches)
AWG 1/2 3/4 1 1 1/4 1 1/2 2 2 1/2 3 3 1/2 4 5 6
14 13 24 39 69 94 154            
12 10 18 29 51 70 114 164          
10 6 11 18 32 44 73 104 160        
8 3 5 9 16 22 36 51 51 106 136    
6 1 4 6 11 15 26 37 37 76 98 154  
4 1 2 4 7 9 16 22 22 47 60 94 137
3 1 1 3 6 8 13 19 29 39 51 80 116
2 1 1 3 5 7 11 16 25 33 43 67 97
1   1 1 3 5 8 12 18 25 32 50 72
0   1 1 3 4 7 10 15 21 27 42 61
00   1 1 2 3 6 8 13 17 22 35 51
000   1 1 1 3 5 7 11 14 18 29 42
0000   1 1 1 2 4 6 9 12 15 24 35
250     1 1 1 3 4 7 10 12 20 28
300     1 1 1 3 4 6 8 11 17 24
350     1 1 1 2 3 5 7 9 15 21
400       1 1 1 3 5 6 8 13 19
500       1 1 1 2 4 5 7 11 16

Ampacity Correction Factors for more than 3 conductors in Raceway

No. Conductors 4 to 6 7 to 9 10 to 20 21 to 30 31 to 40
Factor  0.8 0.7 0.6 0.45 0.4

Why do I need larger gauge wire to carry more current?

The larger the copper wire size the less resistance and hence the more current it can carry without over-heating. Resistance impedes to the flow of electrons and causes a voltage drop across the wire.  You want to avoid voltage drops on your wiring as much as possible because they generate heat and wasted energy.  The calculator below will help determine how much voltage drop you will get with a given copper wire and its associated resistance.

Voltage Drop Calculator

This calculator determines the voltage drop for an aluminum or copper wire of any gauge.  You should generally target less than 3% voltage drop in a given circuit. Wire resistances are based on NEC 2008, Table 8 at 75oC.
 

Select Copper or Aluminum 

Select American Wire Gauge (AWG) Size 

Select Phase 

Enter one-way circuit length in feet (calc uses round trip distance) 

Enter Volts  

Enter Load in amps


Voltage drop

Voltage at load end of circuit 

Per Cent voltage drop 

Wire cross section in circular mils 

Voltage Drop Calculator Download

The following Excel spreadsheet is a voltage drop calculator that is a little more advanced.  It can be used to determine recommended wire gauges, maximum distances, or maximum amperage.

Voltage Drop Calc (.xls, 650KB)

 

Grounding Conductor Size Calculator

Select rating of circuit breaker ahead of equipment, conduit, etc.
Not Exceeding (Amperes)
Required size of Copper
grounding conductor
Required size of Aluminum
grounding conductor
*Calculator based on 2008 NEC Table 250-122.  Calculator specifies minimum size grounding conductors required for grounding a raceway or equipment. Do not use this calculator for sizing hot conductors.

     

Common Electrical Wiring Diagrams

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Wire Gauge and Voltage Drop Calculator

NEC Reference Tables (2011, 2008, 2005, 2002, and 1999)

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Click the icons below to get our NEC® compliant Electrical Calc Elite or Electric Toolkit for Android and iOS.  The Electrical Calc Elite is designed to solve many of your common code-based electrical calculations like wire sizes, voltage drop, conduit sizing, etc.  The Electric Toolkit provides some basic electrical calculations, wiring diagrams (similar to those found on this website), and other electrical reference data. 

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