Wire Sizing

The Formula:               K  x   I   x  L   = G
                                       E x  1000

K  -   Is the constant resistance of copper. It varies by the temperature of the environment in which the wire will operate. Use one of the following:
                                                               11.2 for up to 105F/40C
                                                               12.2 for up to 150F/65C
                                                               13.2 for up to 200F/93C

I  -  Is the current in amps that the wire will carry.

L  -   Is the length of the complete system, in feet. This would be the length of the positive wire and the length of the negative path added together.

E  -   Is allowable voltage drop. For 12vt systems 3% = 0.36 which is a good figure. The OEM standard is 4.2% which equals  0.50 . Remember that as the wire ages and is heated repeatedly while in service this figure will natural increase. Another fact to consider is whether the wire is used in the starting circuit or  operating circuit. A 0.5vt drop on a 12vt circuit increases to a 0.62vt drop when used in a 14.7vt circuit. In a 24vt system 3%= .72 and 4.2%= 1. In a 6vt system 3%= 0.18 and 4.2%= 0.25.

G  - Gauge code (see chart)

Example:     A 300amp at 12vt load with 4 ½ft. positive wire and 4ft. negative wire length in  150°F engine compartment, with a desired voltage drop of no more than 3%.
Step 1.)     12.2 (K) times 300 (amps) times 8.5 (positive leg + negative leg) = 31110
Step 2.)     .36 (E) times 1000 = 360
Step 3.)     Divide 31110 by 360 = 86.42 (Gauge code)
Step 4.)     Check 86.42 in chart = 0 (1/0) AWG

Chart:                             Gauge Code         Use AWG#
                                         2.5 or less                 16
                                         4.1   "                         14
                                         6.5   "                         12
                                       10.4 "                        10
                                       16.5 "                          8
                                       26.3 "                          6
                                       41.7 "                          4
                                       66.4 "                          2
                                       83.7 "                          1
                                     106.0  "                           0
                                       133.0 "                        00
                                       168.0 "                      000
                                       212.0 "                      0000