Wire gauge and fusing

Gauge numbers run backwards

Wire thickness in North America is given as an AWG number (American Wire Gauge), and a smaller number means thicker wire. Thicker wire has less resistance, carries more current and heats less. Two regularities make it easy to remember: every 3 gauges doubles or halves the cross-section area, and every 6 gauges doubles or halves the diameter.

The numbers

AWG	Diameter	Area	Resistance (copper)	Building-wire rating
22	0.64 mm	0.33 mm²	16.1 Ω per 1000 ft	signal wiring only
20	0.81 mm	0.52 mm²	10.2 Ω per 1000 ft	signal wiring only
18	1.02 mm	0.82 mm²	6.4 Ω per 1000 ft	light loads only
16	1.29 mm	1.31 mm²	4.0 Ω per 1000 ft	light loads only
14	1.63 mm	2.08 mm²	2.5 Ω per 1000 ft	15 A
12	2.05 mm	3.31 mm²	1.6 Ω per 1000 ft	20 A
10	2.59 mm	5.26 mm²	1.0 Ω per 1000 ft	30 A
8	3.26 mm	8.37 mm²	0.63 Ω per 1000 ft	40 A
6	4.12 mm	13.3 mm²	0.40 Ω per 1000 ft	55 A

Diameters and resistances come from the formula above, for solid copper at 20 °C. The ratings are the conservative figures used for building (mains) wiring. A rating also depends on insulation, bundling and heat, so it is a ceiling for planning rather than a promise. Stranded wire is slightly less conductive than solid for the same gauge.

Fuses protect the wire

A fuse or breaker exists to open the circuit before the wire gets hot enough to melt its insulation or start a fire. So size it to the thinnest wire in the circuit: a 20 A breaker needs at least 12 AWG, a 15 A breaker at least 14 AWG. A bigger fuse than the wire rates for lets the wire overheat first.

• Put the fuse at the source (for a car battery, close to the battery, in the positive lead) so the whole wire is protected, including a short anywhere along it.
• On AC, fuse only the hot wire. Never fuse neutral or the safety ground. See Electrical safety.
• The fuse must be above normal load and below the wire's rating. Too small and it blows in use; too large and it does not protect.
• Radio fuses are extra protection for the radio. They do not replace the fuse at the battery.

Voltage drop matters for radios

A 100 W HF radio on 13.8 V can draw 20 A or more when transmitting. Even wire that is rated for the current wastes voltage, and a drop at the radio makes it distort or shut down. Voltage is lost on the way out and back, so the resistance counts twice the length.

At 20 A over a 10 ft run (20 ft of wire in the loop): 14 AWG loses about 1.0 V (7 %), 12 AWG 0.64 V (4.6 %), and 10 AWG 0.40 V (2.9 %). (14 AWG is already past its rating at 20 A.) Use thicker wire, or shorten the run, until the drop is small.

Related

• Wire, cable and gaugeAWG, current ratings, and where each wire type belongs.
• Switches, relays and fusesControlling and protecting circuits.
• Electrical safetyShock, fusing and wiring.
• Shack power suppliesSizing and choosing a 13.8 V supply.
• Power and energyWatts, joules and kilowatt-hours; P = E × I.
• Ohm's lawThe relationship that links them, and how to use it.