Calculating Wire Diameter and Area:
Here in the US, we use the AWG (American Wire Gauge), circular mils and square mils. In most other parts of the world, they use mm2.
Solid Wire Diameter:
This section will address the diameter of solid wire. Stranded wire has air spaces between conductors and different combinations of different gauge strands will result in different overall diameters. Keep in mind that, in the following description, we are talking about the area of the wire in a circular shape. This means that the total cross sectional area is doubled when the diameter is increased by a factor of 1.414.
For a reference that's relatively easy to remember, lets use 10g wire again. It's ~0.1" in diameter. If we go up in wire size 6 sizes again (to 4g), the diameter is going to be double the 10g wire. The multiplier is ~1.123 per gauge.
Diameter = .1*1.123^(difference between 4g and 10g) Diameter = .1*1.123^6 Diameter = .1*2.005 Diameter = approximately 0.2" in diameter
These are the recommended Generally Safe maximum fuse ratings for the corresponding wire size.
Maximum Fuse Size
At some point in time, you may need to determine the resistance in a length of wire but you may not have a reference book available. This section will help you to calculate resistance for different wire sizes and lengths. To make quick calculations with an easy reference I use 10g wire as the starting point. It's resistance is approximately 1 ohm per thousand feet of wire length which makes it easy to remember. For non critical calculations, I round it to 1 ohm/1000 ft or 0.001 ohms/foot of wire. If you have a 15 foot run of 10g wire and want to determine the resistance in that run of wire, you simply multiply the resistance per foot by the length of the wire.
Resistance = .001*15 Resistance = .015 ohms
For 4g wire:
Resistance = 1/1.26^(difference between 4g and 10g) Resistance = 1/1.26^6 Resistance = 1/4 ohms per 1000 feet of wire Resistance = .25 ohms per 1000 feet of wire (or 0.00025 ohms per foot)
for 16g wire:
Resistance = 1*1.26^(difference between 16g and 10g) Resistance = 1*1.26^6 Resistance = 1*4 ohms per 1000 feet of wire Resistance = 4 ohms per 1000 feet of wire (or 0.004 ohms per foot)
for 20g wire:
Resistance = 1*1.26^(difference between 20g and 10g) Resistance = 1*1.26^10 Resistance = 1*10 ohms per 1000 feet of wire Resistance = 10 ohms per 1000 feet of wire (or 0.01 ohms per foot)
Voltage Drop = current flow * (length of wire in feet * resistance per foot) Voltage Drop = 150 * (15*.00025)
Voltage Drop = 150 *.00375 ohms Voltage Drop = .563 volts at 150 amps of current
Common Wire Designations
Thermoplastic insulator (generally PVC)
-Dry location - household/building wire (generally the first 'H' if there are 2 Hs)
-High temperature (second H if there are 2 Hs)
Nylon outer insulator (protects against abrasion)
-Silicone rubber (if used at beginning of designation) Generally used in high temp applications.
-Switchboard wire (if used at end of designation)
Rubber (non specific)
Specific Wire Designations
High temperature (90°C 194°F max.) thermoplastic for use in dry locations like building
wiring with a nylon outer insulator.
Standard temperature (75°C 167°F) thermoplastic for use in wet or dry locations with
a nylon outer coating.
Moisture resistant rubber
Rubber moderate temperature (75°C 167°F)
Rubber high temperature (90°C 194°F)
Thermoplastic insulator with braided cover
(generally used for switchboard applications)
Heat resistant latex rubber