 Recently, there has been numerous questions on this board
concerning the proper type or size of AC power cable to use with
different amounts of equipment. It is very important to use the
correct size cable to insure all the power will be available
to your equipment and there is no danger of a fire or short from
your cables.

 Here is a Cable/Current table to help you select the proper
one to use in your application.

 Wire Size (AWG)

 2 Conductor

 3 Conductor

 4 Conductor

 10

 30Amp

 25

 20

 12

 25

 20

 16

 14

 18

 15

 12

 16

 13

 10

 8

 18

 10

 7

 6


 Notice that the smaller the AWG number, the more current
it can handle. All Extension Cords are required to list the wire
gauge. That will tell you the amount of current they can safely
handle.
 The wire in the above example is Copper type and of the same
temperature rating. All currents listed are for Ambient temperature.
Keep in mind that there are also many different type of insulation
material that will determine the temperature rating. The wire
may not be pure copper but an alloyed of aluminum, nickel, tin
and copper.
 Standard cable, as used in home and general construction,
is classified by the wire size, number of wires, insulation type
and dampness condition of the wire environment.

 Example: a cable with the code "12/2 with Ground –
Type UF – 600V – (UL)" has the following specifications:
 1. Wire size is 12 gauge (minimum required size for homes
today).

 2. The "/2" indicates there are two wires in the
cable.

 3. "Ground" indicates there is a third wire in
the cable to be used as a grounding wire.

 4. "Type UF" indicates the insulation type and
acceptable dampness rating.

 5. "600V" means the wire is rated at 600 volts
maximum.

 6. "UL" indicates the wire has been certified by
Underwriters Laboratory to be safe.

 Standard wire color codes are very different between electronic
circuitry and household 110 Volt AC wiring.


 Household wiring (or other AC applications in the 100+ volt
range) use the following color codes:
 BLACK "Hot" wire. Connected to Brass colored
terminal.
GREEEN "Ground"
wire. Also called chassis ground.
RED "Traveler"
wire. Used for 3ways switches.
WHITE "Neutral" wire. Connected to silver colored terminal.

 VOLTAGE DROP vs. WIRE SIZE
 Voltage drop is the amount of voltage lost over the length
of a circuit. Voltage drop changes as a function of the resistance
of the wire and should be less than 2% if possible. If the drop
is greater than 2%, efficiency of the equipment in the circuit
is severely decreased and life of the equipment will be decreased.
As an example, if the voltage drop on an incandescent light bulb
is 10%, the light output of the bulb decreases over 30%!
 Voltage drop can be calculated using Ohms’s Law, which
is:

 Voltage Drop = Current in amperes x Resistance in ohms.

 For example, the voltage drop over a 200 foot long, #14 copper
wire, power line supplying a 1000 watt floodlight is calculated
as follows:

Current = 1000watts/120volts = 8.33 amperes 
Resistance of #14 copper wire = 2.58ohms/1000feet 
Resistance of powerline=2 x 200ft x 0.00258ohms/ft=1.032ohms 
Voltage drop = 8.33 amperes x 1.032 ohms = 8.60 volts 
Percent voltage drop = 8.60volts/120volts = 7.2% 
 The 7.2% drop is over the maximum 2% so either the wattage
of the bulb must be decreased or the diameter of the wire must
be increased (a decrease in wire gauge number). If #9 copper
wire were used in the above example, the voltage drop would have
only been 2.2%.

 A more commonly used method of calculating voltage drop is
as follows:


K x 2 x Wire length in ft. x Current in amperes 
Voltage Drop = 
 

Wire area in circular mils 

 K = Specific resistivity in ohm – circular mils/foot
K = 11 for copper wire loaded at 50% of capacity.
K = 12 for copper wire loaded to 50100% capacity.
K = 18 for aluminum wire

 Using values from the Ohm’s Law example above: #14 copper
wire has an area of 4110 circular mils, then voltage drop = (11
x 2 x 200 x 8.33) / 4110 = 8.92volts = 8.92volts/120volts = 7.4%.

 An interesting corollary to the above example is that if
the line voltage doubles (240 volts instead of 120volts), the
voltage drop decreased by a factor of 4. That means that a line
can carry the same power 4 times further! Higher voltage lines
are more efficient. That’s why voltage is so high (50,000volts)
for power transmission lines.
 I hope this help explain many questions about AC power and
cables.
