Hi narlo,
Welcome to ETO.
I see you are from the US: which state. Care to put it by 'Location' in your user page so that it displays in the window at the left of your posts.
There are three main aspects to cable selection: safety, reliability, and electrical performance.
(1) Safety
There are standard tables for ensuring safety of cables in different environments. This basically involves maximum allowable current and voltage.
(2) Reliability
There are standard tables for ensuring reliability (life) of cables in different environment: temperature, humidity, vibration, etc
(3) Electrical performance
This is the area which is often overlooked and people are included to select a cable from the standard cable tables based on voltage and current, but this is a misnomer.
To select a suitable cable to perform adequately in an application you need to establish how the cable characteristics relate to the intended function.
The main parameter is to establish an acceptable voltage drop under maximum current conditions. To do this you need to work out the total voltage drop from the voltage source to the equipment, taking into account the resistance of the cable (3mili Oms per foot for 14AWG copper wire), both supply and return (earth), and the resistance of any switches, solder joints, fuses, connector contacts etc.
When a voltage drop budget is complete you will normally find that cable of a much thicker cross section than specified in standard cable tables for a particular current is required.
An interesting exercise is to measure the actual voltage across the source terminals and compare that with the actual voltage across the sink (equipment) terminals.
To give you a clue, 2 * 6 feet = 12 feet of 14 AWG copper wire at 15.8 Amps will give a drop of 12 feet * 3 mili Ohms * 15.8 Amps= 569mV, and that does not include other voltage drops.
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