Impedance is an important consideration for DIY Audio loops. Too much impedance in the Loop Cable will make the system weak and not effective. Too little impedance will overload the power amplifier and may shorten it's life or result in distortion.
Impedance (Z) is determined by the Resistance, (R) of the conductors (wire) and the Inductive Reactance (XL). Inductive reactance can be a very low number for loop cables, depending on the number of conductors and how tightly they are packed together.
A 20 gauge wire will have a resistance value of about 1 ohm in 100 feet.
Therefore, a loop cable that has four conductors, each wired in series, and is 100 feet long, will have a resistance of about 4 ohms. Resistance can be easily measured with an ohmmeter. Inductive reactance is more difficult to measure, but for the type of cable that is specified in these pages, it is also about 4 ohms.
Impedance is not just the sum of the two values, but you have to use the square root of the sum of the squares. (The old Pythagorean theorem trick.) Therefore a R value of 4 and a XL value of 4 gives an impedance of 5.6 ohms.
Bigger gauge wire (smaller numbers) will have less resistance so if a bigger gauge wire is used it may overload the amplifier, unless a longer length loop cable is used to increase the resistance. Smaller gauge wire will have more resistance.
It is advisable to measure the total resistance of the completed loop cable. With that figure, you know the impedance is at least that much. Then depending on the total number of conductors, you may estimate the impedance to be from 1 to 3 ohms more than the R value. Choose a tap on the power amplifier that comes closest to that estimation.