Lately we’ve been getting hit with the common question “what speaker wire gauge should I use”? American wire gauge (AWG) is a system of numerical designations of wire diameters where the thicker and less resistive the wire is, the lower the gauge number. More often than not, we’ve seen Audiophiles choose a higher gauge esoteric wire over a lower gauge generic zip cord cable simply because they were sold some marketing nonsense from the exotic cable vendor that their wire will yield higher dynamics and better Mojo from your system than standard generic cable.
Only the truth will set you free, so its time to clear your head of marketing slogans and get a good dose of reality about the single most important aspect of speaker cables – RESISTANCE. Anytime you are driving power into a low impedance load such as a loudspeaker, the dominant metric of concern is DC resistance (measure of restriction of conductivity in a circuit). Certainly there are secondary concerns which are also importance such as inductance, and capacitance. But, as we’ve proven in our various cable articles and reviews, if you cannot first select a cable with low enough DC resistance, the losses will be so great between the amplifier and loudspeaker that all other parameters about the cable will become a wash. This article will focus solely on cable resistance and will serve as a guide for choosing the right wire gauge (AWG) for your installation to ensure you realize the maximum potential of your setup.
The two primary things resistance affects in the amplifier to loudspeaker connection are: insertion loss and damping factor both of which are dependent upon cable resistance.
Insertion loss is the measure of the loss of load power at the speaker due to excessive resistive losses of the cable or any additional component attached between the amplifier and the loudspeaker. We typically express this loss in decibels (dB) using the following formula:
ILoss = 20* log
[ (Rload / (Rload + Rcable) ]
Note: For simplicity sake, we will assume the output impedance of the amplifier is zero which represents an ideal amplifier.
What happens when you have too much insertion loss?
You will experience power loss in the speaker cable resulting in the speaker not playing as loudly. But it takes a whole lot of loss here to become audible. For example, it would take nearly 1 ohm of cable resistance to drop the signal level down 1dB for an 8 ohm speaker. More importantly however, as the series resistance caused by the speaker cable increases, it makes the amplifier look more like a current source which will in turn cause the speakers frequency response to follow the rise and fall of its own impedance curve.
In order to minimize the potential of audibility here, I have established a guideline of the maximum acceptable degree of insertion loss that a cable should present to the system so that when a real world amplifier and loudspeaker are connected, the cable resistance will have a negligible contribution to the overall system response. I plotted the Insertion Loss (dB) vs Distance (ft) for various cable gauges from 18AWG to 10AWG using an 8 ohm speaker load.
If you are truly concerned with top notch fidelity, than:
ILoss < -0.2 dB
Figure 1. Insertion Loss vs Distance for Various AWG Cables
As you can see in the graph, once your distances exceed about 10 feet, you should consider speaker wire 16AWG or lower. If you are using a loudspeaker that is rated at 4 ohms, the critical distance becomes about half that. It is really important to use thicker wire for long cable runs, especially when driving a lot of power into low impedance loudspeaker systems.
Damping factor is a ratio of rated loudspeaker impedance (ZL) to the source impedance (Zs). In this case our source impedance is (Rcable + Ramplifier). Since we set the amplifier impedance to zero, we will focus solely on the source impedance of the speaker cable and see how it affects the damping factor of the system.
Damping Factor = ZL / Zs
What happens when a system exhibits poor damping factor?
Damping factor is often an abused term in the industry much like contrast ratio is in video. Beyond a certain number, it becomes meaningless. The problem however is when the system damping factor is too low, it will have an affect on system linearity and again cause the same issues we noted with excessive insertion loss and speaker frequency response variation. If the source resistance is extremely high, it can be noticed audibly by boomy bass response.
In order to minimize the potential of audibility here, I have established a guideline of the minimum acceptable System Damping Factor between a cable and a loudspeaker so that when a real world amplifier and loudspeaker are connected, the cable resistance will have a negligible contribution to the overall system response. I plotted the Damping Factor vs Distance (ft) for various cable gauges from 18AWG to 10AWG using an 8 ohm speaker load and correlated the results with the data we gathered on Insertion Loss in Figure 1.
If you are truly concerned with top notch fidelity, than:
System Damping Factor > 50
Figure 2. Damping Factor vs Distance for Various AWG Cables
As you can see, lower the speaker cable resistance greatly improves the system damping factor and allows you to run longer lengths without being penalized with significant losses.
Tabulated below are the recommended cable distances (+20%) one should use for various speaker cable gauges and speaker loads.
8 Ohm Load
4 Ohm Load
Table 1. Recommended Cable Distances vs Gauge
Realize once you attach a real world amplifier (non zero output impedance) and loudspeaker (reactive load), the overall system losses will be greater but the goal here is to ensure the speaker cable is as transparent as possible.
Cable inductance, AC resistance due to skin effect (to a much smaller extent), will further increase these losses but they are secondary concerns to primary DC cable resistance. Although I have measured some exotic cables to produce proportionally lower AC resistance than ordinary zip cord, most of these cables had such high DC resistance that even at the frequency extremes; standard 12AWG zip cord had lower overall resistance!
Let me make it clear that the cable length guidelines I set forth here are done so for audiophiles whom are particularly critical on system performance and transparency. For casual listeners or installers setting up distribution whole house audio, you can certainly run cable lengths much longer than I recommend in Table 1. The degree of audibility of running cables longer than these lengths depends on the quality of electronics and loudspeakers, neutrality of the room, and sensitivity of the listener.