How to calculate maximum input power on a speaker?
The ability of speaker wire to carry the current involved is rarely much of an issue except a few quite unusual loudspeakers. The most notable in this regard was probably the Apogee Scintilla, with a rated impedance of one (1) ohm. That extremely low impedance translated to low voltage and quite high current. But, those are still remembered (thirty years later) primarily because they were uniquely difficult to drive, in large part specifically because of that exceptionally low impedance.
With most reasonably normal loudspeakers, the motivation behind keeping the impedance of speaker wire extremely low is completely different. The real reason is that the amplifier can play a significant role in damping the speaker.
Consider what happens when you play some music with, say, a loud &quot;thump&quot; on a bass drum. That translates to a fairly short transient signal being sent to the loudspeaker. That, in turn, moves the cone of the loudspeaker. So far, all is fine and well.
But then we get to part two: as soon as that transient is over, we want the speaker cone to quit moving. But a speaker cone is a physical thing with inertia. Once it starts moving in a particular direction, its tendency is to keep moving that direction.
But, what started it moving was the magnetic signal in the voice coil interacting with a static magnetic field. Once that incoming signal stops, continued movement of the cone actually generates electricity in the voice coil.
That electricity is then carried back to the output stage of the amplifier, and flows through the amplifier's output impedance. So, the lower the amplifier's output impedance, the more it damps that extra movement of the cone.
But, it's not just the amplifier's output impedance that's involved. If the amplifier's output impedance is (say) 0.1 ohms, and the speaker wire adds another 0.1 ohms, then damping is determined by the (roughly) 0.2 ohm total impedance. A solid state amplifier typically has an extremely low output impedance--the 0.1 ohms I mentioned above is realistic, but at the upper end of the typical range. More powerful amplifiers typically use more output transistors in parallel to achieve their higher power output. That also translates to lower output impedance, so if you have a 140 watt amplifier, chances are your amp's output impedance is considerably lower than that.
As such, to ensure that the speakers are damped as well as the amplifier is capable of doing, you want to ensure that the impedance you're adding in series with the amplifier is considerably lower than that of the amplifier itself. Just for example, you might want something like 10 milliohms (0.01 ohms) as a decent target.
Doing a quick check, the specs you've chosen (16 gauge wire for 10 feet) works out to 0.04 Ohms of DC resistance1 (0.08 for the total of 20 feet for the round trip). While 16 gauge wire is certainly plenty to carry the current involved, that resistance is high enough that it probably has at least some effect on the damping of the system as a whole. If your total round trip is 10 feet, a 10 gauge wire gives a resistance of 0.01 ohms. If you're talking 10 feet distance (so a total of 20 feet of wire), you'd need 7 gauge wire to meet the same target.
In fairness, the difference between the two certainly won't be obvious, and depending on the amplifier you're using (specifically its output impedance) it may be essentially irrelevant. On the other hand, especially if it has particularly low output impedance, it could make a bigger difference.


1. In theory, we should care about impedance, not just DC resistance. In this case, however, we're mostly concerned with the part of the signal that goes to the woofer. That's usually restricted to under 1 KHz (and often less than 500 Hz), where the impedance of a wire is usually very close to its DC resistance.
Also: thank you to Bruce Abbot for pointing out that as originally posted, the resistances/gauges needed that I quoted were wrong.