I would not worry about it but if concerned you could always find a slightly larger gauge. Not that I have experimented much with long lengths but would think at 50 - 100 ft or longer might be of some concern due to resistance.
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Yes, typically a run of say 20 feet will be no problem, but it is dependent on the impedance characteristics of your speakers, as well as on the wire's gauge and other characteristics.
The constraints are:
1)The total resistance of the round-trip run (40 feet for a 20 foot cable) should be a very small fraction of the impedance of the speaker, at the lowest point on the speaker's impedance vs frequency curve.
For 14 gauge wire, a 20 foot run (a 40 foot round-trip for the signal) has a resistance of about 0.1 ohms. That would be acceptably small in relation to the impedance of nearly all speakers which are not named Infinity Kappa 9 or Apogee. :)
2)The inductive reactance of the cable at 20kHz (the worst case frequency within the audio band) should be significantly less than the impedance of the speakers at 20kHz. Otherwise the upper treble will be attenuated slightly. As a rough approximation, speaker cables of reasonable gauge and conventional construction will have an inductance of around 0.3 microHenries per foot, which would be 12 uH for 40 feet, which would be about 1.4 ohms of inductive reactance at 20kHz. That would be fine in relation to an 8 ohm speaker impedance at 20 kHz, and within reason even in relation to 4 ohms.
See the following thread for further information, links to some calculators, and discussion of cable selection for long runs:
I have to use, currently, lengthy runs of speaker cables myself.
Using various speakers, amps, and speaker cables from DIY to premium cables of 20-30ft. I've experienced no perceived attenuation of upper band information. Wire gauge was kept fairly large though.... but not immense. #8 AWG stranded, being the largest Ive tried out right..
If Im actually losing something, either it isnt much or it doesnt matter to me apparently.
I've run combinations of amps & speakers from 60 wpc into 4 ohm units that drop lower by half, and other more powerful amps and easier to run speakers. No problems occurred I am aware of.
I did notice spending more on speaker wire helped improve the sound.... and now use SR Sig 10 x2 active biwires. I believe a good reason for that is the termination, and dialectric. The Sig 10s too, are a bit beefy, though I dont know their actual AWG, outright or even its effective overall gauge.
The technical info Al input I'm sure is quite valid and I'm not disputing it, but you must see for yourself how the changes in speaker lengths with your gear might be. There are also devices which can protect your cables from being smashed which lay on the floor and the wires run right thru them. Wiremold among others make such things, and that could save you 10ft or more right there in wire lengths.
A hole in the wall will too. Ultimately, thats what I did and still must use 21ft cables . Until I reroute some power ckts. Im in no hurry to do it though.
If you could make, rent or borrow the same but shorter cables and just reposition the gear temporarily, youd see for yourself what is truly the diff by comparing the short ones to the longer ones.
Some perspective should be gained here for your own now and future benefit, if nothing else but to keep in mind some rule of thumb for yourself.
My longest runs are 36FT. Until recently my Onkyo 805 receiver was driving them and the center ch without trouble, yielding pretty good results with Canare 4S11 cabling.
Audio nuts go to extremes sometimes. Now and then, the issues are only in the mind of the nut doing the task, and not demonstrably noteable if done in a more practical fashion in reality. We do like overkill.
Shorter wire paths are better, no doubt. They arent always possible however. We can only do what we can do, and little more if at all. Id keep the AWG up somewhat as Al points to here and past that, be more mindful of the fact cabling from maker to maker will produce sound differently more so than will the lengths if and as well, are kept appropriate.
Id not lose any sleep over it though, given it cant be helped much now. I'd look to protect the cable while on the floor if that will shorten or ease the strain some.
I wonder too sometimes, if all the rhetoric on cable lengths wasnt promoted by their sheer costs . Wherein, shorter equates to cheaper too and especially with speaker cables. Lol Usually youll find long ICs + shorter speaker wire less costly, than doing things the other way around. Good luck.