Speaker wire isn't carrying the voltage of mains cable. Unless the amp is stupidly powerful -
and I'm talking real current driving capacity and not the magic Watts claims of a lot of manufacturers - it's going to max out at roughly 30V DC and probably a lot less than that in reality. What you should pay attention to though is the
current the wire will be carrying.
There are two areas to think about. The first is the power loss through the cable.
This is determined by cable construction i.e. is it pure copper (good) or the cheaper
copper
coated
aluminium (CCA = bad cable), and then by length and the thickness. e.g. If you only need to run 100W through 10m of cable within a single room then it's not going to make much of a difference if you run 1.5mm or 4mm cable, so spending the extra probably isn't justified. However, if you're starting to run cable all around a house like I do for multi-room speakers then the power loss ramps up and becomes significant. 100W in an 8 Ohm rated system will deliver over 90% of the power to the speaker over 40m of pure copper cable @ 4mm, but only a shade over 70W through a 1mm cable. It's far cheaper to buy thicker cable and avoid the power loss than it is to buy a much more powerful amp to compensate for the power loss in thin cables. The current in both cases is over 3 Amps. That's why you don't use phono plugs. They're not rated for the sorts of Amperages; 2A is commonly the quoted maximum and that presumes a well made plug. There's a lot of rubbish out there that won't deal well with speaker-level Amperages because phono plugs are really a line-level connection where the voltage rarely exceeds 1V and the input impedance of each item in the chain is so high (10,000+ Ohms) that the current is negligible.
The second area is system and speaker matching. This can have a big impact on the current flowing through the speaker cables.
Good quality home audio gear runs at 6-8 Ohms. That means the speakers present a 6-8 Ohm nominal load to the amp, and the amp is designed to drive an 8 Ohm load. With a system matched in this way your 10m 1.5mm pure copper cable is carrying around 3.5 A presuming it's a full frequency signal and the amp is pumping out 100W. Now watch what happens if the system matching changes. Let's say you have one of the all-in-one home cinema kits - you know, it has a built-in DVD/Blu-ray drive and a million Watts output according to the manufacturers specs. The speakers and amp are rated at (typically) 3-4 Ohm. The same 10m 1.5mm is now carrying anything up to 5.5 A. It gets even worse if you mix and match. Put 3 Ohm speakers on the end of an 8 Ohm amp and the current rises to almost 9 Amps.
So, big question - Every all-in-one home cinema kit comes with crappy thin bell wire and generally use 3-4 Ohm loadings, so why don't the cables melt and the amp erupt in a ball of flames?
Simple, the amp isn't outputting anything like the claimed power and the speakers aren't full range so they aren't drawing the current. Manufacturer's power claims are a master class in smoke and mirrors. The figures you'll see on the spec sheet or box are based on driving one single channel - therefore all the power is funnelled to that one channel - and it's working at a very narrow frequency band (1kHz) and for a fraction of a second (peak power output) so the amp doesn't have to recover and continue supplying power. The result is that you're lucky to see 20% of the claimed power output in real world conditions. Add to that that the amp will rarely be running at full power so the current being drawn by the speaker will be even lower.
What are the take-aways then from all this guff?
Speaker cables don't have the same safe zone requirements as mains voltage cables. Sticking a nail through a speaker wire is highly unlikely to kill someone unless something is very very wrong with the amplification or the way it is connected
Decent cable is worth it - to a point - and that point is governed more by the cheapness of the gear (not a great phrase, I know; but accurate) than you might think. 1~1.5 will be adequate unless you plan to hang some full range surrounds off the ends of the cables in future
CCA cable is crap. No two ways about it. Aluminium isn't a good conductor for low voltage signals. Pure copper is what you want. You mentioned 42 strand. That's a good shout. Yes you can use T&E but the difference in price is literally £3-£4 on a 25m roll and I prefer to see something that looks like speaker wire if I lift a board or open a wall
Don't use phono plugs. Go for speaker wall plates with binding posts
Good luck with the job
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