Speaker wire gauge question

[txstrat]

Hi all,

in most of my amps I'm using 24 gauge (0.5 mm) stranded wire from speaker jack to the speaker.
I've never given too much thought about this - and also never had a fault caused by this.
Now I wonder if the gauge is too thin for the signal to the speaker.
How can I calculate the voltage and current running through those wires?

thanks

Matt

[R.G.]

How can I calculate the voltage and current running through those wires?

Power in a sine wave driving a resistor is equal to the (RMS) voltage squared divided by the resistance. You can solve this for voltage by multiplying power by load resistance and taking the square root.

Your amp is rated for sine wave (RMS power). Music is not sine waves, nor are speakers resistors, but it's not too bad an approximation.

So if you have a 100W amp driving 4 ohms, P = 100, R = 4, and then voltage is V = SQRT(100*4) = SQRT(400) = 20VRMS. The current is 20V/4 ohm = 5A RMS.

The voltage is no big deal, any insulation will carry that. The question is then whether the 24Ga wires (for instance) will carry 5A with no bad effect. I consulted some wire tables. 24Ga is 25millohm per foot, there and back, or 50milliohm per foot to the speaker. 18 Ga is 1/4 of that. The rated current carrying capacity of 24Ga is 3.5A in free air. The wire adds some resistance to the source resistance of the amplifier, and lowers the damping factor. This may matter with SS amps but probably not much with tube amps.

24Ga is probably too small to carry more than 50W reliably. I'd go with 18Ga at least for 100W.

[txstrat]

Thank you so much R.G.
Well, my most powerful amp has a 50-60w OT for two 6L6. I'm already using 18 gauge wire for it.
All other amps are 20w or 35w. Think I'll be fine with the 24 gauge for them.
Thanks again.

Matt

[Enzo]

And that 5A of the 100 watts represents a steady 100 watt output. Even screaming guitar doesn't put out a steady wall of 100 watts of sound. Peaks maybe, but not steady state. Your 24ga wire is probably just fine for the small amps.

But I would prefer 18ga or 16ga simply because the wire will be physically more sturdy. Vibration is a given in a speaker cab or combo amp, and if you tuck your power cord and stuff in the back of a combo amp, you likely will be knocking around the speaker wires then too. I don't NEED the heavier wire electrically, but I feel it is more likely to hold up over time mechanically.

[txstrat]

I don't NEED the heavier wire electrically, but I feel it is more likely to hold up over time mechanically.

Yeah, that sounds reasonable. Up to now I've tied the cable to the speaker frame with cable ties just for strain relief. I'll buy some stronger silicon wire with my next order. My supplier has up to 15 gauge available.
Thanks for your thoughts.

Matt

[GregS]

How about for very short wire lengths of only a few inches - like from the impedance switch to the speaker jack? Could you use a gauge that would normally be too small for the current you expect if it is only a few inches in length?

Greg

[R.G.]

Sure if...
er, well, if it's not too long, where "too long" means that the length prevents the ends from doing any cooling effects in the middle, and it's not too much too small, and if you're willing to let it get hot and ... and... and...

There's a long, long series of ifs, so the exact details matter. The rated current carrying capacity of a wire is not a fixed number. It's based on heating and voltage drop, as well as resistance in the speaker case, and a huge number of things affect heating. As well as music having a 10-20db average-to-peak power ratio, so the maximum power/current is not flowing through the wires all the time.

Can you? Sure. But as you get closer to bad effects, the details matter. How close can you get? Depends on the details.