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I just bought a couple of Planet Waves cables. They call them "Speaker Cables" Is there a difference between speaker cables and instrument cables?

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Yes, there’s a difference. Instrument cables are shielded, speaker cables are not. They also have different gauged wires.
 

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What everyone said about the question asked in the BODY of the OP: speaker and instrument cables ARE NOT the same and NOT interchangeable.

However, the OP SUBJECT asks a completely different question: “effects cable” and instrument cables ARE the same thing and ARE interchangeable.
 

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Amplifiers, whether tube-based or solid-state, are designed with a particular speaker-load, or small range of loads, in mind. Although we don't normally think of cables as being resistors/"loads", they are. An instrument cable may insert 10 ohms, or whatever, between your guitar and pedalboard, by virtue of its length. But that pales beside the simple resistance of your guitar's volume pot, so we think of it as being like a zero-resistance path.

In contrast, if the cable running between an amplifier and the speaker load were to add 10 ohms, that amp is essentially trying to drive a 14, 18 or 26-ohm load, once you add the cable and speaker/s. What means nothing at the input, means a lot more on the output. That's why one will generally see much thicker and shorter cable between amplifier and speaker/s. Twelve-gauge will be better than 14-gauge and 14 better than 18, simply because the thicker the wire, the lower the linear resistance.

I do have to say, however, that instrument and speaker cable CAN be interchanged. That is, used within reason, nothing is going to blow up. But unshielded speaker cable will pick up EMI/hum, and amplifying that will not be pleasant to the ear. Similarly, a guitar cable could be used to connect amp and speakers in a pinch. But the linear resistance will likely degrade the sound and amplifier performance; not to mention that such cable is not spec'd to carry the current an amp puts out. Mind you, the wire gauge in instrument cable is probably comparable to, or at least not that much thinner than, what is used for the voice coil in the speaker.

As guitarman2 noted, this is a different topic than "instrument vs effect cables". However, the OP seems to have simply erred in choice of subject headers. It happens.
 

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Of course, I assumed it was a simple typo/error as well. But this is one of those [sarcasm] rare [/sarcasm] questions where the answer is completely different depending on the actual question :).

And a mild comment to add to @mhammer ‘s: at low volumes/power levels and short runs, using an instrument cable between speaker and amp isn’t going to damage anything, but at higher levels, you can melt the instrument cable which can have a cascade effect of running an unloaded tube amp which can result in amp (OT) damage.
 

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@Doug B A friend of mine bought a speaker cable for his brother (both are guitar players) as a birthday gift because it looked "thick and strong and like it would last a long time" (as a guitar/instrument cable). His brother (the recipient) never liked his tone with the new cable but felt obligated to use it. I had to break the news to both of them at the same time.
They were both good about it and laughed.
 

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And a mild comment to add to @mhammer ‘s: at low volumes/power levels and short runs, using an instrument cable between speaker and amp isn’t going to damage anything, but at higher levels, you can melt the instrument cable which can have a cascade effect of running an unloaded tube amp which can result in amp (OT) damage.
I think this is somewhat overblown on the internet.

Example: a 22 gauge multi-stranded center conductor of a shielded cable (the limiting factor) is rated for around 2 amps. At 8 ohms, that is around 30 watts. And 30 watts can be kinda loud, with a speaker rated at 100 dB (1watt*1m). Using an instrument cable for a short period of time, or in a pinch/emergency or whatever, isn't the end of the world as some would have you believe.

Then again, with the technical misinformation I see on the internet, probably safe rather than sorry. Better just to have a blanket proviso - never use an instrument cable as a speaker cable.

My Sunn T50C (and some other 90s Fender amps) use a speaker cable to connect a 3 button footswitch. This usually requires at least 4 conductors - so one more than a TRS cable and requiring a more complicated connector. They are using diodes to step the voltage down (8 different combinations) and an instrument cable will not work (but won't damage anything either - no appreciable current involved). I guess the capacitance of shielded cable messes up the voltage steps. But still, a darn elegant solution for their footswitches.
 
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Example: a 22 gauge multi-stranded center conductor of a shielded cable (the limiting factor) is rated for around 2 amps. At 8 ohms, that is around 30 watts. And 30 watts can be kinda loud, with a speaker rated at 100 dB (1watt*1m). Using an instrument cable for a short period of time, or in a pinch/emergency or whatever, isn't the end of the world as some would have you believe.
You are technically correct, but most people would not do a careful calculation and evaluation in this case (or any case). Your example is a pretty good typical case, but instrument cables come all the way down to 24 gauge and at multi-core, can be rated as low as 1A, which is about 8W in your example (or even just 4W for a classic Fender 4ohm load or, even worse, 2W for some bass setups). And many speakers are worse than 100dB efficiency, so maybe 8W or 4W (or just 2W for bass) isn't all THAT loud. So as you say, perhaps it's overblown for a typical case, but caution is in order when it's so easy to deal with it and if you're playing outside your basement (or even in your basement after having lost most of your hearing). We really don't want people blowing their cool amps.

P.S. Also, if playing outside in the weather we've been having, you gotta apply the 0.82 current carrying capacity reduction factor for 30C+. 24 gauge 8W suddenly becomes 5.4W and etc.
 

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P.S. Also, if playing outside in the weather we've been having, you gotta apply the 0.82 current carrying capacity reduction factor for 30C+. 24 gauge 8W suddenly becomes 5.4W and etc.
So...Be extra cautious and safe and play outdoors ONLY on the coldest days of the winter. Correct?!
 
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