Shielding
Guitar cables require wire shielding to filter out radio frequencies, while speaker cables do not. Using speaker cables with a guitar results in loud humming and possible interception of radio waves, which makes the sound quality unacceptable.
Wire Core
Both guitar and speaker cables use the two-wire cable. Speaker wires carry voltage to the speakers, so a larger wire core size is necessary. Because guitars produce very low voltage, thinner wire core sizes are used.
Wire Design
The two speaker wires are of equal size, as both positive and negative are required to carry the same electrical voltage. Guitar cables use a single ‘hot’ positive wire and a packaged or braided negative ‘earth’ wire, which doubles as a shield. The hot and grounded cables on guitar cables are different sizes.
External appearance
Speakers and guitar cables are very similar in appearance. Both use external insulation and 1/4 inch telephone plugs, which often makes it impossible to identify from the outside. Although some cables are externally labeled, many are not, or the labels may be difficult to read and decipher.
Identification
Because external cable identification marks are sometimes difficult to read or non-existent, the wires can be identified by unscrewing the plug. If both cables are wrapped in insulation, they are speaker cables. If one wire is wrapped in insulation, and the other wire is not, it is a guitar cable.
Risks
Using speaker cables with a guitar makes noise, but there is no danger to the guitar, amplifier or cable. Using guitar cables for speaker connections not only results in lower sound levels, but can also short-circuit the cables, causing possible amplifier or speaker damage. There is also a risk of fire, as the guitar cable is not thick enough to handle higher voltages. There is a fundamental difference between guitar cables and speaker cables in their internal design and use. To beginners, the differences may seem trivial at first, but the proper use of each cable is quite important.
Source:danspela.com