Instrument cable is used to connect guitar, bass, keyboard, or another low-signal instrument to an amplifier. It is made of an outer shield that grounds it and a positive wire, which carries a low DC signal. It has a high resistance to signal flow coming through the wire and is built to be more flexible than speaker cable to allow for more movement.
Speaker cables contain two wires, which are designed to carry a high AC signal to speakers. Because the wires carry a high signal, the wires are thicker than the one found in the instrument cable. It has a low resistance to signal flow, causing the signal to flow through the cable to the speakers more easily.
The risk of using an instrument cable as a speaker cable is greatest when working with high signal. When using an instrument cable to run from an amp to speaker, the cable has the potential to overheat, causing damage to the speaker and/or the cable. This happens because the instrument cable has a high resistance to signal flow, causing the high power from the amp to turn into heat. When using a speaker cable as an instrument cable, the speaker cable will have the tendency to pick up a lot of excess noise from fluorescent lights and external AC power supplies because the speaker cable is unshielded.
Instrument cables most commonly use ¼” connectors. Speaker cables use ¼” frequently as well, but also use speakon, banana wire, or binding post. The connectors are at the end of each cable, and are used to plug into an instrument, amp, or speaker depending on the situation.
Because both cables can use a ¼” connector, it can be difficult to tell whether the cable is a speaker or instrument cable. To identify which it is, unscrew the connector on either end to look at the wiring. If the cable has one wire, it is an instrument cable. If it has two wires, it is a speaker cable.