Not all lamp cable is suitable for speaker use. The copper is fine, but some insulators are better than others. Vinyl and rubber corrode copper. Teflon is great, polypropylene works very well, and I'm sure there others that are acceptable. Good speaker wire can be cheap, but not all cheap wire is good for use with speakers.

Corrosion may not matter if you use solder or gas-tight crimp connectors. But it does matter for the usual clip or compression type speaker connectors.

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