10 gage electrical wire for speaker cables?

Why not just buy a pair of Speltz Anti-cables? They're very inexpensive and made of better wire (magnet wire) which is insulated with a tough polymer coating. And No, they won't damage your speakers, although if you use (uninsulated) hardware store wire and they touch (short out) you could damage your amp ;~)

Can't see any reason why it would harm anything. If you want something similar and a little more finished in appearance, I can recommend Belden 10-gauge 5T00UP speaker cables from Blue Jeans Cable -- no-nonsense, good quality, neutral-sounding (to my ears) speaker wire.

It wont harm anything, but I suggest looking thru the archives using keyword "zip cord" and you will find many opinions on the subject of electrical wire for amp to speakers.
IMO, you will find much better speaker cables at Blue Jeans Cable as Jhold has suggested. They will outperform ordinary electrical wire.

Forgot... Nsgarch states an important point regarding wires shorting out your amp. Also if the 2 bare ends of the wire touch.

hardware store 10 gage electrical wire

Not sure what this is exactly but when you say "electrical wire" I will assume insulated copper, either stranded or solid.

I have tried 10 gage wire,

probably more than large enough for most typical run lengths

one wire for - and one for +

so far so good, as long as the actual conductors don't touch each other as previously mentioned

I just tried it out and it sounds good

ahh....that is the key, if it sounds good then save your money by sticking with what you have, and don't sweat it just because it didn't come in a package that said "audio" on it. There are folks out there who are perfectly happy using outdoor extension cord in 14 or 16 awg sizes as speaker cable, so I am sure your wire is fine. Don't change anything just because your wire was purchased in a hardware store.

wasn't sure if this could harm the system

The answer is "no," unless the wires become shorted.