10 gage electrical wire for speaker cables?

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.

i still have a couple of runs of Esoteric OFC wires; two different gauges- one heavy and one medium. i had spade lugs soldered to the ends by the store i bought this stuff from (for a small fee). it couldn't have been more than 5-10 bucks a foot. but they didn't carry "speaker cables", although they did have interconnects starting at $30 and going up to $125/M. It's fair to say that there is no reason to go much beyond what this store's owner felt was adequate wires
to get excellent results from most equipment. i went over to his house a couple of times and listened to his modified Thorens turntable playing music over his Quad speakers. it sounded really great- not on the same level as the Rowland dealer's setup, or the Wilson dealer, etc. but he had a glorious album collection
and i doubt he felt his system was lacking anything.
but it all depends on how critical you become about linking ever-more-expensive gear together.