Can I Use a 12V Relay on a 120 Volts Circuit?

I work on a project, not a car one, but I need a relay that can support 15 amp at 120 volts.I already have car relay (40 amp) at home and I want to know if I can use that car relay for that. The car relay coil will be activated from a thermostat with a 12 volts souce and I want to connect a heater (12.5 amp at 120 volts) on the switch portion of the relay. Hope you understand what I mean.

No, automotive relays are not rated to handle 120V (I am assuming AC). The contact would either burn out or fuse together in a short time.This information is provided only as a reference.

All circuits should be verified with a digital multi-meter prior to making any connections.

Not to mention the risk of electrocution (creep distances, or post-fusion bridges), or damage from exploding parts after arcing or over-voltage. Only approved relays for mains/domestic power should (must!!) be used for such AC purposes.

Tell me what's wrong with using AC at 120v on the switch if you're using 12v at the coil. I can't see the problem. AC or current handling or both?

It's primarily one of safety, but more so Regulations and approvals. The "creep" distances must be suited - ie, the gaps between conductors to ensure no arcing, and no tracking into the 12V coil and back to the 12V circuit. Same too for fingers etc, the case insulation must be appropriately rated. Whereas 12V only has a 12V peak or 16V etc in vehicles etc, 120VAC is a peak of 170V and that's full-time; hence needing at least 10x the inter-conductor spacing required for 12VDC & 16VDC. The case and insulation etc must not breakdown under the higher AC voltage. The contacts may also not suit the higher voltage, and there I'm thinking of the contacts needed for 24V contacts (eg, silver or whatever) as opposed to what 12V can get away with, though most 12V relay contacts are now constructed as if 24V etc. Current-wise there is IMO no practical difference. In fact AC is probably easier or better because of its net-neutral electroplating, plus it is easy breaking the current due to its zero-crossing voltage. (Try breaking very high-current DC at any voltage!) And back to liability... If you do find that all conditions are met but something then happens. it's up to you to prove to the insurance company and the Judges that you were (more) qualified to employ the use of an unapproved AC device without the warnings or approved testing that accompany approved products.