relay ratings

If a relay has a coil voltage of 12vdc and a contact rating of 120vac @ 30 amps (example) would it still be possible to use the contacts with dc? A relay is a relay is a relay right??

Yes you can use it with DC but the maximum current is likely to be lower.

Not only the amperage is lower, the max. DC voltage is lower by a lot too.

See attached image about relay rating from a well known relay manufacturer.

If you do the maths, your 120V AC @30A relay might be able to switch a DC load of 18A at 15V, but it is just speculation. The sure way to go is to look for DC rating printed on the relay casing, if there is any.

If the load is inductive then it's even worse.

An unsnubed DC inductive load is probably as bad as it gets as it will arc really badly when the contacts open.

Capacitive loads are also pretty bad as the contacts might be welded together when they close.