Greetings, as much as I play around with electronics thusfar I havent touched relays, therefore I know almost nothing about them. I have a box of 12 relays originally used for fire-trucks, on the top it says: 14VDC 30/40A NC30A NO40A now I believe this means that at 14V the Normally Open set of contacts can handle 40 Amps of current though the Normally closed contacts can only handle 30 Amps. now iirc W=IxV so the normally closed contacts can handle 420 Watts however I have never before worked with AC! I dont know if these can handle AC current, if my wattage calculation is correct then at 120 VAC they should be able to handle 3.5 Amps max. now my plans are a light box, basically its a bunch of relays in a box, 120VAC comes in from the wall and each relay is attatched to an outlet. I will plug one light in per outlet. can this be done or should I break donw and buy more relays (they're kinda expensive when your buying 10)
If they don't have a rating for mains, it's unlikely they're suitable for that use. The contact spacing may not be sufficient enough to extinguish an arc at high currents, and the contacts may not be made of the correct material either. If they have a part number, you may be able to find out more about them.
Relay contact ratings don't work as wattages; what you'll find is that a 120V AC current rating is pretty close to the 12V DC rating. For example, one in front of me is 5A @24V DC, 3A @240V AC on inductive loads, 10A 30V DC/240V AC on resistive loads. A problem with high DC voltages and currents is contacts tend to arc when they break, as any arc welder will tell you. which is a good thing in a switch. But SteveyG's right, car relays are built for low voltage/high current switching - some of Maplin's say 24V max, which covers a pair of batteries. Get some that say at least 120V AC on the label.
