# Properties of relays [closed]

i know how a relay works, my question is this. i have a load that needs to be turned off and on remotely. The piece of equipment is 220volts at 30 amps. is there such a thing as being able to use a small, low voltage of 12 or 24vac or dc to actuate the relay? It sounds simple, and what relays are for, but i can't find relays on line that can do that. this is in a commercial building application. thank you.

## closed as off-topic by tcrosley, PeterJ, placeholder, Daniel Grillo, rdtscMay 17 '16 at 16:54

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• Questions seeking recommendations for specific products or services are off-topic in this forum and will be closed. – tcrosley May 17 '16 at 1:11
• S/he hasn't asked for a product recommendation. It's a design question asking what sort of component to use. – Transistor May 17 '16 at 6:14
• If you have access into a common domestic shower water heater, pls pry open and see inside. You will get what you want. – soosai steven May 17 '16 at 6:38

## 3 Answers

What you're looking for is called a contactor. An example of a contactor meeting your requirements can be found at Digikey, but any electronics distributor will have the sort of thing you need.

Be aware, though, that high-power relays need high-power coils in order to move the contacts quickly through a long throw to quench any arcs. The units I linked, for instance, need a 24VDC coil, but pull nearly 3 amps. And yes, that's about 70 watts of coil power.

They also are not cheap, and you should figure on a ballpark figure of 100 to 150 dollars.

Old-school mechanical relays (or "contactors" as they are called in larger sizes) that are physically large enough to switch 30A typically take more to turn-on (or "pull-in") than the small amount of power available in a low-voltage control circuit.

Certainly, you could use a simple and inexpensive SSR (Solid State Relay) to control the larger relay/contactor. That would allow you to use very low power (essentially "logic-level") to control the 120 or 240VAC to the large relay.

For that matter you could use a SSR to control the 240V, 30A directly without the need for the large mechanical relay/contactor. SSRs are available up to probably over 100A.

Note, however, that any kind of mains wiring in a commercial building is probably regulated to be designed, specified and installed by licensed professional electricians. For that reason, it might be a better trade-off to leave the high-current mains wiring (including the relay/contactor) to the licensed people who are responsible for the safety, and then you can use an SSR as a safe interface between your low-voltage control signal and the 120V (or whatever) it takes to activate the large relay/contactor.

I would not attempt to do this myself in a commercial installation, nor would I recommend it as a DIY project to anyone else.

You're on the right track. For projects like this, you'll find a robust and inexpensive parts bin intended for air conditioning units, furnaces and thermostats. These systems use 24VAC sent to a thermostat to switch contactors which operate air conditioning compressors, dehumidifiers, etc.

For a contactor, look to 30-40A DPST contactors intended for air conditioning units. $13 to$25.

For low voltage power, use a 120V to 24V transformer intended to power thermostats. $10 to$20. Many of them are physically packaged for easy NEC compliance, e.g. the transformer mounts on the cover of a junction box, or in a 1/2" knockout, and put the 120V contacts inside the junction box, and the 24V contacts outside.