Low volage to control relays.

[Twoskinsoneman]

Hi. Amazing forum. Such an incredible wealth of knowledge.

My question is concerning relays and low voltage to control them. My job is to design electrical systems for government command and communications vehicles. We as a company only wire to the highest safety standards but we also aren't technically governed by anyone including the NEC. We frequently use approved electromechanical relays to switch on and off 120vac loads. We often use relays such as these http://www.magnecraft.com/type_results.php?id=1

Anyway we use 12vdc to control these relays because it is so safe. You very rarely have to worry about getting shocked from 12 volts (only under prime conditions). Is this type of configuration acceptable for residential wiring? I've tried to research diligently in NEC but as far as I can see it specifically mentions low voltage lighting. Does it distinguish from control circuits?

Specifically I am currently wiring my garage and I was thinking it would be cool if I could turn on my air compressor from outside without unlocking the garage and going in. A low voltage circuit on a switch outside could control a relay inside the circuit breaker panel.

Is this to code? It seems very safe. Especially with a regulated power supply to control the relay. You never have to worry about a fire if your power supply can only put out 300ma even when shorted.

Thanks very much.
Jack

 

[mdshunk]

Sure. The GE RR7/RR9 low voltage switching scheme has been used in higher end homes and commercial occupancies for probably 40 years. They operate with rectified or AC low voltage.

 

[LarryFine]

Jack, I recommend wiring traditionally, since you won't be living there forever, and instead look into home-automation. You can even get wireless control pads and key-fob transmitters. I can recommend stuff for you.

Give me an example of the equipment you have in mind. For example, the compressor: voltage and amperage, and plug type, or hard-wired? How do you want to control it? Wall-mounted pad, hand-held, or key-chain?

 

[Twoskinsoneman]

Actual it is just a regular old craftsman 50 gallon compressor. I was going to give it a dedicated permanent outlet on a dedicated circuit. The compressor only pulls 10 amps max according to specs. I is well within the ratings of the relays. An automated system would be cool but I honestly can not afford it. The bill of materials would basically be a $15 relay, $5 sealed toggle switch hidden under a pneumatic hose outside, and the cheap low voltage wiring to convey the 100ma to engage the relay.

 

[QES]

I have seen those electromechanical relays fail on me before, its stay close. I replaced with a 30 A solid state relay.

 

[LarryFine]

The bill of materials would basically be a $15 relay, $5 sealed toggle switch hidden under a pneumatic hose outside, and the cheap low voltage wiring to convey the 100ma to engage the relay.

Don't forget the transformer, wire, and labor.

Okay, how about a simple wireless system that requires zero installation, total $59.95:

First, an in-your-pocket key-chain remote RF transmitter, $13.50:
http://homeautomationnet.com/shopping/shopexd.asp?id=230

Then, a plug-n RF receiver, that has a built-in switched receptacle, $26.50:
http://homeautomationnet.com/shopping/shopexd.asp?id=103

Finally, a 20a, 240v plug-in appliance module, $19.95:
http://homeautomationnet.com/shopping/shopexd.asp?id=134

Granted, more expensive than your simple system, but with simple plug-in installation, and able to control a second load.

 

[Twoskinsoneman]

You know Larry I guess more than anything I naturally stick to what I know. But thank you for those links, I am always eager to learn new ideas if they are benificial. And I am not afraid of change.

Those Magnecraft relays have rarely failed us but they are normally used for lighting loads. Actually I have never heard of one of them failing us but I haven't been there forever, just two years.

 

[Twoskinsoneman]

I have seen those electromechanical relays fail on me before, its stay close. I replaced with a 30 A solid state relay.

That's a great suggestion. Actually I have been experimenting with a couple of 30a solid state relay. They are silent and you only need 5vdc to control them!

 

[mdshunk]

Up into those higher current capacities for pure resistive loads, I prefer SSR's and Mercury Contactors for reliability's sake. For inductive/motor loads, a mechanical (NEMA style) contactor still fits the bill.