Relay contact ratings


Old 04-11-07, 04:21 PM
Thread Starter
Join Date: Apr 2005
Location: Houston, TX
Posts: 74
Relay contact ratings

I thought I knew a fair amount about electrical, but this has me scrathing my head.
There several applications where I need to use relays but I'm having difficulty rating the contacts to the application.
Example: 1,500 watt 120V fan assisted heater - the relay I'm looking at is rated 15A Tungsten, 20A Ballast, 1 HP.
Example: Noisy ice machine 240V 12A - this will be a timed OFF application where activation of the DPDT relay will BREAK power to the machine. These contacts are rated 30A but I know that breaking contact under full load can create much more arcing.
Is there somewhere I can find more info on how to determine if the contacts can handle various situations?
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Old 04-11-07, 04:32 PM
Join Date: Apr 2006
Location: New Bern, NC
Posts: 1,623
You were looking at a lighting relay. That is why it said tungston and ballast.

The way relay contacts are actually rated is Resistive, or Inductive.

Resistive is a load that has only resistance, like a tungsten light bulb, or heat element. Inductive is a load that has a coil like a motor, ballast, or ice machine compressor.

All you need to do to size the relay is to make sure that the rating of the relay contacts are greater than the rating of the appliance. In your case use the inductive rating of the relay contacts (this will always be the lower of the two ratings)

There is no need to do any further calculation, the mfg did that before they printed the lable for the relay.

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