Choosing a Proper Relay Amperage

How to calculate for the Correct Relay

Relay Ratings and Limits

Relays often have two ratings: AC and DC.  These rating indicate how much power can be switched through the relays. This does not necessarily tell you what the limits of the relay are. For instance, a 5 Amp relay rated at 125VAC can also switch 2.5 Amps at 250VAC. Similarly, a 5 Amp relay rated at 24VDC can switch 2.5 Amps at 48VDC, or even 10 Amps at 12VDC.

Volts x Amps = Watts - Never Exceed Watts!

An easy way to determine the limit of a relay is to multiply the rated Volts times the rated Amps. This will give you the total watts a relay can switch.  Every relay will have two ratings: AC and DC.  You should determine the AC watts and the DC watts, and never exceed these ratings. 

Example Calculations
AC Volts x AC Amps = AC Watts DC Volts x DC Amps = DC Watts
Example: A 5 Amp Relay is Rated at 250 Volts AC.  5 x 250 = 1,250 AC Watts Example: A 5 Amp Relay is Rated at 24 Volts DC.
5 x 24 = 120 DC Watts
If you are switching AC Devices, Make Sure the AC Watts of the Device you are Switching DOES NOT Exceed 1,250 when using a 5A Relay. If you are switching DC Devices, Make Sure the DC Watts of the Device you are Switching DOES NOT Exceed 120 when using a 5A Relay.

Resistive and Inductive Loads

Relays are often rated for switching resistive loads.  Inductive loads can be very hard on the contacts of a relay.  A resistive load is a device that stays electrically quiet when powered up, such as an incandescent light bulb.  An inductive load typically has a violent startup voltage or amperage requirement, such as a motor or a transformer.

Startup and Runtime Loads

Inductive loads typically require 2-3 times the runtime voltage or amperage when power is first applied to the device.  For instance, a motor rate at 5 Amps, 125 VAC will often require 10-15 amps just to get the shaft of the motor in motion.  Once in motion, the the motor may consume no more than 5 amps.  When driving these types of loads, choose a relay that exceeds the initial requirement of the motor.  In this case, a 20-30 Amp relay should be used for best relay life.

Induction Suppression Capacitors

Controlling inductive loads requires the use of induction suppression capacitors.  The purpose of this capacitor is to absorb the high voltages generated by inductive loads, blocking them from the contacts of the relay.  Without this capacitor, the lifespan of the relay will be greatly reduced.  Induction can be so severe that it electrically interferes with the microprocessor of the board possibly requiring the board to be power cycled.

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