Choosing the Proper Amperage

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 Volt 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
When switching AC Devices, make sure the AC watts of the device you are switching DOES NOT exceed 1,250 when using a 5-amp 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 5-amp relay.

Resistive & 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 & 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 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.