Enter your Load Current, Load Voltage, Coil Supply Voltage, Safety Factor, and Load Type into the Relay Sizing Calculator to find your Required Relay Rating, Estimated Coil Power, and Contact Switching Power so you always pick the right relay for the job.
Required Relay Rating
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Minimum Rating (No Safety Factor)
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Estimated Coil Power
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Contact Switching Power
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Multiply your load current by a safety factor (typically 1.5× for automotive applications). This ensures the relay can handle inrush currents and provides a safety margin for reliable operation.
Use 1.25× for light duty applications, 1.5× for automotive and general use, and 2.0× for critical applications or high inrush loads like motors and lamps.
Different load types have varying inrush characteristics. Inductive loads like motors can have 6-8× inrush current, while resistive loads are more predictable. Tungsten lamps can have 10-15× inrush current.
Yes, using a higher rated relay is generally acceptable and provides additional safety margin. However, ensure the coil voltage matches your control circuit requirements.
AC ratings are typically higher because AC current naturally crosses zero, making it easier to interrupt. DC relays must physically separate contacts to break the arc, requiring more robust construction.
Coil voltage must match your control circuit voltage. Common automotive voltages are 12V and 24V. Industrial applications often use 24V, 120V, or 240V coil voltages.
An undersized relay may experience contact welding, reduced life, or complete failure. The contacts can overheat and stick together, causing the relay to fail in the closed position.