Relay Sizing Calculator

A relay is an electrically controlled switch used to safely operate high-power devices from a low-power control circuit — choosing the wrong size can cause overheating or failure. Enter your load current, load voltage, coil supply voltage, load type, application type, and safety factor into the Relay Sizing Calculator to find the required relay rating. Secondary outputs include minimum rating (without safety margin), estimated coil power, and contact switching power.

A

Current drawn by the device being switched

V

Voltage of the circuit being switched

V

Control voltage available for relay coil

Safety margin for relay rating

Type of load affects inrush current

Results

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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Frequently Asked Questions

How do I calculate the correct relay amperage for my application?

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.

What safety factor should I use for relay sizing?

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.

Why does load type affect relay selection?

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.

Can I use a higher rated relay than calculated?

Yes, using a higher rated relay is generally acceptable and provides additional safety margin. However, ensure the coil voltage matches your control circuit requirements.

What's the difference between AC and DC relay ratings?

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.

How does coil voltage affect relay selection?

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.

What happens if I undersize a relay?

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.