24 VDC Power Supply Sizing Calculator

List your 24 VDC loads, and get the total current and the next standard power supply size with margin for growth.

Supply and margin

A common starting point is 20-25% margin so the supply is not run at its limit, plus extra for later additions. Your own standard may differ.

Loads (one per line: name, quantity, load)

Write the load as amps (0.15), milliamps (150mA) or watts (3.6W). Watts are converted using the supply voltage. Use the worst-case or maximum current from the datasheet.

Check more than the total current. Inrush current from solenoids, relays, motors and capacitive loads can briefly exceed the supply's rating, and many supplies derate at high temperatures. Check the supply's datasheet for its derating curve, overload behavior and whether it needs spacing for cooling.

How it works

The total load is the sum of every load's current multiplied by how many there are. The supply must provide at least that current, plus margin so it runs cooler and lasts longer, and room for the additions every panel gets over its life. The calculator then picks the next standard size up.

I = P ÷ V (for loads rated in watts)
total = Σ (quantity × current)
required = total × (1 + margin) × (1 + growth)

The sizes considered are 1, 2.5, 5, 10, 20, 40 and 60 A, which are common DIN-rail sizes. Your supplier may offer other ratings.

Worked example

The sample loads add up to 1.2 + 1.2 + 0.6 + 0.32 + 0.625 = 3.95 A. With 25% margin and 20% for growth, the requirement is 3.95 × 1.25 × 1.2 = 5.9 A, so the next standard size is a 10 A supply, which would run at about 39% load.

Common questions

Should sensors count at their no-load current or their maximum?

A three-wire sensor draws a small current for itself plus whatever its output switches. Add both, using the maximum output current the sensor will actually deliver to its load.

Do I need separate supplies for the PLC and the field devices?

Often yes. Many designs power the PLC and electronics from one supply and noisy loads like solenoids and relays from another, so a short on a field device does not take the controller down. Follow your design standard.

Calculations are provided for convenience and education. Verify results against the datasheets of the loads and the power supply before specifying equipment.