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Guide

How to size a 24 V DC power supply for a control cabinet

Size a 24 V DC power supply correctly: add up the loads, allow inrush and headroom, derate for temperature, and decide whether you need redundancy.

  • 5 min read
  • Updated

Step by step

  1. List each 24 V load and its current: PLC and I/O, sensors, relays and contactor coils, HMI, valves, drives' control supply.
  2. Add the currents to get the continuous load.
  3. Allow for inrush and peaks: contactors, solenoid valves and capacitive loads draw more when switching on.
  4. Add 20 to 30 percent headroom so the supply is not running at its limit.
  5. Choose a standard supply rating at or above that figure (for example 5 A, 10 A or 20 A).
  6. Check derating: many supplies lose output at high ambient temperature, so confirm the rating at your cabinet temperature.

Protection and layout

  • Use electronic circuit breakers or fuses for each branch so one fault does not take down the whole machine.
  • Keep sensitive 24 V circuits separate from relay and valve circuits where possible.
  • Leave clearance around the supply for cooling.

Redundancy

For critical machines, use two supplies with a redundancy module so one can fail without stopping production. Size each supply to carry the full load alone.

Frequently asked questions

How much headroom should a power supply have?

A margin of 20 to 30 percent is common. It lowers heat, extends life and covers load added later.

What is inrush current?

It is the brief higher current drawn when a load switches on, for example a capacitor or a solenoid. The supply must handle it without tripping.

Do I need a redundancy module?

Only when losing 24 V would cause a stop you cannot accept. Many machines run on a single supply.

This guide is general engineering guidance. Always confirm part-specific details, ratings and compatibility against the manufacturer’s datasheet. We confirm price, stock and specifications for the exact part in a written quotation.