Industrial Automation · PLC · Power Supply · 24V DC
24 Volts in PLC Systems: 8 Proven Reasons Engineers Trust It
Walk into almost any control panel and you will find the same number stamped on the power supply. This guide explains why 24 volts in PLC systems became the near universal standard, covers all eight practical reasons behind it, and gives you a voltage drop calculator for real wiring runs.
Why Does 24V DC Dominate PLC Panels?
Electrical wiring in a panel generally falls into two groups, power and control. Power wiring carries the heavier current needed to run motors and large equipment. Control wiring runs the PLC, sensors, relays, and small instruments. While power circuits often stay on higher AC voltages, control circuits overwhelmingly use 24 volts DC, and that choice is deliberate rather than accidental.
The same logic that makes a redundant power supply worth building around 24V also explains why the voltage was chosen in the first place. It is low enough to be safe, yet high enough to reliably power sensitive electronics across an entire panel.
Why 24V Became the Standard: 4 Steps in the Logic
Sensors, relays, and small instruments never needed the heavy current that high voltage AC provides.
→Keeping the whole control circuit at 24V removes the danger of an engineer mistakenly connecting a high voltage source.
→A single SMPS design can accept AC power from almost any country and still deliver a consistent 24V output.
→Every instrument in the panel receives the same stable 24V, simplifying design, sourcing, and troubleshooting.
Four Advantages Behind 24 Volts in PLC Systems
Low voltage means little to no shock risk if a wire is touched accidentally, unlike higher AC voltages.
Impact: fewer required protective procedures for routine panel work.
A single SMPS design can accept a wide range of AC input voltages and still output a fixed 24V.
Impact: one product design works worldwide, regardless of local grid voltage.
Lower DC voltage is far less prone to electromagnetic interference than higher AC voltages carrying harmonics.
Impact: cleaner, more reliable signals reaching the PLC.
DC voltage stays constant, unlike AC voltage which naturally varies, which matters for sensitive electronics.
Impact: more consistent, predictable device behavior over time.
How 24V DC Compares to Mains AC Voltage
8 Proven Reasons for Using 24 Volts in PLC Systems
24V DC Voltage Drop Formula
Where:
Current = load current drawn by the device (A)
Cable Resistance = round trip resistance of the wiring run (ohms)
Example: 2 A load, cable resistance 0.6 ohms round trip Voltage Drop = 2 × 0.6 = 1.2 V, which is 5% of 24V A voltage drop that feels small in absolute terms can be a large percentage at 24V, unlike at 230V where the same drop barely matters. This is exactly why long cable runs need careful sizing on low voltage DC systems.
24V DC vs 230V AC for Control Circuits
Where 24V DC Powers the Panel
The controller itself typically runs directly on a 24V DC supply.
Most field sensors and transmitters expect a 24V DC loop power source.
Interposing relays commonly use 24V coils for switching larger loads safely.
Operator display panels are almost always powered from the same 24V rail.
Many pneumatic solenoid valves use 24V DC coils for their control signal.
Distributed junction boxes typically distribute 24V DC out to nearby instruments.
Working With 24V DC Systems Correctly
- Size the SMPS for full expected load: including margin for future expansion in the panel.
- Check voltage drop on long cable runs: a small drop at 24V can be a large percentage of total voltage.
- Keep DC and AC wiring separated: in the panel to reduce interference and confusion during maintenance.
- Follow extra low voltage safety practice: even though 24V is relatively safe, good habits still matter.
- Don't run 230V AC directly to small instruments: it defeats the entire purpose of standardizing on 24V.
- Don't ignore voltage drop on long runs: undersized cable can starve distant devices of usable voltage.
- Don't undersize the SMPS current rating: to save cost, since it risks unreliable operation under full load.
- Don't mix voltage levels without clear labeling: confusion between 24V and higher voltage wiring is a common source of mistakes.
24V DC Voltage Drop Calculator
Enter your load current, cable length, and cable resistance to check voltage drop on a 24V run.
Quick FAQs: 24 Volts in PLC Systems
External References
- Wikipedia: Extra Low Voltage
- IEC 61131: Programmable Controllers Standard
- Phoenix Contact: 24V DC Power Supply Reference Guide
What we learn today
- 24 volts in PLC systems became the standard because it balances safety, reliability, and practicality far better than higher AC voltages.
- The eight main reasons range from improved safety and reduced noise interference to smaller boards, lower heat, and stable voltage output.
- Voltage drop matters more at 24V than at higher voltages, so cable sizing deserves careful attention on long runs.
- Keeping control circuits standardized on 24V DC simplifies design, sourcing, and troubleshooting across an entire plant.