24vdc power supply sizing is a load-budget project, not a quick search for the nearest DIN-rail part number. If you’re designing a control panel for production use, the supply must carry the steady load, survive startup inrush, stay within its temperature limitation, and still provide enough volts at the farthest device terminals. This guide offers a worksheet-style approach to selecting 24 VDC power supplies for PLCs, sensors, relays, HMIs, solenoids, and small automation enclosures.
- What 24VDC power supply sizing means
- Numbers to collect before you calculate
- Nominal load in amps and watts
- Inrush and capacitive startup load
- Derating, temperature, and margin
- Wire length, voltage drop, and protection
- 2.5 A vs 5 A vs 10 A vs 20 A selection matrix
- Worked PLC panel example
- DC OK, redundancy, and backup power
Quick Specs: 24VDC Power Supply Sizing
- Basic equivalency: watts = volts x amps, amps = watts / volts.
- nominal 24 VDC is typical control voltage, a number many industrial supplies provide (some may have an adjustment feature for 22-28 VDC).
- continuous allowance: start by multiplying the calculated nominal load x 1.20 for a long life, then increase allowance if enclosures generate heat or there’s expansion planned.
- startup check: compare startup inrush current separately from operating current; a supply may pass the steady-load test and sag during startup.
- field measurement: measure voltage at device input pin, not just power supply output pin.
What 24VDC Power Supply Sizing Actually Means
Output current and output power determine a 24VDC supply size: a 60 W at 24 VDC offers 2.5 A, because 60 W / 24 V = 2.5 A. That number simply sets a starting point. In practice, control cabinets send changing loads as relays pull in, sensors heat up, contactor coils energize, and capacitors inside modules charge.
Consider the supply as an ampere budget: each device “spends” a tiny bit of the supply budget. PLC CPUs might consume a few hundred milliamps. A bank of digital inputs might draw less. An HMI, Ethernet switch, solenoid valve, or relay coil can increase the total more rapidly. Final supply rating must support the load after margin and derating, not the sum of the first set of devices.
Technical Note: If a datasheet reports wattage instead of amperage, convert to amps before including on the load table. At 24 VDC 12 W equals 0.50 A, 24 W equals 1.00 A, 60 W equals 2.50 A, 120 W equals 5.00 A, 240 W equals 10.00 A. Use the same unit throughout the entire calculation, so the margin multiplication is not applied twice.
Numbers to Collect Before You Calculate
Create a list before selecting a supply: every 24 VDC device powered from the same branch needs to be listed, including module name, quantity, nominal current, startup/inrush current, duty cycle, wire length, and whether the branch requires a separate branch fuse or breaker. Forgetting one solenoid coil or indicator light might make a small 2.5 A supply seem correct in theory, but fail prematurely in the machine.
| Load-list field | Why it matters | Example value |
|---|---|---|
| Device name | Separates PLC, sensor, relay, HMI, and switch loads | PLC CPU |
| Quantity | Prevents one sensor value from standing in for all sensors | 8 sensors |
| Nominal current | Feeds the steady-load calculation | 0.08 A each |
| Inrush current | Tests startup sag and surge capacity | 0.50 A for 200 ms |
| Duty cycle | Shows whether loads are continuous or intermittent | Continuous PLC, intermittent relay |
| Wire length | Feeds the voltage drop check | 18 m round trip |
| Ambient temperature | Changes the usable output current in warm enclosures | 45 C cabinet air |
| Protection branch | Shows where fuses or breakers divide loads | PLC branch, field I/O branch |
| Future expansion | Reserves spare amp capacity for added modules | 20 percent spare I/O |
Step 1: Add Nominal Load in Amps and Watts
Estimate the steady load, remaining after startup. Add the nominal current for each 24 VDC device activated simultaneously. Divide wattage by 24 V if only watt ratings available. If a device operates in multiple modes, choose the current value during the normal mode of operation, not the idling mode.
Here is a simple control-panel example. Values are sample planning numbers, substitute actual module datasheets for a realistic panel.
| Device | Qty | Nominal current each | Total nominal current |
|---|---|---|---|
| PLC CPU and base modules | 1 | 0.60 A | 0.60 A |
| Digital input module | 1 | 0.15 A | 0.15 A |
| Digital output module | 1 | 0.20 A | 0.20 A |
| Photoelectric sensors | 8 | 0.04 A | 0.32 A |
| Relay coils | 4 | 0.08 A | 0.32 A |
| Small HMI | 1 | 0.45 A | 0.45 A |
| Unmanaged Ethernet switch | 1 | 0.25 A | 0.25 A |
| Panel indicator lights | 5 | 0.02 A | 0.10 A |
| Total before margin | - | - | 2.39 A |
Begins to look barely enough for a 2.5 A supply. The problem is, service-life margin, temperature derating, inrush current, and voltage drop could not yet be added. For a smaller cabinet, a 2.5 A unit such as the Omron S8VK-C06024 switch mode power supply may be compressed, however this example needs the next rating following margin.
Step 2: Check Inrush Current and Capacitive Startup Load
Inrush current is the short startup current that draws when coils pull in or capacitors charge. It may last only milliseconds but the supply must survive without creating a DC bus collapse. Treat inrush as a separate test from nominal load. Add together the worst startup loads that can happen simultaneously, then compare to the supply's surge or peak current value rating.
Some datasheets publish a specific peak-current window such as a short-time current value at constant intervals. Others state only overload behavior, foldback, or hiccup protection. Those are not interchangeable. During startup, a supply entering protection may make a PLC reboot, drop an output module, or create a DC OK fault even when continuous load is under the rating plate.
Do I need to account for inrush currents due to capacitors?
Yes. Inside drives, I/O modules, Ethernet switches, HMIs, and DC-DC converters, capacitors can pull a momentary inrush when energized. If several initialize together, the startup load can easily be several times higher than the operating current. When a datasheet does not publish inrush current, stage startup where possible, split branches, or choose a unit with enough peak-current headroom for capacitive loads.
Pro Tip: When troubleshooting, measure the 24 VDC rail at the load during startup with the real panel load attached. Although the voltage at the supply terminals looks stable, that does not guarantee the farthest I/O module has enough voltage during the same event.
Step 3: Apply Derating, Temperature, and Safety Margin
After knowing the nominal load, add margin. Eaton's publicly published sizing example bumps that calculated nominal load by 20 percent to ensure a reliable power supply life. In reality, 20 percent is a minimum for a tidy, cool, fixed load cabinet. Warm environments, future I/O deviations, long cable runs, or unpredictable startup behavior can often mean warranted margin.
| Condition | Suggested margin | Reason |
|---|---|---|
| Stable cabinet load, cool enclosure, known devices | 20 percent | Matches common extended-life sizing practice |
| Likely I/O expansion within 12-24 months | 30 percent | Leaves room for extra sensors and modules |
| Warm enclosure above normal room temperature | 30 percent or datasheet derating curve | Heat lowers practical current capacity |
| Large capacitive load or multiple coils at startup | Select by surge rating first | Continuous current alone is not the limit |
| Machine downtime is expensive | Redundant or monitored supply | DC OK contacts and backup modules improve fault visibility |
For the 2.39 A example, applying a 20 percent margin results in 2.87 A. Increasing to 30 percent yields 3.11 A. Either number pushes the design out of a 2.5 A power supply. It comes down to practically picking a 5 A supply, or a 10 A power supply when the panel has warm environment switches or future modules envisaged.
Step 4: Check Wire Length, Voltage Drop, and Branch Protection
Nevertheless, correct tailoring at the panel does not necessarily indicate correct voltage at the load. Excessive 24 VDC wiring length commonly causes this error. When low-voltage circuitry has narrow margins, an excessively high voltage drop may exist: in that case a 1 V drop can present more than 4 percent of a 24 V rail.
Buy 24V power supplies - for long runs, check the wire length, conductor size, current and return path. Then measure the device input terminals under load. If the voltage at the field device is too low, an increased wire size, shorter branch length, local fused distribution block or moving the supply closer to load is the better fix. Do not solve a voltage drop limit by raising the supply output unless every other connected device remains within input allowance.
Should each 24 VDC branch have its own fuse or breaker?
Not usually. Branch protection relieves many fault traces, and prevents a single field short from dropping the entire control rail. Exact selection depends on panel standard, supply listing, available fault current and if the circuit is designed to be Class 2. UL 508A is used for industrial control panels, and IEC 60204-1 for machine electrical equipment; consider both as planning documents, not as after-the-fact labels.
I want a fuse added after a non-class 2 supply, where is the listed Class 2 branch? If class 2 status is relevant to your panel, verify supply and branch design per desired listing path.
24VDC Power Supply Selection Matrix: 2.5 A vs 5 A vs 10 A vs 20 A
Once power budget is established, select the next fuse rating that exceeds the nominal load, power-up load, temperature reduction, and field voltage. Use the following table for a guide to common 24 VDC power supplies—not as a stand-alone product data review.
| Calculated load after margin | Common supply rating | Typical use case | Selection note | iTrustBot path |
|---|---|---|---|---|
| 0.5-1.5 A | 2.5 A / 60 W | Small PLC, a few sensors, no HMI | Check startup if relays or capacitive devices are present | Omron S8VK-C06024 |
| 1.6-2.1 A | 2.5 A / 60 W | Compact cabinet with fixed load | Leave little room for expansion | Omron power supplies |
| 2.2-4.0 A | 5 A / 120 W | PLC plus HMI and several field devices | Good next step after a 2.5 A unit is too tight | Request a quote |
| 4.1-7.5 A | 10 A / 240 W | Larger I/O rack or multiple branches | Use branch fusing to isolate field faults | Omron S8VK-G24024 |
| 7.6-8.5 A | 10 A / 240 W | Known load with limited future growth | Check derating curve if enclosure runs hot | Omron S8VK-C24024 |
| 8.6-15 A | 20 A / 480 W | Many outputs, valves, relays, or distributed I/O | Consider splitting into protected branches | Mean Well collection |
| 15.1-17 A | 20 A / 480 W | High-load cabinet with known devices | Little reserve remains after margin | Industrial automation quote |
| Any load with critical uptime | Redundant pair or backup module | Process cell, remote station, hard-to-service panel | Specify DC OK contacts and failure indication | Send a load list |
| Any load with long field wiring | Rating depends on voltage drop | Sensors or valves far from the cabinet | Voltage at device terminals decides the final wire plan | Sensor collection |
Example Calculation: PLC, Sensors, Relays, and an HMI
The 24VDC Amp Budget below continues to follow the full method used in earlier sample cabinet. This is set to demonstrate the calculation pathway, so please substitute your actual product data prior to ordering a supply.
| Budget step | Value | Pass/fail check | Action |
|---|---|---|---|
| Nominal load total | 2.39 A | Below 2.5 A nameplate, but too close | Do not select yet |
| 20 percent margin | 2.87 A | Exceeds 2.5 A | Move to at least 5 A |
| 30 percent margin | 3.11 A | Still within 5 A | 5 A remains suitable if startup is mild |
| Estimated startup group | PLC + HMI + switch + relay bank | Needs datasheet inrush review | Check peak-current behavior |
| Temperature check | 45 C cabinet air | Needs supply datasheet derating curve | Use 5 A only if derated capacity still clears load |
| Wire length check | 18 m round trip on sensor branch | Measure terminal voltage under load | Increase conductor size if voltage is low |
| Branch protection | PLC, field I/O, HMI branches | Fault isolation needed | Add branch fuses or breakers per panel design |
| Expansion reserve | 4 spare sensor inputs planned | 5 A has room; 2.5 A does not | Keep spare amp budget visible in BOM |
| Final practical rating | 5 A / 120 W minimum | Pass if inrush and derating checks clear | Use 10 A when heat, future I/O, or peak load is uncertain |
For this sample, 5 A power supply is the best-case result. Opt for a 10 A choice if the cabinet will gain more I/O, operate warmer, or run longer field branches. For buyers evaluating available industrial automation catalog parts, iTrustBot offers to quote a specific 24 VDC DIN-rail selection from the Omron and Mean Well ranges.
What Is Changing: DC OK Contacts, Redundancy, and Backup 24 VDC Power
Basic amp sizing is no longer the only decision. Control-panel design now includes faster fault reaction, remote troubleshooting support, and recovery after short power events without masking weak 24 VDC distribution rails. DC OK contacts, hold-up time, redundancy modules, and 24 VDC backup hardware now appear more often in modern automation systems.
Implement a diagnostic or redundant architecture when a 24 VDC distribution rail fault could halt production, ruin work-in-process inventory, or necessitate a site visit. DC OK contacts could warn a PLC before the 24 VDC vanishes completely. Redundancy modules would allow fault isolation of a failing power supply. Backup power supply or UPS hardware could ensure control system functionality through temporary outages. These strategies do not replace accurate load calculation; instead, they provide point-of-failure information after the calculation has been performed.
| Advantages | Limitations |
|---|---|
| A 20-30 percent margin gives spare current for device aging, added sensors, and moderate expansion. | Margin does not fix inrush current, voltage drop, or hot-enclosure derating by itself. |
| Branch fusing can keep one field fault from removing the whole 24 VDC rail. | Protection choices must match the panel standard and the available fault current. |
| DC OK contacts help a PLC detect power supply trouble before a silent shutdown. | DC OK signaling is a warning point, not backup power. |
| Larger 10 A supplies can support future I/O without redesigning the cabinet power rail. | Oversizing without branch protection can make fault isolation harder. |
Submit your 24 VDC load list to iTrustBot for a power supply quote
FAQ
Q: How is 24VDC power supply sizing calculated?
Sum the nominal current for as many 24 VDC could simultaneously run, switch to amps where watts, add margin, and verify inrush current; initial conversion: amps=watts/24 V: Sum; then verify temperature derating, wire voltage drop, branch protection, and spare capacity.
Q: How many amps should a 24VDC power supply have for a PLC panel?
Small PLC panels are typically in 2.5 A to 5 A range, but a panel with HMI, Ethernet switch, multiple sensors, or multitude of output loads will easily require 10 A or more. Correct size should be determined from load list. If calculation using margin results in loading comparable to supply rating, then move up to next amp size range.
Q: Is a 20 percent safety margin enough for a 24VDC power supply?
Twenty percent is a good initial value for a stable, cool, fixed-load panel. Go higher if the enclosure will be warm or in future I/O is expected, if start-up current is uncertain, or if the consequences of downtime are high. Check the manufacturer's derating curve before trusting the nameplate current.
Q: Do inrush currents matter for 24VDC power supplies?
Yes. Capacitors, coils, HMIs, and DC-DC converters could pull more current in the initialer starting phase than in the steady state. If those loads were to turn on the 24 VDC rail could sag even though the steady state current total is below the supply nominals. Verify the OVERLOAD or PEAK-CURRENT profile of the supplies.
Q: What is voltage drop in a 24VDC control circuit?
Voltage drop is the difference between the power supply output and the device input terminals. It varies with current and cable length. Since the 24 V is a low control voltage, then a small absolute drop can be a significant percentage of the rail. In use measure at the device, not just at the cabinet supply.
Q: Can I use a desktop power supply instead of an industrial DIN-rail supply?
Use a listed and panel-packaged supply for industrial control panels. DIN-rail units provide cabinet mounting, industrial input ranges, protection functions, temperature ratings and terminal arrangements that desktop supplies do not carry. An unlisted supply can also lead to listing and maintenance problems.
Q: When should I use a redundant 24VDC power supply?
Use redundancy where loss of the 24 VDC rail can shut down a line, damage material, or require difficult service access. Redundant power also gives remote service teams a clearer failure signal. Add it after the normal load, inrush, derating, and protection checks have been made.
About This Sizing Method
This guide uses public standards pages, manufacturer's sizing samples, and iTrustBot products-page specs to develop a workable 24VDC power supply sizing procedure. It is no substitute for an independently certified panel design review, UL 508A assessment, IEC 60204-1 review, or local electrical code inspection. With the 24VDC Amp Budget, load estimate, derating, safety-factor, and choice principle remain visible prior to the buyer making a request for a quote.
Related Articles
- Programmable Logic Controller Fundamentals - useful context for PLC power loads
- Introduction to PLC Troubleshooting - how power faults appear in control systems
- Industrial Automation and Control Systems - where 24 VDC rails fit in machine control
- What Is Fieldbus - why distributed devices change cabinet power planning
- SIMATIC S7-1500 - PLC platform context for control cabinet buyers
References & Sources
- UL 508A Summary of Requirements, Third Edition - UL Solutions
- IEC 60204-1:2016+A1:2021 CSV - International Electrotechnical Commission
- 29 CFR 1910.333, Selection and Use of Work Practices - Occupational Safety and Health Administration
- 29 CFR 1910.303, General Electrical Requirements - Occupational Safety and Health Administration
