To size a 24V DIN rail power supply, add the maximum simultaneous operating current and planned expansion, then divide by the project’s loading target. Check that the candidate can deliver this continuous current at the actual cabinet conditions, and verify startup events and voltage at the loads separately.
In the worked example below, 6.00 A of simultaneous load + 1.00 A of expansion, divided by an 80% project loading target, requires 8.75 A of available continuous output. A nominal 10 A supply is a candidate to check against those requirements.
목차
Start with the maximum simultaneous load
List each device supplied from the 24 V circuit, its quantity, and its documented continuous current. Identify the operating state with the greatest combined demand. Use the selected components’ requirements rather than a typical current taken from a different device.
The following values are assumptions for a worked example. All listed loads operate together; no reduction for non-simultaneous operation is applied.
| Load | Quantity | Assumed current per unit | Total current |
|---|---|---|---|
| PLC and I/O electronics | 1 set | 0.80 A | 0.80 A |
| HMI | 1 | 0.60 A | 0.60 A |
| Industrial Ethernet switch | 1 | 0.40 A | 0.40 A |
| Sensors | 12 | 0.05 A | 0.60 A |
| Relay coils | 10 | 0.04 A | 0.40 A |
| Solenoid valves | 8 | 0.40 A | 3.20 A |
| Total simultaneous operating load | 6.00 A / 144 W | ||
If the machine sequence prevents some devices from operating together, document those permitted operating combinations before reducing the total. Include credible simultaneous events, such as restart after a power interruption; an unsupported diversity factor can hide the worst case.
Count each load once. A PLC or I/O specification may already include current supplied to external devices. For coils and other switched loads, record any documented pull-in and holding requirements separately rather than applying one startup multiplier to every device.
Download the blank load schedule (CSV) 또는 view this worked example (CSV). These are static records to fill in and review; they do not calculate results automatically.
Calculate continuous output with expansion and headroom
Use the same calculation for your own panel:
Irequired, available = (Isimultaneous + Iexpansion) ÷ u
Here, u is the chosen maximum load as a fraction of the supply’s available continuous output at the project conditions. For an 80% target, use 0.80. The simultaneous and expansion currents must refer to the same 24 V supply and the operating combination being evaluated.
This example reserves 1.00 A for planned expansion. The three steps below show how that future load and the chosen loading target affect the required available output.
Include expansion before applying the loading target so that the target still holds when those additional loads are installed. Expansion is a future load commitment; operating headroom is the capacity retained around that load.
The 80% target is a project choice for this example, not a universal WEHO requirement. It requires a supply capacity of 1 ÷ 0.80 = 1.25 times the design load; multiplying the load by 1.20 would not give the same result. Set the actual target using the project requirements and applicable model documentation.
Check whether a 10 A candidate meets the cabinet conditions
WEHO’s public NDR-240 specification lists the NDR-240-24 at 24 V and 0–10 A. That nominal rating makes it a candidate for this example; the selection still depends on its available output in the proposed installation.
- Listed nominal output
- 24 V DC
- Listed output current
- 0–10 A
View NDR-240 product details ↗
View NDR-240 specification PDF ↗
The continuous-current screening threshold is 8.75 A. For a nominal 10 A candidate, this is 87.5% of its nominal current. This ratio is a calculated selection threshold, not a temperature rating or a measured derating characteristic.
| Available continuous current at project conditions | Design load ÷ available current | Meets the chosen 80% target? |
|---|---|---|
| 9.00 A | 7.00 ÷ 9.00 = 77.8% | Yes, for this continuous-load check |
| 8.50 A | 7.00 ÷ 8.50 = 82.4% | No; review the installation, candidate or loading target |
Missing the project’s 80% target does not, by itself, prove that the supply cannot operate the 7 A load. It means that it does not meet this selected capacity criterion.
Check the air temperature around the supply, input conditions, mounting orientation, ventilation and clearances against the applicable model instructions. An operating-temperature range alone does not establish full-load output throughout that range. The public one-page NDR-240 PDF does not provide the installation-specific derating information needed to close this check. Request confirmation for the intended conditions before freezing the production BOM.
For the general distinction between a rated output and an output reduced by operating conditions, see the referenced explanation of power-supply derating and follow our power-supply derating guide to evaluate temperature, altitude and mounting limits together.
When comparing enclosure heat loads, use the candidate’s efficiency at the relevant operating point. The efficiency and power-factor guide separates heat loss, real input power and input-current budgeting.
Verify startup and switching events separately
A steady-state current total does not describe power-up, subsystem switching or restart after an interruption. For each significant simultaneous event, establish the peak current, duration, repetition rate and the connected equipment’s acceptable input-voltage range.
Compare those requirements with the exact candidate’s documented transient capability and protection behavior. An overload-protection threshold is not a guaranteed usable peak-output rating. The AC input inrush of the power supply is also a different quantity from the startup current demanded by its DC loads; if multiple supplies start on one AC branch, review the multi-unit inrush current guide. The referenced external design guide discusses peak-load applications.
During sample evaluation, observe supply-terminal and critical-load voltages during the relevant events. Record any load reset, supply shutdown and recovery behavior against the project’s acceptance criteria. Do not assume that a nominal 10 A rating establishes successful startup.
Check voltage at the load as well as current at the supply
Available current is only one part of selection. Cable and connection losses can leave a critical device below its minimum input voltage while the supply terminals remain near 24 V. Check the complete outgoing and return path, including shared conductors carrying several branches.
Use the load’s documented voltage limits, the candidate’s output tolerance and the distribution losses to establish an acceptance budget. Check startup events separately from the steady-state calculation. Follow the 24 V control-power voltage-budget example for a worked branch calculation. For fault isolation, use a separate 24 V DC branch-protection review; headroom alone does not establish protective-device coordination.
Give purchasing a specification it can compare
A request for a “24 V, 240 W DIN rail power supply” does not define the conditions or acceptance requirements. Attach the load schedule and record supplier exceptions against the same specification.
| Item to confirm | Information to request |
|---|---|
| Product identity | Complete model, output variant and applicable document revision |
| Continuous output | At least 8.75 A for this example at the specified input, local temperature and installation conditions |
| Startup and recovery | Applicable transient limits, protection behavior and recovery requirements |
| Mechanical integration | Dimensions, mounting instructions, clearances and terminal access |
| Project documentation | Installation instructions and evidence for any approvals required by the project |
| Procurement conditions | Sample availability, MOQ, lead time, warranty and change-notification arrangements to confirm with the supplier |
When comparing an alternative, also check its interfaces and recovery behavior. The industrial power-supply replacement checklist covers differences that equal voltage and wattage ratings can conceal. Use the 24 V OEM power supply RFQ checklist to carry those requirements into comparable supplier quotes.
Validate the sample before releasing the production BOM
Set the acceptance criteria before testing and retain the results with the exact model and document revision. A practical evaluation record includes:
| Check | Record and compare |
|---|---|
| Operating conditions | Applied input, local temperature around the supply, mounting and ventilation |
| Continuous load | Actual simultaneous current, expansion budget and documented available continuous output |
| Voltage acceptance | Supply-terminal and critical-load voltages against the connected equipment’s limits |
| Operating events | Relevant startup and switching conditions, voltage response, resets and recovery behavior |
This record gives engineering, purchasing and production a traceable basis for the selected model and later design changes.
If the connected equipment also imposes a ripple limit, define the measurement method and location before comparing results. The power-supply ripple acceptance guide explains how to keep those comparisons consistent.
