The short answer
How to Size a Control Panel Power Supply
Size a control panel power supply by totaling the steady-state current of every load at the supply voltage, adding the largest expected simultaneous inrush, applying a headroom factor of at least 25%, and then verifying the resulting heat dissipation against the enclosure thermal budget. Undersized supplies produce intermittent faults that are extremely difficult to diagnose.
Key points
- Total steady-state load first, from real datasheets rather than estimates.
- Inrush from relays, contactors, and capacitive inputs can far exceed steady-state draw.
- Add at least 25% headroom, more if future expansion is expected.
- Derate for the actual maximum ambient temperature in the enclosure.
- Every watt the supply is inefficient by becomes heat inside the panel.
What you need
- Bill of material for the panel with datasheets
- PLC and I/O module current draw figures at the relevant backplane voltage
- Field device current requirements, including loop-powered transmitters
- Maximum expected ambient temperature at the installation
- Enclosure dimensions for the thermal check
Step 1: total the steady-state load
List every device drawing from the supply and its current at the supply voltage. Use datasheet values. Estimating from experience is where the error enters, because the devices that surprise you are the ones you did not think to list.
| Load | Quantity | Each | Total |
|---|---|---|---|
| PLC processor | 1 | 350 mA | 350 mA |
| Digital input module | 2 | 80 mA | 160 mA |
| Digital output module | 2 | 120 mA | 240 mA |
| Analog input module | 1 | 150 mA | 150 mA |
| Loop-powered transmitters | 3 | 22 mA | 66 mA |
| Interposing relays, energized | 8 | 25 mA | 200 mA |
| Ethernet switch | 1 | 400 mA | 400 mA |
| Cellular modem | 1 | 500 mA | 500 mA |
| Panel light and misc | — | — | 150 mA |
| Steady-state total | about 2.22 A |
Step 2: account for inrush
Steady-state is not the worst case. Relay and contactor coils, capacitive input filters, and switch-mode devices all draw substantially more current at the instant they energize. A supply that is adequate in steady state can drop out of regulation on a simultaneous energization, and the resulting symptom is a controller that resets occasionally for no visible reason.
- Identify loads that energize together. A sequence step that pulls in six relays at once is a single simultaneous event.
- Use the inrush figure from the datasheet where given; several times steady-state is common.
- Check whether the supply specifies a peak or surge capability and for how long. Many supplies tolerate a brief overload well beyond their continuous rating.
- Stagger energization in logic where a large simultaneous inrush is unavoidable. A few hundred milliseconds of separation costs nothing.
Step 3: apply headroom
Supply_Rating ≥ Steady_State_Total × 1.25
- Use 1.25 as a minimum
- Use 1.5 or more where spare I/O slots exist or expansion is anticipated
With a 2.22 A steady-state total, 25% headroom calls for at least 2.8 A, so a 5 A supply is the sensible catalog choice and leaves genuine room for the field devices that always get added later. The cost difference between a 3 A and a 5 A supply is small compared to a return visit.
Step 4: derate for temperature
Power supply ratings are stated at a reference ambient temperature and derate above it. An outdoor panel in Florida sun can reach internal temperatures far above the rating point, and a supply rated 5 A at 25 degrees Celsius may be rated considerably less at 55.
- 01Find the maximum ambient at the installation, including solar gain for outdoor enclosures.
- 02Add the internal temperature rise from the enclosure heat calculation.
- 03Read the derating curve in the supply datasheet at that temperature.
- 04Confirm the derated rating still exceeds your load plus headroom. If not, choose a larger supply, a higher-temperature-rated one, or add cooling.
Step 5: check the heat you added
A supply is not perfectly efficient, and the difference becomes heat inside the enclosure you just sized it for.
Heat_Watts = Output_Watts × (1 ÷ Efficiency − 1)
- Output_Watts — actual delivered load, not the supply rating
- Efficiency — from the datasheet, typically 0.85 to 0.93
At 2.22 A and 24 V the output is about 53 W. At 88% efficiency the supply dissipates roughly 7 W into the enclosure. That is modest on its own and worth adding to the panel heat total alongside the drives and the controller.
Redundancy and backup
- Redundant supplies
- Two supplies through a redundancy module so either can carry the full load. Worth it at unattended critical sites where a supply failure means a truck roll and a station offline.
- DC UPS
- A 24 VDC uninterruptible module with a battery, holding the controller and communications through a power interruption so the site can report the outage. Frequently more valuable than a full AC UPS at a remote station.
- Separate control and field supplies
- Keeping the controller on its own supply, separate from field device power, means a field short does not take down the processor. This is cheap insurance and it also simplifies troubleshooting enormously.
Frequently asked questions
- How much headroom should a control power supply have?
- At least 25% above the calculated steady-state load, and more where spare I/O capacity exists. The step to the next catalog size is usually inexpensive relative to the cost of diagnosing an intermittent brownout later.
- What are the symptoms of an undersized supply?
- Intermittent controller resets, communication modules dropping out, analog readings shifting when relays energize, and faults that cluster around specific sequence steps. All of these look like other problems, which is why they take so long to find.
- Should the PLC and field devices share a supply?
- Preferably not. Separating them means a shorted field wire does not take down the processor, and it makes fault isolation far quicker. Where a single supply is used, distribute through individually protected branches.
- Do I need to derate for altitude?
- Above roughly 2000 metres, yes, because reduced air density lowers convective cooling. Check the datasheet. For most water and wastewater work in low-lying areas this is not a factor, but it matters for mountain installations.
Related topics
- Panel Power SuppliesThe 24 V DC supply behind everything in a modern panel: sizing with real inrush and duty, single versus redundant with diode modules, how a switch-mode supply behaves under a short, distribution and fusing per branch, monitoring, and the failure modes that take out a whole panel at once.
- Enclosure Heat CalculationsEstimating the temperature inside a control panel from the power its contents dissipate and the surface that carries it out, what to do when the answer is too hot, and why drives change everything.
- PLC PowerPowering the controller section of a panel: the control transformer and 24 volt supplies, separating controller power from field I/O power, sizing with margin, the UPS and what rides through, protection and monitoring, the DC ground, and restart sequencing.
- No Control Power in PanelThe panel is dead: no pilot lights, no controller, no HMI. The order of checks from the disconnect through the control transformer and its fuses, the control breaker, the emergency stop chain, and the DC supply, with the readings that locate the break.
- How to Select Surge ProtectionSelect the surge protective devices for a control panel and its field circuits: list every conductor that enters, classify its exposure, choose the device type and ratings for the power, the loops, and write it into the drawings and the I/O list.
- How to Size an EnclosureSize a control panel enclosure so that everything fits, cools, and can be worked on: lay out the components with their clearances and wireway, add the spare space the plant will need, and mounting before ordering.
Direct contact
Have a controls question?
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