How to Size a DIN-Rail Power Supply for a PLC Control Panel

Here’s the field failure that teaches every panel builder the same lesson once. The control panel passes factory acceptance testing. It runs fine through commissioning. Then one day in production, several solenoid valves fire simultaneously — and the 24V power supply trips offline, taking the whole control system down with it. The panel builder sized the power supply for the steady-state load, which was correct on paper, but ignored the inrush current when multiple solenoids energize at once. The supply was technically “big enough” for normal operation but couldn’t handle the real-world peak.

This is the most common DIN-rail sizing mistake, and it’s entirely preventable. Proper sizing isn’t just “add up the wattages” — it accounts for which loads operate simultaneously, the inrush current when inductive and capacitive loads energize, the derating that reduces capacity at high panel temperatures, and the headroom margin for reliability and future expansion. Get these factors right and the panel runs reliably for decades. Get them wrong and you get intermittent failures that are maddening to diagnose because they only happen under specific load combinations.

This guide is the complete sizing methodology for PLC control panel power supplies. It walks through building the load inventory, applying simultaneity factors, handling inrush current, accounting for temperature derating, adding appropriate headroom, and selecting the right supply — with a full worked example using real numbers. Whether you’re a panel builder, controls engineer, or machine OEM, this gives you the framework to size power supplies that work in the field, not just on the spreadsheet.

How do I size a DIN-rail power supply for a PLC panel?

Size a DIN-rail power supply for a PLC panel in six steps: (1) inventory all connected loads with their power consumption, (2) apply a simultaneity factor for loads that don’t all operate at once, (3) account for inrush current from inductive and capacitive loads, (4) apply temperature derating for the panel’s operating environment, (5) add 20-30% headroom for reliability and expansion, and (6) select a standard supply that meets the calculated requirement. For most PLC panels, this results in a 24V supply sized at roughly 1.3-1.5× the realistic peak load.

The sizing formula:

Required power = (Sum of loads × simultaneity factor + inrush allowance) × headroom factor ÷ derating factor

Each factor matters. Skipping any of them leads to either undersizing (field failures) or excessive oversizing (wasted cost and space).

Step 1 — Build the load inventory

The foundation of sizing is a complete inventory of every device drawing power from the supply. Missing loads leads to undersizing. List every device, its quantity, and its power consumption.

Categorize the loads

Group loads by type, because different types have different characteristics:

Continuous loads (always on):

• PLC CPU and power
• HMI displays
• Indicator lights
• Communication modules

Intermittent loads (switch on/off):

• Solenoid valves
• Relays and contactors
• Motor starters

Variable loads (changing demand):

• Variable outputs
• Modulating valves

Example load inventory

For a typical machine control panel:

This is the worst-case total if everything operates simultaneously. But not everything does — which is where simultaneity comes in.

Getting accurate power data

For accurate load data:

• Use manufacturer specifications (not estimates)
• Account for both steady-state and inrush
• Include all auxiliary loads
• Don’t forget small loads (they add up)

For a panel with 178W connected load, missing 20W of small loads is an 11% sizing error.

What is the simultaneity factor in power supply sizing?

The simultaneity factor (also called diversity factor) accounts for the fact that not all loads operate at the same time. In a control panel, some loads are continuous (always on) while others are intermittent (operating only when activated). The simultaneity factor reduces the total connected load to a realistic peak demand. A factor of 1.0 means all loads can operate simultaneously; a factor of 0.7 means realistic peak is 70% of the connected total.

How to determine simultaneity

Analyze the control sequence:

• Which loads are always on? (continuous — factor 1.0)
• Which loads operate together? (simultaneous group)
• Which loads are mutually exclusive? (never together)

Calculating realistic peak

For the example panel:

Continuous loads (always on):

• PLC, I/O, HMI, sensors, lights, comms = 12+24+18+12.8+9.6+9.6+6 = 92W

Intermittent loads (analyze simultaneity):

• 12 solenoid valves: but realistically, maybe 6 operate simultaneously in the worst sequence
• 14 relays: maybe 10 operate together

Realistic peak intermittent:

• 6 solenoids × 6W = 36W
• 10 relays × 1W = 10W
• Subtotal: 46W

Realistic peak total: 92W (continuous) + 46W (peak intermittent) = 138W

This is much less than the 178W worst-case, but more realistic. Note: be conservative — if you’re unsure whether loads operate simultaneously, assume they do.

When to use full simultaneity (factor 1.0)

Use full simultaneity (assume everything on at once) when:

• The control sequence could activate all loads
• Safety requires worst-case sizing
• You can’t determine the actual sequence
• The cost of oversizing is acceptable

For safety-critical or uncertain applications, conservative full-simultaneity sizing is the safe choice.

When reduced simultaneity is justified

Use reduced simultaneity when:

• The control sequence is well-defined
• Loads are clearly sequenced (not simultaneous)
• You understand the actual operation

Even then, be conservative — underestimating simultaneity causes field failures.

How do I account for inrush current when sizing?

Account for inrush current by identifying loads with high startup current (solenoid valves, motors, capacitive inputs, lamps) and ensuring the power supply can handle the peak when these loads energize simultaneously. Inductive loads (solenoids) draw 2-3× steady-state on startup; capacitive loads draw high initial charging current. The power supply needs either sufficient continuous rating to handle inrush, or peak/boost capability to provide short-duration extra current.

Inrush by load type

Different loads have different inrush characteristics:

How power supplies handle inrush

Quality DIN-rail supplies handle inrush through:

Peak/boost capability: Many supplies provide 120-150% rated power for a few seconds, handling brief inrush without tripping. For example, a 100W supply with 150% peak can deliver 150W for 3-5 seconds.

Sufficient continuous rating: Sizing the supply with enough continuous capacity to handle inrush without exceeding rating.

Sequenced startup: Designing the control sequence to stagger inrush (not all loads energize simultaneously).

Calculating inrush allowance

For the example panel, the worst-case inrush scenario:

• If 6 solenoids energize simultaneously: 6 × 6W × 2.5 inrush = 90W peak (vs 36W steady-state)
• This 90W inrush peak lasts ~50ms

If the supply has 150% peak capability, a 100W supply handles 150W peak — sufficient for the 90W inrush plus continuous load briefly.

Alternatively, size the continuous rating high enough, or sequence the solenoid activation.

Inrush sizing strategy

Two approaches:

Approach A — Peak capability: Size for continuous load, rely on the supply’s peak/boost for inrush. Most cost-effective if the supply has good peak capability.

Approach B — Continuous oversizing: Size the continuous rating to cover inrush directly. Simpler but requires larger supply.

For most panels, Approach A (using peak capability) is cost-effective if the supply specifications include adequate peak/boost.

How does temperature derating affect power supply sizing?

DIN-rail power supplies derate (reduce maximum output) at high ambient temperatures because cooling becomes less effective. A supply rated for full power up to 60°C might deliver only 70-80% of rated power at 70°C. For panels in warm environments (enclosed cabinets, hot factories, outdoor enclosures), you must account for derating by selecting a supply with enough capacity at the actual operating temperature, not just the rated temperature.

Why supplies derate at high temperature

Fanless DIN-rail supplies cool by convection. At higher ambient temperature:

• Convection cooling becomes less effective
• Internal components run hotter
• To prevent overheating, the supply must reduce output

The derating protects the supply from thermal damage at high ambient.

Typical derating curve

A typical DIN-rail supply derating:

• Full rated power: up to 55-60°C ambient
• Linear derating above: -2.5%/°C typical
• Zero output: at maximum temperature (70°C typical)

For example, a 100W supply:

• 100W at 60°C
• 75W at 70°C (25% derating over 10°C)

Accounting for derating in sizing

For the example panel:

• If the panel ambient reaches 50°C: minimal derating, full capacity available
• If the panel ambient reaches 65°C: significant derating

If your 138W realistic peak load is in a 65°C environment:

• A supply rated 150W at 60°C might only deliver 125W at 65°C
• This is insufficient — you need a larger supply or better cooling

For warm panels, either:

• Select a larger supply (accounting for derating at operating temperature)
• Improve panel cooling (ventilation, larger enclosure)
• Use a supply rated for higher temperature

Measuring panel temperature

Panel internal temperature is higher than ambient:

• Equipment generates heat inside the enclosure
• Sealed panels trap heat
• Internal temperature can be 10-20°C above external ambient

Measure or estimate the actual internal temperature where the power supply mounts, not just the room temperature.

How much headroom should I add to my power supply sizing?

Add 20-30% headroom over the realistic peak load (after simultaneity, inrush, and derating considerations). This headroom provides margin for reliability (operating below maximum extends life), future expansion (adding loads later), component aging (capacity may decline over years), and measurement uncertainty (load estimates aren’t perfect). For critical applications, use 30%+ headroom; for cost-sensitive standard applications, 20% is acceptable.

Why headroom matters

Operating a power supply near 100% capacity continuously:

• Runs hot (reduces lifespan)
• No margin for load variations
• No room for expansion
• Premature aging

Operating at 70-80% capacity (with headroom):

• Runs cooler (extends life)
• Margin for variations and inrush
• Room for future loads
• Better reliability

Headroom calculation

For the example panel:

• Realistic peak load: 138W
• Apply 30% headroom: 138 × 1.3 = 179W
• Required supply: 180W+ (or account for derating)

Headroom vs derating

Don’t confuse headroom and derating:

• Derating reduces available capacity at high temperature
• Headroom is the margin you add for reliability

Both must be accounted for:

• Required = (peak load × headroom) ÷ derating factor

For 138W peak, 30% headroom, in a 65°C environment with 0.85 derating factor:

• Required = (138 × 1.3) ÷ 0.85 = 211W

This shows why warm environments need significantly larger supplies.

Balancing headroom and cost

More headroom = more reliability but more cost. Balance based on:

• Application criticality (critical → more headroom)
• Future expansion plans (expansion → more headroom)
• Cost sensitivity (cost-sensitive → minimum adequate headroom)

For most applications, 25-30% headroom balances reliability and cost.

A complete worked example — machine control panel

Let’s size a power supply for the example machine control panel from start to finish.

Step 1 — Load inventory

From the inventory table:

• Total connected load: 178W
• Continuous loads: 92W
• Intermittent loads: 86W (full) / 46W (realistic peak)

Step 2 — Apply simultaneity

• Continuous: 92W (always on)
• Realistic peak intermittent: 46W (6 solenoids + 10 relays)
• Realistic peak total: 138W

Step 3 — Account for inrush

• Worst-case solenoid inrush: 6 × 6W × 2.5 = 90W peak (vs 36W steady)
• Inrush adds ~54W briefly above steady-state
• Strategy: rely on supply peak capability (150%)

Step 4 — Temperature derating

• Panel internal temperature estimate: 50°C
• At 50°C: minimal derating, ~full capacity
• Derating factor: ~0.95

Step 5 — Apply headroom

• Realistic peak: 138W
• 30% headroom: 138 × 1.3 = 179W
• Account for derating: 179 ÷ 0.95 = 188W

Step 6 — Select the supply

Requirements:

• 24V output
• ~190W continuous capacity at operating temperature
• 150% peak capability for inrush
• EN62368-1 certified

Selection: A 240W 24V DIN-rail supply (e.g., 24V/10A) provides:

• 240W continuous (well above 188W requirement)
• Peak capability for solenoid inrush
• Margin for reliability and expansion

Alternatively, a 150W supply might work if simultaneity is lower and peak capability is strong — but the 240W provides comfortable margin.

Verification

Verify the selection:

• Continuous load (138W) is 58% of 240W rating → good margin
• Inrush peak (with solenoids) within peak capability → handled
• Operating temperature derating accounted → adequate
• Future expansion room → available

This supply will run reliably without the field-tripping problem from the opening scenario.

When should I use multiple power supplies?

Use multiple power supplies when the total load exceeds a single unit’s practical capacity, when you need to separate noisy and clean loads, when different voltage rails are required, or when redundancy is needed for critical systems. Splitting loads across multiple supplies can improve reliability, isolation, and maintainability — but adds cost and complexity. For most panels, a single appropriately-sized supply is simplest.

Reasons to use multiple supplies

Total load exceeds single unit: For large panels exceeding ~480W (largest practical single DIN-rail unit), multiple supplies are necessary.

Load isolation: Separating noisy loads (motors, solenoids) from sensitive loads (analog sensors, communication) on different supplies improves signal quality.

Multiple voltage rails: If the panel needs 24V, 12V, and 5V, separate supplies (or a multi-output supply) provide each voltage.

Redundancy: Critical systems use redundant supplies (N+1) so one failure doesn’t down the system.

Single vs multiple decision

For most standard panels: single supply (simplest, most cost-effective). For large, critical, or multi-voltage panels: multiple supplies (better isolation, redundancy, flexibility).

Common DIN-rail sizing mistakes

Five mistakes that cause field failures and project problems:

Mistake 1 — Sizing for steady-state, ignoring inrush

The opening scenario: panel works until multiple solenoids fire simultaneously, then trips. The supply was sized for steady-state but couldn’t handle inrush.

Fix: Always account for inrush. Use supplies with peak capability, or size continuous rating to cover inrush, or sequence load activation.

Mistake 2 — Ignoring temperature derating

Supply sized at rated capacity, but installed in a 65°C panel where it derates 25%. The supply can’t deliver the needed power.

Fix: Account for derating at the actual operating temperature. Size for capacity at temperature, not just rated capacity.

Mistake 3 — Forgetting small loads

Engineer sizes for major loads but forgets indicators, sensors, and small auxiliaries. The cumulative small loads cause undersizing.

Fix: Inventory ALL loads, including small ones. They add up.

Mistake 4 — No headroom

Supply sized at exactly the calculated load. No margin for variations, aging, or expansion. The supply runs hot and ages prematurely.

Fix: Always add 20-30% headroom. Operating below maximum extends life and provides margin.

Mistake 5 — Overestimating simultaneity reduction

Engineer assumes low simultaneity to use a smaller supply, but the actual operation activates more loads together than expected. Field failures result.

Fix: Be conservative with simultaneity. When unsure, assume loads operate together.

FAQs

Related guides

• DIN-Rail Power Supply: The Complete Guide for Industrial Control Panels Pillar guide covering DIN-rail fundamentals.
• 12V vs 24V vs 48V DIN-Rail Power Supply: Which Voltage? Selecting the output voltage before sizing.
• Why 24V DC is the Industrial Standard Why 24V dominates control panels.
• Redundant DIN-Rail Power Supply Setup (N+1) Multiple supplies for critical systems.
• How to Wire a DIN-Rail Power Supply Step by Step Installation after sizing.
• DIN-Rail EMC and EMI Compliance Compliance and temperature considerations.
• MEAN WELL vs Phoenix Contact vs PULS vs Chinese DIN-Rail Brand selection after sizing.
• DIN-Rail Power Supply with Battery Backup (DC-UPS) Backup power for critical panels.

References and further reading

1. IEC 62368-1 — Audio/Video, Information and Communication Technology Equipment Safety.
2. UL 508A — Standard for Industrial Control Panels.
3. IEC 61131 — Programmable Controllers (PLC standards).
4. NEC Article 409 — Industrial Control Panels.
5. NEMA ICS — Industrial Control and Systems standards.
6. IEC 60364 — Low-voltage electrical installations.
7. UL 60950-1 / UL 62368-1 — Power supply safety standards.