Industrial Power Supply Sizing: How to Calculate Load and Select the Right Unit

Selecting the wrong industrial power supply causes downtime, nuisance tripping, and premature equipment failure. The fix is straightforward: calculate your true load demand, apply a safety margin, and match voltage and current ratings to your application. This guide walks you through the exact calculations and selection criteria so you can specify the right unit the first time.

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Why Accurate Power Supply Sizing Matters

An undersized power supply cannot deliver enough current under peak load, leading to voltage sag, system resets, or thermal shutdown. An oversized unit wastes energy, costs more upfront, and may operate inefficiently at low load. In industrial environments—where PLCs, sensors, actuators, and fieldbus devices run 24/7—precision matters.

Key consequences of poor sizing:

• Intermittent equipment resets and data loss in controllers
• Reduced service life of capacitors and switching components
• Overheating in enclosures, shortening the lifespan of adjacent components
• Unplanned maintenance and production stoppages

The goal is to match the power supply’s continuous output rating to your system’s worst-case steady-state load, plus a safety buffer for inrush currents and future expansion.

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Step 1: Calculate Your Total Load Demand (Power Supply Load Calculation)

Start by listing every device powered by the supply. For each device, record:

• Nominal voltage (e.g., 24 VDC)
• Steady-state current draw (in amps)
• Peak/inrush current (if known, especially for motors, solenoids, or capacitive loads)

Example calculation table:

| Device | Voltage (VDC) | Current (A) | Peak Current (A) |

|——–|—————|————-|——————|

| PLC CPU | 24 | 1.2 | 1.5 |

| 16 digital inputs | 24 | 0.4 | 0.4 |

| 8 relay outputs | 24 | 0.8 | 2.0 (inrush) |

| HMI panel | 24 | 1.5 | 2.5 |

| Total | | 3.9 A | 6.4 A |

Formula:

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Total load (A) = Sum of all steady-state currents

Peak load (A) = Sum of all peak currents (use worst-case simultaneous operation)

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If your system has devices with different voltages (e.g., 5 VDC and 24 VDC), calculate each voltage rail separately and select a dedicated supply for each rail.

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Step 2: Apply the Safety Margin and Derating Factors

Never size a power supply to 100% of its rated capacity. Industrial power supplies run best at 70–80% of their maximum continuous output. This buffer accounts for:

• Inrush currents during startup (motors, capacitive loads)
• Temperature derating—most supplies lose output capacity above 50°C ambient
• Load transients from simultaneous switching
• Future expansion (adding sensors or I/O modules)

Recommended sizing formula:

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Required supply rating (A) = (Total steady-state load × 1.25) / Derating factor

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Practical example:

• Total steady-state load = 3.9 A
• Safety margin (1.25) = 4.875 A
• If ambient temperature is 50°C and derating is 80%, divide by 0.8 → 6.1 A minimum

In this case, a 7.5 A or 10 A supply would be appropriate. Never round down to the nearest standard rating—always round up.

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Step 3: Select Voltage, Current, and Form Factor

Once your load calculation is complete, match the following specifications:

1. Output voltage — Must exactly match your system’s nominal voltage (typically 24 VDC for modern automation). Check the tolerance range of your devices (±10% is common).

2. Output current — Must meet or exceed your calculated requirement (with margin). Look at both continuous and peak ratings.

3. Input voltage range — Verify your facility’s supply (e.g., 230 VAC single-phase, 400 VAC three-phase, or DC input).

4. Form factor — DIN rail mount is standard for control panels. Check the width in “HP” units (e.g., 45 mm, 60 mm) to fit your enclosure.

5. Protection features — Overload, overvoltage, short-circuit, and thermal protection are essential. For critical processes, consider redundant (parallel) operation capability.

If you need a compact, cost-effective solution for basic control circuits, the LRS power supply series offers reliable 24 VDC output in slim packages. For applications requiring higher efficiency, wide input ranges, or enhanced surge protection, the NDR power supply series provides robust performance in demanding environments.

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Step 4: Account for Environmental and Installation Factors

Industrial power supplies are rated for specific operating conditions. Before finalizing your selection, verify:

• Ambient temperature — Most units are rated for -20°C to +70°C, but output current derates above 50°C. Check the datasheet curve.
• Mounting orientation — Side-by-side mounting reduces airflow. Maintain recommended clearance for convection cooling.
• Humidity and contamination — For washdown or dusty areas, choose units with conformal coating or IP-rated enclosures.
• Input voltage stability — If your plant experiences voltage dips, a supply with a wide input range (e.g., 85–264 VAC) prevents nuisance shutdowns.

Derating example:

A 120 W supply rated at 5 A at 40°C may only deliver 4 A at 60°C. If your enclosure runs hot, calculate the effective current at your actual ambient temperature—not the nominal rating.

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Step 5: Verify Peak Load Handling and Inrush Current

Some loads—like DC motors, contactors, or large capacitor banks—draw several times their steady-state current for milliseconds. If your power supply lacks sufficient peak current capability, the output voltage will dip, causing logic resets.

Check the supply’s datasheet for:

• Peak current capability (e.g., 150% of rated current for 5 seconds)
• Dynamic response (voltage recovery time after a load step)
• Output capacitance — Higher capacitance helps ride through short interruptions

If your peak demand exceeds what a single unit can handle, consider:

• Using a larger supply (e.g., 10 A instead of 5 A)
• Adding a buffer module (capacitor-based) for momentary high loads
• Parallel operation of two supplies (if the model supports it)

For most industrial control panels, a standard 24 VDC supply with 120–150% peak capability is sufficient. The NDR power supply series is specifically designed for high inrush applications and offers excellent dynamic load response.

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Conclusion: Get the Right Supply the First Time

Sizing an industrial power supply is a systematic process: calculate your total load, add a 25% safety margin, apply derating for temperature, and verify peak current handling. By following these steps, you avoid the costly mistakes of undersizing and oversizing.

Recap checklist:

• [ ] Sum all steady-state currents (per voltage rail)
• [ ] Identify worst-case peak current
• [ ] Apply 1.25× safety factor
• [ ] Check temperature derating curve
• [ ] Verify input voltage range and form factor
• [ ] Confirm protection features and peak capability

Still unsure about your load calculation or application requirements? Contact Canuri for a free quotation. Our team can help you select the right power supply from our extensive inventory of industrial automation spare parts—including both the LRS and NDR series—and ensure your system runs reliably for years.

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FAQ: Industrial Power Supply Sizing

#### H3: What happens if my power supply is undersized?

An undersized supply cannot maintain rated voltage under load. Symptoms include voltage dips, system resets, erratic sensor readings, and thermal shutdown. In severe cases, it can damage connected electronics due to undervoltage conditions.

#### H3: Can I use a power supply with a higher current rating than needed?

Yes, oversizing is generally safe. A higher-rated supply will simply deliver only the current the load requires. However, avoid extreme oversizing (e.g., 40 A for a 2 A load) as efficiency drops at very low load percentages, and the unit may not regulate as precisely.

#### H3: How do I calculate power supply load for a PLC system?

List every component: PLC CPU, I/O modules, fieldbus couplers, sensors, actuators, and HMI. Sum the current draw of each at the operating voltage (typically 24 VDC). Add the peak currents of any inductive loads (relays, solenoids). Then apply the 1.25× safety factor.

#### H3: What is the difference between LRS and NDR power supplies?

The LRS series is a budget-friendly, compact option for standard 24 VDC applications with moderate load requirements. The NDR series offers higher efficiency, a wider input voltage range, and better peak current handling—making it suitable for demanding industrial environments with inrush loads or unstable mains power.

#### H3: Do I need a redundant power supply for critical applications?

If a power failure would cause production loss or safety risks, yes. Redundant configurations (two supplies in parallel with decoupling diodes) provide seamless failover. Ensure the selected model supports parallel operation and has a “DC OK” signal for monitoring.