24VDC Power Supply Sizing: 5 Crucial Tips to Prevent Failure

flowchart LR
    A[Power Supply Sizing] --> B[Continuous Loads]
    A --> C[Inrush Currents]
    A --> D[Temperature Derating]
    A --> E[Load Segregation]
    B & C & D & E --> F[Panel Stability]

A machine suddenly halts mid-cycle. The HMI is dark, and the PLC has rebooted, yet no breakers tripped. This phantom fault is one of the most common headaches in industrial automation, and it almost always points to one culprit: voltage sags. Nailing your 24VDC power supply sizing is the only way to eliminate these mysterious resets, intermittent sensor communication errors, and premature hardware failures. Instead of guessing, let’s look at the five hard engineering rules to calculate an airtight power budget.

Why Correct 24VDC Power Supply Sizing is Essential for Panel Stability

Walk up to any modern automation panel, and you’ll find a massive array of 24VDC loads—from the PLC CPU and high-density IO to power-hungry solenoids and field transmitters. Unlike AC distribution, 24VDC loops are highly vulnerable to voltage drops and transient current spikes. If your 24VDC power supply sizing ignores the peak consumption of these devices, the supply will enter current-limiting foldback mode. Once that voltage dips below the controller’s minimum operating threshold (often right around 20.4VDC), the CPU shuts down to protect itself. You lose control, and you lose production.

flowchart LR
    direction LR
    subgraph Power Sizing Workflow
        Load[Calculate Base Load + 25%] --> Peak[Account for Inrush]
        Peak --> Temp[Apply Temp Derating]
        Temp --> Segregate[Isolate Logic/Actuators]
    end

1. Calculate Continuous Current Loads with a Power Budget

You can’t size what you haven’t counted. The first step in proper 24VDC power supply sizing is building a strict power budget. You need to sum the maximum continuous current ratings for every single component in the panel: PLC modules, Ethernet switches, indicator lights, and field sensors. Dig into the manufacturer datasheets for the worst-case continuous currents, especially for high-density analog output cards and remote IO nodes. Once you have that base number, add a 20% to 25% safety margin. This industry-standard buffer accommodates future expansion—because someone will inevitably add another sensor later—and keeps the supply from running at 100% capacity, which drastically shortens its lifespan.

2. Estimate and Account for Inrush and Peak Transient Currents

Inductive loads—like solenoid valves, mechanical contactors, and even the capacitive inputs of certain drives—are notoriously greedy when first energized. They draw brief, massive current spikes that can hit 5 to 10 times their continuous operating rating. When calculating your 24VDC power supply sizing, you need a unit capable of riding through these peak transients without immediately tripping its internal overcurrent protection. Always specify power supplies that feature dynamic power boost (for instance, the ability to deliver 150% of rated current for 3 to 5 seconds). This guarantees solenoid switching won’t crater your control voltage.

3. Implement Temperature De-rating Factors

Control panels on the factory floor regularly bake in environments where internal ambient temperatures push past 50°C. Power supplies generate their own heat, and as that ambient temperature climbs, their maximum safe output power drops. Ignoring temperature de-rating factors during 24VDC power supply sizing is guaranteed to cause summer-time panel shutdowns. Always check the manufacturer’s de-rating curve before buying. For example, if a 10A supply de-rates by 2.5% per degree Celsius above 40°C, installing it in a 50°C enclosure means you’re only getting 7.5A out of it. If your continuous load is 9A, the supply will thermally trip.

4. Segregate Control Logic from Actuator Power

Mixing clean logic power with noisy actuator power is asking for trouble. To build a bulletproof system, physically segregate the sensitive electronics from the heavy lifters. Use one dedicated power supply for the PLC, HMI, and critical analog sensors, and a completely separate supply for inductive loads like solenoids, relays, and motor starter coils. This hard isolation prevents inductive kickback and electrical noise from corrupting the PLC’s memory or dropping network packets. On larger systems, separating logic and load power isn’t just good practice—it’s heavily pushed by safety standards like the NFPA 70 National Electrical Code.

5. Select the Right Protection Devices

Your power supply is only as reliable as the circuit protection sitting directly downstream. Old-school glass fuses are simply too slow to protect sensitive DC electronics from short circuits. Instead, modern panels rely on electronic circuit protectors (ECPs) or highly sensitive thermal-magnetic miniature circuit breakers. But there’s a catch: your 24VDC power supply sizing must ensure the supply can deliver enough instantaneous short-circuit current to trip that breaker immediately. If the supply is undersized, it will fold back its voltage before the breaker trips, leaving the dead short active and potentially cooking your field wiring.

Using AutomationView Store Tools for Electrical Sizing

Building out a bulletproof power budget by hand takes time, and simple math errors can result in expensive hardware failures down the road. To streamline this process, take advantage of the engineering calculators available on the AutomationView Store. These web-based tools handle the heavy lifting—estimating total continuous current, inrush peaks, and even conductor voltage drops. By relying on precise calculations, you can design, build, and commission control panels that run flawlessly for years.