Power Supply Sizing & Inrush Estimator

The Load-Budget Approach for 24V DC Power Supplies

Sizing a 24V DC power supply for industrial automation requires more than just adding up steady-state currents. A methodical load-budget approach is essential to handle startup inrush, thermal headroom, and future expansion.

1. Calculating Steady-State Current

Begin by listing all devices on the 24V DC bus—PLCs, sensors, relays, and HMIs. Sum the continuous current draw based on manufacturer datasheets. Remember that while passive devices like pushbuttons don’t consume power, the PLC inputs they energize do.

Once you have the total, apply a safety margin of at least 20% to 30%. This provides crucial thermal headroom, ensures voltage stability under transient loads, and leaves room for future I/O expansion.

2. Managing Inrush Current

Inrush current is the high-magnitude surge that occurs when a device is first powered on—often 3 to 10 times the steady-state current. High-capacitance devices and inductive loads are the primary culprits.

• If the combined inrush exceeds the power supply’s peak capacity, the supply may enter current-limiting mode, causing voltage sags or failure to start.
• Mitigation strategies include sequential startup, using slow-start power supplies, or installing inrush current limiters (ICLs) for high-surge components.

3. Best Practices for Power Design

Never put all automation devices on a single circuit. Segment your loads using electronic circuit protectors to create logical branches (e.g., separating PLC logic from high-power solenoids). This ensures a localized short circuit won’t drop the entire system. Utilizing a Power Supply Sizing & Inrush Estimator helps guarantee your 24V DC architecture is robust, scalable, and reliable.