Instrument Power Supply Load Calculator

This calculator assists instrumentation engineers in determining the appropriate power source sizing for their field devices. Accurately calculating the total current and power draw helps prevent overloading, ensures stable operation, and aids in selecting the correct power supply unit (PSU) or distributed control system (DCS) power module.

Global System Parameters

Load Input Method

Connected Field Devices

Engineer's Guide to Power Supply Sizing & Selection

In any instrumentation or control system, the power supply unit (PSU) is the heart. A failure here brings the entire system down. Correctly sizing a power supply isn't just about matching voltage; it's a critical engineering task that balances load, efficiency, safety, and future-proofing. An undersized PSU will lead to nuisance trips and instability, while a grossly oversized PSU is a waste of capital and energy. This guide covers the essential principles for getting it right.

Key Concepts in Power Supply Sizing

Before calculating, it's essential to understand the language of power supplies.

  1. Voltage (V): This is the "electrical pressure." For instrumentation, 24VDC is the near-universal standard. This calculator assumes you're working with a DC system.
  2. Current (A or mA): This is the "flow" of electricity. It's the primary value you need to calculate. Every device in your system draws a certain amount of current (e.g., a transmitter at 20mA, a solenoid at 150mA).
  3. Power (W): This is the total rate of energy consumption, calculated as $P = V \times I$. While current is often used for sizing, power (in Watts) is crucial for understanding heat load and overall energy use.
  4. Inrush Current: The momentary, high-current surge that some devices (like solenoids, actuators, and the PSU itself) draw when first powered on. This peak can be 10-20 times the steady-state current. The PSU must be able to "survive" this surge without tripping its internal protection.
  5. Efficiency (%): No power supply is 100% efficient. A 90% efficient PSU drawing 100W from the wall will only *deliver* 90W to your devices. The other 10W is lost as heat inside your panel. This is critical for thermal management.

The 4-Step Sizing Calculation

A robust power calculation follows a clear, conservative process. This calculator automates these steps.

  1. Step 1: Sum the Continuous Base Load

    First, find the "worst-case continuous" load. This means assuming every device is operating at its maximum *normal* draw. For a 4-20mA transmitter, this is 20mA (or ~22mA in a fault condition). For a solenoid, it's its energized holding current. Make a list of every device and its quantity.

    $$I_{base} = (N_{tx} \times I_{tx}) + (N_{sol} \times I_{sol}) + (N_{other} \times I_{other}) + ...$$

  2. Step 2: Apply the Safety & Future-Proofing Factor

    This is the golden rule: Never size a power supply to 100% of its base load. You *must* add a safety margin. This margin accounts for:

    • Future Expansion: The inevitable "one more instrument" you'll be asked to add next year.
    • Unknowns: Small loads that were missed (e.g., panel lights, interface relays).
    • PSU Health: A power supply running at 100% load 24/7 will have a significantly shorter life than one running at 75%.

    A typical safety factor is 20-25%. This tool uses 20% as a default.

    $$I_{required} = I_{base} \times (1 + \frac{SafetyFactor\%}{100})$$

  3. Step 3: Check Peak & Inrush Loads

    Now, consider the "worst-case peak" load. What's the chance all your solenoids will energize at the *exact* same millisecond? Low, but possible. The PSU's "peak load capability" (often listed for a few seconds) must be able to handle this. This calculator uses a simple "Peak Current Multiplier" to estimate this. Your calculated $I_{required}$ is the *continuous* rating your PSU needs, and $I_{required} \times Multiplier$ is the *peak* rating it must be able to handle.

  4. Step 4: Calculate Input Power

    Finally, to size the upstream breaker (e.g., the 120VAC or 230VAC supply *to* the PSU), you must account for efficiency. The power *drawn* from the wall is higher than the power *delivered* to your instruments.

    $$P_{input} = \frac{P_{output\_required}}{Efficiency\%}$$

Practical Selection Criteria

Once you have your required current (e.g., "I need a 24VDC, 5A continuous PSU"), you're not done. You must select the right *type* of PSU.

  • Switched-Mode (SMPS) vs. Linear: 99% of modern industrial PSUs are SMPS. They are highly efficient (85-95%), lightweight, compact, and accept a wide range of input voltages (e.g., 85-264VAC), making them globally compatible. Linear PSUs are simple and have very clean (low-noise) output, but are massive, heavy, inefficient (30-60%), and have fixed input taps.
  • Redundancy (1+1 vs. N+1): For critical systems (like a plant's main DCS or ESD system), you can never have a single point of failure. 1+1 Redundancy uses two identical PSUs, each capable of handling 100% of the load. Their outputs are combined using a "diode-or" or "redundancy module." If one PSU fails, the other takes the full load instantly with zero interruption.
  • Environmental Rating: Where will the PSU live?
    • Temperature: All PSUs have a "derating curve." A 5A PSU might only be a 4A PSU at 60°C. Check the datasheet!
    • IP Rating: Is it in a clean panel (IP20) or a washdown area (IP67)?
    • Hazardous Area: Does it need to be in an explosion-proof (Ex d) box or is it powering Intrinsically Safe (Ex i) devices via a barrier?
  • Protection Features: A good industrial PSU will have built-in protection for overload, short-circuit, and over-voltage. This is non-negotiable for protecting your expensive instruments.

Critical Installation & Safety Pitfalls

A perfectly sized PSU can be rendered useless by poor installation.

  • Voltage Drop: This is the silent killer. You have a perfect 24.0V at the PSU, but after 200 meters of thin 0.5mm² wire, the transmitter only receives 19.5V and fails. You must calculate voltage drop ($V_{drop} = I \times R_{wire}$) for your longest, highest-current runs and ensure all devices receive their minimum required voltage. The fix is using thicker gauge wire.
  • Grounding: This is a massive topic, but two rules are key: 1) The PSU chassis *must* be connected to protective earth (safety ground) to prevent shock. 2) The 0VDC (common) side of your 24VDC loop is often connected to this same earth at a *single point* to create a stable signal reference and prevent noise.
  • Heat Dissipation: That 10W of heat from efficiency loss has to go somewhere. PSUs need to breathe. Follow the manufacturer's clearance specs (e.g., "2 inches above and below"). Do not pack them tightly between two cable ducts. Heat is the #1 killer of electronics.
  • Circuit Protection: Each 24VDC circuit *leaving* the PSU should be individually protected by a fuse or circuit breaker. If a single transmitter shorts to ground, you want that one fuse to blow—not your main 5A PSU, which would shut down the entire panel.