When to use: Use this when sizing the DC power supply for a PLC rack so it can carry the processor backplane draw plus every digital input/output, analog channel, communications card, and field device powered from the supply. Sum the module and field loads, apply a 25% spare-capacity margin, convert to amps, then round up to the next standard PSU rating. Undersizing leads to brownouts and erratic I/O; this confirms the supply has adequate continuous current and wattage.
The PLC DC power supply must continuously deliver current to the CPU backplane, every I/O module, every communications card, and all field devices powered from the 24 VDC supply rail, with a 25% spare-capacity margin for growth and inrush. Undersizing causes voltage sag, erratic I/O behavior, and nuisance faults; this calculator sums all loads precisely and selects the next standard power supply rating.
The total current requirement is the sum of all module and field loads: I_total = I_CPU + (n_DI Γ I_DI) + (n_DO Γ I_DO) + (n_AI Γ I_AI) + (n_AO Γ I_AO) + (n_comms Γ I_comms) + I_field_devices. All currents are in milliamps at the supply voltage (typically 24 V DC). The design current is the total multiplied by a margin factor (1.25 for 25% spare capacity per NFPA 79 good practice). Power in watts = V Γ I_design.
Field devices powered from the PLC supply (2-wire transmitters, solenoid valve indicators, proximity switches) add their current directly. Some designs use a separate 24 V DC field power supply independent of the PLC backplane supply; in that case, field device current is not included in the PLC supply sizing.
NFPA 79 (Electrical Standard for Industrial Machinery) Article 7.2.6 requires that power supplies for industrial control systems have sufficient capacity for the maximum load with appropriate safety margins. IEC 60204-1 (Safety of Machinery β Electrical Equipment) Section 6 addresses power supply sizing and requires that the power supply be rated for the total connected load plus margin. For safety-instrumented systems, IEC 61511 requires redundant power supplies for SIL 2 and above safety functions, with automatic changeover on loss of the primary supply.
The 25% spare capacity margin serves two purposes: it provides headroom for the addition of future modules without replacing the power supply, and it ensures the supply operates at no more than 80% of its rated capacity, which is the standard derating for continuous-duty DC power supplies. Operating a power supply at 100% rated capacity reduces its service life significantly β SMPS (switch-mode power supply) manufacturers typically specify MTBF at 50% and 75% load, not 100%.
For critical applications (safety systems, continuous process control), dual redundant power supplies with automatic changeover are required. The redundant configuration uses two power supplies each rated for 100% of the load, connected with ideal-diode ORing or a diode ORing module so that loss of one supply is bumpless. Each supply in a redundant pair should be sized independently at the full 25% margin without sharing credit for the other supply.
Enter the CPU backplane current draw from the PLC data sheet (typically 300β800 mA for a mid-range CPU). Enter the count and per-point or per-channel current for each I/O type. Enter the count and per-card current for communications modules. Enter the total current of all field devices powered from this supply (from instrument data sheets or loop diagrams). Select the supply voltage (24 V DC is most common; some systems use 12 V or 48 V) and the margin percentage (25% is the standard minimum). The calculator outputs the load breakdown, total design current, and the recommended standard PSU rating.
CPU current draw at 24 V DC varies significantly by platform and processor type: compact PLCs (Allen-Bradley Micro820, Siemens S7-1200) draw 200β500 mA. Mid-range PLCs (AB CompactLogix, Siemens S7-1500 small) draw 300β700 mA. High-end PLCs (AB ControlLogix, Siemens S7-1500 advanced) draw 500β1200 mA. Always use the data sheet value for the specific CPU model β the spread is too large to estimate reliably.
A 16-point 24 V DC sinking digital input module typically draws 50β200 mA total from the 24 V supply depending on whether all inputs are energized and the internal circuit design. Per-point current is approximately 7β10 mA for standard IEC 61131-2 compliant inputs. For the power budget, use the worst-case (all inputs energized) total module current from the data sheet, not the per-point times the number of points.
The 25% margin (operating at no more than 80% of rated capacity) is the industry standard for continuous-duty electronic power supplies for two reasons: it provides headroom for load additions without replacing the supply, and it ensures the supply operates in the safe thermal region where component stress and failure rate are minimized. Switch-mode power supplies are typically rated for continuous output at 100%, but MTBF data shows significantly longer life at 70β80% load.
Best practice is to use a separate 24 V DC field power supply for devices that switch frequently (solenoid valves, indicator lamps) or that draw significant current (2-wire transmitters in large numbers). This isolates switching noise from the PLC logic supply and ensures that a field-side short circuit cannot collapse the PLC backplane voltage. The PLC power supply then only feeds the backplane (CPU, I/O modules, comms cards) where clean, stable voltage is critical.
Redundant power supplies are required for: SIL 2 and above safety functions per IEC 61511, continuous process applications where a brief control outage causes a plant upset or safety event, and any application where scheduled maintenance cannot include PLC power-down without a process shutdown. For SIL 2, the two power supplies must be from separate distribution panels on separate MCCs to avoid common-cause failure from a single upstream breaker trip.
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