Why the Backplane Itself Has a Real Power Budget
Every module installed in a PLC chassis — I/O modules, the CPU, communication modules — draws electrical current from the chassis backplane to power its own internal electronics, distinct from and in addition to whatever field-side voltage and current the module's own I/O channels handle for connected field devices. This backplane current has to be supplied by the chassis power supply, and the total current draw across every installed module directly determines the minimum power supply capacity required for that chassis.
Why Different Module Types Draw Meaningfully Different Current
Backplane current draw varies by module type and function, reflecting the different amount of internal electronics and processing each module type actually requires: simple digital I/O modules (DI, DO) commonly draw on the order of 100 to 150 mA, analog I/O modules (AI, AO) draw somewhat more, commonly in a similar 120 to 150 mA range reflecting their more complex signal conditioning circuitry, communication modules draw meaningfully more (commonly around 500 mA, reflecting their more substantial onboard processing for network communication), and the CPU module itself typically draws the most of any single module type (commonly around 700 mA), reflecting its role as the system's primary processing unit.
Why the Total Has to Be Summed Across Every Installed Module
Because every module contributes its own backplane current draw independently, the total chassis current requirement is the sum of every individual module's draw — the CPU's draw, plus every I/O module's draw, plus every communication module's draw, all added together. This is exactly the calculation this site's PLC I/O Sizing Tool performs automatically once module counts by type are determined from the I/O point counts and selected module density, ensuring no module type's contribution to the total is inadvertently omitted from the power budget.
Why a 25% Margin Is Applied on Top of the Summed Total
Beyond simply summing the calculated per-module current draws, standard practice applies an additional margin — commonly 25 percent — on top of this summed total before selecting an actual power supply. This margin accounts for several real sources of uncertainty and variability: actual module current draw can vary somewhat from published typical or representative values depending on the specific module revision and operating conditions, future spare I/O modules (installed per the spare capacity guidance covered in the companion article) need power budget included even before they are actively connected to field devices, and general engineering practice favors headroom above a bare calculated minimum rather than specifying a power supply with zero margin for any variability.
Why Power Supply Selection Rounds Up to a Standard Rating
Once the margined total current requirement is calculated, actual power supply selection — similar to the standard-size rounding pattern seen in other equipment sizing contexts across engineering — selects the smallest available standard power supply rating that meets or exceeds this calculated requirement, since power supplies (like transformers or circuit breakers) are manufactured in discrete standard current ratings, not continuously variable custom values. This site's PLC I/O Sizing Tool performs this rounding automatically, selecting from each supported platform's actual standard power supply product options rather than returning only a raw calculated current figure.
Why This Calculation Is Performed on a Per-Chassis Basis, Not System-Wide
Because each chassis (whether a local CPU chassis or a remote/expansion chassis, as covered in the companion local-versus-remote I/O article) has its own dedicated power supply, the backplane power budget calculation has to be performed separately for each individual chassis based on that specific chassis's own installed module complement, not as a single system-wide total divided evenly across however many chassis a system happens to have. A chassis with a heavier concentration of high-current-draw modules (more communication modules, for example) needs a correspondingly larger power supply than an otherwise similar chassis with a lighter module mix, even within the same overall system.
Why Actual Datasheet Values Should Eventually Replace Representative Estimates
The representative current draw values commonly cited for each module type (and used as defaults in early sizing tools) are useful, broadly applicable approximations for preliminary design, but actual module current draw varies by specific model and manufacturer within each general module category — a final, permit-ready or purchase-ready power budget calculation should confirm actual current draw values from the specific selected modules' own manufacturer datasheets, rather than relying on general representative figures for the final specification, exactly as this site's tool's own reference notes explicitly recommend.