Utilities & Energy

Electrical Load Calculation and Transformer Sizing for Industrial Laundry Plants

Undersized electrical infrastructure is one of the more expensive mistakes a new laundry plant can make, because a transformer or incoming supply sized for name-plate connected load rather than realistic demand either gets replaced within a few years of growth or throttles voltage every time a large motor starts.

Published July 6, 2026 — Stalwart Engineering Technical Notes

Connected load versus demand load

Connected load is the sum of the name-plate rating of every piece of electrical equipment in the plant: washer-extractor motors, ironer drive and heating elements where electrically heated, dryer motors and burners, compressors, pumps, and lighting. Demand load is what the plant actually draws at any given moment, which is always lower than connected load because not every machine runs at full rated power simultaneously. The ratio between demand and connected load, the diversity factor, is what makes sizing incoming supply on connected load alone wildly oversized and costly; a realistic industrial laundry diversity factor typically falls between 0.5 and 0.7 depending on how synchronized the plant's shift and cycle patterns are.

Motor starting current: the sizing trap

The load calculation that matters most for transformer and switchgear sizing is not steady-state running load but the transient current draw when large motors start. A direct-on-line started motor can draw six to eight times its rated running current for the first second or two of starting, and if several large machines happen to start within the same short window, the transformer must be able to supply that combined inrush without excessive voltage sag, or every other machine on the same supply experiences a voltage dip that can trip sensitive electronic controls. This is precisely why motor starting method selection, star-delta, soft starters, or variable frequency drives, is as much an electrical infrastructure decision as a machine-level one: reducing individual motor inrush current directly reduces the transformer capacity margin the plant must carry for starting transients alone.

Sizing the incoming transformer

A practical sizing approach starts from calculated demand load, adds a margin for the largest single motor's starting transient superimposed on that demand (since the worst case is a large motor starting while the rest of the plant is already near typical running demand), and then adds a growth margin for planned future capacity, commonly 20 to 30 percent, since utility transformer upgrades involve lead time, cost, and often civil work that is far cheaper to avoid by sizing correctly the first time. Central Electricity Authority regulations and the local electricity distribution utility's technical standards govern transformer capacity, power factor, and demand metering requirements for industrial connections, and the utility's sanctioned load approval process is where this calculation gets formally reviewed before a new connection or capacity increase is approved.

Power factor and reactive load

Induction motors, which dominate laundry machinery load, draw lagging reactive power in addition to real power, and a plant with a poor power factor, commonly below 0.85 without correction, both draws a heavier current for the same real power delivered and typically faces a power factor penalty charge from the distribution utility. Installing capacitor banks sized to the plant's reactive load profile, reviewed periodically as machine mix changes, corrects power factor toward the 0.95 to unity range utilities typically target, reducing both the penalty charge and the effective current loading on transformers and cabling.

Practical planning checklist

List every motor and heating load with name-plate kW and expected simultaneous operating pattern by shift, not just a sum of all name-plates.
Identify the two or three largest motors and their starting method, since these dominate the transient sizing case regardless of total connected load.
Apply a realistic diversity factor validated against actual demand data from a comparable existing plant rather than a generic industry rule of thumb alone.
Build in growth margin before the transformer or incoming supply agreement is finalized, since retrofitting capacity later is materially more expensive than sizing correctly at commissioning.

Ongoing demand monitoring

A load calculation done once at commissioning has a limited shelf life in a growing plant. Installing a permanent demand meter or power quality analyzer at the incoming supply, rather than relying solely on the utility's monthly billing meter, lets plant engineering track actual peak demand and power factor trends over time and flag when the plant is approaching its sanctioned load or transformer thermal limit well before a nuisance trip or a utility notice forces the issue. This same monitoring point is the natural place to trend voltage sag events tied to motor starting, giving an early, quantified signal that a starting-method upgrade or a transformer capacity review is due before it becomes an operational problem.