How Much Transformer kVA Does a Load in kW Require?

Divide the electrical input power in kilowatts by the load’s true power factor to obtain apparent power in kilovolt-amperes. For example, 800 kW at a power factor of 0.80 requires 1,000 kVA at that operating point. A buyer bringing this number to Zisheng Electric has a starting load requirement. Selecting the transformer also requires knowing which loads run together and under what conditions.

Check what the kW column measures

A motor schedule often lists shaft output. A utility meter measures electrical input. Both may display kW, but they describe different sides of the motor. For a motor with 90 kW of shaft output and 90 percent efficiency at the stated duty, electrical input is 100 kW. At a true power factor of 0.80, the corresponding load is 125 kVA. These numbers are hypothetical. Use the motor and drive supplier’s input data at the intended load when preparing a real enquiry.

Applying motor efficiency again to a measured electrical input would count the loss twice. Conversely, treating shaft output as electrical demand would understate the requirement. Add a column to the load list identifying the measurement boundary. It costs little to clarify that boundary on paper and can change the transformer capacity being priced.

Cross-check the current on the correct side

For a balanced three-phase system, apparent power in kVA is approximately 1.732 times line-to-line volts times line amperes divided by 1,000. An assumed 1,000 kVA at 400 V corresponds to roughly 1,443 A. That figure belongs to the 400 V side. Comparing it with the current at a medium-voltage primary would give a misleading discrepancy. Schneider Electric publishes the single-phase and three-phase kVA formulas with their voltage and current inputs.

Power factor describes the ratio of real power to apparent power. Use true power factor when non-sinusoidal currents matter. A displacement power factor quoted for a motor can differ from the true input power factor of a complete drive. Request data at the supply terminals. Where variable-frequency drives form a material part of the load, the harmonic load data guide explains what the designer needs to evaluate additional heating.

Build one row for each operating case

Imagine two identical pumps, one duty and one standby. The purchase list contains two motors, but an interlocked arrangement might allow only one to run continuously. Ask the process engineer whether both can operate during changeover, testing or an emergency. Keep the continuous demand and the temporary overlap in separate rows with their durations. An assumption about simultaneity should have a named owner.

For each case, collect coincident input kW and the applicable power-factor basis. For sinusoidal loads with different power factors, an engineer can combine their active and signed reactive power before deriving the total apparent power. An arithmetic average of the individual power factors generally gives the wrong result. For heavily distorted or unbalanced loading, provide measured or calculated phase currents and the underlying load data for engineering review.

Move from calculated demand to a purchase rating

A continuous kVA figure leaves several questions open. Does the owner require the same output with one transformer out of service? Is future expansion already in the duty list? What site temperature, altitude and cooling arrangement apply? Separate a stated expansion allowance from a bidder’s rounding to a standard size. Otherwise procurement can add the same margin twice and compare two offers against different requirements.

Motor starting also needs its own check. A brief high-current event can cause unacceptable voltage drop even when the continuous load fits. Give the engineer the starting method, simultaneous running load and allowable voltage dip. Purchasing a larger unit solely on the starting kVA figure can miss constraints elsewhere in the network.

For a 2500 kVA oil-immersed transformer quotation or another project-specific rating, send Zisheng Electric the electrical load list, operating cases, voltages and site conditions. Ask for the continuous rating and the assumptions behind it. An evaluator can then trace each offered kVA value to a plant duty and see immediately which uncertainty still needs a decision.