# Why Building Owners Overload Transformers Without Realizing It

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A transformer nameplate says 500 kVA. An electrical contractor adds up the building loads, gets 480 kW, and confirms there is room to spare. Within weeks of occupancy, breakers start tripping.

The problem is the unit mismatch between what the transformer delivers and what the equipment consumes.

## kVA is not kW

kVA is apparent power — the total demand the transformer must supply, including both real power (useful work) and reactive power (magnetic fields in motors).

kW is real power — the portion that performs useful work.

The relationship:

**kW = kVA × Power Factor**

Power factor ranges from 0 to 1.0 depending on the type of loads connected. Resistive loads (heaters, incandescent lights) run at PF close to 1.0. Induction motors run at 0.75 to 0.90. Mixed commercial buildings typically fall between 0.80 and 0.90.

## What 500 kVA actually delivers

At power factor 1.0 (all resistive): 500 kW. This almost never happens in real buildings.

At power factor 0.90 (mixed commercial): 450 kW.

At power factor 0.85 (motors + mixed): 425 kW.

At power factor 0.80 (motor-heavy): 400 kW.

The building with 480 kW of load and a 500 kVA transformer is 55 kW over capacity at PF 0.85. The overcurrent protection trips because the transformer sees 480 / 0.85 = 565 kVA of apparent power against a 500 kVA rating.

## Why this keeps happening

Three reasons:

**First**, the nameplate shows one number (kVA) and most people read it as capacity in kW. The distinction is not intuitive unless you work with power systems regularly.

**Second**, load schedules in building design often list equipment in kW without converting to kVA. The mechanical engineer specifies 150 kW of chiller load. The electrical engineer sees 150 kW and allocates 150 kVA on the transformer schedule. But the chiller runs at PF 0.85, so it actually demands 176 kVA. Every motor-driven load is underestimated by the same ratio.

**Third**, buildings add loads after construction. A tenant installs a server room. Another adds a commercial kitchen. Each addition is checked against the "remaining kW capacity" without recalculating the kVA demand with the new composite power factor.

## Additional derating factors

Power factor is not the only reason a transformer delivers less than its nameplate:

*   **Ambient temperature** above 40C (104F) requires derating per ANSI standards. A transformer in an unconditioned mechanical room in a hot climate may lose 5 to 10 percent of capacity.
    
*   **Harmonic loads** from VFDs, LED drivers, and switching power supplies generate non-sinusoidal current that causes additional heating. Transformers serving heavy harmonic loads need K-rated designs or derating factors of 10 to 15 percent.
    
*   **Altitude** above 1,000 meters reduces cooling effectiveness due to thinner air. IEEE recommends derating above this elevation.
    

A 500 kVA transformer in a hot room with significant VFD loads at PF 0.85 might effectively deliver 350 kW — 30 percent less than the nameplate suggests.

## The correct sizing method

Start with the total kW demand. Divide by the expected power factor to get the required kVA. Then select the next standard transformer size above that number.

For 480 kW at PF 0.85:

Required kVA = 480 / 0.85 = 565 kVA

Standard sizes: 300, 500, 750, 1000 kVA.

Select 750 kVA. That gives 565 kVA demand against 750 kVA capacity — 25 percent margin for future load growth and derating factors.

Selecting a 500 kVA unit saves money on the transformer but costs far more in troubleshooting, load shedding, and eventual replacement when the building fills up.