Transformer Sizing Calculator
Calculate the required transformer kVA rating based on load requirements.
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How to Use This Calculator
Enter the total connected load in kVA or kW. If entering kW, provide the power factor to convert to kVA using the formula kVA = kW / power factor. A typical commercial power factor is 0.85 to 0.90. Select the voltage configuration: primary and secondary voltages such as 480V to 208/120V for commercial three phase or 7200V to 240/120V for residential single phase. Enter the demand factor if applicable, which reduces the connected load to the actual expected demand. The calculator determines the minimum transformer kVA rating and recommends the next standard transformer size. For example, with a connected load of 75 kW at 0.85 power factor, the apparent power is 88.2 kVA. With a demand factor of 0.80, the demand is 70.6 kVA. The calculator recommends a 75 kVA transformer as the next standard size.
Understanding the Concept
Transformers convert electrical power from one voltage level to another and must be sized to handle the full demand of all circuits they supply. Undersized transformers overheat, experience voltage regulation problems, and have shortened lifespans. Oversized transformers waste capital and have higher no load losses from magnetizing current. Transformer sizing is based on apparent power measured in kVA (kilovolt amperes), not real power in kW, because the transformer must supply both the real and reactive components of the load current. The relationship between kW and kVA depends on the power factor of the load. Standard transformer sizes follow a series: 15, 25, 37.5, 50, 75, 100, 112.5, 150, 225, 300, 500, 750, and 1000 kVA for dry type three phase units. NEC 450.3 governs transformer overcurrent protection, with different requirements for primary only protection versus primary and secondary protection. The transformer must also be derated for high ambient temperatures and high altitude installations per manufacturer specifications.
The Formula Explained
The transformer sizing formula is Required kVA = (Total Connected Load in kW x Demand Factor) / Power Factor. The demand factor accounts for the fact that not all loads operate simultaneously, similar to the service load calculation. Typical demand factors range from 0.70 to 0.90 depending on the facility type. Power factor converts the real power demand to apparent power. Once you have the required kVA, select the next standard transformer size that meets or exceeds the demand. For three phase systems, the line current on each side is calculated as I = kVA x 1000 / (V x 1.732). For single phase, I = kVA x 1000 / V. NEC 450.3(B) allows primary only overcurrent protection for transformers rated 600V or less at a maximum of 125% of the transformer primary full load current. If 125% does not correspond to a standard fuse or breaker rating, the next higher standard size is permitted up to 250% for fuses.
Frequently Asked Questions
What size transformer do I need for a 200 amp panel?
For a 200A, 208/120V three phase panel, the maximum load is 200 x 208 x 1.732 = 72.1 kVA. A 75 kVA transformer would handle this load. However, it is better practice to apply the actual calculated load with demand factors rather than sizing based on panel amperage alone. The panel rating represents the maximum capacity of the bus, not the actual load. A load calculation per NEC 220 often shows the actual demand is 60 to 80% of the panel capacity.
What is the difference between kW and kVA?
kW (kilowatts) measures real power, which is the actual energy consumed and converted to useful work or heat. kVA (kilovolt amperes) measures apparent power, which is the total power the electrical system must deliver, including reactive power used to maintain magnetic fields in motors and transformers. The relationship is kVA = kW / power factor. A facility with a 0.85 power factor consuming 85 kW of real power requires 100 kVA of apparent power from the transformer. Transformers are always rated in kVA because they must handle the full current regardless of power factor.
Can I run a transformer at full rated capacity continuously?
Dry type transformers are rated for continuous operation at their nameplate kVA in a 40C ambient environment. However, loading a transformer to 100% continuously shortens its lifespan due to sustained thermal stress on the insulation. Best practice is to size transformers at 80% of their rated capacity for continuous loads, leaving margin for load growth and occasional peak demands. Liquid filled transformers have better cooling and can handle overloads more gracefully, but sustained overloading still degrades insulation life.
How do I calculate transformer overcurrent protection per NEC?
NEC 450.3(B) covers transformers rated 600V or less. With primary only protection, the overcurrent device must not exceed 125% of the transformer rated primary current. If 125% does not match a standard device rating, the next higher standard rating is permitted. With both primary and secondary protection, the primary device is set at a maximum of 250% and the secondary at 125% of the respective rated currents. For a 75 kVA, 480V primary transformer, the primary current is 90.2A, so primary only protection would be set at 90.2 x 1.25 = 112.8A, rounded up to a 125A breaker.
What is transformer impedance and why does it matter?
Transformer impedance, expressed as a percentage (typically 2% to 6% for dry type), represents the voltage drop across the transformer at full load and determines the maximum available fault current on the secondary side. Lower impedance means better voltage regulation but higher available fault current, which requires higher rated downstream equipment. Higher impedance limits fault current but causes more voltage drop under load. The available fault current on the secondary is approximately calculated as Secondary Full Load Amps / (Percent Impedance / 100). This value is critical for selecting properly rated breakers and equipment per NEC 110.9 and 110.10.