A transformer changes voltage and current in opposite directions. For the same apparent power, raising voltage lowers current; lowering voltage raises current. The current value produced from transformer kVA and voltage is used to establish a starting point for feeder or secondary-conductor sizing, raceway planning, overcurrent-protection review, voltage-drop calculations, and equipment load coordination.
A step-down transformer supplies lower voltage on its secondary, so its secondary conductors carry more amps than its primary conductors. A step-up transformer does the reverse. The transformer does not create additional apparent power in the ideal calculation; it exchanges voltage for current.
Transformer Voltage Ratio
The voltage ratio compares the primary voltage with the secondary voltage:
\(\displaystyle \text{Voltage Ratio} = \frac{V_P}{V_S}\)
Where:
- \(V_P\) = primary voltage
- \(V_S\) = secondary voltage
For an ideal transformer, current changes by the inverse of that voltage ratio:
\(\displaystyle \frac{I_S}{I_P} = \frac{V_P}{V_S}\)
Where:
- \(I_P\) = primary current
- \(I_S\) = secondary current
If primary voltage is 480 V and secondary voltage is 208 V:
\(\displaystyle \frac{480}{208} = 2.3077\)
The secondary current is therefore about 2.31 times the primary current. The lower-voltage side has the higher current.
Use the Transformer Voltage Ratio Calculator to calculate and verify the voltage relationship between the transformer’s primary and secondary sides.
Current From kVA and Voltage
Calculate transformer current separately at each voltage. The voltage ratio is a useful check, but the kVA-and-voltage calculation produces the actual amp value needed for circuit planning.
For a single-phase transformer:
\(\displaystyle I = \frac{\text{kVA} \times 1{,}000}{V}\)
For a three-phase transformer using line-to-line voltage:
\(\displaystyle I = \frac{\text{kVA} \times 1{,}000}{\sqrt{3} \times V}\)
The same transformer kVA is used on both sides. Only the voltage changes, so the calculated current changes with it.
Use the Transformer Current Calculator to calculate amps from the transformer rating and the applicable voltage. Use the Transformer kVA Calculator when kVA must be checked from the known volts and amps.
Three-Phase Calculation Example
Consider a 75 kVA, 480 V to 208 V, three-phase transformer.
Primary current:
\(\displaystyle I_P = \frac{75{,}000}{\sqrt{3} \times 480}\)
\(\displaystyle I_P = 90.2\text{ A}\)
Secondary current:
\(\displaystyle I_S = \frac{75{,}000}{\sqrt{3} \times 208}\)
\(\displaystyle I_S = 208.2\text{ A}\)
| Transformer side | Voltage | Apparent power | Calculated current |
|---|---|---|---|
| Primary | 480 V | 75 kVA | 90.2 A |
| Secondary | 208 V | 75 kVA | 208.2 A |
Now compare the voltage and current ratios:
\(\displaystyle \frac{V_P}{V_S} = \frac{480}{208} = 2.3077\)
\(\displaystyle \frac{I_S}{I_P} = \frac{208.2}{90.2} = 2.3077\)
The ratios match. Reducing voltage from 480 V to 208 V increases current from 90.2 A to 208.2 A at the same 75 kVA.
That secondary current is the starting electrical load value for the secondary feeder or transformer secondary conductors. It also affects conductor AWG or kcmil selection, equipment terminal space, lug ratings, raceway fill, pull planning, bend layout, and voltage-drop review.
Primary and Secondary Conductor Planning
Primary and secondary conductors cannot be selected by transformer kVA alone. The voltage on each side determines the current, and each side must be evaluated using its own calculated amp value.
For the 75 kVA example:
- The 480 V primary is based on approximately 90.2 A.
- The 208 V secondary is based on approximately 208.2 A.
- The secondary conductor set will generally require substantially more ampacity and physical raceway capacity than the primary conductors.
- Parallel conductors, larger raceways, larger wire-bending space, and higher available fault-duty considerations may become more relevant on the lower-voltage, higher-current side.
A transformer secondary serving motor loads also requires a separate review of motor full-load current, starting characteristics, feeder loading, voltage drop, and the selected overcurrent-protection arrangement. Transformer full-load current is not automatically the final design current for every connected load condition.
Field Verification
The ideal voltage-current relationship is appropriate for checking transformer ratios and estimating full-load current from kVA. Actual transformer and installation decisions require additional review.
Verify:
- Transformer nameplate kVA, primary voltage, secondary voltage, phase, and connection.
- Whether the stated voltage is line-to-line or line-to-neutral for the calculation being performed.
- Actual system voltage and expected voltage regulation under load.
- Conductor ampacity using the applicable insulation temperature rating, terminal rating, correction factor, adjustment factor, and number of current-carrying conductors.
- Secondary conductor routing, raceway fill, physical bending space, and voltage drop.
- Manufacturer instructions, available fault current, protective-device coordination, and AHJ requirements.
Transformer losses, impedance, regulation, winding connection, harmonics, and manufacturer limitations are not represented by the ideal inverse voltage-current relationship. Use the calculated amps as the electrical starting value, then complete the equipment and installation review using the actual project conditions.