For the same apparent power in a balanced three-phase system, current is inversely proportional to line-to-line voltage. A 400 V circuit therefore carries 20% more current than a 480 V circuit at the same kVA. Looking in the opposite direction, the 480 V current is 16.67% lower than the 400 V current. Both statements are correct because their reference values differ.
This is a calculation comparison, not advice to connect 400 V equipment to 480 V or a claim that one voltage is universally better. The equipment and installation must be assessed for their intended supply conditions.
The formula and the inputs that must match
For the balanced three-phase calculation, line current in amperes is:
I = S × 1,000 ÷ (√3 × VLL), where S is apparent power in kVA and VLL is line-to-line voltage in volts.
Schneider Electric’s transformer kVA and amperage FAQ gives the equivalent relationship between three-phase kVA, voltage and current. The Electrical Installation Guide identifies phase-to-phase voltage for its balanced three-phase expression. Those voltage labels matter: entering a line-to-neutral value into a field expecting line-to-line volts answers a different question.
To compare the voltages fairly, hold apparent power, phase arrangement and the meaning of the voltage measurement constant. If the actual equipment draws a different kVA under the alternative supply, the same-kVA comparison no longer describes that equipment’s operating change.
Worked example: 1,000 kVA at both voltages
At 400 V, current = 1,000 × 1,000 ÷ (√3 × 400) = approximately 1,443.4 A. At 480 V, current = 1,000 × 1,000 ÷ (√3 × 480) = approximately 1,202.8 A. The difference is approximately 240.6 A. These are line-current results, not currents to multiply by three again.
| Balanced three-phase apparent power | Current at 400 V | Current at 480 V |
|---|---|---|
| 100 kVA | 144.3 A | 120.3 A |
| 500 kVA | 721.7 A | 601.4 A |
| 1,000 kVA | 1,443.4 A | 1,202.8 A |
| 2,000 kVA | 2,886.8 A | 2,405.6 A |
Values are rounded to one decimal place. To reproduce a row, select three-phase in the kVA to amps calculator, enter the same kVA twice, and change only voltage from 400 to 480. Use the unrounded values if comparing small differences between scenarios.
Why 20% higher is not the same as 20% lower
The current ratio is I400 ÷ I480 = 480 ÷ 400 = 1.20. Relative to the 480 V result, the extra current is therefore 20%. For the reverse comparison, I480 ÷ I400 = 400 ÷ 480 = 0.833333. The reduction relative to the 400 V result is 1 − 0.833333 = 16.67%.
This ratio applies to every row in the table because the kVA term cancels. Writing the denominator beside a percentage prevents a common reporting mistake. A project note should say either “20% higher than the 480 V case” or “16.67% lower than the 400 V case,” not just “a 20% improvement.”
If you start with kW, power factor matters
The table starts from apparent power. It does not need an additional power-factor multiplier. If instead you know electrical input kW, first calculate kVA = kW ÷ PF using the appropriate power factor for the scenario. Do not use mechanical output kW without accounting for the electrical-input boundary.
For an original example, 900 kW at PF 0.90 is 1,000 kVA, so it uses the 1,000 kVA row. The same 900 kW at PF 1.00 is 900 kVA: approximately 1,299.0 A at 400 V and 1,082.5 A at 480 V. Keeping kW constant while changing power factor changes kVA and current. The kW to kVA calculator makes that intermediate assumption explicit.
What the comparison cannot decide
Lower calculated current does not by itself choose a cable, breaker, transformer or switchgear assembly. This calculation contains no installation method, conductor temperature limit, short-circuit duty, protection settings or equipment compatibility data. It also assumes a balanced three-phase load; individual phase currents need separate attention when the load is unbalanced.
Do not infer a facility energy-saving percentage from the current ratio. Energy consumption has not been calculated here. Similarly, a higher-voltage current result cannot be used as permission to reuse equipment marked for another voltage. Frequency, connection arrangement and the manufacturer’s documented operating range remain outside this arithmetic comparison.
A concise comparison record
- State the load in kVA, or retain the kW and power-factor inputs used to derive it.
- Label both voltages as line-to-line and confirm the balanced three-phase assumption.
- Keep the calculated current separate from any candidate equipment rating.
- Record whether a percentage uses the 400 V case or the 480 V case as its denominator.
- Send the scenario and actual equipment data for qualified engineering review before making installation changes.
For transformer research, continue with the transformer sizing calculator. It separates a load-side requirement from the rated current of a candidate nameplate size, which is a different question from the operating-load comparison above.
Sources & further reading
- Schneider Electric: calculating transformer kVA or amperage capacity ↗
- Electrical Installation Guide: Installed apparent power (kVA) ↗