To convert a benchmark 100 kVA to amps on a standard 480V three-phase system, the exact answer is 120.28 amps. The formula used is I = (kVA × 1000) / (√3 × V), which substitutes as I = (100 × 1000) / (1.732 × 480). If your system voltage or phase configuration differs, the amperage shifts inversely. This guide provides the exact mathematical framework, a quick-reference table for neighboring transformer sizes, and the critical assumptions that dictate whether your overcurrent protection will hold or trip under load.
The Core kVA to Amps Formula for Three-Phase Systems
The universal formula for converting apparent power (kVA) to current (Amps) in a balanced three-phase system relies on the line-to-line voltage and the square root of 3.
I = (S × 1000) / (1.732 × VL-L)Where I is current in Amps, S is apparent power in kVA, and VL-L is line-to-line voltage.
The constant 1.732 (the square root of 3) accounts for the 120-degree phase shift between the three conductors. Because kVA measures apparent power, this calculation assumes a balanced load across all three phases. The primary assumption fixing this answer is that the voltage variable (V) represents the line-to-line voltage (e.g., 480V), not the line-to-neutral voltage (e.g., 277V). Confusing these two values is the most common reason DIYers and junior technicians miscalculate wire and breaker sizing.
Quick Reference Table: 80 to 120 kVA at 480V
When sizing feeders or primary overcurrent protection for a 480V three-phase transformer or generator, you rarely deal with a single static number. Below is a reference table covering a ±20% range around the common 100 kVA benchmark.
| Transformer / Generator Size (kVA) | Full Load Amps (3-Phase @ 480V) | NEC 125% Continuous Load Max | Typical Copper Wire Size (THHN, 75°C) |
|---|---|---|---|
| 80 kVA | 96.23 A | 76.98 A | 3 AWG |
| 90 kVA | 108.26 A | 86.60 A | 2 AWG |
| 100 kVA | 120.28 A | 96.22 A | 1 AWG |
| 110 kVA | 132.31 A | 105.84 A | 1/0 AWG |
| 120 kVA | 144.34 A | 115.47 A | 1/0 AWG |
Note: Wire sizes assume 75°C terminations and standard 30°C ambient temperature per NFPA NEC Article 310. Always verify local ampacity derating if bundling more than three current-carrying conductors in a single raceway.
How the Answer Shifts Across Common 3-Phase Voltages
Amperage is inversely proportional to voltage. If you take that same 100 kVA load and apply it to different three-phase system voltages, the current shifts dramatically. Presenting a single-voltage answer as universal is a critical error in electrical design.
- At 120V (3-Phase): 481.12 Amps. Context: True 120V three-phase systems are virtually non-existent in modern commercial wiring. You will usually see 240V Delta systems with a 120V center-tapped "wild leg" for single-phase loads. If you are measuring 120V line-to-neutral on a 208V Wye system, you must use 208V in the formula, not 120V.
- At 208V (3-Phase): 277.58 Amps. Context: Standard for commercial office buildings and light retail in North America. Requires significantly thicker conductors (e.g., 300 kcmil copper) than higher voltage systems.
- At 230V / 240V (3-Phase): 240.56 Amps (at 240V). Context: Common in European industrial settings (230V/400V Wye) and older North American manufacturing. Note that in a 400V Wye system, 230V is the line-to-neutral voltage; you must use 400V in the three-phase formula, which drops the current to 144.34 Amps.
- At 480V (3-Phase): 120.28 Amps. Context: The North American industrial standard. Higher voltage allows for smaller, cheaper copper conductors over long feeder runs, which is why large facilities step down from 480V to 208V/120V at the point of use.
When the Conversion is Meaningless: The kW vs kVA Trap
The kVA to Amps conversion becomes mathematically meaningless—and practically dangerous—under two specific conditions:
- You actually have kW, not kVA, and lack the Power Factor (PF). kVA is apparent power (the total power supplied). kW is real power (the work actually done). If your equipment nameplate lists 100 kW, you cannot use the formula above without knowing the Power Factor. The formula for kW requires multiplying the denominator by the PF:
I = (kW × 1000) / (1.732 × V × PF). If a motor has a PF of 0.80, a 100 kW load draws 150 Amps, not 120 Amps. Sizing a breaker based on kVA math when you actually have kW data will result in an undersized breaker and immediate tripping. For a deeper dive into this relationship, refer to Electrical4U's power conversion guides. - You confuse Line-to-Line with Line-to-Neutral voltage. If you measure 277V to ground on a 480V system and plug 277 into the three-phase formula, your calculated amperage will be artificially inflated by a factor of 1.732. The three-phase formula strictly requires the phase-to-phase (line-to-line) voltage.
Frequently Asked Questions
How do I calculate 3-phase kVA to amps if I only have line-to-neutral voltage?
If you only know the line-to-neutral voltage (VL-N) of a balanced Wye system, you must first calculate the line-to-line voltage by multiplying by the square root of 3 (1.732). For example, if your line-to-neutral voltage is 277V, your line-to-line voltage is 277 × 1.732 = 480V. You then use 480V in the standard three-phase formula. Alternatively, you can use the modified single-phase equivalent formula: I = (kVA × 1000) / (3 × VL-N).
Does power factor matter when converting kVA to amps?
No. Power factor (PF) is entirely irrelevant when converting kVA to amps. kVA already represents the apparent power, which includes both the real power (kW) and the reactive power (kVAR). The power factor is only required when you are converting from kW (real power) to amps. This is a common point of confusion for hobbyists and junior engineers reading motor nameplates.
What is the kVA to amps formula for a single-phase system?
For a single-phase system, the √3 constant is removed because there is no phase shift to account for. The formula simplifies to: I = (kVA × 1000) / V. For example, a 50 kVA single-phase transformer at 240V yields: (50 × 1000) / 240 = 208.33 Amps.
Why is my breaker tripping if my calculated kVA amps are below the rating?
If your math is correct but the breaker still trips, you are likely dealing with inrush current or a continuous load violation. Transformers and large inductive motors draw massive inrush currents (often 8 to 12 times the full load amperage) for the first few cycles upon energization. Standard thermal-magnetic breakers may interpret this as a fault. Furthermore, under NEC Article 210.20, if your 120.28A load runs for 3 hours or more (a continuous load), you must size the breaker at 125% of the load (150A minimum). A standard 125A breaker will eventually trip on a continuous 120A load due to thermal buildup in the bimetallic strip.
