A 100 kVA load on a standard 480V 3-phase system draws exactly 120.28 Amps. If you are working with a 208V 3-phase system, that same 100 kVA load draws 277.58 Amps. The universal formula to find this is Amps = (kVA × 1000) / (√3 × Voltage). Substituting our baseline values: 120.28 = (100 × 1000) / (1.732 × 480). You do not need to know the power factor (PF) for this calculation, because kVA measures apparent power, not real power (kW).
The Core Formula and What Fixes the Answer
When converting kilovolt-amps to amperes in a 3-phase system, the math is anchored by two fixed assumptions: line-to-line voltage and phase count. The square root of 3 (approximately 1.732) is a geometric constant derived from the 120-degree phase shift between the three legs.
Amps = (kVA × 1000) / Voltage) for 3-phase equipment. Forgetting the 1.732 divisor will result in an ampacity calculation that is 73% too high, leading you to massively oversize your conductors and breakers.
The formula is strictly:
I = (S × 1000) / (√3 × V)
- I = Current in Amps
- S = Apparent Power in kVA
- V = Line-to-Line Voltage (e.g., 480V, 208V)
- √3 = 1.732
Notice what is missing: Power Factor (PF). Because kVA is the vector sum of real power (kW) and reactive power (kVAR), it already accounts for the total apparent current flowing through the wires. According to the NFPA National Electrical Code (NEC), conductor ampacity and overcurrent protection must be sized on the actual current flowing (Amps derived from kVA), regardless of how much of that current is doing useful work.
3-Phase kVA to Amps Reference Chart (±20% Range)
Below is a quick-reference spec sheet for common dry-type transformer sizes and heavy 3-phase motor loads, centered around the 100 kVA benchmark. This covers the standard ±20% variance you will encounter when sizing feeders for fluctuating industrial loads.
| Apparent Power (kVA) | Amps @ 480V 3-Phase | Amps @ 208V 3-Phase | Standard Breaker Size (480V)* |
|---|---|---|---|
| 80 kVA | 96.2 A | 222.1 A | 125 A |
| 90 kVA | 108.3 A | 249.9 A | 150 A |
| 100 kVA | 120.3 A | 277.6 A | 175 A |
| 110 kVA | 132.3 A | 305.3 A | 175 A |
| 120 kVA | 144.3 A | 333.1 A | 200 A |
*Breaker sizes assume a continuous load multiplier (125%) per NEC Article 210.20, rounded up to the next standard size per NEC 240.6.
How the Answer Shifts: 120V vs 230V vs 3-Phase
Voltage is the denominator in your equation. As voltage drops, current spikes to deliver the same apparent power. Here is how a fixed 50 kVA load behaves across different common service entrances:
- 120V Single-Phase:
(50 × 1000) / 120= 416.7 Amps. (Requires massive parallel conductors; rarely used for this capacity). - 240V Single-Phase:
(50 × 1000) / 240= 208.3 Amps. (Common for large residential or light commercial split-phase panels). - 208V 3-Phase:
(50 × 1000) / (1.732 × 208)= 138.8 Amps. (Standard commercial wye configuration). - 480V 3-Phase:
(50 × 1000) / (1.732 × 480)= 60.1 Amps. (Standard industrial delta/wye configuration).
The shift from 208V to 480V 3-phase cuts the current by more than half. This is why industrial facilities step up to 480V for distribution: it allows the use of significantly smaller, cheaper copper wire and reduces I²R (heat) losses over long conduit runs.
Decision Path: Sizing Your Breaker and Wire
Knowing the ampacity is only step one. To actually terminate this in a panel, follow this decision tree to select your overcurrent protective device (OCPD) and conductor size. We will use our baseline 100 kVA @ 480V (120.28A) as the continuous load example.
Step 1: Is the load continuous (running 3 hours or more)?
- YES: Multiply the calculated Amps by 1.25. (120.28A × 1.25 = 150.35A).
- NO: Use the base Amps. (120.28A).
Step 2: Select the Breaker (OCPD)
- IF your Step 1 value is exactly a standard size (e.g., 150A), THEN pick that breaker.
- IF your Step 1 value falls between standard sizes (like our 150.35A), THEN round UP to the next standard NEC 240.6 size. Result: 175A Molded Case Breaker.
Step 3: Select the Wire (Based on Breaker and Terminal Ratings)
- IF your breaker and equipment lugs are rated 75°C (standard for modern industrial gear), THEN look at the NEC 310.16 75°C column.
- IF the breaker is 175A, THEN select the wire with an ampacity of at least 175A. Result: 2/0 AWG Copper THHN (rated 175A at 75°C).
Final Concrete Pick: For a 100 kVA continuous 3-phase load at 480V, purchase a 175A 3-pole breaker and pull 2/0 AWG copper THHN conductors (plus an appropriately sized equipment grounding conductor). For deeper transformer-specific secondary protection rules, refer to the Electrical Technology kVA to Amps Guide and NEC Article 450.
When This Conversion is Meaningless (The kW Trap)
The kVA to Amps conversion becomes entirely meaningless if the value you were handed is actually kW (kilowatts), not kVA.
In the real world, equipment nameplates and utility bills often mix these up. kW measures real power (the work actually being done, like turning a motor shaft). If your load is specified in kW, you must know the Power Factor (PF) to find the current. The formula shifts to:
Amps = (kW × 1000) / (√3 × Voltage × PF)
If a manufacturer tells you a machine uses '50 kVA' but they actually mean '50 kW at a 0.8 PF', the true apparent power is 62.5 kVA. If you size your wire for 50 kVA (60 Amps at 480V) instead of 62.5 kVA (75 Amps at 480V), your 1/0 AWG wire will run hot, the voltage drop will exceed 3%, and your breaker will nuisance-trip under load. Always verify if the nameplate reads kVA (apparent) or kW (real) before pulling wire.
