If you are sizing a panel for a 100 kVA load at 480V 3-phase, the exact current is 120.28 Amps. If that same 100 kVA load is on a 208V 3-phase system, it pulls 277.58 Amps, and on a 240V single-phase line, it draws 416.67 Amps. The governing formula for 3-phase is Amps = (kVA × 1000) / (√3 × Volts), which substitutes to Amps = 100,000 / (1.732 × 480) = 120.28A. For single-phase, simply drop the √3 (1.732) multiplier. Notice what is entirely missing from these formulas? Power factor. Unlike kW (real power), kVA is apparent power, meaning your conversion to Amps is fixed strictly by voltage and phase configuration, regardless of how inductive the load is.
The Core kVA to Amps Formulas (And the Power Factor Trap)
The most common mistake DIYers and junior techs make when sizing transformers or feeders is trying to factor in Power Factor (PF) when converting kVA to Amps. Apparent power (kVA) is the vector sum of real power (kW) and reactive power (kVAR). Because the kVA value already accounts for the phase angle difference between voltage and current, you do not need to adjust for PF to find the amperage.
Single-Phase Formula:
Amps = (kVA × 1000) / Volts
Three-Phase Formula:
Amps = (kVA × 1000) / (1.732 × Volts)
100 kVA Reference Table (480V 3-Phase ±20%)
Commercial and light industrial environments heavily rely on 480V 3-phase power. Below is a quick-reference table for a 100 kVA baseline, expanding ±20% to cover common neighboring transformer sizes (80 kVA to 120 kVA). We have also included the 125% continuous load multiplier required by NEC Article 215.2 for sizing conductors that will carry the load for 3 hours or more.
| Transformer Size (kVA) | Base Amperage (480V 3Φ) | 125% Continuous Load Amps | Standard Breaker Size (Next Size Up) |
|---|---|---|---|
| 80 kVA | 96.23 A | 120.28 A | 125 A or 150 A |
| 90 kVA | 108.26 A | 135.32 A | 150 A |
| 100 kVA | 120.28 A | 150.35 A | 175 A |
| 110 kVA | 132.31 A | 165.38 A | 175 A |
| 120 kVA | 144.34 A | 180.42 A | 200 A |
How Voltage and Phase Shift the Answer
Amperage has an inverse relationship with voltage. When you step down voltage, your current spikes proportionally. This is why a 50 kVA transformer feeding a 120/240V residential split-phase panel requires massive secondary conductors compared to its 480V primary feed.
- 120V Single-Phase: Used for standard receptacles. A 10 kVA load here pulls a massive 83.33 Amps. This is why we don't run high-kVA loads on standard 120V branch circuits.
- 230V/240V Single-Phase: Common for residential ranges, dryers, and subpanels. A 10 kVA load drops to 41.67 Amps, easily handled by 6 AWG copper and a 50A breaker.
- 208V Three-Phase: Standard for commercial wye-configured panels. A 10 kVA load draws 27.76 Amps. The √3 multiplier does the heavy lifting to keep current manageable.
- 480V Three-Phase: The industrial standard. A 10 kVA load pulls just 12.03 Amps, allowing for long feeder runs with minimal voltage drop and smaller wire gauges.
Decision Tree: Sizing Your Breaker and Wire
Knowing the exact amperage is only half the job. You must translate that number into physical copper and a protective device. Use this decision path to terminate your math into a concrete parts list.
| Step | Condition / Calculation | Action / Concrete Pick |
|---|---|---|
| 1. Calculate Base Amps | 100 kVA @ 480V 3Φ = 120.28 A | Record 120.28 A as your baseline. |
| 2. Apply NEC 125% Rule | Is the load continuous (3+ hours)? | If YES: 120.28 × 1.25 = 150.35 A. If NO: Stay at 120.28 A. |
| 3. Select Conductor | Need wire rated for 150.35 A (75°C column) | Pick: 1/0 AWG THHN Copper (Rated 150A at 75°C, but bump to 1/0 for terminal temperature limits and voltage drop margin on runs over 50ft). |
| 4. Select Breaker | Next standard size up per NEC 240.4(B) | Pick: 175A 3-Pole Molded Case Breaker (Standard sizes: 150, 175, 200. 150A is too close to 150.35A continuous limit). |
When the Conversion Becomes Meaningless
There is one specific scenario where attempting to calculate Amps from your available data is mathematically meaningless: when you only have a kW rating and the Power Factor is unknown.
If you are staring at a motor nameplate or an old HVAC spec sheet that lists 50 kW, but the PF is rubbed off or unlisted, you cannot accurately convert to Amps. The formula kVA = kW / PF requires that denominator. Guessing the PF can lead to catastrophic undersizing. An industrial motor might have a PF of 0.85, while a heavily loaded server UPS might sit at 0.98.
The Fix: If PF is unknown, do not guess. You must either clamp the line with a True-RMS power analyzer (like a Fluke 435) to measure the actual kVA and Amps directly under load, or you must assume a conservative, worst-case industrial PF of 0.80 to size your conductors safely. Assuming 0.80 forces your calculated kVA higher, which in turn sizes your wire thicker, keeping you on the safe side of thermal limits.
Quick FAQ
Do I need to derate for ambient temperature?
Yes. The 1/0 AWG THHN pick above assumes an ambient temperature of 30°C (86°F). If your panel is in a boiler room or on a roof where ambient temps hit 40°C (104°F), NEC Table 310.15(B)(16) requires a derating factor of 0.82 for 90°C wire. You would need to step up to 2/0 AWG copper to maintain your ampacity.
What size transformer do I need for a 200 Amp panel?
Working backward: 200 Amps at 208V 3-phase is (200 × 208 × 1.732) / 1000 = 72.05 kVA. The next standard commercial transformer size up is 75 kVA. If your panel is 480V 3-phase, 200 Amps equates to (200 × 480 × 1.732) / 1000 = 166.27 kVA, requiring a 167 kVA (often sold as 150 kVA or 225 kVA depending on manufacturer standard increments) transformer.
