Since your search for convert kva to amps per phase didn't specify a transformer or generator size, I am using a standard commercial baseline of 100 kVA. On a 480V 3-phase system, 100 kVA equals exactly 120.3 Amps per phase. On a 208V 3-phase system, that same 100 kVA yields 277.6 Amps per phase.
The formula used to derive the 480V answer is: Amps = (100 × 1000) / (1.732 × 480) = 120.28A. If you are working with a different kVA rating, the math scales linearly, but your voltage and phase topology are the fixed assumptions that dictate the final number. Below is the complete breakdown of how to calculate this, how the numbers shift across common voltages, and exactly what breaker and wire to pull for your build.
The Core Formula and Fixed Assumptions
To convert kVA (kilovolt-amperes) to amps, you only need two fixed assumptions: system voltage and phase topology (single-phase vs. three-phase).
Here are the exact formulas with our 100 kVA baseline substituted:
- 3-Phase Formula:
I = (kVA × 1000) / (√3 × V)
Substituted:I = (100 × 1000) / (1.732 × 480) = 120.28A - 1-Phase Formula:
I = (kVA × 1000) / V
Substituted:I = (100 × 1000) / 240 = 416.67A
The square root of 3 (1.732) accounts for the 120-degree phase shift between the three legs in a 3-phase system, effectively distributing the load and lowering the amperage required per individual conductor compared to single-phase.
Neighboring Values Reference Table (±20% Range)
If your equipment isn't exactly 100 kVA, use this ±20% reference table to find your amperage per phase without reaching for a calculator. These values assume a standard commercial 3-phase supply.
| Transformer/Generator Size (kVA) | Amps Per Phase @ 208V 3-Phase | Amps Per Phase @ 480V 3-Phase |
|---|---|---|
| 80 kVA | 222.1 A | 96.2 A |
| 90 kVA | 249.9 A | 108.3 A |
| 100 kVA (Baseline) | 277.6 A | 120.3 A |
| 110 kVA | 305.3 A | 132.3 A |
| 120 kVA | 333.1 A | 144.3 A |
How the Answer Shifts: 120V vs 240V vs 3-Phase
Voltage is the denominator in your equation. As voltage drops, amperage spikes to deliver the same apparent power. Here is how a 50 kVA load shifts across common residential and light-commercial topologies:
- 120V Single-Phase:
(50 × 1000) / 120= 416.7 Amps. (Requires massive parallel conductors; rarely used for 50 kVA loads). - 240V Single-Phase:
(50 × 1000) / 240= 208.3 Amps. (Standard for large residential backup generators). - 208V 3-Phase:
(50 × 1000) / (1.732 × 208)= 138.8 Amps. (Common in commercial strip malls and office parks). - 480V 3-Phase:
(50 × 1000) / (1.732 × 480)= 60.1 Amps. (Standard industrial and heavy commercial).
The takeaway: pushing the same kVA through a 3-phase 480V system requires roughly 70% less current per leg than pushing it through a 1-phase 240V system. This is why industrial facilities use high-voltage 3-phase—it drastically reduces copper costs and I²R heating losses.
When This Conversion is Meaningless
Blindly converting numbers without context leads to tripped breakers or melted lugs. This conversion is entirely meaningless in two specific scenarios:
- When your nameplate says kW, not kVA: If a motor or heater bank is rated in kilowatts (real power), you must know the Power Factor (PF) to find the current. As noted by Fluke's electrical training guides, a 50 kW motor with a 0.85 PF actually draws 58.8 kVA of apparent power from the grid. If you calculate amps based on 50 kVA, your wires will overheat.
- When the system voltage is unknown or unverified: A 100 kVA transformer tapped for a 240V delta secondary will output 240.6A per phase. That exact same physical transformer, re-tapped for 480V, outputs 120.3A. Never assume voltage based on the kVA rating alone; always verify with a multimeter at the lugs before sizing wire.
Decision Path: Sizing Your Breaker and Wire
Knowing the amps per phase is only step one. To actually build the circuit, you must apply NFPA 70 (National Electrical Code) derating and continuous load rules. Use this decision tree to terminate your math into a concrete hardware pick.
| Step | Logic / Calculation | Result for 100 kVA @ 480V 3-Phase |
|---|---|---|
| 1. Base Amps | Calculate raw amps per phase (from formula) | 120.3 A |
| 2. Continuous Load Multiplier | If load runs ≥3 hours, multiply by 1.25 (NEC 210.20) | 120.3 × 1.25 = 150.37 A |
| 3. Breaker Sizing | Round UP to next standard breaker size (NEC 240.6) | 175 A Breaker (or 150A if non-continuous) |
| 4. Wire Ampacity | Select wire rated ≥ breaker size at 75°C column | 175 A requires 2/0 AWG Copper |
| 5. Voltage Drop Check | If run > 100ft, upsized wire may be required | Assume <100ft for baseline |
Quick Conversion Edge Cases (FAQ)
Q: Do I count the neutral wire when calculating amps per phase?
A: No. In a balanced 3-phase wye system, the neutral carries near-zero current. The 'amps per phase' calculation applies only to the three hot legs (L1, L2, L3). However, the neutral must still be sized per NEC 220.61 based on the maximum unbalanced load.
Q: My generator says 100 kVA, but the breaker keeps tripping at 110 Amps on a 480V system. Why?
A: Generators are rated for apparent power (kVA), but their engine prime movers are limited by real power (kW). If your load has a terrible power factor (e.g., 0.7), you are pulling 120A (100kVA) but the engine is only producing 70kW of usable work. The alternator might be hitting its thermal limit or the voltage regulator is collapsing. Check your PF and add power factor correction capacitors if necessary.
Q: Can I use aluminum wire instead of copper for these ampacities?
A: Yes, but you must increase the gauge. For the 175A continuous requirement above, you would need to step up to 4/0 AWG Aluminum XHHW-2 (rated 180A in the 75°C column). Always verify terminal lug ratings; many breakers under 100A are only rated for 60°C copper, which severely restricts aluminum use.
