100 kVA represents 100,000 volt-amperes of apparent power, and the exact amperage it delivers depends entirely on the system voltage and whether the supply is single-phase or three-phase. For a standard 3-phase 480V system, 100 kVA equals 120.3 amps, while a 3-phase 208V system yields 277.6 amps. This specific amperage value is the foundational metric that dictates your secondary conductor AWG, overcurrent protective device (OCPD) sizing, and busbar thermal limits in commercial and industrial installations. The most common confusion occurs when specifiers mix up kVA (apparent power) with kW (real power), falsely assuming a 100 kVA source can deliver 100 kW of pure resistive load without accounting for the system's power factor.
100 kVA to Amps Reference Table
Because apparent power (kVA) is a product of voltage and current, the physical amperage drops as your system voltage increases. The table below provides the exact Full Load Amps (FLA) for a 100 kVA source across standard North American commercial and industrial voltages. Use this as your baseline before applying NEC derating factors.
| System Configuration | Line-to-Line Voltage | Line-to-Neutral Voltage | Full Load Amps (FLA) | Common Application |
|---|---|---|---|---|
| Single-Phase (1P) | 120/240V | 120V | 416.7 A | Large residential / Small commercial split-phase |
| Single-Phase (1P) | 277V | 277V | 361.0 A | Commercial lighting arrays |
| Three-Phase (3P) Wye | 208Y | 120V | 277.6 A | Standard commercial office panels |
| Three-Phase (3P) Delta | 240V | 120V (High-leg) | 240.6 A | Legacy industrial motor loads |
| Three-Phase (3P) Wye | 480Y | 277V | 120.3 A | Modern commercial / Data center UPS |
| Three-Phase (3P) Wye | 600Y | 347V | 96.2 A | Canadian commercial / Heavy industrial |
Single-Phase:
I = 100,000 / VThree-Phase:
I = 100,000 / (V × √3) (where √3 ≈ 1.732)
Worked Numeric Example: Sizing a 480V Three-Phase Secondary
Let’s walk through a real-world installation scenario. You are wiring the secondary side of a 100 kVA, 3-phase, 480V dry-type transformer (such as a Square D EX480V series) feeding a continuous commercial HVAC load. Here is how you move from the kVA rating to the physical copper and breakers required by the 2026 NEC.
Step 1: Calculate the Base Full Load Amps (FLA)
Using the three-phase formula, we divide the total volt-amperes by the line-to-line voltage multiplied by the square root of 3:
I = 100,000 / (480 × 1.732) = 100,000 / 831.36 = 120.28 Amps
Step 2: Apply the 125% Continuous Load Rule
Under NEC Article 215.2(A)(1), conductors supplying continuous loads (those expected to run for 3 hours or more) must be sized at 125% of the base FLA.
120.28 A × 1.25 = 150.35 Amps
Your conductors must have an allowable ampacity of at least 150.35A after all correction and adjustment factors are applied.
Step 3: Select the Conductor (AWG)
Assuming you are using THHN copper wire in a raceway with an ambient temperature of 30°C (86°F), you must look at the 75°C column of NEC Table 310.16, as most commercial breaker and transformer lugs are rated for 75°C terminations.
- 1/0 AWG Copper: Rated for exactly 150A at 75°C. Because 150A is less than our required 150.35A, this wire is technically undersized by a fraction of an amp.
- 2/0 AWG Copper: Rated for 175A at 75°C. This is the correct, code-compliant choice.
Step 4: Size the Overcurrent Protective Device (OCPD)
Per NEC 240.4(B), you are permitted to round up to the next standard breaker size if the conductor ampacity does not match a standard fuse/breaker rating. The standard sizes (NEC 240.6) are 150A and 175A. Since our calculated continuous load requirement is 150.35A, we step up to a 175A 3-pole breaker.
Where You Meet 100 kVA in Practice
A 100 kVA capacity sits at a critical threshold in modern electrical infrastructure. It is too large for standard residential service but serves as the workhorse for mid-sized commercial and specialized IT applications.
Data Center and Server Room UPS Systems
In IT infrastructure, a 100 kVA Uninterruptible Power Supply (like the Eaton 9PX series or Vertiv Liebert EXL S1) is a standard building block for server rows. However, data center loads are rarely purely resistive. If the UPS operates at a 0.9 Power Factor (PF), a 100 kVA UPS will only deliver 90 kW of real power. If your server racks draw 95 kW, the UPS will overload and drop the load, even though the kW number seems close to the kVA number. Always size UPS systems based on the lesser of the kW or kVA rating.
Commercial EV Charging Depots
With the rapid expansion of Level 3 DC Fast Charging (DCFC) in 2026, a single 150 kW charger requires massive apparent power. A 100 kVA transformer is frequently deployed as a dedicated step-down unit to feed a bank of Level 2 AC chargers (typically 19.2 kW each) or to act as a buffer for a single derated DCFC unit. The high harmonic distortion from EV rectifiers often requires specifying a K-4 or K-13 rated transformer to handle the excess heat generated by non-linear loads.
Step-Down Transformers for Lighting and Receptacles
Large commercial buildings often distribute power at 480V to minimize voltage drop and copper costs, then use a 100 kVA step-down transformer to create a 208Y/120V subpanel. This specific transformer size comfortably handles roughly 40 to 50 standard 20A/120V office receptacle circuits, assuming standard diversity factors.
Think of kVA as the total physical size of a beer glass, while kW is the actual liquid beer inside. The foam (reactive power, kVAR) takes up space in the glass but doesn't quench your thirst. Your utility wires and transformers (the glass) must be sized for the total volume (kVA), even if the load only does useful work (kW) on a portion of it.
Frequently Asked Questions & Edge Cases
Do I need to size the primary breaker for 100 kVA the same way as the secondary?
No. The primary amperage is entirely dependent on the primary voltage. If your 100 kVA transformer has a 480V primary and a 208V secondary, the secondary draws 277.6A, but the primary only draws 120.3A. Furthermore, NEC 450.3(B) allows primary-only overcurrent protection to be sized up to 250% of the primary FLA to accommodate transformer magnetizing inrush current, which can briefly spike to 10 or 12 times the FLA when the core is first energized.
Can I use the 90°C column for my THHN wire ampacity to save money?
Rarely. While THHN wire is rated for 90°C in the conduit (which helps with ambient temperature derating), NEC 110.14(C) requires you to use the 75°C column for termination ampacity unless the transformer lugs and breaker lugs are explicitly marked for 90°C. Most standard dry-type transformers and molded case breakers are only rated for 75°C terminations. Using the 90°C column to select your base wire size will result in a code violation and overheated lugs.
What happens if my load exceeds 100 kVA by a small margin, like 102 kVA?
Transformers and UPS systems have a thermal mass that allows for brief overloads, but continuous operation above the nameplate kVA will degrade the insulation. For dry-type transformers with Class 220 insulation, a continuous 5% overload might be survivable in a cold environment, but it voids manufacturer warranties. For UPS systems, exceeding the kVA limit by even 1% will typically trigger an immediate static bypass, transferring the load to raw utility power and defeating the battery backup.
How does altitude affect a 100 kVA transformer's ampacity?
Air density drops at higher elevations, reducing its ability to cool the transformer windings. Standard dry-type transformers are rated for a maximum ambient temperature of 40°C at elevations up to 3,300 feet (1,000 meters). For installations above 3,300 feet, you must derate the transformer capacity by roughly 0.3% for every 330 feet of additional elevation, meaning your 100 kVA unit might only be legally allowed to carry 95 kVA of continuous load in Denver, Colorado.
