15kVA in amps is the electrical current drawn by a 15 kilovolt-ampere load, calculated by dividing 15,000 volt-amps by the system voltage (and by the square root of 3 for three-phase systems). When a maker, electrician, or facility manager asks what 15kVA is in amps, they are almost always trying to size a breaker, select a wire gauge, or specify a disconnect switch for a heavy piece of equipment. Getting this conversion right is the difference between a safely running workshop subpanel and a melted terminal lug on a dry-type transformer.
The Core Math: Converting 15kVA to Amps
Apparent power (kVA) dictates the actual current flowing through your conductors, regardless of how much of that power is doing real work. To find the amperage, you need to know your system voltage and phase configuration. The formulas are straightforward:
- Single-Phase: Amps = (kVA × 1000) ÷ Voltage
- Three-Phase: Amps = (kVA × 1000) ÷ (Voltage × √3)
You are installing a 15kVA isolation transformer for a sensitive CNC machine on a 240V single-phase supply.
1. Multiply kVA by 1000: 15 × 1000 = 15,000 VA.
2. Divide by voltage: 15,000 ÷ 240 = 62.5 Amps.
The primary and secondary windings will carry 62.5A under full load.
Here is a reference chart for the exact amperage of a 15kVA load across standard North American and IEC voltages:
| System Voltage | Phase | Calculated Amps | Common Application |
|---|---|---|---|
| 120V | 1-Phase | 125.0A | Large temporary event power distribution |
| 208V | 3-Phase | 41.6A | Commercial IT server room UPS systems |
| 240V | 1-Phase | 62.5A | Residential/Light commercial subpanels, EV charger arrays |
| 480V | 3-Phase | 18.0A | Industrial motor control centers, large HVAC |
Where You Meet 15kVA in Practice
You will rarely see '15kVA' printed on a standard household appliance. This rating lives in the realm of infrastructure and heavy equipment. Here is where this specific capacity shows up on the jobsite or in the workshop:
- Workshop Subpanels: A 15kVA transformer or feeder is the exact size needed to run a serious hobbyist or small commercial woodshop, supporting a 5HP table saw (approx. 30A at 240V), dust collection, and lighting simultaneously.
- Multi-Bay EV Charging: If you are installing three Level 2 EV chargers (each drawing roughly 40A at 240V) with load-sharing software, the aggregate peak demand hovers right around 15kVA.
- Standby Generators and UPS: A 15kVA Uninterruptible Power Supply (UPS) is a standard rack-mounted or floor-standing unit used to keep network switches, telecom gear, and critical servers online during brownouts.
What 15kVA Changes in Your Installation
Crossing the threshold into a 15kVA load fundamentally changes your physical installation requirements. You are no longer dealing with standard 12 AWG or 10 AWG branch circuits. A 15kVA load dictates:
- Feeder-Level Conductors: You will be pulling heavy gauge wire (typically 4 AWG to 1 AWG copper), which requires larger conduit (minimum 3/4-inch, usually 1-inch EMT or PVC) and specific pulling lubricants to avoid damaging the insulation.
- Terminal Torque: At 60+ amps, loose connections generate significant heat due to I²R losses. You must use a calibrated inch-pound torque screwdriver or torque wrench to terminate these wires, adhering strictly to the manufacturer's terminal torque specifications (often between 35 and 50 in-lbs for smaller lugs, up to 250 in-lbs for larger mechanical lugs).
- Thermal Management: A 15kVA dry-type transformer or a high-amperage UPS generates substantial waste heat. NEC-style guidance requires specific clearances (often 6 to 12 inches minimum) around the equipment for ventilation, and you may need to upgrade the room's HVAC tonnage to compensate for the BTU output.
Decision Tree: Sizing Breakers and Wire for a 15kVA Load
Let's walk through the exact decision path for sizing the overcurrent protection and conductors for the most common 15kVA scenario: a 240V, single-phase, continuous load (like a server rack UPS or a transformer supplying continuous lighting). According to the NFPA 70 National Electrical Code, a continuous load is one expected to run for 3 hours or more.
| Step | Action / Rule | Calculation / Result |
|---|---|---|
| 1. Base Current | Calculate raw amps from kVA | 15,000 ÷ 240 = 62.5A |
| 2. Continuous Load Rule | Multiply by 125% (NEC 210.20) | 62.5A × 1.25 = 78.125A |
| 3. Breaker Sizing | Select next standard size (NEC 240.6) | Next standard size above 78.1A is 80A |
| 4. Wire Ampacity | Wire must carry the continuous load (78.1A) | Look up 75°C column in NEC 310.16 |
| 5. Conductor Selection | Match wire to 80A breaker / 78.1A load | 4 AWG Copper THHN (rated 85A at 75°C) |
Note: If your 15kVA load is non-continuous (e.g., a motor starting up or a welder with a low duty cycle), you can drop the 125% multiplier, allowing you to use a 70A breaker and 4 AWG wire, but defaulting to the continuous sizing is the safest bench and jobsite practice unless the equipment nameplate explicitly states otherwise.
Common Confusions: kVA vs. kW and Amps
The most frequent mistake DIYers and junior technicians make is confusing kVA (apparent power) with kW (real power) when sizing wire. This confusion stems from Power Factor (PF).
If your 15kVA load is a bank of servers with a power factor of 0.85, the real power doing the computing work is only 12.75 kW (15 × 0.85). A common error is calculating the wire size based on the 12.75 kW figure. This is wrong and dangerous.
Wire ampacity and breaker sizing do not care about power factor; they only care about the physical current (Amps) generating heat in the copper. Even if the load is only doing 12.75 kW of real work, the conductors are still carrying the full 62.5 Amps of apparent current. Always size your breakers, fuses, and wire based on the kVA rating, and reserve the kW rating for calculating your utility bill or sizing the mechanical cooling (HVAC) required to remove the heat generated by the equipment. For deeper dives into transformer nameplate ratings and PF, Schneider Electric's transformer sizing guides provide excellent manufacturer-level context.
FAQ: 15kVA Load Questions
Can I use a 60A breaker for a 15kVA 240V load?
No. A 15kVA load at 240V draws 62.5A. A 60A breaker will trip immediately upon reaching full load, and if it is a continuous load, the NEC requires the breaker to be rated for 125% of the draw (78.1A). You must step up to an 80A breaker.
Does the power factor of my 15kVA UPS change the wire size I need?
No. Power factor changes the real power (kW) and your energy costs, but the physical current (Amps) flowing through the wire is dictated entirely by the kVA and the voltage. Size the wire for the kVA-derived amps.
What size ground wire do I need for a 15kVA 240V circuit on an 80A breaker?
According to NEC Table 250.122, an 80A overcurrent device requires a minimum 8 AWG copper equipment grounding conductor. If you upsized your ungrounded conductors (e.g., using 3 AWG instead of 4 AWG to mitigate voltage drop over a long distance), you must proportionally upsize the ground wire as well.
Is 15kVA enough to run a whole house?
For an average 2,000 sq ft home without electric resistance heating or an electric oven, 15kVA (roughly 62.5A at 240V) can run lights, refrigerators, a TV, and a gas furnace blower. However, it will not support central AC, electric water heaters, and EV charging simultaneously. Most modern US homes have a 200A (48kVA) main service panel.
