Amps to kVA Calculator

What kVA measures that amps do not

An amp measures current, the number of electrons moving past a point each second. A volt measures the push behind them. Multiply the two and you get volt-amps, the total size of the load. Divide by 1,000 and you have kVA, the unit printed on transformers, generators and UPS cabinets.

Engineers call kVA apparent power. It adds the current that does work to the current that charges motor windings and capacitors and then flows back. Wires and windings carry both, so equipment is sized on kVA rather than on the part that turns into motion.

This page starts with amps and volts and ends with kVA. The amps to watts calculator stops one step earlier, at real power in watts, and the kVA to kW calculator starts from kVA and divides by the power factor. NIST's guide to SI units and conversion factors lists the volt, the ampere and the watt, and one kVA is exactly 1,000 volts times 1 amp.

The formulas, one for each system

Single phase covers most of a house: 120 V receptacles, and 240 V dryers, water heaters and air conditioners. Volts times amps, divided by 1,000.

Three phase covers motors and commercial panels. The formula keeps that shape and adds the square root of 3, about 1.732, because the legs sit 120 degrees apart. On a 480 V system each leg is 277 V to neutral, and 277 times 1.732 comes back to 480. Enter the line to line number, not the line to neutral one. Going the other way, amps equal kVA times 1,000 divided by volts, or by 1.732 times volts on three phase.

Direct current has no phase and no power factor, so kVA and kW are the same number.

Leaving out the 1.732 on a 480 V, 100 A load gives 48 kVA instead of 83.14 kVA, a 42 percent shortfall.

Example: 30 amps at 240 volts, single phase

A 30 A, 240 V circuit is a common size for a dryer or a small water heater. Volts times amps gives 7,200 volt-amps, and dividing by 1,000 gives 7.2 kVA.

The power factor turns that into real power. A resistive heater runs near 1.0, so its 7.2 kVA is also 7.2 kW. A dryer with a motor and electronic controls runs closer to 0.9, so 7.2 times 0.9 is 6.48 kW of useful work. The rest, 3.14 kVAR, shuttles between the supply and the motor windings. Check it: 6.48 squared plus 3.14 squared is 51.85, and 7.2 squared is 51.84.

A load that runs three hours or more counts as continuous, and section 210.20(A) of the 2023 edition of NFPA 70, the National Electrical Code, requires the overcurrent device to carry 125 percent of it. That is 30 times 1.25, or 37.5 A, and the next standard breaker size is 40 A. On a 120 V circuit the same 30 A would be 3.6 kVA.

Example: 100 amps at 480 volts, three phase

Take a 100 A feeder at 480 V, three phase. Multiply 1.732 by 480 by 100 and you get 83,138 volt-amps, or 83.14 kVA.

At a power factor of 0.85 the real power is 83.14 times 0.85, or 70.67 kW, with 43.8 kVAR left over. The triangle closes again: 70.67 squared plus 43.8 squared is about 6,912, and 83.14 squared is about 6,912 too.

Two checks are worth running. Without the 1.732 the answer would be 48 kVA, and a 48 kVA transformer on an 83.14 kVA load runs hot and trips its own protection. At 125 percent for continuous duty the current is 125 A, a standard size, so a 125 A breaker and conductors rated at least 125 A fit this feeder.

Generators are sold the same way. A 100 kVA generator at a 0.8 power factor is an 80 kW machine, so an 83.14 kVA load needs 103.9 kVA of machine and the next standard size is 125 kVA.

Power factor is the bridge to kW

Power factor is real power divided by apparent power, always between 0 and 1. A factor of 1.0 means every amp does work. A factor of 0.5 means half the current does work and half circulates.

That ratio explains a surprise: at a lower power factor, the same kW load pulls more amps. A 10 kW load at 1.0 needs 10 kVA. At 0.5 it needs 20 kVA, so every wire in the path is twice as large for the same output. Reactive power is the third side of the triangle: kVAR equals the square root of kVA squared minus kW squared.

Those are typical published ranges, not a specification for any one unit. The nameplate, the manual or a clamp meter with a power factor reading beats a table every time. The power factor calculator goes the other way, starting from kW and kVA to find the factor and the capacitor size needed to correct it.

Sizing a generator or transformer from motor amps

Equipment is rated in kVA because heat in a winding depends on voltage times current, not on how much of that product becomes torque. That makes kVA the honest number for a transformer, a generator or a UPS.

A 10 hp, 460 V, three phase motor has a full-load current of 14 A in Table 430.250 of the 2023 National Electrical Code. Put that through the three phase formula and you get 11.15 kVA.

Section 430.6(A)(1) says conductors and short-circuit protection are sized from that table value, while the nameplate current sets the overload relay. Mixing those up is the second most common motor mistake after dropping the 1.732.

Efficiency sits between shaft horsepower and electrical input. Ten horsepower is 7.46 kW at the shaft. At 88 percent efficiency the motor pulls 8.47 kW from the line, so its power factor is 8.47 divided by 11.15, or about 0.76.

A motor can pull six to eight times its running current for a fraction of a second, so an 11.15 kVA motor can look like 70 to 90 kVA as it starts. Generators have to absorb that dip without stalling.

The 125 percent rule and the listing exception

Section 210.20(A) is short: the breaker must be at least the noncontinuous load plus 125 percent of the continuous load. A continuous load is one expected to run three hours or more, per Article 100.

The 125 percent is not a margin someone invented. Breakers and wires run warmer inside a closed panel than in open air, and a continuous load asks more of them.

One exception is a listing question, not a math question. If the breaker and its enclosure are listed as an assembly for 100 percent operation, the 125 percent does not apply to the breaker rating. That listing is what UL 489 and the panelboard standards cover, and it is printed on the equipment label. If the label does not say 100 percent rated, use 125 percent. UL Solutions describes the UL 489 evaluation for molded-case breakers if you want the scope in plain print.

Keep the units apart: convert kVA to amps first, then apply 125 percent, because the breaker only sees amps. The 2023 code text is free to read through NFPA's guide to accessing NFPA 70.

Reading a nameplate without adding 125 percent twice

Manufacturers print several current numbers, and they are not interchangeable. FLA is the running current for that unit. FLC usually means the code table value for a motor of that horsepower and voltage. MCA is the smallest conductor the manufacturer allows, and it already includes the 125 percent factor for the largest motor. MOP is the largest breaker they allow.

Because MCA already carries the 125 percent, multiplying it again double counts. A heat pump with an MCA of 24 A at 240 V, single phase, is 5.76 kVA. Its MOP might be 40 A, and that 40 A is a breaker limit, not a load to convert.

Four mistakes cover most of the wrong answers. Using kW where kVA belongs, which understates the load by the power factor. Dropping the 1.732 on three phase. Feeding a line to neutral voltage into a three phase formula. And adding 125 percent to a number that already includes it, such as MCA or a nameplate worst case.

When two methods disagree, trust the nameplate for that unit and the code table for the wire and breaker. The watts to amps calculator and the Ohm's law calculator cover the neighboring steps once you have the current.