kVA to Amps Calculator

3-phase: Amps = (kVA x 1000) / (V x 1.732). Single-phase: Amps = (kVA x 1000) / V. Example: a 1,500 kVA, 480V 3-phase transformer secondary delivers (1,500,000) / (480 x 1.732) = 1,804 A. This is the basis for sizing the secondary main circuit breaker and conductor per NEC 450.3. For the primary side, use the same formula but with the primary voltage: the same 1,500 kVA transformer with a 13.8 kV primary draws (1,500,000) / (13,800 x 1.732) = 62.8 A on the primary side.

In a 3-phase system, apparent power S = sqrt(3) x V_L-L x I_L. Solving for line current I_L: I_L = S / (sqrt(3) x V_L-L). The sqrt(3) = 1.732 is the geometric consequence of the 120-degree phase angle between the three phase voltages. Using the single-phase formula for a 3-phase system overestimates current by 73.2%, which would lead to oversized (wasteful) but safe conductor sizing. The reverse -- using the 3-phase formula (with sqrt(3)) for a single-phase system -- underestimates current by 42% (1/1.732 = 0.577), which is dangerous: a 50 kVA single-phase 240V transformer correctly draws 208.3 A, but the erroneous 3-phase formula gives 50,000/(240 x 1.732) = 120.3 A -- a 42% underestimate that would lead to severely undersized secondary conductors.

No. kVA already represents apparent power, the vector sum of real (kW) and reactive (kVAR) power. The conversion from kVA to amps is independent of power factor -- 500 kVA at PF = 1.0 and 500 kVA at PF = 0.7 both result in the same 601.4 A at 480V 3-phase. This is the key advantage of working in kVA for electrical infrastructure design: it tells you the total current the transformer, generator, or switchgear must carry, regardless of the load's power factor. If you start from kW, you must first divide by PF to find kVA before calculating amps. A 400 kW load at PF = 0.8 becomes 400/0.8 = 500 kVA, drawing 601.4 A at 480V -- not the 400,000/(480 x 1.732) = 481.1 A you would calculate by mistakenly omitting PF.

kW (kilowatts) measures real power -- the power that actually performs useful work (turning motors, heating elements, illuminating spaces). kVA (kilovolt-amperes) measures apparent power -- the total electrical power the utility or generator must supply, including both real power and reactive power (the magnetizing current that motors, transformers, and some lighting require but that produces no useful work). Equipment that carries current (transformers, generators, cables, circuit breakers, switchgear bus bars) is rated in kVA because their heating and voltage drop depend on total current, which is proportional to kVA, not kW. Equipment that consumes or converts energy (motors, heaters, UPS batteries) is rated in kW because kW measures the rate of energy conversion. The ratio kW/kVA equals the power factor -- a measure of how efficiently the current is being used to deliver real power. For generator sizing, a common error is adding all loads in kW and comparing to the generator's kW rating, while ignoring that the generator's alternator has a kVA limit that may be exceeded by low-power-factor loads.

First calculate the primary full-load amps: I_primary = (kVA x 1000) / (V_primary x sqrt(3)) for 3-phase. Example: 500 kVA, 13.8 kV primary: I = 500,000 / (13,800 x 1.732) = 20.9 A. Per NEC Table 450.3(A), for a transformer with primary-only protection (no secondary breaker) and primary voltage > 1,000V, the primary overcurrent device maximum is 300% of rated primary current for supervised locations: 20.9 x 300% = 62.7 A, next standard size up is permitted per Note 1. For unsupervised locations, the limit is 250%: 20.9 x 250% = 52.3 A. The primary protection must also coordinate with the transformer inrush current, which can be 8-12 times full-load current for 0.1 seconds during energization -- a time-delay fuse or electronic trip unit with inrush restraint is typically required.

A generator has two limiting ratings: the engine's mechanical power limit (kW), and the alternator's thermal current limit (kVA). A typical standby diesel generator is rated at 0.8 PF, meaning the kW rating is 80% of the kVA rating (e.g., 1,000 kW from a 1,250 kVA alternator). The engine limit is set by the diesel or gas engine's rated shaft power. The alternator limit is set by the maximum stator current its windings can carry without exceeding the insulation temperature class (typically Class H, 180C rise). If the load power factor drops below 0.8, the generator reaches its kVA limit before reaching its kW limit: a 1,250 kVA set with an 0.7 PF load can only deliver 875 kW before the 1,250 kVA limit is reached, even though the engine is rated for 1,000 kW. This is why critical facility generator loading studies must evaluate both kW and kVA, not just kW.