Enter power and voltage to get current, or a measured current to get power, for single-phase, line-to-line or three-phase loads, with power factor, efficiency and winding connection.
Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.
Line current for the same real power, with power factor from 0.5 to 1.0.
Current for the same real power connected line-to-neutral, line-to-line and three-phase on the same system.
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Divide electrical power by √3 times voltage times power factor: I = P ÷ (√3 × V × PF). For a motor, electrical power is the rating divided by efficiency. In the example, 50 hp is 37.3 kW at the shaft; at 94.1% efficiency and 0.86 PF on 480 V, current is 55.4 A.
They are all power units. 1 hp is 745.7 W and 1 metric hp (CV or PS) is 735.5 W, about 1.4% less. On a motor nameplate, hp and kW are mechanical, shaft power; to find what the motor draws from the supply, divide by efficiency.
kW is real power, which does work and shows on the energy bill. kVA is apparent power, which includes the reactive power used to magnetize motors and transformers. kW = kVA × power factor. Conductors, breakers and transformers are sized by kVA, since it sets the current.
The voltage and current in each winding. In wye, each winding sees line voltage ÷ √3 and carries line current; in delta, line voltage and line current ÷ √3. In the delta example, each winding sees 480 V and carries 32.0 A while the line carries 55.4 A. Total power does not change.
Choose power from current, enter the clamp-meter current, voltage and power factor. The calculator returns kW, kVA and kvar and, with efficiency, shaft power. Without a power analyzer, use nameplate PF; the result is approximate, especially at light load.
NEC 430.6 requires motor branch-circuit conductors and protection to be sized from Tables 430.247 to 430.250, which are conservative values. For a 50 hp, 460 V three-phase motor the table gives 65 A, above the 55.4 A this formula gives for a premium motor at 480 V.
It assumes sinusoidal voltage and a balanced load. It does not handle harmonics from drives and electronic supplies, nor phase unbalance. For motors, the manufacturer nameplate current is the reference for overloads.
Important notice: Indus Metrics tools produce estimates and pre-sizing results from the data you enter and from published engineering methods. They do not replace design, calculations or reports signed by a licensed professional engineer (PE), nor verification against the applicable codes, standards and regulations, such as OSHA rules and the ASME codes. Material and fuel properties vary: check manufacturer data or your own process analyses before deciding. By using this site you agree to the Terms of Use.
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