Electrical Calculators

Voltage drop is the loss along the conductor between the source and the load. NEC informational notes recommend keeping branch-circuit drop under 3% and the combined feeder-plus-branch drop under 5% for reasonable efficiency.

It matters more on HVAC than almost anywhere else. A compressor motor seeing low voltage draws higher current for the same work, runs hotter, and fails sooner. Long runs, undersized conductors, and loose terminations all show up as voltage drop long before they show up as an outright failure.

You size to the nameplate, not to the breaker. Use the Minimum Circuit Ampacity (MCA) for conductor sizing and the Maximum Overcurrent Protection (MOCP) for the disconnect and breaker — both are printed on the unit rating plate.

• Conductors for a hermetic refrigerant motor-compressor are sized at 125% of the rated load current, plus the other loads on the circuit (NEC 440.32 / 430.24).
• Never upsize the breaker to stop nuisance tripping — find out why it's tripping first.
• Verify ampacity against the conductor's insulation rating and the ambient temperature, and confirm against local code.

Unbalance is the maximum deviation from the average of the three phase voltages, divided by the average, times 100:

% unbalance = (maximum deviation from average ÷ average voltage) × 100

Motors are the reason this matters. A small voltage unbalance produces a much larger current unbalance, and the extra losses show up as heat in the windings — even 2% unbalance pushes winding temperatures up noticeably, and 5% or more will shorten a motor's life dramatically. If you measure unbalance, check the supply, the connections, and the other loads on the service before condemning the motor.

• Single-phase power: watts = volts × amps × power factor.
• Three-phase power: watts = volts × amps × power factor × √3.
• Resistance: R = V ÷ I — useful for checking a heating element or a motor winding.
• Horsepower: 1 hp ≈ 746 W — handy for sanity-checking nameplate numbers against measured amps.