Voltage Regulation Calculator
Calculate voltage drop, drop percentage, and receiving-end voltage for power cables with resistance and reactance.
About this calculator
Long cable runs don't just carry current, they lose voltage along the way, and this calculator quantifies exactly how much. It uses the standard AC voltage-drop equation, VD = 2 × I × L × (R × cos φ + X × sin φ) ÷ 1000, where the factor of 2 accounts for the round trip through the conductor and back (the single-phase / two-wire case), I is load current, L is the one-way cable length, and R and X are the cable's resistance and reactance per kilometer. The cos φ and sin φ terms come from the load's power factor — sin φ is derived internally as √(1 − PF²) — because a lagging power factor means the reactive part of the impedance contributes to the drop too, not just the resistive part. Dividing by 1000 reconciles the per-km resistance/reactance figures with a cable length entered in meters.
The result is expressed three ways: the raw Voltage Drop in volts, that drop as a percentage of Source Voltage, and the Receiving End Voltage the load actually sees. The percentage is checked against a 3% threshold, which mirrors the NEC's recommended limit for branch circuits (5% is the typical ceiling for feeder plus branch combined) — exceeding it usually means upsizing the conductor, shortening the run, or raising the source voltage. A common mixup is entering resistance/reactance in ohms rather than ohms per kilometer, or entering total round-trip length instead of the one-way distance the formula expects.
Inputs
NEC Ch. 9 Table 9 (Cu 75°C): 12 AWG 6.56; 10 AWG 4.13; 4 AWG 1.31; 1/0 AWG 0.524; 4/0 AWG 0.328 Ω/km
Results
Voltage Drop
9.68 V
Voltage Drop
2.02%
How to Use This Calculator
- Enter the Load Current in amperes and the Cable Length (one way) in meters from source to load.
- Enter the Cable Resistance in Ω/km from the manufacturer datasheet or NEC Chapter 9 Table 9 — e.g., 95 mm² copper ≈ 0.193 Ω/km.
- Enter the Cable Reactance in Ω/km — typically 0.06–0.10 Ω/km for cables in conduit.
- Enter the Load Power Factor and Source Voltage in volts.
- Read the Voltage Drop in volts and as a percentage — NEC recommends a maximum of 3% for branch circuits and 5% total (feeder + branch).
- If the drop exceeds 3%, increase cable size, reduce run length, or raise source voltage to bring the load within specification.
How the result changes with Load Current
| Load Current | Voltage Drop | Voltage Drop |
|---|---|---|
| 50 | 4.84 V | 1.01% |
| 75 | 7.26 V | 1.51% |
| 150 | 14.52 V | 3.02% |
| 250 | 24.19 V | 5.04% |
What each input means
- Load Current
- Current flowing through the cable in amperes.
- Cable Length (one way)
- One-way distance from source to load.
- Cable Resistance
- AC resistance per kilometer of cable per NEC Chapter 9 Table 9 (copper, 75°C, in conduit). 12 AWG: 6.56 Ω/km; 10 AWG: 4.13 Ω/km; 8 AWG: 2.60 Ω/km; 4/0 AWG: 0.328 Ω/km; 350 kcmil: 0.183 Ω/km.
- Cable Reactance
- Inductive reactance per kilometer of cable. Typical: 0.06-0.10 Ω/km.
- Load Power Factor
- Power factor of the load. Unity PF for resistive loads.
- Source Voltage
- Voltage at the source (sending end).
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersLoad Current = 100, Cable Length (one way) = 100, Cable Resistance = 0.524, Cable Reactance = 0.073 = 6 input(s) provided
- Calculate Voltage DropVoltage Drop = Math9.68 = 9.68
- Calculate Voltage DropVoltage Drop = Math2.02 = 2.02
- Calculate Receiving End VoltageReceiving End Voltage470.32 = 470.32
- Calculate ≤3% Drop?≤3% Drop?1 = 1
Engine last updated . Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the formula multiply by 2 instead of using the cable length as-is?
The factor of 2 in VD = 2 × I × L × (R × cosφ + X × sinφ) ÷ 1000 accounts for the fact that current has to travel the cable length twice — out to the load and back through the return conductor — for a single-phase, two-wire circuit. That's why the Cable Length input is explicitly labeled 'one way': the calculator applies the round-trip factor internally, so entering the round-trip distance would double-count it.
Why does power factor show up in both a cosφ and a sinφ term?
cos φ (the power factor itself) weights the resistive part of the cable impedance, while sin φ — derived internally as √(1 − PF²) — weights the reactive part. A lower power factor means more of the load current is reactive, and since the cable's reactance (X) also drops voltage, that reactive current needs its own term; ignoring sin φ would understate voltage drop for any load that isn't unity power factor.
What does exceeding the 3% threshold on Voltage Drop actually mean I should do?
The isAcceptable check compares your computed voltage drop percentage against 3%, which mirrors NEC's recommended limit for branch circuits (the commonly cited ceiling is 5% for a branch circuit plus feeder combined). If your result exceeds 3%, the practical fixes are upsizing the conductor (lowering R and X), shortening the run, or raising the source voltage — all three directly reduce the VD formula's output.
Why do I enter cable length in meters but resistance and reactance in ohms per kilometer?
The formula divides by 1000 specifically to reconcile these two units — R and X are per-kilometer figures pulled from cable datasheets or NEC Chapter 9 Table 9, while cable runs are more naturally measured and entered in meters. If you mistakenly enter resistance/reactance already in ohms (not per-km), the division by 1000 will make the calculated voltage drop far too small.
Related Calculators
The questions that sit next to this one — chosen by subject, including calculators filed under a different category.
Motor Starting Current Calculator
Calculate motor full load amps, starting inrush current, voltage drop during starting, and starting KVA demand.
Electrical EngineeringCable Ampacity Calculator
Determine cable ampacity with NEC derating factors for temperature, conduit fill, and continuous loads.
Power GridVoltage Drop Distribution Calculator
Calculate voltage drop across a distribution feeder.
Science & EngineeringVoltage Drop Calculator
Calculate voltage drop in electrical wiring based on wire gauge, length, current, and material. Ensure your circuits meet NEC recommendations.
Electrical & PlumbingVoltage Drop Calculator (Residential)
Calculate voltage drop for residential wiring and check compliance with the NEC 3% recommendation.
More in Engineering.