Voltage Regulation Calculator
Calculate voltage drop, drop percentage, and receiving-end voltage for power cables with resistance and reactance.
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 |
|---|---|---|
| 500 | 48.39 V | 10.08% |
| 1,750 | 169.35 V | 35.28% |
| 3,250 | 314.51 V | 65.52% |
| 4,500 | 435.47 V | 90.72% |
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
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