Voltage Drop Calculator for AC Wiring
Calculate voltage drop for AC residential and commercial circuits. Enter system voltage, circuit type, conductor size and one-way cable length to get voltage drop, wire resistance and power loss, checked against the NEC 3% and IEC 60364 guidelines.
Last updated: May 2026
Enter circuit voltage, conductor size, length and current to calculate voltage drop.
Single-phase: Vdrop = 2 × ρ × L × I / A | Three-phase: Vdrop = √3 × ρ × L × I / A
Why the 3% voltage drop guideline matters
Every conductor has resistance. When current flows through that resistance, voltage is lost along the way; the load at the end of the circuit receives less than the supply voltage. For AC building wiring, both IEC 60364 (used in Europe and most of the world) and the NEC (used in the US) set a 3% maximum voltage drop for branch circuits. Exceeding this limit causes problems for voltage-sensitive equipment: motors run hot and fail sooner, dimmers flicker, and sensitive electronics may malfunction or shut down.
The resistivity (rho) values used are 0.0172 ohm·mm²/m for copper and 0.0282 ohm·mm²/m for aluminium at 20 °C. At operating temperature, resistance is slightly higher; real-world drop is typically 5 to 10% above the calculated value. Size up when you are close to the 3% limit.
This page vs. the DC voltage drop calculator
The DC voltage drop calculator is designed for low-voltage DC systems: 12 V and 24 V LED strips, solar panel wiring, automotive accessories and bench-top electronics. This page focuses on AC residential and commercial circuits at 120 V, 230 V, 240 V and higher, where the NEC and IEC building wiring codes apply.
NEC Article 210-19 (US) vs. IEC 60364 (Europe)
Under NEC Article 210-19(A)(1), the conductor for a branch circuit must be sized so the voltage drop does not exceed 3% of the branch circuit supply voltage. A further informational note recommends keeping the combined feeder plus branch circuit drop below 5%. Under IEC 60364-5-52, the recommended maximum is 3% for lighting circuits and 5% for other uses, measured from the origin of the installation.
In practice, the 3% rule is applied conservatively: target 2% to leave headroom for temperature de-rating, future load growth and connection resistance.
Typical conductor sizes for a 16 A single-phase circuit at 230 V (copper)
| One-way length | Min. conductor | Voltage drop | Drop % |
|---|---|---|---|
| 5 m | 1.5 mm² | 1.83 V | 0.79% |
| 10 m | 2.5 mm² | 2.20 V | 0.96% |
| 20 m | 4 mm² | 2.75 V | 1.19% |
| 30 m | 6 mm² | 2.75 V | 1.19% |
| 50 m | 10 mm² | 2.75 V | 1.19% |
Calculated using Vdrop = 2 × 0.0172 × L × 16 / A. Conductor sizes chosen so drop remains below 3% of 230 V (6.9 V).
Frequently Asked Questions
Does the 3% rule apply to the whole circuit or just the branch?
Both standards allow total drop to be shared across the feeder and the branch circuit. Under NEC, the informational note recommends the feeder drop be kept to 2% and the branch circuit drop to 3%, giving 5% total from the transformer to the load. In practice, keep each segment within 3% and you will always stay within the 5% combined limit. IEC 60364 similarly treats 3% as the branch circuit limit from the origin of the installation.
My circuit runs 20 m but the conduit bends add extra length. Do I count that?
Yes. The calculation uses actual conductor length, not straight-line distance. Add the extra length for bends, risers, conduit routing and any slack left at terminations. A good rule of thumb: add 10 to 15% to your straight-line measurement for a typical installation with several bends. For critical circuits or long runs, trace the actual conduit path.
Can I use aluminium wiring to reduce cost?
Aluminium is allowed for fixed wiring but requires careful consideration. It has higher resistivity than copper (0.0282 vs 0.0172), so you need one or two standard sizes larger to match the voltage drop and ampacity of a copper equivalent. Aluminium also requires anti-oxidant compound at terminations and rated aluminium-compatible connectors; standard copper terminals can cause poor contact and overheating over time. Most electricians do not recommend aluminium for branch circuits below 35 mm²; it is most common in main supply cables and service entrance conductors.
What is the difference between voltage drop and voltage regulation?
Voltage drop is the resistive loss in the conductor under a specific steady-state load. It is fixed as long as the current stays constant. Voltage regulation is the change in voltage from no-load to full-load, measured at the load terminals; it includes both the resistive drop and any reactive (inductive) effects in the supply. For purely resistive loads on residential circuits, the two are nearly equal. For motor circuits, voltage regulation matters more because motors draw high inrush current at start-up, causing a momentary voltage dip that can trip under-voltage protection or cause contactors to drop out.
Next step in this workflow
Drop within limits: now size the overcurrent protection for your circuit.