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).
Checking the drop after the gauge is chosen
The wire gauge calculator (step 1) picks the minimum cross-section that satisfies ampacity and keeps drop within the limit for a given current. This page is step 2: you now know the conductor size, so you run the actual route length through the drop formula and confirm the number sits below 3% before you commit to the cable. Once you have that confirmation, step 3 is sizing the overcurrent protection with the fuse calculator, which rates the breaker to the conductor rather than to the load. Step 4 is the conduit fill calculator, which checks that bundling the cables in a conduit does not push the fill ratio over code. On the supply side, the power supply calculator adds up the connected load, and the power factor calculator corrects watts to VA for reactive loads. After the circuit is live, the appliance running cost and electricity cost calculators turn the load into a monthly figure. You are here: step 2, confirming the drop on the actual run. A long feed to a shed or outbuilding is where drop quietly fails: the socket at the far end reads low even though the cable is rated for the current, because the route length is two or three times what you might assume from the straight-line distance.
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.
I measured the straight-line distance to my outbuilding, but the conduit has to go around the eaves and drop down the wall. Which length do I enter?
Enter the actual conductor length, not the straight-line distance. Trace the conduit route: wall drop, eave run, underground section, any rise inside the outbuilding. Then add roughly 1 m per termination for slack at each end. A common mistake on subpanel feeds is using the property-line distance; the conduit often travels 30 to 40 percent further once it follows the building. The extra length can push a borderline 2.5 mm² run over 3% drop, making 4 mm² the right call.
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.
Methodology and sources
This tool computes the steady-state resistive voltage drop in an AC branch circuit from the conductor's resistivity, length, cross-section and load current, then expresses it as a percentage of supply voltage and checks it against the 3% guideline.
- Method: Single-phase Vdrop = 2 × ρ × L × I / A; three-phase Vdrop = √3 × ρ × L × I / A, where ρ is conductor resistivity, L the one-way length, I the load current and A the cross-section. The factor 2 accounts for the out-and-return conductors of a single-phase loop. Drop percent = Vdrop / Vsupply × 100.
- Standards and sources: NEC Article 210.19(A) (US, 3% branch-circuit recommendation) and IEC 60364-5-52 (Europe and most of the world, 3% lighting / 5% other). Resistivity values ρ = 0.0172 Ω·mm²/m for copper and ρ = 0.0282 Ω·mm²/m for aluminium at 20 °C are standard annealed-conductor figures.
- Assumptions and limits: Resistance-only model at 20 °C; it ignores conductor reactance, so it under-reads at high frequency or with large inductive runs. At operating temperature, real drop is typically 5 to 10% above the calculated value, so size up when close to 3%. Uses actual one-way conductor length (add length for bends and risers), not straight-line distance.
Reviewed and maintained by Rick Oosterling, who builds and wires 12 V, solar and EV systems hands-on. Last reviewed: June 2026. This calculator is a planning aid, not a substitute for a qualified electrician or your local wiring and building code; verify all conductor and protection sizing against the code that applies to your installation.
Next step in this workflow
Drop within limits: now size the overcurrent protection for your circuit.