Fundamentals calculator
Voltage Drop Calculator
Single- and three-phase voltage drop for copper and aluminium conductors, with the smallest conductor that meets your limit. NEC and IEC units.
BasicsBasis: NEC Ch. 9 Table 8 · IEC 60228 · IEC 60364-5-52
Results
- Enter values to calculate
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Formula
Single-phase / DC: Vd = 2 · I · R · L / n
Three-phase: Vd = √3 · I · R · L / n
Vd% = 100 · Vd / V
Iload current (A),Lone-way length,nparallel setsRconductor resistance per unit length. NEC: Chapter 9 Table 8 at 75 °C. IEC: IEC 60228 at 20 °C, corrected to operating temperature withRT = R20 · [1 + α (T − 20)], α = 0.00393 (Cu) or 0.00403 (Al).
Worked example
NEC: 480 V three-phase feeder, 100 A, 250 ft of 1/0 AWG copper (0.122 Ω/1000 ft): Vd = 1.732 × 100 × 0.122 × 250 / 1000 = 5.28 V, which is 1.10 % of 480 V.
IEC: 230 V single-phase circuit, 32 A, 40 m of 6 mm² copper at 70 °C: R = 3.08 × [1 + 0.00393 × 50] = 3.685 Ω/km; Vd = 2 × 32 × 3.685 × 0.040 = 9.43 V, which is 4.10 %.
Assumptions and standards
- Resistance-only (DC) method. On AC circuits with conductors above about 1/0 AWG or 50 mm², reactance adds to the drop, especially at low power factor; use the NEC Chapter 9 Table 9 effective impedance or the IEC 60364-5-52 Annex G formula for those.
- NEC values are uncoated stranded copper and aluminium at 75 °C. IEC values are the IEC 60228 maximum for Class 2 conductors.
- Common targets: NEC 210.19(A) and 215.2(A) informational notes suggest 3 % for a branch circuit or feeder and 5 % in total. IEC 60364-5-52 Annex G suggests 3 % for lighting and 5 % for other uses on a public supply.
Formula and data last verified: October 2026.
Frequently asked questions
Is the NEC voltage-drop limit mandatory?
For most circuits it is not. The 3 % and 5 % figures appear in informational notes, which are recommendations. Some cases, such as sensitive electronic equipment and certain fire pumps, have enforceable limits, and local energy codes may also require them.
Why does the calculator suggest a larger conductor?
It checks every standard size from smallest to largest and shows the first one whose drop is within your allowed percentage, using the same system, length and number of parallel sets.
Should I use one-way or round-trip length?
Enter the one-way length from source to load. The formula already accounts for the return path (factor 2 for single-phase, √3 for three-phase).
Can I use these results for final design?
Use them for preliminary design and checking. Final designs must be verified by a qualified engineer against the code edition and standards adopted where the work is installed.
Is my data sent anywhere?
No. Every calculation runs in your browser. Nothing you type is uploaded or stored on our servers.