Question 1

How do I calculate voltage drop in a wire?

Accepted Answer

Voltage drop is Vd = k · I · R1, where I is the current, R1 = ρ · L / A is the one-way conductor resistance (ρ is resistivity, L one-way length, A cross-section), and k is 2 for DC two-wire and single-phase AC or √3 for balanced three-phase. For example copper, 3.31 mm² (12 AWG), 30 m, 10 A, single-phase gives R1 = 0.0172 × 30 / 3.31 = 0.156 Ω and Vd = 2 × 10 × 0.156 = 3.12 V, or 2.6 % of 120 V.

Question 2

What is an acceptable voltage drop?

Accepted Answer

A common rule of thumb is to keep the drop under about 3 % on a branch circuit and 5 % overall from the source to the load. Japan's internal wiring code (内線規程) is stricter for many cases. These are guidelines — the governing installation code always takes priority, and motors or sensitive electronics may need tighter limits.

Question 3

Does conductor material change the voltage drop?

Accepted Answer

Yes. Copper has a resistivity of about 0.0172 Ω·mm²/m at 20 °C while aluminium is about 0.0282 Ω·mm²/m, roughly 64 % higher. For the same size and length an aluminium run drops about 1.64 times the voltage of copper, so aluminium conductors are usually sized one or two gauges larger.

Question 4

How does temperature affect the result?

Accepted Answer

Metal resistance rises with temperature: ρT = ρ20 · (1 + α(T − 20)), with α about 0.00393 per °C for copper and 0.00403 for aluminium. A conductor at 70 °C carries roughly 20 % more resistance than at 20 °C, so the voltage drop grows accordingly. Enter the expected operating temperature for a realistic figure.

Question 5

How do I convert AWG to a cross-section in mm²?

Accepted Answer

The diameter is d = 0.127 × 92^((36 − AWG) / 39) in millimetres, and the area is A = π/4 · d². For 12 AWG that gives d ≈ 2.053 mm and A ≈ 3.31 mm². Each 3 AWG step roughly doubles or halves the area. This tool does the conversion for you when you pick an AWG size.

Voltage DropCalculator

Voltage Drop
Calculator