Question 1

What is the formula for voltage drop?

Accepted Answer

For single-phase: VD = 2 × K × I × L ÷ CM (US) or VD = 2 × ρ × I × L ÷ A (metric), where K is resistivity (12.9 for copper, 21.2 for aluminum in CM·Ω/ft), I is current, L is one-way length, and CM is the wire area in circular mils.

Question 2

What is the maximum allowed voltage drop?

Accepted Answer

The NEC recommends a maximum voltage drop of 3% for branch circuits and 5% for the total of feeder plus branch circuit. Many electricians use 3% as the practical limit for optimal performance.

Question 3

How do I calculate wire size from voltage drop?

Accepted Answer

Rearrange the voltage drop formula: CM = 2 × K × I × L ÷ VD. Solve for the minimum circular mils and pick the next-larger standard AWG. This tool does the lookup automatically for AWG 0000 through AWG 40.

Question 4

Why does aluminum have more voltage drop than copper?

Accepted Answer

Aluminum has about 61% of copper's conductivity, so a K value of 21.2 vs copper's 12.9. To carry the same current with the same voltage drop, an aluminum conductor must be roughly one AWG size larger.