What is the NEC 3% rule for voltage drop?

The National Electrical Code (NEC) recommends that voltage drop on branch circuits not exceed 3% of the source voltage under full load, with a combined feeder and branch circuit drop not exceeding 5%. Exceeding these limits can cause equipment malfunction, overheating, and reduced efficiency.

Why is copper better than aluminum for wiring?

Copper has a resistivity of 1.724×10⁻⁸ Ω·m compared to aluminum's 2.65×10⁻⁸ Ω·m. Copper conductors are about 61% more conductive, meaning less voltage drop for the same wire size. Aluminum is lighter and cheaper, so it is used for large feeders and utility distribution.

AWG stands for American Wire Gauge, a standardized system for wire sizes. Lower AWG numbers indicate thicker wire with lower resistance. Common household circuits use AWG 14 (15A) or AWG 12 (20A).

Voltage Drop Calculator

When electrical current flows through a wire, it loses some voltage along the way due to the resistance of the conductor. This is called voltage drop. Understanding and controlling voltage drop is essential for safe, efficient, and code-compliant electrical installations. This calculator uses the NEC (National Electrical Code) 3% guideline and the fundamental physics of conductor resistance to determine whether your cable sizing is adequate.

Why Voltage Drop Matters

Excessive voltage drop causes real problems:

Equipment malfunction: Motors running at reduced voltage draw higher current, overheat, and fail prematurely.
Dimming and flickering lights: Voltage-sensitive LEDs and fluorescent fixtures are noticeably affected at drops above 3–5%.
Overheating conductors: Undersized wires carry excess current (due to the load drawing more current at lower voltage), causing insulation damage and fire risk.
Reduced efficiency: Power lost as heat in conductors is wasted energy that adds to operating costs.

The NEC recommends a maximum of 3% voltage drop on branch circuits and a combined feeder + branch circuit drop of no more than 5%.

The Formula

For a single-phase AC or DC circuit, the voltage drop across both conductors (supply and return) is:

ΔV = (2 × ρ × I × L) / A

Where:

ΔV = voltage drop (V)
ρ = resistivity of conductor material (Ω·m): copper = 1.724×10⁻⁸, aluminum = 2.65×10⁻⁸
I = current (A)
L = one-way length (m)
A = cross-sectional area (m²)

The percentage drop is: ΔV% = (ΔV / Vsource) × 100

AWG Wire Sizes

American Wire Gauge (AWG) is the US standard for wire diameter. Lower numbers mean thicker wire. Common residential wire sizes:

AWG	Area (mm²)	Typical Use
14	2.08	15A branch circuits
12	3.31	20A branch circuits
10	5.26	30A dryer/AC circuits
8	8.37	40–50A range circuits
6	13.3	60A sub-panels

For metric installations, wire size is specified directly in mm².

Copper vs Aluminum

Copper is the preferred conductor for most residential and commercial wiring because:

Higher conductivity (lower resistivity) means less voltage drop for the same wire size.
Better mechanical properties — easier to terminate and more resistant to vibration.

Aluminum is used for:

Large-capacity feeders and service entrance conductors.
Overhead utility distribution lines.
Cost savings on very large conductors (aluminum is significantly cheaper than copper per kilogram, though requires a larger size for equivalent performance).

Aluminum conductors require special connectors and anti-oxidant compound at terminations.

Worked Examples

Example 1 — 20A Branch Circuit

A 20A kitchen circuit (AWG 12 copper) runs 25 m to a distant outlet. System voltage is 120 V.

A = 3.31 mm² = 3.31×10⁻⁶ m²
ΔV = (2 × 1.724×10⁻⁸ × 20 × 25) / 3.31×10⁻⁶
ΔV ≈ 5.21 V → 4.3% drop → Warning (exceeds 3% NEC limit)
Solution: upgrade to AWG 10 (5.26 mm²) → ΔV ≈ 3.28 V → 2.7% ✓

Example 2 — European 230 V Circuit

A 6 mm² copper cable runs 50 m carrying 20A at 230 V.

ΔV = (2 × 1.724×10⁻⁸ × 20 × 50) / 6×10⁻⁶ ≈ 5.75 V → 2.5% → OK

Three-Phase Circuits

This calculator assumes single-phase (or DC) circuits. For balanced three-phase systems, the formula changes:

ΔV₃φ = (√3 × ρ × I × L) / A

The three-phase voltage drop is approximately 86.6% of the single-phase result for the same wire size, length, and current.

Reference

National Electrical Code (NEC), NFPA 70, Article 210.19(A)(1) FPN No. 4. IEEE Std 399 — Recommended Practice for Industrial and Commercial Power Systems Analysis.