Electrical Wiring Guide: Wire Sizing, Voltage Drop & NEC Basics

Correct wire sizing is a safety-critical calculation. An undersized conductor overheats under load, degrading insulation and creating fire risk. An oversized conductor wastes material cost. Voltage drop determines whether equipment at the end of a circuit receives adequate power. Together, these calculations form the foundation of every electrical installation — from a single outlet to a commercial distribution system.

This guide covers the AWG gauge system, NEC ampacity tables, the voltage drop formula for single-phase and three-phase circuits, conduit fill rules, and the derating adjustments that apply when conductors run in groups. All calculations are referenced to the 2023 National Electrical Code.

American Wire Gauge (AWG) is the US standard for wire sizes. The system is counterintuitive: smaller AWG numbers mean larger, heavier conductors. This inverse relationship traces to the manufacturing process — the AWG number roughly indicates how many wire-drawing steps produced the wire.

Key dimensions (copper, approximate):

• 18 AWG — 0.040 in / 1.02 mm diameter — low-voltage wiring, extension cords
• 14 AWG — 0.064 in / 1.63 mm — 15 A circuits, lighting
• 12 AWG — 0.081 in / 2.05 mm — 20 A circuits, receptacles
• 10 AWG — 0.102 in / 2.59 mm — 30 A circuits, dryers, water heaters
• 8 AWG — 0.128 in / 3.26 mm — 40–50 A circuits, ranges
• 6 AWG — 0.162 in / 4.11 mm — 55–65 A, subpanels, EV chargers
• 4 AWG — 0.204 in / 5.19 mm — 85 A service conductors
• 2 AWG — 0.258 in / 6.54 mm — 95–115 A service entrance
• 1/0 AWG — 0.325 in / 8.25 mm — 125–150 A service entrance
• 4/0 AWG — 0.460 in / 11.7 mm — 200–230 A service entrance

Each 3-gauge step roughly halves the cross-sectional area and doubles the resistance per unit length. Each 6-gauge step doubles the diameter.

AWG to mm² conversion: metric systems express wire size in cross-sectional area (mm²). Common equivalencies:

• 14 AWG ≈ 2.1 mm²
• 12 AWG ≈ 3.3 mm²
• 10 AWG ≈ 5.3 mm²
• 8 AWG ≈ 8.4 mm²
• 6 AWG ≈ 13.3 mm²

Ampacity is the maximum continuous current a conductor can carry without exceeding its temperature rating. NEC Table 310.16 lists ampacity values for copper and aluminum conductors in various conditions.

NEC Table 310.16 — copper conductor ampacity (selected rows):

• 14 AWG — 15 A (60°C), 20 A (75°C), 25 A (90°C)
• 12 AWG — 20 A (60°C), 25 A (75°C), 30 A (90°C)
• 10 AWG — 30 A (60°C), 35 A (75°C), 40 A (90°C)
• 8 AWG — 40 A (60°C), 50 A (75°C), 55 A (90°C)
• 6 AWG — 55 A (60°C), 65 A (75°C), 75 A (90°C)

In practice, the terminal temperature rating of connected equipment governs. Most residential devices (breakers, outlets, switches) are rated for 60°C or 75°C terminals. You may use 90°C-rated wire (like THHN), but you must still apply the 60°C or 75°C column ampacity at the terminations.

Load calculation: total circuit load must not exceed 80% of breaker rating for continuous loads (loads energized for 3 hours or more — lighting, motor loads). A 20 A breaker serving a continuous load circuit must not exceed 16 A of continuous current.

The Wire Size and Voltage Drop Calculator applies these table values and derating factors automatically.

Voltage drop is the reduction in voltage between the panel and the end of the circuit, caused by the resistance of the conductors. Even NEC-legal wire sizes can cause unacceptable voltage drop on long runs.

Single-phase / DC voltage drop formula:

VD = (2 × K × I × L) ÷ CM

• VD = voltage drop (volts)
• K = resistivity constant: 12.9 for copper, 21.2 for aluminum
• I = current draw (amperes)
• L = one-way circuit length (feet)
• CM = conductor area in circular mils (from AWG tables)

Three-phase voltage drop:

VD = (√3 × K × I × L) ÷ CM ≈ (1.732 × K × I × L) ÷ CM

Voltage drop percentage: VD% = (VD ÷ Source Voltage) × 100

Example — 20 A, 120 V circuit, 100-foot run, 12 AWG copper:

• CM for 12 AWG = 6,530 circular mils
• VD = (2 × 12.9 × 20 × 100) ÷ 6530 = 51,600 ÷ 6530 ≈ 7.9 V
• VD% = (7.9 ÷ 120) × 100 ≈ 6.6% — exceeds the 3% recommendation
• Solution: upsize to 10 AWG (CM = 10,380) → VD ≈ 4.97 V → 4.1% — still marginal
• Or: upsize to 8 AWG (CM = 16,510) → VD ≈ 3.1 V → 2.6% — within limits

NEC recommendations:

• Branch circuits: 3% maximum recommended
• Feeder circuits: 3% maximum recommended
• Combined (feeder + branch): 5% maximum recommended

These are in NEC Informational Notes (not mandatory), but are widely treated as the design standard. Some sensitive equipment (hospitals, data centers) uses tighter limits of 1–2%.

NEC Chapter 9 limits how many conductors can share a conduit to prevent heat buildup and wire damage during installation pulls.

Fill percentage limits (NEC Table 1, Chapter 9):

• 1 conductor: 53% of conduit interior cross-sectional area
• 2 conductors: 31%
• 3 or more conductors: 40%

Calculation method:

• Find the cross-sectional area of each conductor type and size (NEC Table 5)
• Sum the total area of all conductors in the conduit
• Look up the interior area of the conduit size (NEC Table 4 by conduit type)
• Divide conductor total by conduit interior area; compare to the applicable limit

Example — three 12 AWG THHN wires in ½-inch EMT:

• Area per 12 AWG THHN: 0.0133 in² (NEC Table 5)
• Total for 3 wires: 0.0133 × 3 = 0.0399 in²
• ½-inch EMT interior area: 0.122 in² (NEC Table 4)
• Fill: 0.0399 ÷ 0.122 = 32.7% — under 40% limit. Acceptable.

Adding a 4th wire: 0.0133 × 4 = 0.0532 in² → 43.6% — exceeds 40%. Move to ¾-inch EMT (interior area 0.213 in²) → 25% fill.

The Conduit and Wire Fill Calculator automates this lookup and recommends the minimum compliant conduit size.

Two conditions require reducing the ampacity below the NEC table values: elevated ambient temperature and bundled conductors sharing a raceway.

Temperature correction (NEC Table 310.15(B)(1)):

NEC ampacity tables assume a 30°C (86°F) ambient temperature. For higher ambient temperatures, apply a correction factor:

• 41–45°C (106–113°F): multiply by 0.87 for 75°C-rated wire
• 46–50°C (115–122°F): multiply by 0.82
• 51–55°C (124–131°F): multiply by 0.76
• 56–60°C (133–140°F): multiply by 0.71

Attics in southern climates regularly exceed 50°C in summer, making temperature derating important for wiring run through unventilated attic spaces.

Bundling / conductor count adjustment (NEC 310.15(C)):

• 4–6 current-carrying conductors in conduit: multiply ampacity by 0.80
• 7–9 conductors: multiply by 0.70
• 10–20 conductors: multiply by 0.50
• 21–30 conductors: multiply by 0.45

Neutral conductors that carry only unbalanced current are not counted for this adjustment. Bare equipment grounding conductors are never counted.

When both temperature correction and conductor bundling apply, multiply both factors together. A 12 AWG THHN in a conduit with 8 other conductors in a 50°C ambient: 25A × 0.82 (temp) × 0.70 (bundling) = 14.35 A effective ampacity. This may require upsizing to 10 AWG.

Reference wire sizes for standard US residential and light commercial applications:

• General lighting and receptacles: 14 AWG / 15 A breaker (lighting) or 12 AWG / 20 A breaker (receptacles per NEC 210.52)
• Kitchen small appliance circuits: 12 AWG / 20 A — required by NEC for countertop receptacles
• Bathroom receptacles: 12 AWG / 20 A, GFCI protected
• Electric dryer: 10 AWG / 30 A, 240 V, 4-wire circuit
• Electric range: 6 or 8 AWG / 40–50 A, 240 V
• Central AC (3-ton / 36,000 BTU): typically 30–40 A, 8–10 AWG; verify nameplate ampacity rating
• EV charger (Level 2, 48 A): 6 AWG copper / 60 A breaker; upsized to 4 AWG for runs over 75 feet
• 200 A residential service: 4/0 AWG copper or 250 kcmil aluminum

Always verify against the specific equipment's nameplate and confirm with the authority having jurisdiction (AHJ) before installation.

• Electrical Cable/Wire Size and Voltage Drop Calculator — AWG sizing, voltage drop, ampacity, and metric/imperial conversion
• Conduit and Wire Fill Calculator — fill percentage, minimum conduit size, NEC compliance check

Browse the full Engineering Calculators hub for duct sizing and pipe volume tools as well.