NEC Code Reference: Wire Sizing Made Simple
February 2026 · 8 min read
Wire sizing is one of the most common calculations electricians perform — and one of the most commonly done wrong. Undersized wire is a fire hazard. Oversized wire wastes money. This guide breaks down the NEC wire sizing process into clear, actionable steps.
The Three Factors That Determine Wire Size
Every wire sizing decision comes down to three checks. The wire must satisfy all three — use the largest size required by any single factor:
- Ampacity — Can the wire carry the load current safely?
- Voltage drop — Will the voltage at the load be within acceptable limits?
- Overcurrent protection — Is the wire protected by the correct breaker size?
Step 1: Determine Load Current
Calculate the full-load current of the circuit. For motors, use NEC Table 430.248 (single-phase) or 430.250 (three-phase) — not the nameplate. For other loads, use:
I = P ÷ (V × PF)
Where I is current in amps, P is power in watts, V is voltage, and PF is power factor (use 1.0 for resistive loads, 0.8-0.85 for motor loads).
Step 2: Check Ampacity (NEC Table 310.16)
Table 310.16 is the workhorse. It gives allowable ampacities for insulated conductors rated 0-2000V in raceways, cables, or direct burial. Here's a simplified reference:
| Wire Size (AWG/kcmil) | 60°C (TW) | 75°C (THWN) | 90°C (THHN) |
|---|---|---|---|
| 14 | 15A | 15A | 15A |
| 12 | 20A | 20A | 20A |
| 10 | 30A | 30A | 30A |
| 8 | 40A | 50A | 55A |
| 6 | 55A | 65A | 75A |
| 4 | 70A | 85A | 95A |
| 3 | 85A | 100A | 115A |
| 2 | 95A | 115A | 130A |
| 1 | 110A | 130A | 145A |
| 1/0 | 125A | 150A | 170A |
| 2/0 | 145A | 175A | 195A |
| 3/0 | 165A | 200A | 225A |
| 4/0 | 195A | 230A | 260A |
Important: These are base ampacities. You must apply correction and adjustment factors.
Step 3: Apply Correction Factors
Temperature Correction (NEC 310.15(B)(1))
If the ambient temperature exceeds 30°C (86°F), you must derate. This matters in hot attics, mechanical rooms, and rooftop installations.
- 31-35°C: Multiply by 0.91 (75°C column)
- 36-40°C: Multiply by 0.82
- 41-45°C: Multiply by 0.71
- 46-50°C: Multiply by 0.58
Conduit Fill Adjustment (NEC 310.15(C)(1))
More conductors in a conduit = less heat dissipation = more derating.
- 1-3 conductors: No adjustment (100%)
- 4-6 conductors: Multiply by 0.80
- 7-9 conductors: Multiply by 0.70
- 10-20 conductors: Multiply by 0.50
Step 4: Voltage Drop Check
The NEC recommends (not requires) a maximum 3% voltage drop for branch circuits and 5% total for feeder + branch. For a single-phase circuit:
VD = (2 × L × I × R) ÷ 1000
Where L is one-way distance in feet, I is load current, and R is resistance per 1000 feet (from NEC Chapter 9, Table 8).
Voltage drop is especially critical on long runs — parking lot lights, well pumps, outbuildings. When in doubt, go up one wire size.
Skip the Math: Use an App
All of these calculations — ampacity with corrections, voltage drop, conduit fill — are built into Stats WireSpec. Enter your load, distance, conduit type, and ambient temperature, and it gives you the correct wire size instantly.
Or use our free Wire Size Calculator in your browser — no download needed.