Wire Size Guide
A comprehensive engineering reference covering key concepts, formulas, standards, and best practices.
What Is This Calculator For?
Wire sizing selects the minimum conductor cross-section that satisfies both ampacity and voltage drop requirements per NEC.
Key Input Parameters Explained
The accuracy depends on quality of inputs. Below are the key parameters.
| Parameter | Symbol | Typical Range | Notes |
|---|---|---|---|
| Load Current | I | 1–5000 A | ×1.25 for continuous |
| Length | L | 1–5000 m | One-way |
| Material | — | Cu/Al | Copper or aluminum |
Formulas & Calculation Methods
Ampacity: I_req = I_load × 1.25 → NEC Table 310.16
Voltage drop: cmil = (2×K×I×L)/V_drop (1-phase); cmil = (√3×K×I×L)/V_drop (3-phase)
Common Mistakes
- Ampacity-only sizing ignoring voltage drop
- Not applying 125% continuous load factor
- Using wrong column (60°C vs 75°C terminals)
- Not derating for >3 conductors in conduit
Applicable Standards
NEC 310.16, NEC 210.19, IEC 60228, IEEE 835
Frequently Asked Questions
What size wire for 20A?
12 AWG copper (75°C column: 25A). For >50 ft, check voltage drop.
Copper vs aluminum?
Al lighter/cheaper but needs ~1 size larger. Copper for branch circuits.
AWG vs mm²?
12 AWG ≈ 3.31 mm², 10 AWG ≈ 5.26 mm², 1/0 ≈ 53.5 mm².