Wire Size: AWG (NEC) vs mm² (IEC) — HVAC Conductor Sizing Guide
HVAC engineers working across the US (NEC) and international (IEC) markets must translate between AWG and mm² wire sizing systems. This guide provides the reference crosswalk, motor branch circuit sizing rules, insulation rating comparisons, and worked examples for both 480 V / 208 V (US) and 400 V / 230 V (international) systems.
1. AWG to mm² Reference Cross-Reference
| AWG (NEC) | Actual mm² | IEC Standard Size | NEC Ampacity 75°C (conduit) | IEC Ampacity 70°C PVC (conduit) | IEC Ampacity 90°C XLPE (conduit) |
|---|---|---|---|---|---|
| 18 AWG | 0.82 mm² | 1.0 mm² | 7 A | 11 A | 13 A |
| 16 AWG | 1.31 mm² | 1.5 mm² | 13 A | 14 A | 17 A |
| 14 AWG | 2.08 mm² | 2.5 mm² | 20 A | 18 A | 22 A |
| 12 AWG | 3.31 mm² | 4 mm² | 25 A | 25 A | 30 A |
| 10 AWG | 5.26 mm² | 6 mm² | 35 A | 32 A | 40 A |
| 8 AWG | 8.37 mm² | 10 mm² | 50 A | 43 A | 54 A |
| 6 AWG | 13.3 mm² | 16 mm² | 65 A | 57 A | 73 A |
| 4 AWG | 21.2 mm² | 25 mm² | 85 A | 75 A | 95 A |
| 3 AWG | 26.7 mm² | 35 mm² | 100 A | 92 A | 115 A |
| 2 AWG | 33.6 mm² | 35 mm² | 115 A | 92 A | 115 A |
| 1 AWG | 42.4 mm² | 50 mm² | 130 A | 110 A | 140 A |
| 1/0 AWG | 53.5 mm² | 70 mm² | 150 A | 136 A | 170 A |
| 2/0 AWG | 67.4 mm² | 70 mm² | 175 A | 136 A | 170 A |
| 3/0 AWG | 85.0 mm² | 95 mm² | 200 A | 172 A | 215 A |
| 4/0 AWG | 107 mm² | 120 mm² | 230 A | 197 A | 250 A |
| 250 kcmil | 127 mm² | 150 mm² | 255 A | 226 A | 285 A |
| 300 kcmil | 152 mm² | 185 mm² | 285 A | 261 A | 330 A |
| 350 kcmil | 177 mm² | 185 mm² | 310 A | 261 A | 330 A |
| 400 kcmil | 203 mm² | 240 mm² | 335 A | 313 A | 395 A |
| 500 kcmil | 253 mm² | 300 mm² | 380 A | 358 A | 460 A |
Sources: NEC 2023 Table 310.16 (copper, in conduit); IEC 60364-5-52:2009 Table B.52.4 (copper, in conduit, method B2). Ambient temp 30°C (86°F). Three conductors loaded.
2. Key Differences: NEC vs IEC Conductor Sizing
2.1 Sizing System Origin
- AWG (American Wire Gauge) — originated in 1857. Lower numbers = larger wire. Non-linear scale: each 6 AWG decrease roughly doubles the cross-sectional area. Sizes above 4/0 AWG switch to kcmil (thousands of circular mils).
- IEC mm² — metric cross-sectional area in mm². Larger numbers = larger wire. Standard sizes per IEC 60228: 1.0, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300, 400, 500, 630 mm².
2.2 Motor Branch Circuit Sizing Rules
| Rule | NEC (NFPA 70) | IEC (60364-4-43) |
|---|---|---|
| Branch circuit minimum | 125% × FLC (NEC 430.22) | 125% × rated current (continuous duty) |
| FLC lookup | NEC Table 430.250 (3-phase, 460/480 V) | Calculated from motor nameplate: I = P / (√3 × V × η × cos φ) |
| Overcurrent protection (max) | 250% FLC for inverse-time breaker (NEC 430.52) | IEC 60947-4: coord. Type 1 or Type 2 (contactor + overload relay) |
| Conduit fill limit | NEC Chapter 9, Table 1: 40% fill (2+ conductors) | IEC 60364-5-52: grouping factors (Ca, Cg, Ci) |
| Temperature correction | NEC Table 310.15(B)(1) derating factors | IEC 60364-5-52 Table B.52.14 correction factors |
2.3 Insulation Types: NEC vs IEC
| NEC Insulation Type | IEC/BS Equivalent | Max Conductor Temp | Typical Application |
|---|---|---|---|
| TW | PVC 60°C | 60°C | Wet/dry locations, older buildings |
| THW / THHW | PVC 70°C | 75°C | General purpose, wet locations |
| THHN / THWN-2 | XLPE 90°C | 90°C | Conduit, dry/wet, HVAC equipment wiring |
| XHHW-2 | XLPE 90°C (wet) | 90°C | Wet locations, underground, HVAC |
| RHW-2 | EPR 90°C | 90°C | High flex applications, motor leads |
When using NEC 90°C ampacity from Table 310.16, the lower of the conductor rating and equipment terminal rating applies. Most HVAC equipment terminals are rated 75°C — use the 75°C column for final sizing.
3. Worked Examples
Example 1: 15 HP HVAC Compressor Motor, 480 V 3-Phase (NEC)
- From NEC Table 430.250: 15 HP at 460 V 3-phase → FLC = 21 A
- Branch circuit conductors: 125% × 21 A = 26.25 A minimum
- Select conductor: minimum 26.25 A — 10 AWG THHN (35 A at 75°C) ✅
- Overcurrent protection (inverse-time breaker): max 250% × 21 A = 52.5 A → 50 A breaker
- Voltage drop check (100 ft run, 3-phase): VD = (1.732 × 12.9 × 21 × 100) / 10,380 = 4.53 V = 0.94% ✅
Example 2: 11 kW HVAC Fan Motor, 400 V 3-Phase (IEC)
- Motor rated current: I = 11,000 / (1.732 × 400 × 0.88 × 0.85) = 21.2 A
- Branch circuit conductors: 125% × 21.2 A = 26.5 A minimum
- Select conductor: 6 mm² (32 A at 70°C in conduit per IEC 60364-5-52) ✅
- Overload relay setting: 21.2 A (class 10 thermal overload)
- Voltage drop check (30 m run): VD% = (1.732 × 0.0206 × 30 × 21.2 × 0.85 × 100) / (6 × 400) = 1.6% ✅
4. Temperature Derating: NEC vs IEC
Both standards require ampacity correction when ambient temperature exceeds the rated value (30°C / 86°F for most tables).
| Ambient Temperature | NEC Correction Factor (75°C insulation) | IEC Correction Factor (70°C PVC) |
|---|---|---|
| 30°C (86°F) | 1.00 (base) | 1.00 (base) |
| 35°C (95°F) | 0.94 | 0.94 |
| 40°C (104°F) | 0.88 | 0.87 |
| 45°C (113°F) | 0.82 | 0.79 |
| 50°C (122°F) | 0.75 | 0.71 |
| 60°C (140°F) | 0.58 | 0.50 |
Sources: NEC 2023 Table 310.15(B)(1); IEC 60364-5-52:2009 Table B.52.14.
5. Standards Reference
- NEC 2023 Article 310 — Conductors for General Wiring (ampacity tables, installation methods)
- NEC 2023 Article 430 — Motors, Motor Circuits, and Controllers (125% rule, overcurrent protection)
- IEC 60228:2004 — Conductors of insulated cables (standard conductor cross-sections)
- IEC 60364-5-52:2009 — Electrical installations of buildings: Selection and erection of electrical equipment — Wiring systems
- IEC 60502-1/2 — Power cables with extruded insulation (600 V to 30 kV)
- ASTM B3 — Standard Specification for Soft or Annealed Copper Wire (cross-section to CM conversion)
6. Frequently Asked Questions
What is the IEC equivalent of 12 AWG wire?
12 AWG (actual 3.31 mm²) is most closely matched by 4 mm² IEC conductor. For ampacity, 12 AWG rates 25 A at 75°C; IEC 4 mm² rates 25 A at 70°C PVC in conduit. Use 4 mm² when substituting in IEC-jurisdiction installations.
How does NEC wire sizing differ from IEC for HVAC motors?
Both require 125% of rated current for continuous motor circuits. The main differences: NEC looks up FLC from Table 430.250; IEC calculates from nameplate data. NEC specifies overcurrent protection as a percentage of FLC; IEC uses Type 1/Type 2 coordination between contactor and overload relay.
Can I use American AWG wire in IEC-standard countries?
AWG conductors are not certified to IEC 60228 or IEC 60502. For permanent installations in EU, UK, Australia, or Japan, use IEC-standard metric cable. AWG-to-mm² substitution requires local authority approval and is only acceptable for temporary or emergency situations.
What wire size for a 10 HP motor on 480 V 3-phase?
NEC Table 430.250: 10 HP / 460 V → FLC = 14 A. Branch circuit minimum = 125% × 14 = 17.5 A → 12 AWG (25 A at 75°C). Maximum OCPD: 250% × 14 = 35 A → 35 A breaker. For IEC 400 V equivalent (7.5 kW): I ≈ 15.2 A → conductor minimum 19 A → 4 mm² (25 A at 70°C).
What is the difference between NEC THHN and IEC XLPE?
Both are rated 90°C. THHN is a PVC jacket over thermoplastic insulation with a nylon jacket. XLPE is cross-linked polyethylene. For HVAC wiring calculations, both allow the same conductor operating temperature, but NEC limits terminal ratings to 75°C for most equipment. IEC 90°C XLPE tables generally show higher ampacity than NEC 75°C because of this terminal rating constraint.