What wire size do I need for a 10 HP (7.5 kW) HVAC motor on 480 V 3-phase?

On 480 V 3-phase (NEC): 10 HP motor FLC = 14 A (NEC Table 430.250). Branch circuit conductor must be 125% × 14 A = 17.5 A → minimum 12 AWG (rated 25 A at 75°C). On 400 V 3-phase (IEC): 7.5 kW motor rated current ≈ 15.2 A at 0.85 PF, 0.9 efficiency. Conductor minimum = 125% × 15.2 = 19 A → 4 mm² (rated 25 A in conduit at 70°C) or 6 mm² if derating factors apply.

What is the difference between NEC THHN and IEC XLPE insulation ratings?

NEC THHN is a 90°C rated thermoplastic insulation (THHN = Thermoplastic High Heat-resistant Nylon-coated). NEC Table 310.16 allows using 90°C ampacity for derating calculations, but terminal ratings must still be checked (most equipment terminals are rated for 60°C or 75°C). IEC XLPE (cross-linked polyethylene) insulation is also rated 90°C. IEC 70°C PVC (equivalent to NEC TW or THW) uses a lower ampacity. When comparing, 90°C THHN ≈ 90°C XLPE in terms of conductor rating, but conduit fill, de-rating tables, and cable standards differ significantly.

Wire Size: AWG (NEC) vs mm² (IEC) — HVAC Conductor Sizing Guide

Q: What is the IEC equivalent of 12 AWG wire?

12 AWG (NEC) is approximately equivalent to 4 mm² in IEC/metric sizing. The actual conductor cross-section is 3.31 mm². The IEC standard size of 4 mm² (the next standard size up) is the correct substitute when specifying metric cable. For ampacity, 12 AWG is rated 25 A at 75°C in NEC Table 310.16; IEC 60364-5-52 rates 4 mm² copper in conduit at approximately 27 A at 70°C.

Q: How does NEC wire sizing differ from IEC for HVAC motors?

NEC 430.22 requires motor branch circuit conductors rated at 125% of the motor's full-load current (FLC). IEC 60364-4-43 uses a similar overload factor but specifies it differently: conductors must be sized for at least the rated current × 1.25 for continuous duty motors. The key practical difference is the ampacity tables: NEC uses Fahrenheit and 60°C/75°C/90°C insulation ratings; IEC uses Celsius and 70°C PVC or 90°C XLPE ratings. NEC also uses AWG circular-mil sizing while IEC uses mm².

Q: Can I use American AWG wire in IEC-standard countries?

AWG wire is not typically listed or certified for use in IEC-jurisdiction countries (EU, UK, Australia, Japan, etc.). You must use conductors that comply with local standards — IEC 60228 for flexible conductors or IEC 60502 for power cables. In emergency or commissioning situations, an AWG conductor can be substituted using the nearest mm² equivalent, but this requires local authority approval. Always specify IEC-standard cable for permanent installations outside North America.

Calculate Wire Size Free →

HVAC engineers working across the US (NEC) and international (IEC) markets must translate between AWG and mm² wire sizing systems. This guide provides the full cross-reference table, 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 Complete Cross-Reference Table

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.