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United States, Canada, Mexico
NEC (NFPA 70) — the North American standard for electrical installations.
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Wire Sizing Standards
What Are NEC and IEC Wire Sizing Standards?
NEC and IEC are the two dominant frameworks for sizing electrical conductors, and they serve the same job with different rulebooks.
NEC
North American Standard
The National Electrical Code, published by NFPA as standard 70, governs electrical installations across North America. Wire and cable sizing lives in Article 310, where Table 310.16 lists ampacity by conductor size and insulation temperature. Conductors are measured in American Wire Gauge (AWG), which runs backward: a smaller number means a thicker wire. Above 4/0 AWG, sizes are given in thousands of circular mils, kcmil.
IEC
International Standard
The International Electrotechnical Commission governs through IEC 60364, the standard for low-voltage electrical installations. Part 5-52 sets current-carrying capacity, and conductors are measured in square millimetres (mm²) of cross-sectional area. Many countries adopt IEC 60364 directly or as a national derivative: BS 7671 in the UK, AS/NZS 3008 in Australia and New Zealand, and GB standards in China.
Common Physics
Same Engineering
Underneath the different units and tables, both systems rest on the same thermal physics. A conductor heats up under load, and its rating is the current that holds it below the insulation's temperature limit in a given installation. The Neher-McGrath equations and IEC 60287 calculations that produce the tables are the same engineering either way. The divergence comes from how each code packages those results.
Two Standards, One Physics
Both systems rest on the same thermal physics. A conductor heats up under load, and its rating is the current that holds it below the insulation's temperature limit in a given installation. The Neher-McGrath equations and IEC 60287 calculations that produce the tables are the same engineering either way. The divergence comes from how each code packages those results — AWG vs mm², 75°C vs 70°C columns, and different installation correction factors.
Key Takeaways
Know Your Standard
01
NEC = North America
Uses AWG (backward gauge) and kcmil for larger sizes. Article 310 Table 310.16 is the primary ampacity reference.
02
IEC = Rest of the World
Uses mm² cross-sectional area. IEC 60364 Part 5-52 sets the current-carrying capacity tables for most countries outside North America.
03
Same Physics, Different Packaging
Both systems are built on Neher-McGrath and IEC 60287 thermal equations. The standards diverge in units, temperature columns, and correction factors.
04
Always Specify the Standard
A "14 AWG" conductor and a "2.08 mm²" conductor are the same size — but the ampacity values differ by code. Always specify which standard you are sizing to.
Conductor Units
Conductor Units: AWG vs mm²
The first practical difference is the unit of measurement, and the table below is the crosswalk between the two.
AWG to mm² Crosswalk & Typical 60°C Ampacity
| AWG | Cross-Section | Typical 60°C Ampacity |
|---|---|---|
| 14 AWG | 2.08 mm² | 15 A |
| 12 AWG | 3.31 mm² | 20 A |
| 10 AWG | 5.26 mm² | 30 A |
| 8 AWG | 8.37 mm² | 40 A |
| 6 AWG | 13.30 mm² | 55 A |
| 4 AWG | 21.15 mm² | 70 A |
| 2 AWG | 33.62 mm² | 95 A |
| 1/0 AWG | 53.49 mm² | 125 A |
| 4/0 AWG | 107.2 mm² | 195 A |
The Crosswalk Is Approximate — That Matters
You do not substitute a 6 AWG cable for a 13.3 mm² cable directly, because the metric world has no 13.3 mm² conductor. The IEC standard series runs 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, and 300 mm² and up.
A 6 AWG cable therefore maps to the nearest standard size, which is 16 mm², not to a notional 13.3 mm² value.
The rule is: convert the size, then select the nearest standard conductor in the target system, then confirm ampacity against that system's own table. The crosswalk gets you close; the ampacity table closes the gap.
Key Takeaways
How to Crosswalk AWG to mm²
01
Convert, Then Select Standard
AWG converts to a theoretical mm² value. Select the nearest standard IEC size — 6 AWG ≈ 13.3 mm² → use 16 mm².
02
IEC Standard Series
1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300 mm² — these are the actual conductor sizes available in metric markets.
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Crosswalk ≠ Direct Substitution
A theoretical 13.3 mm² conductor does not exist. Always confirm ampacity against the target system's own table.
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Ampacity Closes the Gap
The crosswalk gets you close; the ampacity table closes the gap. Always verify with the standard you are sizing to.
Standards Comparison
Key Differences at a Glance
The table below summarizes where NEC and IEC wire sizing diverge.
| Aspect | NEC (NFPA 70) | IEC (IEC 60364-5-52) |
|---|---|---|
| Conductor unit | AWG / kcmil | mm² |
| Ampacity source | Table 310.16 | Installation-method tables (B.52.x) |
| Temperature basis | 60 / 75 / 90°C columns | 70°C PVC / 90°C XLPE |
| Continuous load | 125% rule (NEC 210.19) | No continuous-load rule |
| Grouping derating | Fixed factors (1–3 = 1.00, 4–6 = 0.80) | Annex B tables by method |
| Voltage drop | 3% branch / 5% total (recommended) | ~3% / 4–5% (clause 525) |
| Cable tray fill | More permissive | More conservative |
| Next-size-up OCPD | Allowed | Not allowed |
The 125% continuous-load rule is the difference most engineers
trip on. NEC 210.19 and 210.20 require that a conductor and its overcurrent
device be sized at 125% of a continuous load, one that runs
three hours or more. A 40 A motor feeder is sized for 50 A. IEC 60364 has no
equivalent rule; its ampacity is defined as a cable property for prolonged
normal service, with correction factors applied only for installation
conditions such as temperature and grouping. The 125% concept in IEC practice
applies to the protective device, not the conductor.
Grouping derating also diverges. NEC uses a short table of adjustment factors, from 1.00 for three conductors down to 0.50 for ten to twenty. IEC 60364-5-52 uses more granular Annex B tables keyed to specific methods and tray layouts. And in cable trays the two are far apart: NEC Article 392 allows many more conductors per tray than IEC, which typically caps a run at around six cables with a derating near 0.82.
Grouping derating also diverges. NEC uses a short table of adjustment factors, from 1.00 for three conductors down to 0.50 for ten to twenty. IEC 60364-5-52 uses more granular Annex B tables keyed to specific methods and tray layouts. And in cable trays the two are far apart: NEC Article 392 allows many more conductors per tray than IEC, which typically caps a run at around six cables with a derating near 0.82.
Side‑by‑Side Comparison
Worked Example: One Load, Two Standards
The same load, sized under both standards, shows where the systems agree and where they part.
Worked Example
100 A, Three‑Phase, 50 m Run — NEC vs IEC
Take a 100 amp, three‑phase, 50 metre run of PVC‑insulated copper in conduit at 30°C ambient. Under IEC 60364-5-52, Reference Method B selects a 35 mm² conductor rated 110 A at 70°C. Under NEC Table 310.16, the design lands on 2 AWG, about 33.6 mm², rated 115 A at 75°C. The two answers are physically almost identical.
The convergence makes sense: a simple indoor conduit run at standard temperature is where the two standards are closest. They diverge in the harder cases. Add a buried run and IEC brings detailed correction tables for burial depth, ground temperature, and soil resistivity, while NEC points to a separate underground table. Load several cables into a tray and NEC allows more conductors per tray than IEC will accept. Run a continuous load and NEC adds the 125% factor while IEC does not.
The lesson is not that one standard is right. It is that the standards agree on the fundamentals and differ on the margins, and the margins are where real installations live.
Market Reference
Which Standard Applies to Your Market?
The decision between NEC and IEC is rarely a choice; it is set by where the equipment will be installed.
02
United Kingdom
BS 7671 — the UK national standard, derived from IEC 60364.
03
Europe
IEC 60364 or national derivatives (e.g., VDE in Germany, NF C 15-100 in France).
04
Middle East, Asia, Africa
IEC 60364 or national derivatives, with local variations in voltage and frequency.
05
Australia, New Zealand
AS/NZS 3008 — the IEC‑derived standard for cable sizing in the region.
06
China
GB standards — IEC‑aligned national standards for electrical installations.
For a manufacturer, this is where the standards question becomes a business question. The same generator set shipped to two customers needs two feeder‑cable specifications, one in AWG for North America and one in mm² for most of the rest of the world. The conductor that is correct under NEC is not automatically the conductor you specify under IEC, even though the underlying physics is identical.
Generator Feeders
From Wire Sizing to Generator Cable Sizing
Of all the conductors in a power system, the generator feeder is the one where NEC vs IEC wire sizing matters most. It carries continuous load, often a large motor‑starting surge, and it runs between the generator and the switchgear where a sizing error is expensive and hard to fix.
A
Under NEC
A generator feeder is a continuous load under NEC, so the 125% rule applies. It is usually a long run, so voltage drop governs. The 125% factor and voltage drop together often push the conductor size up significantly.
B
Under IEC
Under IEC, the same feeder is sized by installation method with no continuous‑load factor. This is why the same generator can carry a different cable size depending on the destination market. Get this right and the generator delivers its full rating; get it wrong and the cable runs hot or the voltage sags under load.
Once the conductor is specified, the next question is whether the generator itself is sized for the load it feeds. The feeder size and generator capacity must be coordinated — and both depend on the same load data and the same destination standard.
Let's talk
About Shandong Huali
Shandong Huali Electromechanical Co., Ltd. manufactures diesel generator sets, dust-suppression equipment, and industrial automation systems, with over 25 years of experience and exports to more than 20 countries. We size power equipment to both NEC and IEC standards every day, and we publish free electrical reference tools to help engineers and buyers specify power systems correctly.
Frequently Ask Questions
Q: What is the difference between NEC and IEC wire sizing?
A: NEC, used in North America, sizes conductors in AWG and lists ampacity by insulation temperature in Table 310.16, with a 125% continuous-load rule. IEC, used globally, sizes conductors in mm² and derives ampacity from the installation method in IEC 60364-5-52, with no continuous-load rule. The two reach similar results in simple cases and diverge in grouped, buried, and continuous-load installations.
Q: What is AWG vs mm²?
A: AWG is the American Wire Gauge, a scale where a smaller number means a thicker conductor. mm² is the metric cross-sectional area used by the IEC. A 10 AWG conductor is about 5.26 mm², and a 1/0 AWG is about 53.5 mm². The two systems use different standard size series, so conversion is approximate.
Q: How do I convert AWG to mm²?
A: Use a crosswalk table. Common conversions are 14 AWG to 2.08 mm², 10 AWG to 5.26 mm², 6 AWG to 13.3 mm², and 1/0 AWG to 53.5 mm². After converting, select the nearest standard metric size and confirm ampacity against the IEC table for the installation method.
Q: Which standard should I use, NEC or IEC?
A: It depends on where the equipment will be installed. North America uses NEC, while the UK, Europe, Asia, Africa, the Middle East, and Australia use IEC or an IEC-derived national standard. The market sets the standard, not preference.
Q: Why does NEC use a 125% continuous-load rule but IEC does not?
A: NEC 210.19 and 210.20 require conductors and overcurrent devices on continuous loads, those running three hours or more, to be sized at 125% of the load. IEC 60364 treats ampacity as a cable property for prolonged service and applies the 125% concept to protective devices instead, not to the conductor itself.
Q: What is IEC 60364?
A: IEC 60364 is the international standard for low-voltage electrical installations. Part 5-52 sets current-carrying capacity for conductors, keyed to installation method, ambient temperature, and grouping. Many national standards, including BS 7671 and AS/NZS 3008, are based on it.