The Complete Cable Sizing Comparison: Every Factor, All Four Standards
A comprehensive reference comparing AS/NZS 3008, BS 7671, IEC 60364, and NEC across every cable sizing factor: current rating, voltage drop, short circuit, derating, and installation methods.
This is the master reference article for engineers who work across multiple standards. Rather than examining a single scenario, this article maps every key cable sizing parameter across all four standards — showing where they align, where they diverge, and what drives the differences.
Installation Method Classification
The first step in cable sizing is determining the installation method. Each standard classifies methods differently:
Scenario
Installation method classification across standards
| Parameter | AS/NZS | BS 7671 | IEC 60364 | NEC |
|---|---|---|---|---|
Enclosed in conduit on wall | Method B2Table 3 | Method BTable 4A2 | Method BTable B.52.1 | Table 310.16Conduit/raceway |
Clipped direct to surface | Method CTable 3 | Method CTable 4A2 | Method CTable B.52.1 | Table 310.16Open air |
On perforated cable tray | Method ETable 3 | Method ETable 4A2 | Method ETable B.52.1 | Table 392Separate tray tableNEC Article 392 |
Buried direct in ground | Method DTable 3 | Method DTable 4A2 | Method DTable B.52.1 | Table 310.60Separate underground tableNEC 310.60 |
Free air / spaced from wall | Method F/VV = ventilated trayTable 3 | Method FTable 4A2 | Method FTable B.52.1 | Table 310.17Free air tableNEC 310.17 |
Conductor Temperature Rating
This is the single biggest driver of ampacity differences:
Scenario
Conductor temperature ratings and their impact
| Parameter | AS/NZS | BS 7671 | IEC 60364 | NEC |
|---|---|---|---|---|
PVC insulation rating | 75°CHigher than IECAS/NZS 3008, Table 1 | 70°CTable 4A1 | 70°CTable 52.1 | 60°C / 75°C / 90°CMultiple columnsTable 310.16 |
XLPE insulation rating | 90°CAS/NZS 3008, Table 1 | 90°CTable 4A1 | 90°CTable 52.1 | 90°CTable 310.16 |
Reference ambient temperature | 30°C (air)25°C in ground | 30°C (air)20°C in ground | 30°C (air)20°C in ground | 30°C (air)20°C in ground |
Example: 16mm² PVC Cu, Method C | 76A75°C ratingTable 13 | 68A70°C ratingTable 4D2A | 68A70°C ratingTable B.52.4 | 65A/75A/90A60°C/75°C/90°CTable 310.16 |
Why AS/NZS Uses 75°C for PVC
AS/NZS 3008 rates PVC-insulated cables at 75°C maximum conductor temperature — 5°C higher than BS 7671 and IEC. This originates from the Australian cable manufacturing standard (AS/NZS 1125) which specifies a PVC compound with a slightly higher thermal rating. This 5°C difference results in approximately 10-12% higher ampacity at any given cable size.
Voltage Drop Limits
Scenario
Maximum allowable voltage drop by circuit type
| Parameter | AS/NZS | BS 7671 | IEC 60364 | NEC |
|---|---|---|---|---|
Lighting circuits | 5% totalAS/NZS 3000, Cl 3.6.2 | 3%From origin to luminaireTable 4Ab | 4% totalIEC 60364-5-52, 525 | 3% branch5% total210.19(A) Note 4 |
Power circuits | 5% totalAS/NZS 3000, Cl 3.6.2 | 5%From origin to loadTable 4Ab | 4% totalIEC 60364-5-52, 525 | 3% branch5% total210.19(A) Note 4 |
Sub-main allocation | 3% + 2%Typical split: sub-main + final | Not prescribedTotal limit only | 1.5% + 2.5%Typical IEC allocation | 2% + 3%Feeder + branch |
Mandatory or advisory? | MandatoryShall not exceed | MandatoryRegulation 525 | RecommendedNational annex may mandate | AdvisoryInformational Note only |
Short Circuit Protection (k-Factor Summary)
Scenario
k-factors for adiabatic equation across conductor/insulation types
| Parameter | AS/NZS | BS 7671 | IEC 60364 | NEC |
|---|---|---|---|---|
Copper / PVC (70-75°C) | k = 143Table 52 | k = 115Table 43.1 | k = 115Table 43A | Per ICEAManufacturer data |
Copper / XLPE (90°C) | k = 176Table 52 | k = 143Table 43.1 | k = 143Table 43A | Per ICEAManufacturer data |
Aluminium / PVC (70-75°C) | k = 94Table 52 | k = 76Table 43.1 | k = 76Table 43A | Per ICEAManufacturer data |
PE conductor (Cu/PVC) | k = 143Same as line conductor | k = 115Table 54.4 | k = 115Table 54.2 | Table 250.122Minimum PE size table |
Practical Summary: When Standards Matter Most
Based on the comparisons above, the standards diverge most significantly in these areas:
1. Long Cable Runs (>50m)
Voltage drop limits become the governing factor. NEC's 3% limit requires cables 1-2 sizes larger than BS/IEC's 4-5% allowance.
2. Grouped Installations (>6 circuits)
NEC's 90°C derating advantage allows smaller cables in grouped trays — up to 2.5× smaller than IEC/BS for heavily grouped configurations.
3. High Fault Level (>10kA)
AS/NZS k-factor (143) gives 54% more withstand capacity than BS/IEC (115). A cable that passes AS/NZS may need to go up two sizes for BS/IEC compliance.
4. Motor Circuits
NEC's multiplier approach to fuse sizing results in larger fuses (and therefore larger cables for coordination) than IEC's gM motor fuse system.
5. Elevated Ambient Temperature (>35°C)
NEC's 90°C base rating retains more capacity at high temperatures (correction factor 0.91 at 40°C vs 0.87 for 70°C-rated cables).
The Multi-Standard Design Strategy
For projects that must satisfy multiple standards simultaneously:
- Use the most conservative voltage drop limit (typically NEC 3%)
- Use the lowest k-factor for short circuit (BS/IEC k=115)
- Use the highest derating factors (BS/IEC starting from 70°C base)
- Verify each check independently — no single standard is universally most conservative
Quick Reference: Which Standard Is Most Conservative?
| Factor | Most Conservative | Most Permissive |
|---|---|---|
| Current rating (PVC) | NEC at 60°C | AS/NZS at 75°C |
| Current rating (XLPE) | All agree at 90°C | — |
| Voltage drop | NEC (3% branch) | AS/NZS (5% total) |
| Short circuit k-factor | BS 7671 / IEC (k=115) | AS/NZS (k=143) |
| Grouping derating | All similar (~0.50 for 12 cables) | NEC (90°C base advantage) |
| Temperature correction | BS/IEC (starts from 70°C) | NEC (starts from 90°C) |
| Motor protection | NEC (largest fuse sizes) | IEC (gM fuse, smallest) |
No single standard is universally the most or least conservative. The "answer" depends on which factor governs your specific circuit.
Related Resources
- Grenfell Tower: Fire-Resistant Cable Sizing — BS 7671 cable sizing with MICC-specific derating factors
- Why Aluminium Cables Are Making a Comeback — How conductor material choice affects sizing across standards
- The Complete Guide to Cable Sizing (All Standards) — Methodology walkthrough from design current to final selection
- Cable Sizing: The 50m Office Feeder — One specific scenario, four standards, full comparison
- Cable Derating: 12 Cables in a Tray at 40°C — NEC's 90°C column advantage under grouping
- Voltage Drop: The 100m Workshop Cable — How voltage drop limits drive cable upsizing differently
- View all worked examples →
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Lead Electrical & Instrumentation Engineer
18+ years of experience in electrical engineering at large-scale mining operations. Specializing in power systems design, cable sizing, and protection coordination across BS 7671, IEC 60364, NEC, and AS/NZS standards.