Cable Sizing Guide: BS 7671, IEC 60364, NEC & AS/NZS [Free]

Cable sizing is the single most common calculation in electrical engineering — and one of the most consequential. An undersized cable overheats, degrades insulation, and can cause fire. An oversized cable wastes copper, increases installation cost, and may not be properly protected by the upstream device. Getting it right requires a systematic, standards-referenced methodology.

This guide walks through the universal cable sizing process used across BS 7671, IEC 60364-5-52, NEC/NFPA 70, and AS/NZS 3008.1.1 — the four major international wiring standards. Whether you design to one standard or work across multiple jurisdictions, the fundamental methodology is the same.

What is Cable Sizing?

Cable sizing is the process of selecting the correct conductor cross-sectional area to safely carry a given load current under specific installation conditions, while also satisfying voltage drop limits and short circuit withstand requirements.

The cable must simultaneously satisfy three independent criteria:

Current carrying capacity — the cable must carry the design current without exceeding its maximum operating temperature
Voltage drop — the voltage at the load terminals must remain within acceptable limits
Short circuit withstand — the cable must survive the thermal stress of fault current until the protective device disconnects

Safety-Critical Calculation

Cable sizing directly affects electrical safety. An incorrectly sized cable can overheat under normal load, cause nuisance tripping, or fail to deliver adequate voltage to equipment. Always verify calculations against the relevant standard and have them reviewed by a competent person.

The Cable Sizing Process (Step-by-Step)

All four major standards follow essentially the same nine-step process. The differences lie in the specific tables, reference temperatures, and safety margins — not in the methodology itself.

Step 1: Determine Design Current (I_b)

The design current is the maximum sustained current the cable will carry under normal operating conditions. For resistive loads, this is straightforward. For motors, transformers, and other reactive loads, power factor must be included.

Three-Phase Design Current

I_b = P / (√3 × V_L × PF × η)

Where:

P = active power (W)
V_L = line-to-line voltage (V)
PF = power factor (typically 0.8–0.95 for motors)
η = efficiency of the load

Single-Phase Design Current

I_b = P / (V × PF)

For motor circuits, the design current should be the motor full-load current (FLC) from the manufacturer's nameplate — not calculated from the rated power, since motor FLC already accounts for power factor and efficiency.

Practical Tip

For motor circuits, always use the nameplate full-load current. The calculated value from rated kW will differ from the nameplate value because manufacturers account for actual efficiency and power factor at rated load.

Step 2: Select Protective Device Rating (I_n)

The nominal rating of the protective device (MCB, MCCB, or fuse) must satisfy:

Protective Device Selection

I_n ≥ I_b

Select the next standard device rating above the design current. For example, if I_b = 38 A, select a 40 A device.

The protective device also determines the minimum cable current carrying capacity, since the cable must be able to carry the device rating continuously without the device tripping.

Step 3: Determine Installation Method

The installation method describes how the cable is physically installed — in conduit, on cable tray, in free air, direct buried, etc. This significantly affects heat dissipation and therefore the cable's current carrying capacity.

Each standard classifies installation methods differently:

Standard	Reference	Methods
BS 7671	Table B.52.1	Reference Methods A1, A2, B1, B2, C, D1, D2, E, F, G
IEC 60364	Table B.52.1	Same as BS 7671 (BS 7671 adopts IEC classification)
NEC	Table 310.15(B)(2)	Raceway, cable, direct buried, free air
AS/NZS 3008	Table 3	29 installation methods (numbered 1–29)

Installation Method Matters

The same cable can carry significantly different currents depending on installation method. A 16 mm² 4-core cable on an open perforated tray might be rated at 73 A, but the same cable in enclosed conduit might only be rated at 57 A. Always select the correct installation method before looking up current ratings.

Step 4: Ambient Temperature Derating

Cable current ratings are tabulated at a reference ambient temperature. If the actual ambient temperature differs, a correction factor must be applied.

Reference ambient temperatures by standard:

BS 7671: 30°C (in air), 20°C (in ground)
IEC 60364: 30°C (in air), 20°C (in ground)
NEC: 30°C (in air), 20°C (in ground)
AS/NZS 3008: 40°C (in air), 25°C (in ground)

AS/NZS 3008 Uses 40°C Base

AS/NZS 3008 uses a 40°C reference ambient — 10°C higher than BS 7671, IEC, and NEC. This means AS/NZS current ratings are lower than the other standards for the same cable at the same temperature. When comparing results across standards, this difference must be accounted for.

The temperature correction factor (k₁) is:

Temperature Correction Factor

k₁ = √((T_max - T_ambient) / (T_max - T_reference))

Where T_max is the maximum conductor operating temperature (70°C for PVC, 90°C for XLPE), T_ambient is the actual ambient, and T_reference is the standard's reference temperature.

For temperatures higher than the reference, k₁ < 1 (the cable must be derated). For temperatures lower than the reference, k₁ > 1 (the cable can carry more current).

Step 5: Grouping (Proximity) Derating

When multiple cables are installed together, they generate mutual heat. This reduces each cable's current carrying capacity. The grouping correction factor (k₂) depends on:

Number of circuits or cables grouped together
Arrangement (touching, spaced, single layer, multiple layers)
Whether cables carry the same or different loads

Typical grouping factors:

Circuits Grouped	Single Layer (touching)	Single Layer (spaced)
2	0.80	0.85
3	0.70	0.79
4	0.65	0.75
6	0.57	0.72
9	0.50	0.69

When Grouping Can Be Ignored

If cables are spaced by at least one cable diameter in free air, or if diversity can be demonstrated (not all circuits loaded simultaneously), grouping factors can be relaxed. Check the specific standard for conditions where reduced grouping applies.

Step 6: Other Derating Factors

Additional derating factors may apply depending on the installation:

Thermal insulation (k₃): If a cable is enclosed in thermal insulation, the current rating may need to be reduced by up to 50%, depending on the length of cable in contact with insulation. BS 7671, Table 52.2 — Reduction factors for cables in thermal insulation
Soil thermal resistivity: For buried cables, the standard assumes a reference soil resistivity (typically 2.5 K·m/W). Different soil conditions require correction. IEC 60364, Table B.52.16
Depth of burial: Cables buried deeper than the reference depth (typically 0.5–0.8 m) dissipate heat less effectively.
Solar radiation: Cables exposed to direct sunlight receive additional thermal load. AS/NZS 3008 provides specific correction methods for this.

Step 7: Calculate Required Current Capacity

The required tabulated current carrying capacity is:

Required Current Carrying Capacity

I_z = I_n / (k₁ × k₂ × k₃ × ...)

Select the cable from the appropriate current rating table where the tabulated value ≥ I_z.

The tabulated current rating tables are the core of each standard:

BS 7671: Tables 4D1A through 4J4A (Appendix 4)
IEC 60364: Tables B.52.2 through B.52.14
NEC: Table 310.16 (0–2000V, most common)
AS/NZS 3008: Tables 13, 14, and 15

Step 8: Verify Voltage Drop

After selecting a cable based on current carrying capacity, you must verify that the voltage drop does not exceed the allowable limit.

Voltage Drop

Vd = (mV/A/m × I_b × L) / 1000

Where mV/A/m is the voltage drop per ampere per metre from the standard's impedance tables, I_b is the design current, and L is the route length in metres.

Maximum allowable voltage drop by standard:

Standard	From Origin to Load
BS 7671	3% lighting, 5% other (from Appendix 12)
IEC 60364	3% lighting, 5% other (typical national values)
NEC	3% branch, 5% total (recommended, not mandatory)
AS/NZS 3008	5% from point of supply to point of use

If the voltage drop exceeds the limit, you must either increase the cable size or reduce the circuit length.

Voltage Drop Often Governs on Long Runs

For circuits longer than about 50 metres, voltage drop often requires a larger cable than current carrying capacity alone. Always check both criteria — the final cable selection is the larger of the two.

Step 9: Verify Short Circuit Withstand

The cable must withstand the thermal stress of fault current until the protective device disconnects. This is verified by the adiabatic equation:

Adiabatic Equation

I²t ≤ k²S²

Where:

I = prospective fault current (A)
t = protective device disconnection time (s)
k = constant depending on conductor and insulation material (BS 7671 Table 43.1: copper/PVC = 115, copper/XLPE = 143)
S = conductor cross-sectional area (mm²)

If the selected cable does not satisfy this equation, a larger cable size must be chosen. In practice, this constraint rarely governs for distribution cables but can be critical for long runs or high fault level installations.

Cable Sizing per BS 7671 (18th Edition)

BS 7671, Section 523 — Current-carrying capacities

BS 7671:2018+A2:2022 is the UK standard for electrical installations, published jointly by the IET and BSI. For cable sizing, the key references are:

Table B.52.1: Installation methods (reference methods A1 through G)
Tables 4D1A–4J4A: Current carrying capacity for different cable constructions and installation methods
Table 4B1: Correction factors for ambient temperature
Table 4C1–4C6: Correction factors for grouping
Table 52.2: Reduction factors for thermal insulation
Appendix 4, Table 4Ab–4Kb: Voltage drop data (mV/A/m)

BS 7671 uses 30°C reference ambient temperature and provides current ratings for common UK cable constructions (flat twin and earth, singles in conduit, SWA multicore, etc.).

The standard requires coordination between the protective device and the cable such that the cable is protected against both overload (Section 433) and fault current (Section 434).

Cable Sizing per IEC 60364-5-52

IEC 60364-5-52, Table B.52.1 — Installation methods

IEC 60364-5-52 is the international cable sizing standard, adopted (often with national variations) in most countries outside the UK, USA, and Australia/NZ. BS 7671 is closely aligned with IEC 60364, so the methodology is very similar.

Key differences from BS 7671:

IEC provides reference tables that national standards may modify
Some installation method definitions differ slightly
Current rating tables may use different cable constructions reflecting local practices
Voltage drop limits are typically set by national annexes

Cable Sizing per NEC/NFPA 70

NEC, Article 310 — Conductors for General Wiring

The National Electrical Code (NEC) uses a different table structure from BS 7671 and IEC 60364:

Table 310.16: Ampacities for 0–2000V conductors (most commonly used table)
Section 310.15(B): Adjustment and correction factors
Table 310.15(B)(1): Temperature correction factors
Table 310.15(B)(2): Adjustment for number of current-carrying conductors
Chapter 9, Table 9: AC resistance and reactance for voltage drop

NEC uses AWG/kcmil conductor sizes rather than metric mm². It provides ampacities at three insulation temperature ratings: 60°C, 75°C, and 90°C. The applicable temperature rating depends on the termination temperature rating of the equipment (typically 75°C for most industrial equipment).

NEC Termination Temperature Rule

Even when using 90°C rated cable (THHN/THWN-2), the ampacity used for conductor selection is typically limited by the termination temperature rating — usually 75°C for equipment rated over 100 A, and 60°C for equipment rated 100 A or less. The 90°C column can only be used for derating purposes.

Cable Sizing per AS/NZS 3008

AS/NZS 3008.1.1, Table 13 — Current-carrying capacity

AS/NZS 3008.1.1 is the Australian and New Zealand standard for cable selection. Its most distinctive feature is the 40°C reference ambient temperature — 10°C higher than other major standards — reflecting the warmer Australian climate.

Key tables:

Table 3: Installation methods (29 methods, numbered 1–29)
Tables 13, 14, 15: Current carrying capacity for different cable types
Table 22: Temperature correction factors (referenced from 40°C)
Table 25: Grouping correction factors
Table 35–42: Voltage drop data (mV/A/m)

The 2025 edition introduces updated ratings, LSZH cable data, and improved IEC harmonisation.

Common Mistakes Engineers Make

Wrong installation method — Selecting "on cable tray" when cables are actually in enclosed trunking. This can overstate the current rating by 10–20%.
Forgetting grouping factors — Multiple circuits sharing a tray or conduit must be derated. This is the most commonly missed derating factor.
Using incorrect base temperature — Mixing AS/NZS tables (40°C base) with BS 7671 correction factors (30°C base) produces incorrect results.
Ignoring voltage drop on long runs — Current capacity is satisfied but the 5% voltage drop limit is exceeded. Always check both.
Not checking short circuit withstand — Particularly on long sub-main cables where fault levels are still significant.
Using 90°C ampacity directly (NEC) — Ignoring the termination temperature limitation and using the full 90°C column for conductor selection.

Worked Example

Problem: Size a cable for a 45 kW, 415 V, three-phase motor with PF = 0.85 and η = 0.92. Installation: multicore XLPE/SWA on perforated cable tray, ambient temperature 35°C, grouped with 5 other circuits (single layer, touching). Circuit length: 80 m. Standard: BS 7671.

Step 1: Design current

Design Current Calculation

I_b = 45,000 / (√3 × 415 × 0.85 × 0.92) = 80.2 A

Step 2: Protective device — Select 100 A MCCB (I_n = 100 A ≥ 80.2 A)

Step 3: Installation method — Multicore on perforated tray = Reference Method E (BS 7671 Table B.52.1)

Step 4: Temperature correction — 35°C ambient, XLPE (90°C max). From Table 4B1: k₁ = 0.94

Step 5: Grouping correction — 6 circuits, single layer touching. From Table 4C1: k₂ = 0.57

Step 6: No other derating factors apply

Step 7: Required current capacity

Required I_z

I_z = 100 / (0.94 × 0.57) = 186.6 A

From Table 4E4A (multicore XLPE/SWA, Method E): 25 mm² = 131 A, 35 mm² = 162 A, 50 mm² = 196 A ✓

Select 50 mm² 4-core XLPE/SWA cable.

Step 8: Voltage drop check

From Table 4E4B, 50 mm² 3-phase: 0.86 mV/A/m

Voltage Drop

Vd = 0.86 × 80.2 × 80 / 1000 = 5.5 V = 1.33% of 415 V ✓

This is well within the 5% limit. Cable selection is confirmed.

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Applies all applicable derating factors from the correct standard tables
Selects the minimum cable size from the published current rating tables
Verifies voltage drop against the standard's limit
Checks short circuit withstand
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