BS 7671 is the wiring regulations of the United Kingdom. AS/NZS 3000 is the wiring rules of Australia and New Zealand, and AS/NZS 3008.1 gives its cable ratings. The two systems size a cable in the same general way. Their numbers do not transfer from one to the other.
This page compares the structure and the reference conditions. It gives no BS rating or factor. AmpSize sizes cables only to AS/NZS 3000 and AS/NZS 3008.1.
Where the ratings are
BS 7671:2018 keeps its current ratings in Appendix 4. That appendix is informative. The regulations set the requirements, and the appendix gives one method to meet them.
AS/NZS 3000:2018 Cl 3.4 deals with current-carrying capacity. The ratings are in a separate standard, AS/NZS 3008.1. That standard has a part for Australia and a part for New Zealand.
Reference conditions
Each set of ratings applies to one set of site conditions. A rating applies without correction only at those conditions.
Air. BS 7671 rates cables in air at 30 °C, with its correction in Table 4B1. The NZ part of AS/NZS 3008.1 also uses 30 °C. The AU part uses 40 °C.
Ground temperature. BS 7671 rates buried cables at 20 °C, with its correction in Table 4B2. The NZ part uses 15 °C and the AU part uses 25 °C.
Soil. BS 7671 assumes a soil thermal resistivity of 2.5 K·m/W, with its correction in Table 4B3. Both AS/NZS parts assume 1.2 K·m/W. Soil with a higher resistivity removes heat more slowly.
Depth. BS 7671 assumes a depth of 0.7 m, with its correction in Table 4B4. Both AS/NZS parts assume 0.5 m.
The page on the AU and NZ conditions compares the two AS/NZS parts in more detail.
What the conditions alone do
This example uses AS/NZS factors only. It shows how much the reference conditions move a result. It is not a BS 7671 result.
A 32 A single-phase circuit at 230 V runs 25 m. The cable is multicore copper with PVC insulation, buried direct. At the NZ reference of 15 °C soil, 1.2 K·m/W and 0.5 m, the engine selects 4 mm². Its rating is 36 A. Open the NZ reference case.
Now enter the BS 7671 reference ground conditions as site conditions: 20 °C, 2.5 K·m/W and 0.7 m. AS/NZS 3008.1.2:2017 gives a correction factor (derating factor in 2017) for each one. The factors are 0.95 for soil temperature, 0.74 for resistivity and 0.98 for depth. Together they give 0.6889. The cable must now have a tabulated rating of 46.45 A. The engine selects 10 mm², which carries 42.03 A after correction. Open the case at the BS reference ground conditions.
The circuit is the same, but the size goes from 4 mm² to 10 mm². The reference conditions are not a small detail.
Installation methods
BS 7671 Table 4A2 lists installation methods. It gives each one a reference method, from A to G. The current ratings are given for each reference method.
AS/NZS 3008.1 Tables 3(1) to 3(4) group the installation methods that have the same rating. Each entry refers to a column of Tables 4 to 21 and to the correction tables that apply. The names and the groups are different. Do not match a BS reference method to an AS/NZS column by its name.
Grouping and correction factors
BS 7671 gives its grouping factors in Tables 4C1 to 4C6. It calls each multiplier a rating factor, with a symbol for each condition. Ca is for ambient temperature, Cg for grouping, Cs for soil and Cd for depth.
AS/NZS 3008.1 gives its grouping factors in Tables 22 to 26. AmpSize multiplies them with the other correction factors into one total. The page on current-carrying capacity and derating explains how.
The overload rule
Both systems use the same pair of conditions. The design current Ib must not exceed the device rating In. The device rating In must not exceed the cable capacity Iz. The current that makes the device operate must not exceed 1.45 times Iz.
BS 7671 states the rule in Regulation 433.1.1. AS/NZS 3000:2018 states it in Cl 2.5.3.1. The page on Ib, In and Iz works through it. The two rules have the same form, but Iz comes from a different set of ratings in each system.
Voltage drop
BS 7671 Section 525 deals with voltage drop. Appendix 4, section 6.4, gives values for an installation that a public low voltage network supplies. They are 3 % for lighting and 5 % for other uses. The appendix says "should", so these values are guidance.
AS/NZS 3000:2018 Cl 3.6.2 is a requirement. The drop from the point of supply to any point must not exceed 5 %.
The 32 A circuit on 4 mm² above has a drop of 3.54 %. That is less than 5 %, but more than 3 %. At a 3 % limit, the engine gives a maximum length of 21.17 m for that cable. Open the drop at a 3 % limit. The page on voltage drop explains the calculation.
Fault withstand
BS 7671 Regulation 543.1.3 sizes a protective conductor with the adiabatic equation, S = √(I²t) / k. AS/NZS 3008.1.2:2017 Cl 5.3 uses the same equation for conductors in a short circuit. The k values depend on the conductor, its insulation and its temperature limits.
For 4 mm² copper with PVC insulation, AmpSize reads k = 111. A fault of 1 kA for 0.1 s needs at least 2.85 mm². The cable withstands 1.4 kA for that time. Open the fault check.
Never convert a BS rating
Do not multiply a BS 7671 rating by a factor to get an AS/NZS rating. The two sets of ratings use different reference conditions, installation groups and cable constructions. No single factor connects them.
To design to AS/NZS 3000, size the cable again from AS/NZS 3008.1 with the real site conditions. Use the part for the country where the installation is.
This page is a design aid. Verify every value against the current edition of the standard.