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The 80 % rule and the On-Site GuideWhy multiply the table value by 0.8?

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The values in BS 7671 are for a circuit in use — a cable at its operating temperature of 70 °C. Current through copper makes it hot, and hot copper has more resistance. So a circuit that's been running has a higher Zs than the same circuit stone cold.

%When you test a brand new, unenergised circuit, it's cold. Its measured Zs should therefore be no more than 80 % of the tabulated value, leaving 20 % of headroom for the warming-up it will do in service.

Device	BS 7671 tabulated (100 %)	Max measured (80 %)
Type B 50 A	0.87 Ω	0.70 Ω (0.87 × 0.8)
Type B 16 A	2.73 Ω	2.19 Ω (On-Site Guide, Table B6)

You don't have to do the sum. The On-Site Guide already prints the 80 % figures — Appendix B, Table B6 for circuit breakers. Look up type B, 16 A and it gives 2.19 Ω directly (2.73 × 0.8 on the rounded table figure comes to 2.18). That's the number you're checking against with your tester in your hand.

!Which book, which number.
BS 7671 tables = maximum permitted Zs, circuit in use, 100 %.
On-Site Guide tables = maximum measured Zs, new circuit, 80 %.

Spotting which one a question wants

The book uses several phrasings and they are not interchangeable:

Wording in the question	Use
maximum tabulated / permitted / allowed / acceptable	The BS 7671 value, 100 %
maximum measured	The 80 % value — multiply by 0.8

Worked example: "A 32 A BS EN 60898 type C circuit breaker… what is the maximum acceptable measured Zs?" → Table 41.3 gives 0.68 Ω → the word "measured" means multiply by 0.8, or read the type C, 32 A figure from OSG Table B6. The wrong answer 0.68 is sitting right there in the options for anyone who missed the word "measured".

On-Site Guide · Appendix B · Table B6 · 80 % Zs for circuit breakers Table 41.3 · 100 % tabulated values

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