SPARKYSTUDY NOTES
SPARKY STUDY NOTES sparkystudynotes.co.uk/learn/max-permitted-zs-and-the-80-figure

Free notes

Max permitted Zs and the 80% figureWhich maximum Zs do you compare against?

Free from the notes: Sparky’s video on it, the section word for word as members see it, and a PDF to keep.

From Sparky’s full video lesson. Members get the whole lesson on every notes page.

The question at the end: Why do the two Zs columns differ? The answer is in the notes below.

The notes

Exactly as they are on the site.

Download these notesPDF · free to keep

Two figures for the same circuit, from different books for a reason. Only one goes in the schedule's maximum Zs box.

FigureAlso calledComes fromApplies to
Max permitted — the schedule boxMax tabulatedBS 7671 · Tables 41.2–41.4The cable in service, warm
Max measured — your check valueThe 80% valueOn-Site Guide Table B6The cable cold, as you test it
Two Zₛ figures, and why they come from different books because they describe the cable in different states Max permitted also called max tabulated 100% BS 7671Tables 41.2–41.4 the cable in service, warm — current has heated the copper and raised its resistance Max measured the 80% value 80% 20% On-Site Guide Table B6 the cable cold — which is how it is on initial verification, before anything has been used The 20% gap is the temperature difference. Nothing else.
Both figures describe the same circuit, measured in two different conditions. BS 7671's figures assume the cable is in use and warm, which raises its resistance. You test on a cold cable, so your reading has to come in under the lower, 80% figure — and that is the one from GN3 or the On-Site Guide. Comparing your cold reading against the warm table is the mistake.
The 20% gap is the temperature difference

BS 7671's figures assume the cable is in use and current has warmed the copper, raising its resistance. On initial verification the cable is cold — nothing has run through it but your test current. The 80% allowance covers the difference.

On the schedule. The maximum Zs box takes the BS 7671 Chapter 41 table figure — 1.37 Ω for a B32. If you worked to another source, such as the 80% figure, state it in the Remarks (column 31).
Where the 80% values live. On-Site Guide · Table B6 · p.159 (16 A Type B 2.19 Ω, 20 A 1.75 Ω, 32 A 1.09 Ω).
You can also just do the arithmetic. Take the BS 7671 figure and multiply by 0.8. Or go the other way: a 2.19 Ω figure in the 80% table goes back to the 2.73 Ω BS 7671 figure for a 16 A Type B (2.19 ÷ 0.8 gives 2.74 only because 2.19 is itself rounded).

Free section from Sparky Study Notes. The full Level 3 notes, 1,463 practice questions and the video lessons: sparkystudynotes.co.uk

Test yourself on it

The tests are in the members’ area. Get one wrong and it shows you why, and where to revise.

Get access

Get new notes and exam tips from Sparky

About one email a fortnight. Unsubscribe any time. Privacy notice

That was one section. There are 583.

Sparky Study Notes is the whole Level 3 syllabus written up like this: the explanation, the worked example, and where it is in your book. With a video lesson for every part and a practice paper for every exam.

  • 583sections of written notes
  • 35video lessons
  • 1,463practice questions
  • 146diagrams, redrawn
  • 18th Edition
  • Building Regs
  • PAT
  • Fundamental Inspection & Testing
  • Initial Verification
  • Periodic Inspection & Testing
Get access — £100 12 months. One payment. See what’s inside