Cable calculation, step 10: R1 + R2How big is the CPC in 16 mm² twin and earth?

So far we know the circuit works in normal use: little voltage drop, Reg 433.1.1 met, the right cable. The second half asks: will it disconnect in a fault? Imagine a pan boils over, water gets into the oven and the metal case becomes live. We want automatic disconnection of supply (ADS). That needs a good protective conductor path from the cooker case all the way back to the star point of the supply transformer (on a TN system).

BS 7671 · Part 2 · Fig 2.1 · illustration of earthing and protective conductor terms · p.51 Its key names the earthing conductor, the main earthing terminal, the circuit protective conductor and the exposed-conductive-part

T&E CPC sizes. T&E is one of the few cables with a smaller CPC. The standard off-the-shelf sizes are:

• Table B1 (Guidance Note 3, Appendix B): resistance of line + CPC per metre. 16 mm² line with 6 mm² CPC = 4.23 mΩ/m.
• Table B2: don't use it for design. It's for the inspector to correct a test reading from 20 °C to the temperature on the day of testing. It's a test correction, not a design one.
• Table B3: corrects the resistance to the conductor's operating temperature. Faults happen while circuits are working, and warm copper has more resistance. Skip B3 and you don't know whether it will disconnect with the oven on. For 70 °C thermoplastic, use the row "incorporated in a cable or bunched" (the one you'll almost always use): 1.2.

Line 16 mm² · CPC 6 mm² · L = 27 m
R1 + R2 = (4.23 × 1.2 × 27) ÷ 1000
        = 0.137 Ω        (keep the full figure; don't round as you go)

Guidance Note 3 · Appendix B · Table B1 · R1 + R2 per metre (mΩ/m) · p.174 Guidance Note 3 · Table B2 · ambient temperature multipliers (for testing) · p.175 Guidance Note 3 · Table B3 · operating-temperature multipliers (1.2) · p.176