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Proving volt drop without a volt drop testNo volt drop test. Can you still prove it?

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The best thing here. There is no test for volt drop — but as the inspector you can still prove it is adequate, because almost everything you need is already on the certificate.

The scenario

4.0/1.5 mm² twin and earth, 25 m
16 A Type B, immersion heater
Reference method C (clipped direct)
Ze provided = 0.56 Ω

Step 1 — expected R1 + R2, from Table B1

4/1.5 = 16.71 mΩ/m

(16.71 × 25) ÷ 1000 = 0.4178  →  0.42 Ω

Step 2 — the cable's figures, from On-Site Guide · Appendix F · Table F6 (p.193)

4 mm², reference method C:

  It (tabulated current-carrying capacity) = 37 A
  millivolt drop per amp per metre         = 11 mV/A/m

Both are on the same row — the mV figure is the column
right next to the current rating.

Step 3 — the volt drop

volt drop = (mV/A/m × IB × length) ÷ 1000
          = (11 × 16 × 25) ÷ 1000
          = 4.4 V

5% of 230 V = 11.5 V     →  4.4 V is comfortably inside
Why IN can stand in for IB. Because IB ≤ IN. If you are given the device rating but not the design current, the device rating is the safe figure to use.

Step 4 — and the Zs that follows

Zs = Ze + (R1 + R2) = 0.56 + 0.42 = 0.98 Ω

16 A Type B, 80% value from OSG Table B6 p.159 = 2.19 Ω
0.98 Ω is well inside. Pass.
The only thing not on the certificate is the length

And you can get that too: measure R1 + R2, look up the mΩ/m in Table B1, and run the formula backwards. Cable type, reference method, device rating, RCD protection — all of it is on the paperwork already.

So volt drop is verifiable during inspection and testing, even though no instrument measures it. That is a genuinely useful thing to be able to do.

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