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Cable calculation, step 8: voltage dropDoes it pass the 5 % limit?
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You get the mV/A/m from the same table as the current rating. For T&E, Table 4D5 has both. For some other cable types they're split: e.g. single-core 70 °C cable uses Table 4D1A for current ratings and Table 4D1B for mV/A/m.
÷ 1000 turns millivolts into volts. (A ring final divides by 4000.)
Working
16 mm² T&E → 2.8 mV/A/m (Table 4D5, mV/A/m column) VD = (2.8 × 24.5 × 27) ÷ 1000 = 1.85 V (≈ 1.8 V) Max for a non-lighting circuit = 5% of 230 V = 11.5 V 1.85 V ≤ 11.5 V ✓
- The low voltage drop is helped massively by the big cable. A smaller cable has more resistance, so more drop.
- Lighting circuits: 3% (6.9 V). Everything else: 5% (11.5 V).
- Three-phase: work from 400 V, so 5% allows 20 V.
BS 7671 · Appendix 4 · Table 4Ab · voltage drop limits 3% / 5%
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