11.3 Verification of Voltage Drop

Why Voltage Drop Is Verified

Every conductor has resistance, so every conductor carrying load current drops some voltage along its length. If the drop is too large, equipment at the far end receives less than its rated voltage: lamps dim, motors run hot and lose torque, heaters underperform, and electronic equipment may reset.

Regulation 525 sets the requirement — the voltage at the terminals of current-using equipment must be suitable for that equipment — and Regulation 643.11 places its verification at the end of the test sequence.

Note what this is not. Voltage drop is a performance and design requirement, not a shock-protection one. A circuit with excessive voltage drop is not dangerous in the way that a high Zs or a broken protective conductor is dangerous. That distinction drives how it is coded on a condition report.

The Limits

Appendix 4 of BS 7671 gives the limits as a percentage of the nominal voltage of the supply, measured between the origin of the installation and the terminals of the equipment.

For the 230 V single-phase supply that dominates UK work:

For a 400 V three-phase supply the same percentages give 12 V and 20 V respectively.

The greater allowance for a private supply reflects the fact that the consumer controls the whole system from the transformer onwards, so there is no distributor’s allowance to protect.

Why It Is Calculated, Not Measured

You cannot simply put a voltmeter on the far socket and read the difference. To do that you would need the circuit loaded to its design current at the moment of measurement, and a simultaneous reading at the origin — neither of which is practical during verification, and both of which vary with what the rest of the installation is doing.

So Regulation 643.11 allows the verification to be evaluated, and the qualification specification names the method explicitly: voltage drop using the measured conductor resistance method. You already have the measurement you need — the continuity test.

The measured conductor resistance method

For a single-phase circuit:

Vd = (R1 + Rn) × Ib

where R1 + Rn is the measured resistance of the line and neutral conductors of the circuit (the go-and-return path for load current) and Ib is the design current.

Two adjustments matter:

• Use R1 + Rn, not R1 + R2. Load current returns through the neutral, not the protective conductor. On a circuit where the cpc is a smaller cross-sectional area than the line conductor, using R1 + R2 overstates the drop.
• Correct for temperature. Your continuity measurement was taken cold. Under load the conductors reach their operating temperature, and resistance rises by roughly 0.4% per °C. A commonly used multiplier for a thermoplastic cable measured at around 20 °C and operating at 70 °C is about 1.20.

The alternative: tabulated mV/A/m

Appendix 4 also tabulates voltage drop per ampere per metre for each cable type and size, so:

Vd = (mV/A/m × Ib × L) ÷ 1000

where L is the route length in metres. This is the design-stage method; the measured-resistance method is the verification-stage one, because it uses the circuit as actually installed rather than as drawn.

Worked Example

A 230 V radial socket-outlet circuit is wired in 2.5 mm² thermoplastic twin-and-earth. The design current Ib is 20 A. The measured line-plus-neutral resistance, taken cold at about 20 °C with the leads nulled, is R1 + Rn = 0.42 Ω.

Step 1 — correct the resistance to operating temperature.

0.42 Ω × 1.20 = 0.504 Ω

Step 2 — calculate the drop.

Vd = 0.504 × 20 = 10.08 V

Step 3 — express it as a percentage and compare.

10.08 ÷ 230 = 4.38%

The limit for a socket-outlet circuit on a public supply is 5%, or 11.5 V. At 4.38% and 10.08 V the circuit complies, though with little margin — adding a spur or extending the run would push it over.

A second look — same circuit as a lighting circuit. If that cable fed lighting rather than sockets, the limit would be 3% (6.9 V) and the same 10.08 V would be a clear fail, needing a larger conductor or a shorter route.

Recording and Coding

Voltage drop is verified where necessary — typically on long runs, on circuits where the designer has flagged it, and where equipment is reported to be underperforming. On a short domestic circuit it is normally satisfied by the design and the model forms may show it as not applicable.

At periodic inspection, excessive voltage drop is a performance deficiency, not a danger. Where equipment is functioning acceptably it is normally recorded as C3 — improvement recommended. It rises to C2 only where the consequence is genuinely a safety matter, for example where the drop is preventing safety-critical equipment from operating correctly, and the reasoning is recorded.

Do not confuse this with a high Zs, which is coded C2, because that is about whether the protective device disconnects a fault in time.