Max Cable Run Length Calculator: Voltage Drop, Derating & BS 7671 Compliance

Quick Answer: Maximum cable run length is limited by voltage drop (must not exceed 3% for lighting, 5% for power circuits under BS 7671 18th Edition). For a 2.5mm² twin-and-earth cable carrying 16A on a ring final circuit, the maximum run is approximately 57m before voltage drop becomes an issue. For a radial circuit, halve this. Voltage drop = (current × resistance per metre × length × 2) in millivolts. Derating (applying reduction factors for grouped or insulated cables) reduces effective current-carrying capacity and may force a larger cable size.

Summary

Voltage drop and cable derating are the two most important calculations in domestic and light commercial electrical installation design. They are also the most commonly skipped — with the result that circuits are either underrated (causing nuisance tripping) or overrated (creating a fire risk from undersized cable).

This article covers the BS 7671 voltage drop limits, the calculation method, the derating factors that apply to cables in common installation conditions, and worked examples for the most common domestic circuit types.

Key Facts

Quick Reference Table

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Cable Size mV/A/m Max Length at 5% VD, 16A Max Length at 3% VD, 6A
1.0mm² 44 8.2m 88m
1.5mm² 29 12.4m 133m
2.5mm² 18 20m 215m
4mm² 11 33m 350m
6mm² 7.3 49m 530m
10mm² 4.4 82m 880m

These are single-pole lengths — multiply mV/A/m by 2 for two-core cable (there and back)

Derating Scenario Factor
In thermal insulation (fully enclosed) 0.50
Clipped to surface below insulation 0.75
2 circuits grouped 0.80
3 circuits grouped 0.70
4 circuits grouped 0.65
Ambient temperature 40°C (70°C cable) 0.87
Ambient temperature 45°C 0.79
All factors combined (multiply) Product of all applicable

Detailed Guidance

Step-by-Step Voltage Drop Calculation

Formula:

Voltage drop (mV) = mV/A/m × Design current (A) × Length (m) × 2

(The ×2 accounts for the live and neutral conductors both contributing to the total drop)

Step 1: Establish the design current (Ib) For a socket ring final: Ib = maximum demand (after diversity) but typically taken as the protective device rating for calculation purposes; for a 32A MCB ring, Ib = 32A (worst case)

Step 2: Find the cable's mV/A/m value From the table above or from BS 7671 Appendix 4 Table 4F2A (for flat 70°C PVC twin-and-earth)

Step 3: Calculate voltage drop Voltage drop (mV) = mV/A/m × current × length × 2

Step 4: Check against limit Limit = 5% × 230,000 mV = 11,500 mV for power; 3% × 230,000 mV = 6,900 mV for lighting

Worked example — 2.5mm² ring final circuit:

The issue is the ring: for a ring circuit, the worst-case point is the farthest point on the ring, and the current splits between the two routes. The actual calculation uses half the design current for each leg, so:

In practice, for a 32A ring final serving a floor, the guidance is to keep each leg to approximately 20m, so a maximum ring length of approximately 40m.

Derating Calculations

Example: 2.5mm² cable running through 150mm loft insulation

  1. Tabulated current for 2.5mm² clipped to surface (Method C, Table 4D5): 27A
  2. Derating factor for thermal insulation (fully enclosed): Ci = 0.50
  3. Effective current capacity: Iz = 27 × 0.50 = 13.5A
  4. MCB rating must not exceed Iz: 13.5A → maximum 13A MCB; you cannot use a 16A MCB with this installation
  5. For a 16A MCB, you would need at least 4mm² cable (tabulated current 36A × 0.50 = 18A ≥ 16A)

Example: Three cables in same conduit

  1. Tabulated current for 2.5mm² in conduit (Method B, Table 4D5): 23A
  2. Grouping factor for 3 circuits: Cg = 0.70
  3. Effective current capacity: Iz = 23 × 0.70 = 16.1A
  4. For a 16A MCB: 16.1A > 16A — just acceptable
  5. For a 20A MCB: 16.1A < 20A — not acceptable; would need larger cable

Common Domestic Circuit Calculations

1.5mm² lighting circuit (6A MCB):

For a house with ceiling heights of 2.4m and first floor lighting, the distribution board to the furthest light may easily be 15–20m — this is tight on 1.5mm². Consider 2.5mm² for longer lighting runs, or sub-splitting lighting circuits.

6mm² shower circuit (40A MCB):

Shower circuits should have the consumer unit as close to the shower as practical. Over 20m, consider 10mm² cable.

10mm² cooker circuit (40A MCB, 10kW cooker at 43A):

Most domestic cooker runs are under 10m, so 6mm² is usually acceptable unless the consumer unit is far from the kitchen.

Earth Conductor Sizing

The earth (CPC — circuit protective conductor) must be correctly sized per BS 7671 Table 54.7 or by calculation. Common rules:

For flat twin-and-earth cable, the CPC is typically one size smaller than the live: 1.0mm²/1.0mm² T&E has a 1.0mm² CPC; 2.5mm² T&E has a 1.5mm² CPC. Check the specific cable.

Frequently Asked Questions

My lighting circuit is 30m. Is 1.5mm² cable adequate?

Probably not without checking. For a 6A MCB and 1.5mm² (mV/A/m = 29), max length for 3% VD is approximately 20m per the calculation above. At 30m, voltage drop would be approximately 4.5% — exceeding the 3% lighting limit. Options: use 2.5mm² cable; split into two shorter circuits; fit a sub-distribution board closer to the lights; or justify exceeding the 3% limit if the absolute voltage at the load is still adequate (regulatory guidance allows this if you can demonstrate the installation still performs safely).

Do I need to consider voltage drop for a short spur off a socket ring?

Yes, in theory — the spur adds to the voltage drop from the ring to the spur's socket. In practice, short spurs (<3m) from a ring add negligible additional drop and do not require separate calculation.

Does derating apply to armoured cables or outdoor SWA runs?

Yes. Armoured cables (SWA — Steel Wire Armoured) have different current-carrying capacities to flat T&E cables. Use the appropriate table from BS 7671 Appendix 4 for the specific cable type (e.g., Table 4D4A for two-core PVC armoured; Table 4D4B for three-core). Outdoor direct-buried SWA cable (Installation Method D) typically has higher current ratings than clipped to surface (Method C) due to the ground acting as a heat sink. Apply the same derating factors for grouping and temperature but start from the correct base table for the cable and installation method.

Regulations & Standards