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

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

• Voltage drop limits (BS 7671:2018+A2:2022) — from origin to most distant point: lighting circuits: 3% (6.9V at 230V); power circuits: 5% (11.5V at 230V)
• Voltage drop per ampere per metre — for twin-and-earth PVC cable at 70°C (from BS 7671 Appendix 4): 1.0mm²: 44 mV/A/m; 1.5mm²: 29 mV/A/m; 2.5mm²: 18 mV/A/m; 4mm²: 11 mV/A/m; 6mm²: 7.3 mV/A/m; 10mm²: 4.4 mV/A/m
• Ring final circuit calculation — the voltage drop on a ring is calculated assuming the worst-case spur or the worst-case point on the ring (typically the farthest point from the origin where the loop back is equivalent to a radial of half the ring length)
• Derating (correction) factors — cable current capacity must be derated for: ambient temperature above 30°C; grouping with other cables; being clipped to or embedded in thermal insulation
• Ca (ambient temperature) — standard BS 7671 Appendix 4 tables are for 30°C ambient; for higher temperatures apply Ca factor (e.g., at 40°C, Ca = 0.87 for 70°C PVC)
• Cg (grouping) — cables touching or grouped reduce capacity; two circuits together: 0.80; three: 0.70; four: 0.65; five: 0.60 (BS 7671 Table B52.17)
• Ci (thermal insulation) — cable clipped to surface with insulation above: 0.75; cable enclosed in thermal insulation: 0.50 (derate by 50%)
• Effective current capacity — Iz = tabulated current (It) × Ca × Cg × Ci; the design current (Ib) must not exceed Iz; the fuse or MCB rating (In) must not exceed Iz
• Common cable sizes for domestic circuits — lighting: 1.0mm² or 1.5mm²; ring final (sockets): 2.5mm²; radial socket circuit: 2.5mm² (20A) or 4mm² (32A radial); cooker: 6mm² or 10mm²; shower: 6mm² or 10mm²
• Maximum circuit lengths (indicative, for derating factor 1.0) — 1.0mm² lighting (6A MCB): 100m; 1.5mm² lighting (6A MCB): 160m; 2.5mm² ring final (32A MCB, ring): 57m each leg; 6mm² shower (40A MCB): 23m

Quick Reference Table

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

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:

• Cable: 2.5mm² T&E; mV/A/m = 18
• Design current: 32A (MCB rating)
• Ring length: let's find maximum length
• Maximum VD = 11,500 mV
• Rearranging: max length = 11,500 ÷ (18 × 32 × 2) = 11,500 ÷ 1,152 = 10m — this seems short

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:

• Max VD for ring = 11,500 ÷ (18 × 16 × 2) = 11,500 ÷ 576 = 20m per leg
• Total ring length ≤ 40m (20m each leg from CU)

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):

• mV/A/m: 29; current: 6A; VD limit: 3% = 6,900 mV
• Max length = 6,900 ÷ (29 × 6 × 2) = 6,900 ÷ 348 = 19.8m

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):

• mV/A/m: 7.3; current: 40A; VD limit: 5% = 11,500 mV
• Max length = 11,500 ÷ (7.3 × 40 × 2) = 11,500 ÷ 584 = 19.7m

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):

• mV/A/m: 4.4; current: 40A (MCB); VD limit: 5% = 11,500 mV
• Max length = 11,500 ÷ (4.4 × 40 × 2) = 11,500 ÷ 352 = 32.7m

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:

• Up to 16mm² live conductor: CPC = same size as live
• 16–35mm²: CPC = 16mm²
• Above 35mm²: CPC = half the live conductor CSA

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

• BS 7671:2018+A2:2022 (IET Wiring Regulations 18th Edition) — Appendix 4 (current-carrying capacity and voltage drop tables); Regulation 525 (voltage drop limits)

• IET Guidance Note 1 — Selection and erection of equipment; cable sizing worked examples

• BS 7671 Table 4D5 — Current-carrying capacity for flat twin-and-earth PVC cables

• BS 7671 Table B52.17 — Grouping factors for cables in bunches

• IET: BS 7671 Resources — Access to wiring regulations

• Electrical Safety First: Cable Sizing — Guidance notes

• Voltage Drop Calculator (ECA) — Online tool from Electrical Contractors Association

• socket circuits — Ring final and radial circuit design

• testing commissioning — Testing voltage drop in completed installations

• earthing bonding — CPC and earth conductor sizing