Cable Sizing: Current-Carrying Capacity, Voltage Drop and BS 7671

Quick Answer: Cable sizing under BS 7671:2018+A2:2022 means selecting a conductor cross-sectional area (CSA) that satisfies four checks: it carries the design current after applying correction factors (Iz ≥ In ≥ Ib), it allows the protective device to disconnect within the required time (earth fault loop impedance), it keeps voltage drop within limits (3% for lighting, 5% for power, of 230V), and it withstands fault current (adiabatic check). The current-carrying capacity comes from BS 7671 Appendix 4 tables, derated for installation method, ambient temperature, grouping, and insulation. Undersizing causes overheating; ignoring voltage drop causes poor performance on long runs.

Summary

Cable sizing is the most fundamental design calculation in electrical installation work, and the one most often done by rule of thumb when it should be done properly. The cable must safely carry the load current without overheating, allow protective devices to operate correctly under fault, deliver acceptable voltage to the equipment, and survive a short circuit long enough for the fuse or breaker to clear it. BS 7671 Appendix 4 provides the tables and method; the skill is in applying the correction factors correctly, because a cable's rating in free air is very different from the same cable buried in loft insulation alongside five others.

The four checks are not optional alternatives — a compliant cable must pass all of them. A 2.5mm² cable might carry the current fine but fail voltage drop on a 30m run; a 6mm² cable might be ample for current and volt drop but the circuit could still fail if the earth fault loop impedance is too high for the breaker to disconnect in time. Good designers run all four and let the worst-case check govern the cable size.

This article explains current-carrying capacity and the correction factors, the voltage-drop limits and calculation, earth fault loop impedance, and the adiabatic fault check, with worked examples. It is the technical reference behind every circuit on site. For related topics see armoured cable, cable sizing, ev charger installation types and wiring regs overview.

Key Facts

• Ib — design current of the circuit (the load it must carry)
• In — nominal rating of the protective device (In ≥ Ib)
• Iz — effective current-carrying capacity of the cable after correction factors (Iz ≥ In)
• It — tabulated current-carrying capacity from BS 7671 Appendix 4 (before correction)
• Correction factors — Ca (ambient temperature), Cg (grouping), Ci (thermal insulation), Cf/Cc (BS 3036 rewireable fuse factor 0.725 where applicable)
• Voltage drop limit — 3% for lighting (≈6.9V), 5% for other uses (≈11.5V) of 230V nominal (BS 7671 Appendix 12)
• Voltage drop unit — mV/A/m from Appendix 4 tables; VD = (mV/A/m × Ib × length) ÷ 1000
• Earth fault loop impedance (Zs) — must be low enough for disconnection within 0.4s (final circuits ≤32A, TN) or 5s (distribution)
• Adiabatic equation — S = √(I²t) ÷ k; checks the conductor survives fault energy
• Installation methods — Reference Methods A-G in Appendix 4 (e.g. Method C clipped direct, Method 100/101 in insulation)
• 70°C thermoplastic (PVC) vs 90°C thermosetting (XLPE) — different rating tables
• Common domestic sizes — 1mm²/1.5mm² lighting, 2.5mm² ring/radial sockets, 6mm² cooker/shower, 10mm² shower/EV, 16mm² tails/sub-main
• Diversity — Appendix used to assess maximum demand realistically (not all loads coincide)

Quick Reference Table

Typical twin-and-earth (70°C PVC) capacities — indicative, method-dependent

Values are indicative of Appendix 4 tables and vary by exact method and reference; always use the current BS 7671 tables and apply correction factors. "In insulation" dramatically derates a cable.

Voltage drop (approx mV/A/m, twin-and-earth)

Detailed Guidance

The selection sequence

BS 7671 cable sizing follows a logical order:

1. Determine Ib — the design current. For a resistive load, Ib = P ÷ V. A 7kW EV charger ≈ 7000 ÷ 230 ≈ 30.4A (call it 32A continuous).
2. Select In — the protective device rating, where In ≥ Ib. So a 32A device for the EV charger.
3. Find the required It — divide In by the product of all applicable correction factors: It ≥ In ÷ (Ca × Cg × Ci × ...). The correction factors reduce the cable's usable rating.
4. Choose a CSA whose tabulated It (for the installation method) meets or exceeds the required value — this gives Iz ≥ In ≥ Ib.
5. Check voltage drop — recalculate for the actual run length; increase CSA if it exceeds the limit.
6. Check earth fault loop impedance (Zs) — confirm disconnection time is met; increase CSA (lower R₁+R₂) if needed.
7. Check the adiabatic — confirm the conductor survives the prospective fault current for the device's operating time.

The final cable size is whichever check demands the largest conductor.

Current-carrying capacity and correction factors

The tabulated capacity (It) assumes ideal conditions. Real installations derate it:

• Ca — ambient temperature: capacities are tabled at 30°C ambient; hotter environments (near boilers, in roof spaces in summer) reduce capacity.
• Cg — grouping: cables bundled together can't shed heat as well; more cables in a group means a larger derating factor.
• Ci — thermal insulation: a cable surrounded by loft or wall insulation runs much hotter. A cable wholly in insulation for >0.5m can be derated to roughly half its clipped rating — this is one of the most common real-world sizing failures.
• Cf (rewireable fuse): where a BS 3036 semi-enclosed (rewireable) fuse protects the circuit, a factor of 0.725 applies because these fuses are less precise.

Apply the worst-case combination of factors that the cable actually experiences along its route.

Voltage drop — the long-run killer

Even a cable with ample current capacity delivers poor performance if voltage drop is excessive. BS 7671 (Appendix 12) limits voltage drop to 3% for lighting and 5% for other circuits of the 230V nominal — about 6.9V and 11.5V respectively. The calculation:

VD (volts) = (mV/A/m × Ib × L) ÷ 1000, where L is the route length in metres.

Worked example: a 6mm² cable (7.3 mV/A/m) feeding a 32A load over a 25m run: VD = (7.3 × 32 × 25) ÷ 1000 = 5.84V ≈ 2.5% — within the 5% limit. The same load over 50m would give 11.7V (>5%) and require uprating to 10mm². Long runs (EV chargers at the end of drives, outbuildings, garden offices) routinely fail on voltage drop before they fail on current capacity — always check.

Earth fault loop impedance

For the protective device to disconnect quickly under an earth fault, the total earth fault loop impedance (Zs = Ze + R₁ + R₂) must be low enough. BS 7671 gives maximum Zs values per device type and rating for the required disconnection time (0.4s for final circuits up to 32A on TN systems, 5s for distribution circuits). A long, thin cable has high R₁+R₂, raising Zs above the limit and meaning the breaker won't clear the fault in time — requiring a larger CSA (to reduce conductor resistance) or a different protective device. This is verified by measurement at testing, but designed for at sizing.

The adiabatic fault check

Finally, the conductor must survive the heat of a short-circuit or earth fault for as long as the device takes to operate. The adiabatic equation checks this:

S ≥ √(I² × t) ÷ k

where S is the conductor CSA (mm²), I is the fault current (A), t is the device disconnection time (s), and k is a material constant (e.g. 115 for 70°C PVC copper, 143 for 90°C thermosetting copper). If the actual CSA is at least the calculated minimum S, the conductor (and crucially the CPC, which is often smaller) won't be damaged before the device clears the fault. The CPC in twin-and-earth cable, being reduced in size, is the usual limiting conductor for this check.

Frequently Asked Questions

What size cable do I need for a given load?

Start with the design current (load ÷ 230V), pick a protective device rating at or above it, then find a cable whose corrected current-carrying capacity meets that rating for the installation method — and then check voltage drop, earth fault loop impedance, and the adiabatic fault check. The cable size is whichever check demands the largest conductor. There is no single "X amps = Y mm²" answer because installation method, run length, grouping, and insulation all change it.

Why does running cable through insulation matter so much?

Because insulation traps the heat the cable generates, so it can carry far less current before overheating. A cable wholly surrounded by thermal insulation for more than half a metre can be derated to roughly half its clipped-direct rating. This is one of the most common causes of dangerously undersized circuits — a 2.5mm² socket cable buried in loft insulation behaves very differently from the same cable clipped to a joist.

How do I calculate voltage drop?

Use VD = (mV/A/m × Ib × length) ÷ 1000, where the mV/A/m figure comes from the BS 7671 Appendix 4 table for the cable, Ib is the design current, and length is the route length in metres. Compare against the limit: 3% (≈6.9V) for lighting, 5% (≈11.5V) for power, of 230V. Long runs to EV chargers, outbuildings, and garden rooms commonly fail this and need a larger cable than the current capacity alone would suggest.

Can I just use a bigger cable to be safe?

Up to a point — a larger cable always satisfies current capacity and voltage drop more easily, and is the right answer for long runs. But oversizing wastes money, is harder to terminate, and doesn't fix everything: the protective device must still match the circuit, and the CPC must still pass the adiabatic check. "Bigger to be safe" is reasonable for voltage drop margin on long runs, but the design checks still have to be done.

Regulations & Standards

• BS 7671:2018+A2:2022 — Requirements for Electrical Installations (IET Wiring Regulations)

• BS 7671 Appendix 4 — current-carrying capacity and voltage drop tables

• BS 7671 Appendix 12 — voltage drop guidance

• BS 7671 Chapter 43 / 54 — overcurrent protection and protective conductors (adiabatic)

• BS 7671 Chapter 41 — protection against electric shock (disconnection times, Zs)

• BS EN 60228 — conductors of insulated cables

• IET — BS 7671 Wiring Regulations

• IET — On-Site Guide and Guidance Notes

• Electrical Safety First — best practice guides

• BSI — BS EN 60228 conductors

• cable sizing — cable sizing overview

• armoured cable — SWA cable for buried/external runs

• ev charger installation types — high-load circuit sizing example

• wiring regs overview — BS 7671 overview