What Size Cable Do I Need? Electrical Cable Sizing by Circuit Type

Summary

Cable sizing under BS 7671 is a three-stage process: determine the design current (Ib), select a cable whose tabulated current-carrying capacity (It) — after applying correction factors — equals or exceeds Ib, then verify the voltage drop is within permitted limits. For domestic twin and earth (6242Y) installations, Table 4D5A provides current-carrying capacities and Table 4D5B provides voltage drop values in mV/A/m. The installation method (Reference Method from Table 4A1) has a major impact on ratings — a 2.5mm² cable clipped direct to a joist carries 27A, but the same cable enclosed in thermal insulation for over 0.5m drops to as low as 13.5A. Voltage drop must not exceed 3% (6.9V) for lighting or 5% (11.5V) for other circuits per Appendix 12. On longer runs, voltage drop — not current capacity — often becomes the limiting factor that forces upsizing.

Key Facts

• BS 7671:2018+A2:2022 (18th Edition, Amendment 2) is the current standard — all cable sizing must comply
• Table 4D5A covers current-carrying capacity for flat twin and earth (T&E) 6242Y cables; Table 4D2A covers single-core PVC cables in conduit/trunking
• The base ambient temperature is 30°C for cables in air and 20°C for cables in ground — derating applies above these
• A cable totally surrounded by thermal insulation for more than 0.5m is derated to 0.5 of its clipped-direct rating (Regulation 523.7)
• Ring final circuits are the only domestic circuit type where a 32A protective device is permitted with 2.5mm² cable — because current shares between both legs
• Shower and immersion heater circuits carry continuous loads with no diversity — cable must handle 100% rated current
• Cooker circuits benefit from diversity: first 10A at 100%, remainder at 30%, plus 5A if the cooker control unit has a socket (IET On-Site Guide Table 2)
• All EV charger installations require a dedicated circuit and must comply with BS 7671 and Part P of the Building Regulations

Quick Reference Table

Notes on the table:

• Current ratings are for Reference Method C (clipped direct to a non-metallic surface), single circuit, 30°C ambient, no thermal insulation contact
• Max run is calculated at 5% voltage drop (11.5V) for power circuits and 3% (6.9V) for lighting, at the stated current
• All figures assume copper conductors with 70°C thermoplastic (PVC) insulation
• These are starting points — always verify against your specific installation conditions and apply correction factors

Detailed Guidance

How do I size cable for a ring final circuit?

A ring final circuit is unique in UK wiring practice. Because the cable loops from the consumer unit, around the circuit, and back to the consumer unit, current flows in both directions. This effectively halves the load on any section of cable, allowing 2.5mm² T&E to be protected by a 32A MCB.

BS 7671 Requirements (Regulation 433.1.204):

• Minimum cable size: 2.5mm² (copper)
• Protective device: 30A or 32A (Type B MCB standard)
• Maximum floor area served: 100m²
• The cable must form a complete ring — both ends terminated at the same MCB

Spur rules:

• A fused spur (via a fused connection unit with 13A fuse) can serve any number of outlets
• An unfused spur can serve only one single or one double socket outlet
• An unfused spur must be connected at the terminals of a socket on the ring, at a junction box on the ring, or at the origin (consumer unit)
• There is no limit in BS 7671 on the total number of socket outlets on a ring, but good practice limits it to prevent excessive load on the ring

When a ring will not work:

• Floor area exceeds 100m² — use two rings or a 32A radial with 4mm²
• The cable route means the ring cannot form a balanced loop
• High continuous loads (e.g., multiple space heaters) where diversity assumptions do not hold

How do I size cable for a radial socket circuit?

Radial circuits are simpler than rings — the cable runs from the consumer unit to each socket in sequence. The cable must carry the full design current because there is no return path.

Two standard options per IET On-Site Guide:

A 20A radial on 2.5mm² is often the practical choice for a single room extension, garage, or loft conversion where a ring circuit would be excessive.

How do I size cable for a shower circuit?

Electric showers are continuous-rated loads — no diversity applies. The cable must carry 100% of the shower's rated current for the entire duration of use. This makes showers one of the most demanding domestic circuits.

Design current calculation: Ib = Power (W) / Voltage (V)

At 230V nominal:

• 7.5kW = 32.6A
• 8.5kW = 37.0A
• 9.5kW = 41.3A
• 10.0kW = 43.5A
• 10.8kW = 47.0A
• 11.0kW = 47.8A

Cable selection:

• Up to 9.5kW: 6mm² T&E with 40A MCB is acceptable where cable is clipped direct (47A capacity) and runs are short
• 10kW and above: 10mm² T&E with 45A or 50A MCB
• Any shower where the cable passes through thermal insulation: strongly consider 10mm² regardless of kW rating

Critical warning: A 6mm² cable clipped direct carries 47A — adequate for a 9.5kW shower (41.3A). But if even a short section of that cable passes through loft insulation (Method 100/101/102/103), the capacity can drop to 23–34A depending on insulation depth. At 41.3A design current, this is non-compliant. Always check the entire cable route for insulation contact.

How do I size cable for a cooker circuit?

Cooker circuits benefit from diversity because it is unlikely all elements (oven, hob rings, grill) will be at full power simultaneously. The IET On-Site Guide provides a standard diversity formula.

Diversity calculation (Table 2, IET On-Site Guide):

Assessed demand = 10A + 30% x (Full load current - 10A) + 5A (if cooker control unit has a socket)

Worked example — 12kW cooker with socket on CCU:

• Full load: 12,000W / 230V = 52.2A
• Diversity: 10 + 0.3 x (52.2 - 10) + 5 = 10 + 12.7 + 5 = 27.7A
• Cable: 6mm² (47A capacity) on 32A MCB — compliant

Worked example — 15kW range cooker with socket:

• Full load: 15,000W / 230V = 65.2A
• Diversity: 10 + 0.3 x (65.2 - 10) + 5 = 10 + 16.6 + 5 = 31.6A
• Cable: 6mm² on 32A MCB is marginal. If any insulation contact exists on the cable route, upsize to 10mm² on 45A MCB

General guidance:

• Cookers up to approximately 13kW: 6mm² T&E, 32A MCB
• Cookers 13kW–18kW: 10mm² T&E, 45A MCB (recommended for new installations)
• Modern large range cookers (e.g., Rangemaster, AGA): often need 10mm² as standard
• Always check the manufacturer's installation instructions for minimum cable and MCB requirements

How do I size cable for an EV charger?

EV chargers are typically continuous-rated loads (vehicles can charge for many hours). BS 7671 and IET guidance note that a continuous load must be derated — the MCB must be rated at no less than the full load current, and the cable must be rated for continuous operation.

Common domestic setups:

Key considerations:

• Dedicated circuit required — never spur off an existing circuit
• SWA (steel wire armoured) cable is standard for external runs to driveways/garages
• Voltage drop on long runs to detached garages is the most common reason to upsize from 6mm² to 10mm²
• Load management / smart charging may be required if the property's main fuse (DNO cut-out) cannot support the additional load — check maximum demand
• Part P notification is required for a new circuit

How do I calculate voltage drop?

Voltage drop is the reduction in voltage along the length of the cable due to its resistance. BS 7671 Appendix 12 sets maximum limits.

Permitted voltage drop (from the origin of the installation):

• Lighting circuits: 3% of 230V = 6.9V maximum
• All other circuits: 5% of 230V = 11.5V maximum

Calculation formula:

VD (volts) = (mV/A/m x Ib x L) / 1000

Where:

• mV/A/m = voltage drop per ampere per metre from Table 4D5B (T&E cables)
• Ib = design current in amps
• L = one-way cable length in metres

mV/A/m values for T&E cables (Table 4D5B):

Worked example — 10.8kW shower, 10mm² cable, 18m run:

• Ib = 10,800 / 230 = 47A
• VD = (4.4 x 47 x 18) / 1000 = 3.72V
• 3.72V is within the 11.5V limit — compliant

Worked example — Lighting circuit, 1.0mm², 5A load, 30m run:

• VD = (44 x 5 x 30) / 1000 = 6.6V
• 6.6V is within 6.9V limit — compliant but tight. A 35m run would exceed the limit.

When voltage drop forces an upsize: If the voltage drop exceeds the limit, you must either reduce the cable length (not usually possible) or upsize the cable. This is why 10mm² is often used for EV chargers running to detached garages even though 6mm² has sufficient current capacity.

What correction factors do I need to apply?

The tabulated current-carrying capacity (It) in Tables 4D5A/4D2A assumes ideal conditions: 30°C ambient, single circuit, no thermal insulation. Real installations rarely match these assumptions.

The relationship: It >= Ib / (Ca x Cg x Ci)

Where It is the minimum required tabulated rating and Ib is the design current.

Ca — Ambient temperature correction (Table 4B1):

Most domestic installations assume 30°C. Apply correction for loft spaces in summer (can exceed 40°C), airing cupboards, or near heat sources.

Cg — Grouping correction (Table 4C1, bunched and clipped direct):

Grouping only applies where cables are touching or bunched. If the spacing between cables exceeds twice the cable's overall diameter, Cg = 1.00 (no correction needed).

Ci — Thermal insulation correction (Regulation 523.7):

The insulation factor is the most commonly underestimated correction in domestic work. A 6mm² cable rated at 47A drops to just 23.5A when fully enclosed in insulation — less than the 32A MCB protecting a cooker circuit.

Worked example — shower cable through insulated loft:

• 9.5kW shower, Ib = 41.3A
• Cable passes through 200mm loft insulation for 3m (>0.5m, so Ci = 0.5)
• Ambient in loft: 35°C, Ca = 0.94
• Single circuit, Cg = 1.00
• Required It = 41.3 / (0.94 x 1.00 x 0.50) = 41.3 / 0.47 = 87.9A
• 6mm² (47A) and 10mm² (64A) are both insufficient — need 16mm² (85A) or re-route the cable to avoid the insulation

This is why experienced electricians check the entire cable route before selecting a size.

Frequently Asked Questions

Can I use 6mm² for a 10.5kW shower?

No, not safely in most real-world installations. At 230V, a 10.5kW shower draws 45.7A. A 6mm² cable clipped direct to a surface is rated at 47A — technically adequate by only 1.3A. Any thermal insulation contact, grouping with other cables, or ambient temperatures above 30°C will reduce the capacity below the design current. Use 10mm² on a 45A MCB as standard for any shower rated 10kW or above.

Is 6mm² enough for a cooker?

For cookers up to approximately 13kW in a straightforward installation (short cable run, clipped direct, no insulation contact), 6mm² on a 32A MCB is compliant when diversity is applied. However, for new installations, many experienced electricians now default to 10mm² on a 45A MCB to provide headroom for future appliance upgrades and to account for increasingly well-insulated properties where cable routes frequently pass through thermal insulation.

Do I need to apply correction factors for a single cable clipped to joists?

If the cable is a single circuit (Cg = 1.00), in a 30°C ambient (Ca = 1.00), and not in contact with thermal insulation (Ci = 1.00), then no correction is needed — you can use the tabulated value directly from Table 4D5A. This is the ideal scenario and the one the table figures are based on. In practice, always verify all three conditions are met along the entire cable route.

What is the maximum cable run for a 32A ring final?

There is no specific maximum cable length for a ring final in BS 7671. The 100m² floor area limit is the controlling factor. However, you must still verify that the earth fault loop impedance (Zs) at the furthest point does not exceed the maximum permitted Zs for a 32A Type B MCB (1.37 ohms at the DB, 0.5 x (R1 + R2) for the ring). On very large floor areas, R1+R2 and loop impedance — not voltage drop — become the limiting factors.

When should I use SWA instead of T&E?

Steel wire armoured (SWA) cable must be used for any external underground or exposed installation. T&E (twin and earth) is for internal use only — it has no mechanical protection and its flat PVC sheath degrades in UV light. Use SWA for: EV charger supplies to driveways, outbuilding feeds, external lighting runs buried underground, and any cable that will be exposed to mechanical damage. SWA has its own current rating tables (Table 4D4A) with different values from T&E.

What Reference Method do I use for T&E in a stud wall?

T&E cable in a thermally insulated stud wall is covered by Reference Methods 100, 101, 102, and 103 in Table 4D5A, depending on whether the cable is in contact with one side of the plasterboard, touching insulation, or surrounded by insulation. The most conservative assumption (Method 103, surrounded by insulation for >0.5m) gives a Ci factor of 0.50. Always determine the actual installation condition and select the appropriate column from Table 4D5A.

Regulations & Standards

• BS 7671:2018+A2:2022 (IET Wiring Regulations, 18th Edition with Amendment 2) — the primary standard for all UK electrical installations

  • Table 4A1 — Reference methods for cable installation
  • Table 4B1 — Ambient temperature correction factors (Ca)
  • Table 4C1 — Grouping correction factors (Cg)
  • Table 4D5A — Current-carrying capacity for flat T&E cables (6242Y) by reference method
  • Table 4D5B — Voltage drop for flat T&E cables (mV/A/m)
  • Table 4D2A — Current-carrying capacity for single-core PVC cables in conduit/trunking
  • Table 4D4A — Current-carrying capacity for SWA cables
  • Appendix 12 — Voltage drop limits (3% lighting, 5% other)
  • Regulation 433.1.204 — Ring final circuit requirements
  • Regulation 523.7 — Cables in thermal insulation

• IET On-Site Guide (BS 7671) — practical companion to the Regulations

  • Table 2 — Diversity allowances (including cooker diversity formula)

• IET Guidance Note 1 — Selection and erection of equipment (includes cable sizing worked examples)

• Building Regulations Part P — Electrical safety in dwellings; notification required for new circuits

• ENA Engineering Recommendation G100 — Maximum demand and connection capacity (relevant for EV charger maximum demand assessments)

• IET Wiring Regulations (BS 7671)

• IET On-Site Guide

• IET Appendix 4 of BS 7671 — Cable Data Tables

• NICEIC — Calculating Voltage Drop

• SparkyHub — Cable Current Ratings (BS 7671 Tables)

• Total Skills UK — Volt Drop Calculations

• DJT Electrical Training — Correction Factors

• Pro Certs Software — Current Carrying Capacities

• Elec-Mate — Cooker Circuit Guide

• consumer units — Consumer unit standards and RCBO requirements

• bathroom zones — Bathroom electrical zones and IP ratings

• part p notifications — When Part P notification is needed

• ev charger installation — EV charger installation requirements

• rcd protection — RCD and RCBO selection for domestic circuits

• testing and inspection — Verification testing after installation