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Cable size calculator: twin and earth size, voltage drop and length for a UK circuit

Work out a starting cable size for a circuit from the type of circuit, how it is installed and how far it runs, check the voltage drop against BS 7671, and see how much twin and earth to buy. It uses the same approach as TradeCalc in the Hard Hat Way app with the ratings and mV/A/m figures from Table 4D5. It is a guide for planning and pricing: new circuits and other notifiable work must be designed, installed and certified by a registered electrician.

Your job

CircuitCable run (m)

From the consumer unit to the furthest point along the route the cable takes. For a ring, the total length of the loop

Appliance rating (kW)

From the rating plate: showers 7.5–10.5 kW, cookers 6–12 kW

Socket in the cooker switch

Adds 5 A to the diversified load

Lighting MCBLoad (A)

Watts ÷ 230

How the cable is installed

The reference method in BS 7671 Table 4A2. Most house wiring runs above ceilings and in stud walls

Cables bunched together

Circuits touching along the run, for example leaving the board together

Ambient temperaturePasses through thermal insulation

Only for Methods A, B and C; Methods 100–103 already include the insulation

Protective deviceVoltage drop limit

What you need

Sizing

Suggested size (guide): 2.5 mm² T&E a ring final is 2.5 mm² on a 32 A MCB; the rating and length checks below passBS 7671:2018+A2:2022 Table 4D5
Design current Ib: 20.0 Aa ring is protected at 32 A and each leg must carry at least 20 A (Reg 433.1.204)
Device In: 32 AMCB or RCBO
Required cable rating It: 20.0 A20 A a leg ÷ (Ca 1 × Cg 1 × Ci 1 × Cf 1)Tables 4B1, 4C1, 52.2; BS 3036 factor 0.725
Iz of 2.5 mm² by Method 100: 21 Acarries the required currentTable 4D5
Ring length check: 20 mOn-Site Guide maximum 106 m for a 2.5 mm² ring at 5% drop: fineIET On-Site Guide Table 7.1(i)

All sizes (Table 4D5)

1.0 mm²: Fails rating Iz 13 A by Method 100
1.5 mm²: Fails rating Iz 16 A by Method 100
2.5 mm²: OK Iz 21 A by Method 100
4.0 mm²: OK Iz 27 A by Method 100
6.0 mm²: OK Iz 34 A by Method 100
10.0 mm²: OK Iz 45 A by Method 100
16.0 mm²: OK Iz 57 A by Method 100

Materials

2.5 mm² T&E to buy: 23.0 m20 m plus 15% for drops and terminations; sold in 50 and 100 m drums
Cable clips: 67 clipsone every 300 mm where clipped, typical

Rules

Part P: Registered electrician This is a guide for planning and pricing, not a circuit design. Cable size depends on the installation method, grouping, ambient temperature, thermal insulation and the protective device, all checked against BS 7671:2018+A2:2022 Appendix 4. New circuits, consumer unit changes and work in a bathroom are notifiable under Part P and must be designed, installed, tested and certified by an electrician registered with a competent person scheme (NICEIC, NAPIT, ELECSA) or notified to building control.

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How the maths works

The design current Ib comes from the circuit: a ring final is 2.5 mm² on a 32 A MCB with each leg rated for at least 20 A (Reg 433.1.204); a 20 A radial 2.5 mm², a 32 A radial 4 mm², lighting on 6 or 10 A, an immersion on 16 A, an EV charger 32 A. A cooker gets diversity (the first 10 A plus 30% of the rest, plus 5 A for a socket in the switch) on a 32 A MCB or bigger; a shower is its full kW ÷ 230 V with no diversity. The MCB In is the next size up.
The required cable rating It is In ÷ (Ca × Cg × Ci × Cf): ambient from Table 4B1 (30 °C = 1.00, 40 °C = 0.87), grouping from Table 4C1 (2 circuits 0.80, 3 circuits 0.70), thermal insulation from Table 52.2 (100 mm 0.81, 400 mm 0.50) only where the reference method does not already include it, and 0.725 for a BS 3036 rewireable fuse.
The current-carrying capacity Iz of each size comes from BS 7671 Table 4D5 by reference method: 2.5 mm² is 27 A clipped direct (C), 21 A above a ceiling with up to 100 mm of insulation (100), 17 A with more than 100 mm (101) and 13.5 A in a stud wall not touching the board (103). 6 mm² is 47 / 34 / 27 / 23.5 A; 10 mm² is 64 / 45 / 36 / 32 A.
Voltage drop is mV/A/m × Ib × length ÷ 1,000, single phase, with the length one way: 44 mV for 1.0 mm², 29 for 1.5, 18 for 2.5, 11 for 4, 7.3 for 6, 4.4 for 10, 2.8 for 16 (Table 4D5). The limit is 3% (6.9 V) on lighting and 5% (11.5 V) on other circuits, from the origin to the furthest point. A ring is checked against the On-Site Guide maximum of 106 m for 2.5 mm².
The suggested size is the smallest that passes both the rating and the drop, and every size from 1.0 to 16 mm² is listed with why it passes or fails. Flat twin and earth is only tabulated to 16 mm²; above that is SWA or singles and a proper design.
Cable to buy is the run plus 15% for drops and terminations; a twin and earth cable carries line, neutral and earth in one sheath, so it is not doubled. Clips are one every 300 mm where it is clipped, as a typical figure.
This is a guide for planning and pricing, not a circuit design. Cable size depends on the installation method, grouping, ambient temperature, thermal insulation and the protective device, all checked against BS 7671:2018+A2:2022 Appendix 4. New circuits, consumer unit changes and work in a bathroom are notifiable under Part P and must be designed, installed, tested and certified by an electrician registered with a competent person scheme (NICEIC, NAPIT, ELECSA) or notified to building control.

Worked example

A 20 m ring final above the ceilings with up to 100 mm of insulation (Method 100), on its own at 30 °C on a 32 A MCB, is 2.5 mm² T&E: Table 4D5 gives it 21 A by Method 100 against the 20 A each leg needs, and 20 m is well inside the 106 m ring limit. Buy 23 m of cable and about 67 clips. Check the same table for a 20 A radial: 2.5 mm² over 20 m drops 7.2 V, or 3.1%, inside the 5% allowed on a power circuit but over the 3% a lighting circuit gets.

Questions people ask

What size cable do I need for an electric shower?

A 9.5 kW shower draws 41.3 A, so a 45 A MCB, and the trade rule is 10 mm² for anything 8 kW and over. That holds clipped direct (64 A) or above a ceiling with light insulation (45 A), but in more than 100 mm of insulation (Method 101) 10 mm² is only rated 36 A and you are into 16 mm². Pick the shower circuit and the method above and the table shows which sizes pass. The electrician also checks earth fault loop impedance, which this page does not.

What cable is a ring main?

2.5 mm² twin and earth on a 32 A MCB, serving up to 100 m² of floor, with each leg rated at least 20 A and the total ring no longer than about 106 m for voltage drop. Entering 32 A into a plain load calculator gives 6 mm², which is wrong for a ring; that is why the circuit type comes first here. A 20 A radial in 2.5 mm² covers 50 m²; a 32 A radial in 4 mm² covers 75 m².

Do I need an electrician?

Yes for anything notifiable under Part P in England and Wales: a new circuit, a consumer unit change, and additions or alterations in a bathroom or shower room. It must be done by an electrician registered with a competent person scheme (NICEIC, NAPIT, ELECSA) or notified to building control, and you should get an Electrical Installation Certificate. Since 2013 kitchens and outdoor work are no longer notifiable in England on their own, but any new circuit still is. Like-for-like replacement of a socket or light fitting is not notifiable.

What is voltage drop and why does it matter?

Cable has resistance, so the voltage at the far end is lower than at the board, and the longer and thinner the cable the bigger the loss. BS 7671 allows 3% on lighting (6.9 V) and 5% on other circuits (11.5 V). Too much drop makes lights dim, motors run hot and appliances misbehave. At 20 A over 20 m, 2.5 mm² drops 7.2 V (3.1%): fine for sockets, too much for lighting.

Why does the installation method change the rating so much?

A cable buried in loft insulation cannot shed heat, so it is derated: 2.5 mm² carries 27 A clipped to a joist but only 17 A under more than 100 mm of insulation and 13.5 A boxed in a stud wall. Grouping with other circuits and warm lofts derate it further. That is why the same shower can need 10 mm² in one house and 16 mm² in the next, and why the electrician confirms the route before sizing.

How much does a rewire cost?

It depends on the size of the house, how many circuits and points you want, whether the walls are being replastered anyway and where you live, so get a few quotes from registered electricians. Our house rewire cost guide explains what drives the price.

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Built by Stu Crompton — bricklayer, 22 years on the tools. These are the numbers I use on site. If a trade quotes you wildly more material than this, ask why.