Cable size calculator for twin and earth

How the calculator picks a cable size

It does two checks and gives you the smallest size that passes both. First, the cable must carry the design current for the way it is installed. Second, the voltage lost along the run must stay inside the limit: 5% of 230V for most circuits, 3% for lighting.

The design current, written Ib, is the current the load draws. Enter it in amps, or in kW and the calculator works it out as watts divided by 230V. That assumes a power factor of 1, which is right for heating loads such as showers, immersion heaters and kettles. For a motor, enter the full-load current from its rating plate instead.

The current-carrying capacity, written Iz, comes from BS 7671 Table 4D5, the table for flat twin and earth with a bare earth. The cable makers point you to it: Doncaster Cables' H6242Y page says "for current ratings refer to table 4D5 of BS7671". The calculator uses the figures as reproduced by Caledonian Cables, cross-checked against Eland Cables' 6242Y data sheet. Both are for a 30°C ambient and a conductor running at up to 70°C.

Voltage drop is worked out the way NICEIC sets it out: the cable's mV/A/m figure × Ib × the length in metres ÷ 1,000.

Worked example: a 9.5kW electric shower

A 9.5kW electric shower, 15 metres of cable from the consumer unit, clipped direct to the joists along the whole route with no insulation near it. The calculator loads this example when you first open it. An electric shower is the highest-rated item in National Energy Action's appliance table, at 7,000 to 10,500 watts.

• Design current: 9,500W ÷ 230V = 41.30A.
• Current check, clipped direct (Method C): 4mm² is rated 37A, which is too low. 6mm² is rated 47A, which is enough. Smallest size by current: 6mm².
• Voltage drop on 6mm²: 7.3 mV/A/m × 41.30A × 15m ÷ 1,000 = 4.52V.
• As a percentage: 4.52V ÷ 230V = 1.97%, inside the 5% limit of 11.5V. Passes.
• Answer: 6mm² twin and earth, before any correction for grouping, ambient temperature or the protective device.

Why the installation method changes the answer

Cable gets warm when it carries current. Clipped to a joist in open air, the heat escapes. Buried in loft insulation or a packed stud wall, it cannot, so BS 7671 gives the same cable a lower rating. The calculator offers the six methods that Table 4D5 covers for twin and earth.

Run the same 9.5kW shower through each one and the answer moves from 6mm² to 16mm². It is also how a cable that was fine for the last owner's shower can end up too small after someone lays thick loft insulation over it.

The figures the calculator uses

Each line gives the rating in amps for Methods C, 100, 101, 102, 103 and A, then the voltage drop in mV/A/m. Ratings are from BS 7671 Table 4D5 as published by Caledonian Cables. The Method A, Method C and mV/A/m figures match Eland Cables' data sheet for every size it lists, 1.0 to 10mm².

Check them against your own copy of BS 7671 before you sign anything off. The tool covers twin and earth only: singles in conduit, armoured cable and flex have their own tables, and none of them are in here.

• 1.0mm²: C 16A, 100 13A, 101 10.5A, 102 13A, 103 8A, A 11.5A. 44 mV/A/m.
• 1.5mm²: C 20A, 100 16A, 101 13A, 102 16A, 103 10A, A 14.5A. 29 mV/A/m.
• 2.5mm²: C 27A, 100 21A, 101 17A, 102 21A, 103 13.5A, A 20A. 18 mV/A/m.
• 4mm²: C 37A, 100 27A, 101 22A, 102 27A, 103 17.5A, A 26A. 11 mV/A/m.
• 6mm²: C 47A, 100 34A, 101 27A, 102 35A, 103 23.5A, A 32A. 7.3 mV/A/m.
• 10mm²: C 64A, 100 45A, 101 36A, 102 47A, 103 32A, A 44A. 4.4 mV/A/m.
• 16mm²: C 85A, 100 57A, 101 46A, 102 63A, 103 42.5A, A 57A. 2.8 mV/A/m.

The breaker has to fit between the two

Sizing to the load is only half of it. BS 7671 regulation 433.1.1, as NICEIC explains it, needs Ib ≤ In ≤ Iz: the design current must not exceed the rating of the breaker or fuse (In), and the breaker must not exceed what the cable can carry.

So if you add a breaker rating, the calculator checks that the breaker is at least the design current and that the cable is rated for the breaker, not just the load. For the shower on a 45A breaker, clipped direct, 6mm² at 47A still passes. Put the same run in conduit in an insulated wall (Method A) and 10mm² at 44A no longer does, so the answer becomes 16mm². In a stud wall at Method 103, even 16mm² (42.5A) is under 45A. Then no twin and earth in this table will do: change the route, or have the circuit designed properly.

Two cases the calculator does not handle. A rewireable BS 3036 fuse falls outside the simple Ib ≤ In ≤ Iz check, as NICEIC notes, and needs its own design. A ring final circuit is a special case: regulation 433.1.204 allows 2.5mm² on a 30A or 32A device, provided the cable's corrected rating is at least 20A.

What this calculator does not do

This is a guide, not a design. It assumes one circuit on its own, in a 30°C room, with no correction factors. It leaves out grouping (several cables bunched together run hotter and lose rating), ambient temperature above 30°C, and the type of protective device. It does not check earth fault loop impedance, disconnection times, the protective conductor or RCD protection. Any one of those can push the cable up a size or rule the circuit out.

The voltage drop check covers this circuit only. The limit runs from the origin of the installation, so a circuit fed from a sub-main has to share the 5% with it.

A competent person has to design, inspect, test and certify the real circuit under BS 7671. In England, Approved Document P sets out what is notifiable: any new circuit, a replacement consumer unit, and changes to circuits in a special location, for example close to a bath or shower. Notifiable work is signed off by an electrician registered with a competent person scheme, or by building control. Use the answer to sense-check a quote or plan a job, not to skip that.

Common questions