BS7671 Cable Sizing Guide for Electricians

BS7671 Cable Sizing: A Practical Guide

Getting cable sizing right isn't optional — it's the difference between a safe installation and a fire risk. This guide walks through the BS7671 cable sizing process in plain English, with the tables and worked examples you actually need on site.

Why Cable Sizing Matters

An undersized cable carrying too much current overheats. That heat degrades the insulation, and degraded insulation causes faults — or worse, fires. BS7671 (the IET Wiring Regulations, 18th Edition) sets out the rules to prevent this.

Cable sizing also affects voltage drop. Even a correctly rated cable can deliver insufficient voltage to an appliance if the run is too long, causing motors to overheat, LED drivers to flicker, and electric showers to underperform.

The Cable Sizing Process

There are five key steps:

1. Determine the design current (Ib)
2. Select the protective device rating (In)
3. Calculate the tabulated current carrying capacity (It)
4. Check the cable can carry It after correction factors
5. Verify voltage drop is within limits

Step 1: Design Current (Ib)

The design current is the maximum current the circuit will carry in normal service. For a single appliance, it's on the data plate. For a ring final circuit, it's based on the diversity-applied load.

Ib = Power (W) ÷ Voltage (V)

An 8.5kW electric shower on a 230V supply:

8,500 ÷ 230 = 37.0A

Step 2: Protective Device Rating (In)

Select a standard overcurrent device rating equal to or greater than Ib. For our shower:

In = 40A (next standard MCB rating above 37.0A)

Step 3: Tabulated Current (It)

This is where correction factors come in. The current carrying capacity values in BS7671 Appendix 4 assume ideal conditions. Real installations rarely have ideal conditions, so you divide by correction factors to find the minimum tabulated current your cable must achieve.

It = In ÷ (Ca × Cg × Ci × Cf)

Where:

• Ca — Ambient temperature correction factor
• Cg — Grouping correction factor
• Ci — Thermal insulation correction factor
• Cf — Semi-enclosed fuse factor (0.725 for BS 3036 fuses, 1.0 for MCBs)

Correction Factors Explained

Ambient Temperature (Ca)

BS7671 tables assume an ambient temperature of 30°C. If cables run through hotter spaces (boiler rooms, roof spaces in summer), the correction factor reduces below 1.0, meaning you need a larger cable.

| Ambient Temp (°C) | 70°C Thermoplastic (Ca) | 90°C Thermosetting (Ca) | |---|---|---| | 25 | 1.03 | 1.02 | | 30 | 1.00 | 1.00 | | 35 | 0.94 | 0.96 | | 40 | 0.87 | 0.91 | | 45 | 0.79 | 0.87 | | 50 | 0.71 | 0.82 |

Grouping (Cg)

When cables are bunched together or run in the same trunking/conduit, they share heat and each cable's capacity is reduced.

| Number of Circuits | Clipped Direct (Cg) | Enclosed in Conduit/Trunking (Cg) | |---|---|---| | 1 | 1.00 | 1.00 | | 2 | 0.80 | 0.80 | | 3 | 0.70 | 0.70 | | 4 | 0.65 | 0.65 | | 5 | 0.60 | 0.60 | | 6 | 0.57 | 0.57 |

Thermal Insulation (Ci)

If a cable is surrounded by thermal insulation (common in loft spaces and timber frame builds), its ability to dissipate heat is severely restricted.

• Cable touching insulation on one side: Ci = 0.75
• Cable fully enclosed in insulation (up to 100mm): Ci = 0.50
• Cable enclosed in insulation over 100mm: Ci = 0.40 (consult manufacturer data)

This is one of the most commonly underestimated factors. A cable running through loft insulation that was fine 20 years ago may now be undersized because modern insulation is thicker.

Step 4: Select the Cable

With your minimum It calculated, look up BS7671 Table 4D5A (for twin and earth cables clipped direct) or the relevant table for your installation method. Pick the cable size whose current carrying capacity meets or exceeds It.

Step 5: Check Voltage Drop

BS7671 limits voltage drop to 5% of the nominal supply voltage for a total installation (3% for lighting circuits is common practice). On a 230V supply:

• Maximum total voltage drop: 11.5V
• Practical limit per circuit: 3–5% depending on circuit type

Voltage drop is calculated as:

VD = (mV/A/m × Ib × L) ÷ 1,000

Where:

• mV/A/m — millivolts per amp per metre (from BS7671 tables, varies by cable size)
• Ib — design current in amps
• L — cable run length in metres (one way, not total cable length)

If the voltage drop exceeds the limit, you need to go up a cable size — even if the current carrying capacity of the smaller cable was adequate.

Rather than looking up tables manually, the Cable Calculator and Volt Drop Calculator will run through this process for you and flag any issues.

Worked Example: 8.5kW Electric Shower

Let's size the cable for an 8.5kW shower with a 15-metre run from the consumer unit, clipped direct, not grouped, in a room at 30°C ambient temperature, using 70°C thermoplastic (PVC) twin and earth cable.

Design current: 8,500 ÷ 230 = 37.0A

Protective device: 40A Type B MCB

Correction factors:

• Ca = 1.00 (30°C ambient)
• Cg = 1.00 (not grouped)
• Ci = 1.00 (not in insulation)
• Cf = 1.00 (MCB, not BS 3036 fuse)

Minimum It: 40 ÷ (1.00 × 1.00 × 1.00 × 1.00) = 40A

From Table 4D5A (Reference Method C — clipped direct):

• 6mm² T&E = 47A — this meets the 40A requirement

Voltage drop check:

• 6mm² mV/A/m = 7.3 (from Table 4D5B)
• VD = (7.3 × 37.0 × 15) ÷ 1,000 = 4.05V (1.76%)

4.05V is well within the 11.5V limit, so 6mm² T&E on a 40A MCB is correct for this installation.

If the run were 25 metres:

• VD = (7.3 × 37.0 × 25) ÷ 1,000 = 6.75V (2.93%)

Still within limits, but getting closer. On a longer run, or if correction factors applied, you might need to step up to 10mm².

Common Domestic Circuits: Quick Reference

| Circuit | Typical Rating | Cable Size (T&E) | Max Run (Approx.)* | |---|---|---|---| | Lighting | 6A MCB | 1.0mm² or 1.5mm² | 20m / 30m | | Ring final (sockets) | 32A MCB | 2.5mm² | N/A (ring) | | Radial sockets | 20A MCB | 2.5mm² | 25m | | Radial sockets | 32A MCB | 4.0mm² | 30m | | Immersion heater | 16A MCB | 2.5mm² | 25m | | Electric shower (7.5kW) | 32A MCB | 6.0mm² | 30m | | Electric shower (8.5kW) | 40A MCB | 6.0mm² | 25m | | Electric shower (10.5kW) | 45A MCB | 10.0mm² | 35m | | Cooker (up to 13kW) | 32A MCB | 6.0mm² | 25m | | EV charger (7.4kW, 32A) | 32A MCB | 6.0mm² | 25m |

Approximate maximum run lengths assume clipped direct, no grouping, 30°C ambient, no insulation contact. Always calculate for your specific installation.

Common Mistakes to Avoid

• Ignoring insulation contact. Cables in loft spaces are almost always in contact with insulation now. Apply Ci = 0.50 if the cable is enclosed.
• Forgetting grouping at the consumer unit. Even if cables separate after the CU, the section where they're bunched together needs the grouping factor applied.
• Measuring cable length, not route length. Measure the actual route the cable takes — up walls, across ceilings, through floor voids — not the straight-line distance.
• Using old tables. Make sure you're working from the current edition of BS 7671 (18th Edition Amendment 4). Cable ratings can change between editions. The IET BS 7671 guidance page includes the current On-Site Guide and related publications.
• Not accounting for voltage drop on long runs. A cable can have adequate current carrying capacity and still fail on voltage drop. Always check both.

Key Takeaway

For registered electricians, competent person schemes such as NICEIC and NAPIT provide additional technical guidance and self-certification routes for notifiable work.

Cable sizing is a calculation, not a guess. Work through the process methodically — design current, protective device, correction factors, table lookup, voltage drop check — and you'll get it right every time.

If you want to speed up the process on site, the Cable Calculator applies the correction factors and checks voltage drop automatically. It's free and works on your phone — worth bookmarking for quick reference between jobs.