How to Calculate Cable Size and Protective Device Rating: The BS 7671 Procedure

Cable sizing is the calculation that electricians and designers get asked about most often — and it’s the question that appears in almost every IET exam. Get it wrong, and you end up with a cable that either overheats (too small) or costs far more than necessary (too big).

This guide walks you through the full BS 7671 cable sizing procedure, from design current to final verification, with a worked example you can follow along with.

The Key Rule: Ib ≤ In ≤ It

Before diving into the steps, you need to understand the fundamental relationship that governs cable sizing in BS 7671:

The key relationship: Ib ≤ In ≤ It

Ib (design current) — the actual current the circuit needs to carry
In (nominal rating) — the rating of the protective device (MCB, fuse, RCBO)
It (tabulated current) — the current-carrying capacity of the cable

The device rating must be at least as big as the design current, and the cable must be able to carry at least as much as the device rating. This ensures the cable is protected against overload.

The Cable Sizing Procedure — Step by Step

Cable sizing procedure flowchart

Step 1: Determine the Design Current (Ib)

The design current is the maximum current the circuit will normally carry. How you calculate it depends on the type of load:

Resistive loads (heaters, kettles): Ib = P ÷ V (e.g., 3000W ÷ 230V = 13.0A)
Motor loads: Use the motor’s full-load current from the manufacturer’s data
Diversity: For circuits with multiple loads, apply diversity factors per BS 7671 Appendix 1

Step 2: Select the Protective Device (In)

Choose an MCB, fuse, or RCBO with a rating equal to or greater than Ib. Standard MCB ratings are: 6, 10, 16, 20, 25, 32, 40, 50, 63A.

For example, if Ib = 28A, the next standard MCB rating up is 32A, so In = 32A.

Important: You must also consider the type of MCB (B, C, or D) based on the load characteristics. Type B is standard for most domestic circuits.

Step 3: Apply Correction Factors

This is where most exam candidates struggle. The cable’s published current rating (from BS 7671 tables) assumes ideal installation conditions: 30°C ambient temperature, cables not grouped together, no thermal insulation, and an MCB or HRC fuse as the protective device.

If conditions differ from the ideal, you need correction factors that increase the required cable capacity.

Correction factors explained

The four main correction factors are:

Factor	What it accounts for	Typical values	BS 7671 Reference
Ca	Ambient temperature above 30°C	0.94 (35°C) to 0.50 (70°C)	Table 4B1
Cg	Grouping (cables bunched together)	0.65 to 1.00	Tables 4C1–4C4
Ci	Thermal insulation contact	0.50 (surrounded) to 1.00	Reg. 523.7
Cf	BS 3036 fuse (rewirable)	0.725	Reg. 433.1.1

The required tabulated current capacity of the cable is:

It = In ÷ (Ca × Cg × Ci × Cf)

If no correction factor applies, use 1.0 for that factor.

Step 4: Select the Cable from Tables

Look up the appropriate BS 7671 current-carrying capacity table for your installation method (e.g., Table 4D5 for thermoplastic flat cable clipped direct). Find a cable size where the tabulated current (It) is greater than or equal to your calculated minimum It.

Step 5: Check Voltage Drop

Even if the cable can carry the current safely, it may drop too much voltage over a long run. BS 7671 limits the voltage drop to:

3% of 230V = 6.9V for lighting circuits
5% of 230V = 11.5V for other circuits

Voltage drop check

The formula is:

VD = (mV/A/m × Ib × L) ÷ 1000

Where:

mV/A/m = millivolts per amp per metre (from BS 7671 Table 4D5)
Ib = design current in amps
L = cable length in metres (one way, not total)

If the voltage drop exceeds the limit, you need to increase the cable size (a bigger cable has lower resistance per metre and therefore less voltage drop).

Step 6: Check Earth Fault Loop Impedance (Zs)

Finally, verify that the earth fault loop impedance for the circuit is low enough to ensure the protective device will trip within the required disconnection time. This check is covered in detail in our separate guide on testing Zs.

Worked Example

Scenario: A 9.2 kW electric shower is to be installed in a bathroom. The cable run is 18 metres, clipped direct to a wall, at 30°C ambient. The cable passes through 200mm of loft insulation. The circuit is protected by a Type B MCB.

Step 1 — Design current:

Ib = P ÷ V = 9200 ÷ 230 = 40A

Step 2 — Protective device:

Next standard MCB rating ≥ 40A = 40A Type B MCB

Step 3 — Correction factors:

Ca = 1.0 (30°C ambient — no correction needed)
Cg = 1.0 (single circuit — no grouping)
Ci = 0.75 (cable passes through insulation but not surrounded — Reg. 523.7)
Cf = 1.0 (MCB, not a BS 3036 fuse)

Minimum It = 40 ÷ (1.0 × 1.0 × 0.75 × 1.0) = 53.3A

Step 4 — Cable selection:

From Table 4D5 (Method C — clipped direct), a 10 mm² T&E cable has a tabulated current of 64A, which is ≥ 53.3A. ✓

Step 5 — Voltage drop:

From Table 4D5, the mV/A/m for 10 mm² is 4.4.

VD = (4.4 × 40 × 18) ÷ 1000 = 3.17V

3.17V is well within the 5% limit of 11.5V. ✓

Result: Use 10 mm² T&E cable with a 40A Type B MCB.

Common Mistakes

Mistake	Problem	Solution
Forgetting to apply correction factors	Cable undersized — risk of overheating	Always check Ca, Cg, Ci, and Cf
Using the wrong installation method table	Incorrect current ratings selected	Match the table to your actual installation method
Calculating voltage drop with total cable length	VD appears twice as high as actual	Use one-way cable length, not there-and-back
Selecting In less than Ib	Protective device trips under normal load	In must be ≥ Ib — always round up to next standard rating
Ignoring thermal insulation	Cable in loft insulation overheats	Apply Ci = 0.50 (surrounded) or 0.75 (touching one surface)

Key Regulations

Reg. 433.1 — Overload protection: Ib ≤ In ≤ Iz
Reg. 433.1.1 — BS 3036 fuse factor (0.725)
Reg. 523.7 — Cables in thermal insulation
Appendix 4 — Current-carrying capacity and voltage drop tables
Table 4B1 — Ambient temperature correction factors
Tables 4C1–4C4 — Grouping correction factors
Table 4D5 — Current ratings for flat T&E cable
Appendix 12 — Voltage drop limits

Practice and Further Study

Cable sizing calculations are covered under Part 4: Protection for Safety and Part 5: Selection and Erection of Equipment of BS 7671. Test your knowledge:

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