What is the difference between Ib, In and Iz?

Ib is the design current — what the circuit actually needs to carry. In is the rating of the protective device (MCB, MCCB or fuse) selected to protect it. Iz is the actual current-carrying capacity of the installed cable, after correction factors. BS7671 Regulation 433.1.1 requires Ib ≤ In ≤ Iz.

Do I always need to check voltage drop, or only current rating?

Both, always. They are independent checks. A cable can pass the current rating check comfortably and still fail on voltage drop, particularly on longer runs — see the voltage drop guide.

What is the adiabatic check and when do I need it?

The adiabatic check (Regulation 543.1.3) verifies the cable — or its earth conductor — can withstand the heat generated by a fault current for the time it takes the protective device to disconnect, without being damaged: S = (I × √t) / k. It needs a prospective fault current figure, which this quick guide doesn't collect — use the full calculator for short-circuit and adiabatic checks.

Can I use this method for aluminium conductors?

Yes, with a different k factor (76, versus 143 for copper XLPE or 115 for copper PVC, per Table 54.4) and different Appendix 4 tables (4D3A/4D4A for PVC, 4E3A/4E4A for XLPE). Aluminium's minimum practical size is usually 16mm².

BS7671 Cable Sizing Explained | Step-by-Step Guide

The five checks BS7671 requires

Cable sizing to BS7671 isn't a single lookup — it's a sequence of checks that all have to pass simultaneously. Get any one of them wrong and the "correct" size from the others is still non-compliant. This guide walks through each step in order, the way BS7671:2018 Amendment 2 Appendix 4 and Section 525 define it.

1. Determine the design current, Ib

Ib is the current the circuit actually needs to carry under normal load — read directly from equipment ratings, calculated from kW and voltage for resistive loads (Ib = P / V for single-phase), or derived from motor nameplate data with efficiency and power factor for motor circuits.

2. Select the protective device, In

Choose an MCB, MCCB, fuse or RCBO with a standard rating at or above Ib. BS7671 Regulation 433.1.1 requires Ib ≤ In.

3. Read the tabulated rating, It

Using BS7671 Appendix 4, look up the tabulated current-carrying capacity for the cable's insulation (PVC 70°C or XLPE 90°C), configuration (multicore or single-core) and installation reference method (A1 through G — see the full tables). This figure assumes ideal conditions: 30°C ambient, installed alone, no thermal insulation contact.

4. Apply the correction factors to get Iz

Real installations rarely match those ideal conditions, so four factors correct It down to the actual current-carrying capacity, Iz:

Ca — ambient temperature, if not 30°C
Cg — grouping, if the cable shares a route with other loaded circuits
Ci — thermal insulation, if the cable is in contact with insulation
Cc — protective device type (only BS3036 semi-enclosed fuses need a factor below 1.00)

Iz = It × Ca × Cg × Ci × Cc, and Regulation 433.1.1 requires In ≤ Iz — combined with step 2, the full chain is Ib ≤ In ≤ Iz.

5. Check voltage drop

Independently of current rating, BS7671 Section 525 limits voltage drop to 3% of nominal voltage for lighting circuits and 5% for others, calculated from the cable's mV/A/m figure, its length and the design current. See the voltage drop calculator for the full method — this is frequently what actually decides the final cable size on longer runs, not the current rating.

6. Check the adiabatic (short-circuit) withstand

If a prospective fault current is available, Regulation 543.1.3 requires the minimum conductor size to satisfy S = (I × √t) / k, where I is the fault current, t is the protective device's disconnection time, and k is a material and insulation constant (143 for copper XLPE, 115 for copper PVC, 76 for aluminium, per Table 54.4). This step needs fault-level data that a quick sizing tool doesn't collect — it's covered in the full calculator.

BS7671 Cable Sizing, Explained