Free guide · Electrical design
Cable selection: Ib ≤ In ≤ Iz explained
Choosing a cable comes down to one rule: the design current of the load (Ib) must not be more than the rating of the protective device (In), and the device rating must not be more than what the cable can carry where it's installed (Iz).
Change the load, the length and the installation conditions in the interactive visualisation below to see each number change, then read the steps.
Before you startChoosing a cable: Ib ≤ In ≤ IzRead first
What this shows
This interactive visualisation follows the five steps of choosing a cable, from the load's design current to the voltage drop check. Each box at the top feeds its number into the next box.
The line in the middle places Ib, In and Iz on one scale, and the bar at the bottom compares the voltage drop with its limit. The sliders and buttons change the load, the length and the correction factors.
Key words
- Ib
- The design current: the current the load draws.
- In
- The rating of the protective device.
- It
- The table current the cable needs: In divided by the correction factors.
- Iz
- What the cable can carry where the cable is installed: the table rating times the correction factors.
- Correction factors
- Ca (ambient temperature), Cg (grouping) and Ci (thermal insulation): each one lowers what a cable can carry.
- mV/A/m
- The cable's voltage drop in millivolts for each amp and each metre.
What to look for
- Move the Design current Ib slider and watch the device rating jump to the next standard size.
- Press Surrounded under thermal insulation. It doubles, and a bigger cable is chosen.
- Move the Length slider up until the voltage drop bar turns red, then watch the cable upsize to bring the drop back under the limit.
- Read the working box line by line: each line uses the answer from the line above.
On the job
- Always use the current BS 7671 Appendix 4 tables and the cable maker's data. The ratings in this visualisation are illustrative only.
- Ask where the cable will run: through insulation, bunched with other cables, or in a hot loft. Each of these lowers what the cable can carry.
- Check the voltage drop on long runs, such as garages, outbuildings and EV chargers. Voltage drop often decides the cable size on long runs.
- After choosing the cable, check the earth fault loop impedance as well, so that the device still disconnects in time.
Step by step
- 1Find the design current (Ib)Work out the current the load draws. For a single-phase load, Ib = P ÷ V.
- 2Choose the protective device (In)Pick the next standard device rating that is equal to or greater than Ib.
- 3Allow for the conditionsDivide In by the correction factors for grouping, ambient temperature, thermal insulation and the type of device. The answer (It) is the current the cable must be able to carry from the tables.
- 4Pick the cable (Iz)Choose a cable whose tabulated current-carrying capacity is at least It, for the way it's installed.
- 5Check the voltage dropFinally, check the voltage drop over the cable's length is within the limit. If it isn't, go up a cable size.
Always follow the current edition of BS 7671 and its guidance notes.
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