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Sizing a cable: correction factors and ItWhich correction factors happen at the same time?

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What it's for: choosing the cross-sectional area (CSA) of a conductor from the information you have. It's a selection, not measuring a conductor. Before you start you need to know four things:

  1. the cable type (e.g. 90 °C thermosetting singles),
  2. the installation (reference) method,
  3. any correction factors present, and
  4. the device rating, In. On a 6 A circuit breaker, the "6 A" is the In.

Our scenario

90 °C thermosetting unsheathed single-core conductors
protected by a 16 A Type C circuit breaker
ambient temperature 35 °C
passing through 100 mm of thermal insulation
grouped with one other circuit, in conduit (on a surface)
single-phase circuit

→ In = 16 A · Reference Method B (conduit on a surface)

A common mix-up is "70 °C thermosetting". It's 90 °C thermosetting. (70 °C cable is thermoplastic, PVC.) Ambient temperature just means the air temperature around the cable.

Find each correction factor

FactorWhat it's forWhere to find itValue
CaAmbient temperature. Read down the 90 °C thermosetting column to 35 °C.Table 4B10.96
CgGrouping. 2 circuits in total (always count the one you're designing), item 1 "bunched in air, on a surface, embedded or enclosed" matches cables in conduit.Table 4C10.8
CiCable surrounded by thermal insulation, 100 mm length of insulation (e.g. through a cavity wall)Appendix 4 section 2.6 table (it has no table number)0.78 (not used)

BS 7671 · Appendix 4 · Table 4B1 ambient temperature factors (Ca) BS 7671 · Table 4C1 grouping factors (Cg) BS 7671 · Appendix 4 · 2.6 cable surrounded by thermal insulation (Ci), table with no number · p.451

Sparky's tip: the Ci table
The thermal insulation table is in Appendix 4 (p.451), but it has no table number. Tab it so you can find it.

Which factors are happening at the same time?

This is the designer's key question. You only multiply together the factors that affect the cable at the same place, at the same time.

100 mm thermal insulation (Ci 0.78?) conduit on a surface 2 circuits: Cg 0.8 ambient 35 °C: Ca 0.96 (the whole run) Ca and Cg: all along the run, at the same time → use both Ci: the insulation surrounds the conduit, not the cable → left out
Our reasoning. The section 2.6 wording is "where a cable is totally surrounded by thermal insulation", not "a cable or its containment". Here the cables are surrounded by conduit, so we didn't apply Ci. Some designers would disagree and include it. There's a lot of judgement in design.
  • Ca: there's always an ambient temperature.
  • Cg: the two circuits run in the conduit together all the way, in that same ambient temperature. So Ca and Cg are happening at the same time.
  • Ci: crossed out, "not used due to cable being surrounded by conduit, not thermal insulation".
The tailor analogy
Correction factors are like going to a tailor. Instead of buying trousers off the peg that fit a thousand other people, each factor is one of your measurements (waist, leg…). Put them together and you get a cable "made to measure" for its real surroundings. Some people are lucky and the off-the-peg size fits. Most of us need the tailor, and that's the It calculation.

The It calculation

It ≥ In ÷ (Ca × Cg × Ci × Cf …)
Divide the device rating by the factors that apply, multiplied together. It is in amps.

Worked example: It for our scenario

It ≥ In ÷ (Ca × Cg)          (Ci not used)
   ≥ 16 ÷ (0.96 × 0.8)
   ≥ 16 ÷ 0.768
   ≥ 20.83 A

BS 7671 · Appendix 4 · the It ≥ In ÷ (Ca × Cg × Ci × Cf …) formula · p.453 On-Site Guide · Appendix F · the same formula

20.83 A isn't the current flowing in the circuit. It's the rating the cable would need in "perfect" conditions to cope with its real surroundings. People look in Chapter 43 for It, alongside Ib, In and Iz, and can't find it. That's because It is really the "tailoring" step in Appendix 4.

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