Rating Factors and Cable Sizes: How to Use Ca, Cg, Ci and Cf Correctly

Appendix 4 is where a lot of candidates lose their nerve. You turn to it, you see an equation full of letter C’s with subscripts, you see a dozen tables of decimals, and the natural reaction is to close the book and hope the question doesn’t come up. It always comes up.

 

The good news is that the whole thing reduces to one short equation and three or four table look-ups. Once you have done two worked examples the method sticks, and it is the same method whether you are sizing a lighting circuit in a loft or an immersion heater run through a hot bakery.

 

This guide works through it using flat twin and earth cable, because that is what most domestic work uses and because if you can manipulate the equation for twin and earth, every other cable type follows the same pattern.

The Three I Values You Must Get in Order

Before you touch a rating factor, you have to be certain which current is which. Mixing these up is the single most common reason a cable sizing answer comes out wrong.

 

Symbol	Name	Memory Hook	Where It Comes From
Ib	Design current	I built — what the circuit is built for	Calculated from the load
In	Nominal rating of protective device	The number printed on the device	The MCB or fuse rating
It	Tabulated current-carrying capacity	I table — from the tables	Table 4D5 and similar

 

Ib is the design current — the maximum current the circuit has been built to carry. A kitchen lighting circuit might be 4 A. An immersion heater is 13 A. A ring final circuit is an unknown load protected by a 32 A device.

 

In is the nominal rating — the name given to the device. A 13 A fuse is called a 13 A fuse; a 20 A circuit breaker is called a 20 A breaker. That printed number is In.

 

It is the tabulated capacity you look up once the calculation is done, and it tells you which cable size to select.

 

Key point: Under no-fault conditions the device rating must be at least the design current — In ≥ Ib. You would not plug a 13 A kettle into a circuit protected at 6 A. This is the first half of the coordination requirement in Regulation 433.1.1.

 

The Rating Factors and Where to Find Them

The rating factors are listed in Part 2 of BS 7671 in the definitions, each with a short description and a cross-reference to where it is used in the book. For circuits above ground — which is what the exam concentrates on — only four matter.

 

Factor	Applies To	Where to Look	Typical Effect
Ca	Ambient air temperature	Table 4B1	Below 1 above 30°C
Cg	Grouping / bunching of circuits	Table 4C1	Below 1
Ci	Thermal insulation	Insulation tables and reference methods	Below 1
Cf	BS 3036 rewireable fuse	Fusing factor, Appendix 4	Always 0.725

 

Ca — Ambient Temperature

Table 4B1 is straightforward once you know its layout. Find the ambient temperature in question, read across to the column for 70°C thermoplastic insulation (which is what twin and earth is), and read off the factor.

 

Two things to note. First, 30°C is the reference temperature and carries a factor of 1.00 — a factor of 1 changes nothing. Second, 25°C gives a factor greater than 1 (1.03). That is the only occasion where a rating factor makes the cable requirement smaller. Every other factor you will meet pushes the cable size up.

 

Exam tip: The reason twin and earth is limited to 70°C is not arbitrary. Appendix 4 refers you to the BS 6004 cable standard, which sets 70°C as the maximum conductor operating temperature for flat profile twin and earth cable — the same figure that underpins the tabulated values in Table 4D5.

 

Cg — Grouping

Table 4C1 gives the grouping factors. You choose the installation method (bunched in conduit, clipped direct on a surface, on a perforated tray, and so on), then the number of circuits or multicore cables grouped together, and read off the factor. More circuits sharing the same heat means a lower factor and a bigger cable.

 

Ci — Thermal Insulation

Loft insulation is the everyday example. There are two ways this appears in the book, and the exam uses both:

 

Situation	How It Is Handled
Cable totally surrounded by insulation	Use the derating table in Appendix 4 for cables enclosed in insulation (reference method 103 for twin and earth)
Cable in proximity to insulation	Use one of the four twin and earth installation reference methods — 100, 101, 102, 103

 

The four reference methods for twin and earth in proximity to thermal insulation are set out in Table 4A2. Learn where that page is — exam setters love it, and you cannot use Table 4D5 correctly without knowing which of those columns applies.

 

For a fuller treatment of how the reference methods map onto real installations, see our guide to installation reference methods and cable sizing.

 

Cf — The BS 3036 Fusing Factor

A BS 3036 semi-enclosed rewireable fuse is a piece of fuse wire in a carrier, and the melting of that wire is nowhere near as fast or as predictable as the operation of a modern breaker. To compensate, a fusing factor of 0.725 is applied.

 

Remember: Cf applies only where the protective device is a BS 3036 fuse. With a BS EN 60898 breaker or a BS 88 fuse, Cf does not enter the calculation at all. There are still thousands of 1950s rewireable fuses in service, so this is not a museum piece — and it appears in exam questions regularly.

 

The Equation for Circuits Above Ground

Here is the equation, trimmed down to the version that applies to cables installed above ground:

 

It ≥ In / (Ca × Cg × Ci × Cf)

 

In words: the current the cable can carry under standard tabulated conditions must be at least the rating of the protective device, divided by all the conditions the cable is actually exposed to.

 

Important: Factors that do not apply are removed from the equation entirely. Do not enter them as 1 and do not guess a value — if there is no grouping, Cg simply is not there. A tidy equation with only the relevant factors in it is far less likely to produce an arithmetic slip under exam pressure.

 

This is one of the three formulas worth committing to memory for the exam — the others being the adiabatic equation and voltage drop. We cover all three together in three essential formulas for the 18th Edition exam.

 

Why a Bigger Cable for the Same Load?

This is the conceptual hurdle. The load has not changed, so why does the cable need to get bigger?

 

Think of it as a 25 kg sack of potatoes. Ask a scout to carry it across a flat car park and it gets done — slowly, perhaps, but it gets done. Now ask the same scout to carry the same 25 kg up a long steep hill in blazing sun and they will overheat and drop it. Send a soldier instead, someone who trains daily to carry loads up hills in full kit, and it is no problem at all.

 

The sack never changed weight. All that changed was who is carrying it, because the conditions got harder.

 

Key point: The design current does not increase when you apply rating factors. What increases is the required current-carrying capacity — the cable’s ability to shed heat under adverse conditions without exceeding 70°C. The load is the same; the cable is stronger.

 

If the cable does exceed its limiting temperature, the insulation degrades, the protection against electric shock that the insulation provides is compromised, and the rising conductor resistance pushes the circuit’s Zs up as well — a problem that shows up on test as a fail against the maximum Zs tables.

 

Worked Example 1: One Rating Factor

Question: What size live conductors would you select from Table 4D5 when installing twin and earth cable in a conservatory with an ambient temperature of 40°C? The circuit has overload protection from a 20 A BS EN 60898 circuit breaker. Assume installation reference method C throughout. No other rating factors apply.

 

Step 1 — Identify what applies. Only ambient temperature is in play. Cg, Ci and Cf all drop out, leaving:

 

It ≥ In / Ca

 

Step 2 — Find the factors.

Item	Value	Source
In (circuit breaker)	20 A	Given in question
Ca at 40°C, 70°C thermoplastic	0.87	Table 4B1

 

Step 3 — Calculate.

 

It ≥ 20 ÷ 0.87 = 23 A (rounded up)

 

Step 4 — Select from Table 4D5. Go to the reference method C column, work down until you reach a value of 23 A or more. The first entry that satisfies this is 27 A. Read across to the left-hand column: 2.5 mm² twin and earth.

 

Result	Value
Required It	23 A
Nearest tabulated capacity (method C)	27 A
Cable selected	2.5 mm²

 

Worked Example 2: Two Rating Factors

Question: What size live conductors would you select from Table 4D5 when installing twin and earth cable in an old bakery with an ambient temperature of 40°C? The circuit has overload protection from a 20 A BS 3036 semi-enclosed fuse. Assume installation reference method C throughout. Cg and Ci do not apply.

 

Same ambient temperature, but the protective device has changed — so Cf now enters the equation:

 

It ≥ In / (Ca × Cf)

 

Item	Value	Source
In (BS 3036 fuse)	20 A	Given in question
Ca at 40°C	0.87	Table 4B1
Cf (BS 3036 fusing factor)	0.725	Appendix 4

 

It ≥ 20 ÷ (0.87 × 0.725) = 20 ÷ 0.63075 = 31.7 A

 

Turning to Table 4D5, reference method C, the smallest cable with a capacity of at least 31.7 A takes us to 4 mm² twin and earth.

 

Key point: Identical load, identical ambient temperature, identical installation method — and the cable has gone from 2.5 mm² to 4 mm² purely because of the type of protective device. The more adverse conditions you stack on a cable, the larger it has to be. Virtually everything we do to a cable makes its conditions worse, and managing that properly is part of the job.

 

The Exception: When Ib Goes on Top

There is one important variation. The standard equation puts In on the top, but Appendix 4 gives an alternative that uses Ib instead:

 

It ≥ Ib / (Ca × Cg × Ci × Cf)

 

This applies where overload protection is not required or is provided by other means — typically a fixed load that physically cannot draw more than its rated current. The classic case is a hardwired appliance on a dedicated circuit.

 

Worked Comparison

A 13 A space heater is hardwired to a circuit protected by a 20 A breaker. The circuit serves nothing else, so the current can never exceed 13 A. Conditions: 40°C ambient (Ca = 0.87), installation reference method 101 (cable covered by more than 100 mm of insulation).

 

Basis	Calculation	Required It	Table 4D5 (method 101)	Cable
Using Ib = 13 A	13 ÷ 0.87	15 A	17 A	2.5 mm²
Using In = 20 A	20 ÷ 0.87	23 A	27 A	6 mm²

 

Exam tip: That is a jump from 2.5 mm² to 6 mm² — two full cable sizes — decided entirely by which current you put on the top line. Read the question carefully. If it mentions overload protection being provided by the device, use In. If it tells you overload protection is not required, or that the load is fixed and cannot be increased, use Ib.

 

The relationship between design current, device rating and cable capacity is the heart of Regulation 433.1.1, which we work through in more detail in circuit design and overload protection revision notes.

 

Reading Table 4D5 Without Errors

Table 4D5 is the destination for every twin and earth calculation, and it is worth knowing exactly how it behaves.

 

Feature	What to Know
Columns	Reference methods — 100, 101, 102, 103 and method C, among others
Left-hand column	Conductor cross-sectional area (csa) of the live conductors
Capacity values	Choose the first value equal to or greater than your calculated It
CPC size	Not shown — the table gives live conductors only; the cpc is whatever that cable is manufactured with
Voltage drop	Also tabulated here in mV/A/m — needed for the separate voltage drop check

 

Important: Never round your It down to fit a smaller cable. If you calculated 31.7 A and the table offers 27 A and 36 A, you take 36 A. Selecting the smaller cable because “it’s nearly there” is the classic way to fail both an exam question and a real installation.

 

Cable sizing does not finish with current-carrying capacity, either. A cable that passes the It check can still fail on voltage drop or on the adiabatic check for the cpc, and the full six-step procedure is covered in cable size calculations: the six-step method.

 

Common Mistakes in the Exam

 

Mistake	Why It Costs Marks
Multiplying by the factors instead of dividing	The factors go on the bottom. Dividing by a number less than 1 makes the answer bigger — which is the whole point
Applying Cf to a BS EN 60898 breaker	0.725 belongs to BS 3036 rewireable fuses only
Using In when the question says overload protection is not required	Gives an oversized cable and the wrong answer
Reading the wrong reference method column	Methods 100–103 and method C give very different capacities for the same csa
Reading the cpc column as the live conductor size	Table 4D5 lists live conductors; the cpc comes with the cable
Rounding It down	Always select the tabulated value equal to or greater than your calculated It
Forgetting 25°C gives a factor above 1	Candidates assume every factor is below 1 and get the direction wrong

 

Remember: In the exam you only have BS 7671 in front of you. The On-Site Guide has a useful reminder of rating factors in its appendices, and it is worth reading during revision — but it is not permitted in the 18th Edition exam. Do all your practice out of the regulations book itself so that the page turns become automatic.

 

Practise finding Table 4B1, Table 4C1, Table 4A2 and Table 4D5 until you can reach each one in a few seconds. Speed of navigation is worth as much in the exam as knowing the method — a point we make in ten essential BS 7671 tables for the exam.

 

Practice and Further Study

Rating factors sit at the junction of Part 4 (protection) and Part 5 (selection and erection) — together the two highest-weighted parts of the exam. Set yourself calculation questions, change one condition at a time, and check the cable size that results. That habit builds both exam speed and site competence.

 

Test yourself on the relevant topics:

• Part 4 — Protection for Safety quiz
• Part 5 — Selection and Erection of Equipment quiz
• Part 3 — Assessment of General Characteristics quiz

 

Our app includes 690+ practice questions across all 8 parts of BS 7671, with worked explanations that reference the specific table or regulation behind each answer — including a set of Appendix 4 rating factor and cable selection questions. Full timed mock tests use the same weighted question distribution as the real exam, so you can practise working through calculation questions at the two-minutes-per-question pace you will face on the day.

 

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