Design, Special Locations, Lighting & Three-Phase

01Correction factors (Ca, Cg and more)

Yesterday's shower example had no correction factors. In real installations, the environment around a cable can reduce how much current it can safely carry, so we correct for it and usually end up with a bigger cable.

In exam or homework questions you'll be given the temperature and the number of grouped circuits. You don't measure them. In the exam the tutor expects mainly Ca, Cg and Ci.

On-Site Guide · Appendix F · Table F1 ambient temperature factors (Ca) · p.184 new · p.168 old On-Site Guide · Table F3 grouping factors (Cg) · p.186 new On-Site Guide · Iz formula and some factors · p.183 new

BS 7671 · Appendix 4 · list of symbols Ca, Cg, Ci, Cc, Cf (incl. Cf 0.725 and Cc 0.9) · p.452 new · p.424 old Tutor: highlight Cf and Cc

BS 7671 · Table 4B1 ambient temperature factors · p.469 new · p.441 old BS 7671 · Table 4C1 grouping factors · p.471 new · p.443 old

02Worked example: shower with factors

8.5 kW shower · domestic · flat T&E · reference method 102 · 20 m run
BS EN 60898 circuit breaker
NEW: ambient temperature 35 °C · grouped with 2 OTHER circuits (3 in total)

1) Ib = P ÷ V = 8500 ÷ 230 = 36.96 A
2) In = 40 A Type B        (Ib ≤ In ✓)

3) Correction factors
   Ca (70 °C thermoplastic, 35 °C)        = 0.94
   Cg (3 circuits, bunched/enclosed)       = 0.70

4) Iz = In ÷ (Ca × Cg) = 40 ÷ (0.94 × 0.70) = 60.79 A

5–6) T&E, method 102, need It ≥ 60.79 A
   10 mm² → 47 A  ✗
   16 mm² → 63 A  ✓  → 16 mm² twin and earth

7) Voltage drop: 16 mm² → 2.8 mV/A/m (max 5% = 11.5 V)
   VD = (2.8 × 36.96 × 20) ÷ 1000 = 2.07 V ✓

Answer: 16 mm² T&E on a 40 A Type B.
(Without the factors it was 10 mm².)

Remember Ib ≤ In ≤ Iz ≤ It. You don't make the protective device bigger to fix it. You upsize the cable, because the device must still operate before the cable overheats.

BS 7671 · Appendix 4 · Table 4D5 70 °C flat T&E (It and mV/A/m) · p.484 new · p.456 old On-Site Guide · Table F6 (same values, handier) · p.193 new Electrician's Guide to the Building Regs · T&E current ratings table by reference method · p.41 new · p.37 old

Voltage drop limits in BS 7671

BS 7671 · Appendix 4 · Table 4Ab voltage drop: 3% lighting, 5% other uses · p.458 new · p.430 old The VD formula itself is only in the On-Site Guide. Learn it for the 18th Edition exam.

max mV/A/m = (max VD × 1000) ÷ (Ib × L)
           = (11.5 × 1000) ÷ (36.96 × 20) = 15.55 mV/A/m
→ any cable at or below 15.55 mV/A/m passes volt drop on this run.
You won't be asked to do this in the exam. You'll pick the value from the table.

03Ring final circuits and loading

A ring final circuit (on a 32 A device, 2.5 mm² T&E, up to 100 m²) is really two legs of cable from the same breaker. That lets the load spread across both legs, which is why you shouldn't plug a kettle and a toaster into one double socket.

• At the consumer unit, both line conductors go in the same terminal of the one breaker (or RCBO), both neutrals in the same neutral terminal, and both CPCs in the same earth terminal.
• The same happens at every socket. It's still parallel wiring: every socket gets 230 V.
• With 2 lines out and 2 neutrals back there are 4 paths, which is why ring volt drop divides by 4000 instead of 1000.
• A radial has one leg and ends at the last point. Many kitchens now use a 32 A radial in 4 mm² instead of a ring.

On-Site Guide · Appendix H · Table H2.1 socket-outlet circuits · p.218 new · p.210 old BS 7671 · Appendix 15 · Figure 15A ring final circuit arrangements (and the radial figure on the next page) · p.583 new · p.555 old "A really good drawing": ring, junction box, FCU spurs, fixed equipment

Electrician's Guide to the Building Regs · radial circuit figure · p.79 new The new edition shows only the radial, and the old one only the ring

04Spurs: fused and unfused

On-Site Guide · Appendix H · H2.4 Spurs (fused unlimited, unfused ≤ number of sockets) · p.219 new · p.211 old

05Kitchens and accessory heights

• Keep sockets at least 300 mm (edge to edge, horizontally) from a sink.
• The tutor said the cooker switch (or cooker switch with socket) shouldn't be within 100 mm of the edge of the hob, and shouldn't be above it. Check the exact figure in your book.
• Appliance sockets under the worktop (fridge, washing machine, dishwasher) are often unswitched and controlled by a switched FCU above the worktop, so they can be isolated without pulling out the appliance.
• Read the notes under figures. The tutor said exam questions often come from them.
• Approved Document M heights are 450–1200 mm above finished floor level. These are the minimum and maximum, for accessibility.

Electrician's Guide to the Building Regs · kitchen installation figure (sink 300 mm, cooker switch, FCUs) · p.85 new · p.79 old Electrician's Guide to the Building Regs · Section 10 · Part M switch and socket heights · p.186 new

06Cables in hot areas

Cables feeding equipment that gets hot, such as water boilers, immersion heaters, fires and central heating, must withstand at least 90 °C. Ordinary PVC twin and earth is only rated 70 °C, so it isn't suitable for the hot final connection.

• Typical method: run T&E as the fixed wiring to an FCU, then use 90 °C heat-resistant flex from the FCU to the immersion heater or boiler. Look for markings like H07 or XLPE (cross-linked polyethylene) insulation.
• Heat-resistant flex is not fireproof. It's heat resistant and flame retardant.
• FP200 (usually red) is for fire alarms. MICC works too but is very expensive, around £570–£690 per 100 m for 1.5 mm² 2-core.
• Plastic accessories and back boxes are generally made for 70 °C only, so a hot fault can still melt the accessory even if the cable survives.
• A drip loop in a flex lets moisture drip off before it reaches the equipment.

07Old and new cable colours

The UK harmonised its colours with the rest of Europe (not because of Brexit). New colours could be used from 1 April 2004, and from 1 April 2006 only new-colour cable should be installed.

Where old and new colours meet in one installation, a caution label has traditionally been fitted at the consumer unit: "This installation has wiring colours to two versions of BS 7671…". The tutor believed this may no longer be required under the latest amendment. Check Reg 514.14 in your copy before relying on that.

Colour-blind electricians aren't excluded. Apps can identify cable colours. Some rail and aviation roles do require a colour test.

Electrician's Guide to the Building Regs · Chapter 11 · old and new conductor colours table · p.191 new · p.194 old On-Site Guide · Appendix K · identification of conductors (old and new)

08Bathrooms and special locations

A room containing a bath or shower is a special location. That includes a bedroom with a shower enclosure in it. An en-suite is its own room, but it's still a special location. We define zones because the closer you are to water, the higher the shock risk.

• Equipment you'll see: a pull-cord switch (on the ceiling above 2.25 m, so the cord can hang into a zone), ceiling lights, a shaver socket, extractor fans, and an electric shower.
• Where does the shower go? On the wall in zone 1, never in zone 0. Fixed equipment in zone 1 needs at least IPX4, or IPX5 if it will be cleaned with water jets. Allowed fixed equipment in zone 1 includes whirlpool units, electric showers and shower pumps, ventilation, towel rails, luminaires and water heaters.
• Shaver socket: it has a safety isolating transformer inside, with a laminated core to reduce eddy-current losses. Because the output is isolated from earth, touching one conductor won't give an earth-path shock.

Electrician's Guide to the Building Regs · Section 5 · locations containing a bath or shower · p.91 new · p.85 old Electrician's Guide to the Building Regs · walk-in shower / wet room figure · p.89 new Electrician's Guide to the Building Regs · table: minimum IP rating and equipment allowed per zone · p.87 new

On-Site Guide · Section 8 · zone requirements table (IPX4, IPX5 where water jets used for cleaning) · p.118 new · p.104 old BS 7671 · Section 701 locations containing a bath or shower

09IP codes

IP = International Protection (often called "ingress protection"). There are two numbers:

• The 1st number is protection against solid objects: fingers, tools and dust.
• The 2nd number is protection against water.
• X means "not tested / not relevant" for that part.

• IPX4 = splash-proof (the bathroom zone 1 minimum). IP2X = you can't poke a finger in. IP4X = you can't poke a 1 mm wire in. Consumer units use IP2X and IP4X in places, which is covered in the 18th Edition.
• IP66 = dust tight and protected against powerful jets. That's typical for an outdoor socket enclosure, which costs about £10 compared with about £2 for an indoor socket, so you only fit one where it's needed.
• An extra letter can follow, e.g. IPXXB (a finger can't reach live parts) or IPXXD (a 1 mm wire 100 mm long can't reach them).

On-Site Guide · Appendix L · IP codes (only about a page) Code of Practice for In-service Inspection & Testing · Appendix 2 · IP codes table · p.95 The tutor's favourite IP table, but you can't take that book into the Building Regs exam

10Gardens and outbuildings

Electrician's Guide to the Building Regs · buried cables in gardens (depth, marker tape) · p.97 new Electrician's Guide to the Building Regs · outbuilding supply figure

11Lighting basics and the ceiling rose

• Cable: usually 1.0 mm² T&E (carries 16 A clipped direct, Table F6). 1.5 mm² carries 20 A. The tutor said not to over-size for "future proofing".
• Protective device: usually a 6 A breaker. A 3 A is possible for very small loads.
• Switch closed = closed circuit = lamp on. Switch open = open circuit = lamp off.
• A single-pole switch breaks the line only. A double-pole switch breaks line and neutral. Never take a neutral into an ordinary light switch.
• The pendant is Class II, so it uses 2-core flex with no earth.
• Tip: make a bathroom light the last point on a circuit, so fewer cables end up in a fitting exposed to condensation.

12Two-way switching

A two-way switch has 3 terminals: COM (common), L1 and L2. Pressing it doesn't turn it "on" or "off". It just swaps COM between L1 and L2. The two switches are linked by 3-core & earth cable (brown, black, grey plus a bare CPC), called the strappers.

• Sleeve every strapper brown (black and grey), because they're all line conductors. You could buy all-brown 3-core, but then you couldn't tell which is which.
• There's no right or wrong colour for COM, L1 or L2. The rule is that both switches match: whatever colour is in COM on one goes in COM on the other, and the same for L1 and L2. If they don't match, the switches won't "talk" to each other.
• Coiling the CPC neatly in the back box is purely cosmetic. It adds length, and length means resistance.

On-Site Guide · Section 7 · Figure 7.4.4 two-way switching wiring (black in COM in that drawing) · p.107 new · p.95 old

13Intermediate switching

To control a light from 3 or more places, fit intermediate switches between the two two-way switches. You can have as many as you need, e.g. a stairwell over several floors or a big room with several doors. They're mainly commercial, and rare in homes.

Wire it as before, matching colours. The common conductor (whatever colour you used in COM) doesn't connect to the intermediate. It just joins through in a connector in the back box. It isn't an earth.

14ELV and SELV lighting

ELV

Extra-low voltage: not exceeding 50 V AC or 120 V ripple-free DC. "Ripple-free" means a perfectly smooth DC line, not half-wave humps.

LV

Low voltage: above ELV, up to 1000 V AC (1500 V DC). The 230/400 V mains is LV.

SELV

Separated extra-low voltage, supplied through a safety isolating transformer, e.g. 230 V in, 12 V (≤ 50 V) out.

• Where you'll see it: under-cabinet and LED strip lighting, track lighting, media walls and garden lighting.
• ELV isn't automatically safe. You still need the right protective measures and must consider the environment.
• Why the grid steps voltage up: for the same power, higher voltage means less current, so less voltage drop over long distances. That allows thinner cables and fewer transformers.
• Transformer types include step-up and step-down (voltage), current, power and isolating.

15Three-phase supplies and boards

• A generator has 3 windings, making 3 sine waves 120° apart. Line to neutral is 230 V on each phase. Line to line is 400 V, not 460 V, because the phases never peak together.
• Star (the transformer secondary) is unbalanced, because each phase feeds different loads, so it needs a neutral. Delta (usually the primary) is balanced, so it doesn't need a neutral.
• Three-phase is mainly commercial and industrial (big machinery, plant, welders). Very large sites may take an 11 kV supply. One transformer can feed a street, a village or a single factory. A house normally takes one phase, and you usually won't know which.

• Triple-pole (TP) main switch: switches off all three phases at once, so you can't turn off just one. A TP&N (4-pole) switch also breaks the neutral.
• You can mix three-phase and single-phase circuits on one board. Example: 4 three-phase breakers fill 12 ways, or 2 three-phase plus 6 single-phase uses the same space for 8 circuits.
• For a whole supply there are 3 DNO fuses (all the same rating: 63, 80 or 100 A), a three-phase meter with L1, L2, L3 and N in and out, and an isolator. The earth doesn't go through the meter.
• Isolation: for a consumer unit change with no isolator, the DNO must pull the fuse. For work on one circuit, lock off that breaker or the main switch. Safe isolation is covered in detail later.
• Mixed colours: when joining old and new three-phase cables, e.g. red→brown, yellow→black, blue→grey, black→blue, fit a caution label and make sure every conductor is correctly identified.

16How an RCD's test button works

Inside an RCD, the line and neutral pass through a toroid coil (a ring-shaped core). Normally the line and neutral currents are equal, so their magnetic fields cancel. If some current leaks to earth, they don't match, and the coil detects the imbalance and trips the RCD.

The test button connects a small resistor that makes a deliberate imbalance, so the RCD should trip. Pressing it is the first test you'll do when you test an RCD later in the course. Tell an RCD apart from a CB by the test button and the mA rating.

On-Site Guide · Section 11 · RCD construction and operation (toroid, test button)

17All book references from this day

Tab these pages. "New" = orange / green-orange editions, "old" = brown / green-brown.

★ = the tutor specifically said to highlight or tab it, or went through it in detail. Page numbers are as given in the session. Check them against your own copy.

18Self-test

✎ Take the Module 2 test (20 questions)

Unofficial student notes written from the Module 2 Day 2 webinar recordings (17/09/2026), shared free for fellow learners. Not produced or endorsed by the training provider. Where the tutor's statement looked wrong it's flagged (shaver socket voltage, caution label). Always check against your course books, BS 7671 and your tutor. Spotted a mistake? Let us know so it can be fixed for everyone.

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