The Electrician's Cheat Sheet Pack
Every table a working electrician actually needs, on paper, in the van. Printed from electricianseopro.co.uk/tools, where the interactive versions live.
To save: File, Print, Save as PDF.
1. Maximum Zs, MCBs and RCBOs (18th Edition, Cmin 0.95)
Values in ohms. Live version with your own readings: the Maximum Zs calculator.
| Rating | Type B | Type C | Type D |
|---|---|---|---|
| 6 A | 7.28 | 3.64 | 1.82 |
| 10 A | 4.37 | 2.19 | 1.09 |
| 16 A | 2.73 | 1.37 | 0.68 |
| 20 A | 2.19 | 1.09 | 0.55 |
| 25 A | 1.75 | 0.87 | 0.44 |
| 32 A | 1.37 | 0.68 | 0.34 |
| 40 A | 1.09 | 0.55 | 0.27 |
| 45 A | 0.97 | 0.49 | 0.24 |
| 50 A | 0.87 | 0.44 | 0.22 |
| 63 A | 0.69 | 0.35 | 0.17 |
On-site 80% rule: compare cold measured Zs against 0.8 × these values.
2. Voltage drop, mV/A/m (copper, 70°C thermoplastic)
Multiply by Ib and length, divide by 1000. Interactive version: the voltage drop calculator.
| Size | Single-phase | Three-phase |
|---|---|---|
| 1.0 mm² | 44 | 38.1 |
| 1.5 mm² | 29 | 25.1 |
| 2.5 mm² | 18 | 15.6 |
| 4 mm² | 11 | 9.5 |
| 6 mm² | 7.3 | 6.3 |
| 10 mm² | 4.4 | 3.8 |
| 16 mm² | 2.8 | 2.4 |
| 25 mm² | 1.75 | 1.5 |
Limits: 3% lighting (6.9 V at 230 V), 5% other circuits (11.5 V).
3. Twin & earth current ratings, Table 4D5 (amps)
By installation reference method. Do the full sizing job with the cable size calculator.
| Size | C (clipped) | 100 | 101 | 102 | 103 | A (conduit) |
|---|---|---|---|---|---|---|
| 1.0 mm² | 16 | 13 | 10.5 | 13 | 8 | 11.5 |
| 1.5 mm² | 20 | 16 | 13 | 16 | 10 | 14.5 |
| 2.5 mm² | 27 | 21 | 17 | 21 | 13.5 | 20 |
| 4 mm² | 37 | 27 | 22 | 27 | 17.5 | 26 |
| 6 mm² | 47 | 34 | 27 | 35 | 23.5 | 32 |
| 10 mm² | 64 | 45 | 36 | 47 | 32 | 44 |
| 16 mm² | 85 | 57 | 46 | 63 | 42.5 | 57 |
4. SWA current ratings, Table 4E4A (90°C thermosetting, clipped direct, amps)
Armoured cable, clipped direct.
| Size | 2-core | 3/4-core |
|---|---|---|
| 1.5 mm² | 27 | 23 |
| 2.5 mm² | 36 | 31 |
| 4 mm² | 49 | 42 |
| 6 mm² | 62 | 53 |
| 10 mm² | 85 | 73 |
| 16 mm² | 110 | 94 |
| 25 mm² | 146 | 124 |
5. Correction factors
Apply to the tabulated rating before you compare it with In.
| Ambient temperature, Ca (70°C PVC) | |
|---|---|
| 25°C | 1.03 |
| 30°C | 1.00 |
| 35°C | 0.94 |
| 40°C | 0.87 |
| 45°C | 0.79 |
| 50°C | 0.71 |
| Grouping, Cg (bunched) | |
|---|---|
| 1 circuit | 1.00 |
| 2 circuits | 0.80 |
| 3 circuits | 0.70 |
| 4 circuits | 0.65 |
| 5 circuits | 0.60 |
| 6 circuits | 0.57 |
6. Wiring colours
Old to new. Full detail on the UK wiring colours page.
| Single-phase | Old | New |
|---|---|---|
| Live | Red | Brown |
| Neutral | Black | Blue |
| Earth | Green/Yellow | Green/Yellow |
| Three-phase | Old | New |
|---|---|---|
| L1 | Red | Brown |
| L2 | Yellow | Black |
| L3 | Blue | Grey |
| N | Black | Blue |
Warning: old blue = phase, new blue = neutral. Old black = neutral, new black = phase. Test everything.
7. IP ratings quick reference
First digit is solids, second is water.
| First digit | Solids |
|---|---|
| 0 | None |
| 1 | Objects >50 mm |
| 2 | Objects >12.5 mm |
| 3 | Objects >2.5 mm |
| 4 | Objects >1 mm |
| 5 | Dust protected |
| 6 | Dust tight |
| Second digit | Water |
|---|---|
| 0 | None |
| 1 | Vertical drips |
| 2 | Drips at 15° |
| 3 | Spray to 60° |
| 4 | Splashes, any direction |
| 5 | Water jets |
| 6 | Powerful jets |
| 7 | Immersion to 1 m |
| 8 | Continuous immersion |
| 9 | High-pressure hot jets |
| Common rating | Typical use |
|---|---|
| IP20 | Indoors |
| IP44 | Sheltered outdoor, bathroom zone 2 |
| IP65 | Outdoor lighting |
| IP66 | Outdoor sockets |
| IP67 | Ground lights |
| IP68 | Submerged |
8. Bathroom zones
All bathroom circuits: 30 mA RCD protection.
| Zone | Extent | Requirement |
|---|---|---|
| Zone 0 | Inside the bath or shower tray | IPX7, SELV 12 V only |
| Zone 1 | To 2.25 m above the bath or tray | IPX4 (IPX5 with water jets) |
| Zone 2 | A further 0.6 m beyond zone 1 | IPX4, shaver units allowed |
| Outside zones | Rest of the room | Sockets at least 3 m from the zone 1 boundary |
9. Test limits
The numbers you check against on the day.
| Test | Limit |
|---|---|
| Insulation resistance, LV circuits | Minimum 1.0 MΩ at 500 V DC |
| Insulation resistance, SELV/PELV | Minimum 0.5 MΩ at 250 V DC |
| RCD, 30 mA general type | Trip within 300 ms at rated current, 40 ms at 5× |
| Ring continuity (2.5/1.5 T&E) | r1 ≈ rn, r2 ≈ 1.67 × r1 |
10. Common loads (230 V)
Quick conversions. Odd load on the job? Use the watts to amps or kW to amps calculators.
| Load | Power | Current |
|---|---|---|
| Kettle | 3 kW | 13 A |
| Immersion heater | 3 kW | 13 A |
| Oven | 3.5 kW | 15.2 A |
| Hob | 7.4 kW | 32.2 A |
| Shower | 8.5 kW | 37 A |
| Shower | 9.5 kW | 41.3 A |
| Shower | 10.5 kW | 45.7 A |
| EV charger | 7.4 kW | 32 A |
These tables are taken from the published BS 7671:2018+A2:2022 figures. Standards get amended, so verify against the current edition before relying on them for a design or a cert. Want the versions that do the maths for you? Grab the live calculators at the Helpful Tools hub.
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