Question 1

How do I calculate cable size for a circuit?

Accepted Answer

Cable sizing follows BS 7671 Appendix 4. First, determine your design current (Ib). Then apply correction factors for ambient temperature (Ca), grouping (Cg), and thermal insulation (Ci). The minimum current-carrying capacity required is It = Ib ÷ (Ca × Cg × Ci). Select a cable from Table 4D1A or 4D2A where Iz ≥ It. Finally, verify voltage drop compliance.

Question 2

What size cable do I need for a 32A circuit?

Accepted Answer

For a 32A circuit, you typically need 4mm² or 6mm² cable depending on installation method and cable run length. Clipped direct (Method C), 4mm² T&E is rated at 37A. In conduit or trunking, you may need 6mm². Always check voltage drop for runs over 10m and apply correction factors for your specific installation.

Question 3

What's the difference between Ib, In, It, and Iz?

Accepted Answer

Ib is design current (the actual load). In is the protective device rating (MCB/fuse size). It is the minimum cable rating required after applying correction factors. Iz is the actual current-carrying capacity of your chosen cable. The relationship must be: Ib ≤ In ≤ Iz for a compliant installation.

Question 4

Which BS 7671 tables do I use for cable sizing?

Accepted Answer

Table 4D1A covers single-core and multicore thermoplastic (PVC) cables including T&E. Table 4D2A covers multicore armoured cables. Table 4E1A covers single-core thermosetting (XLPE) cables. Table 4E2A covers multicore thermosetting cables. Each table lists current ratings by installation method (Reference Methods A, B, C, etc.).

Question 5

What are the BS 7671 installation methods?

Accepted Answer

Reference Method A: enclosed in conduit in thermally insulating wall. Method B: enclosed in conduit on wall or in trunking. Method C: clipped direct to surface. Method D: in underground ducts. Method E: in free air on cable tray. Method F: touching on cable tray. The method affects current-carrying capacity significantly.

Question 6

How does ambient temperature affect cable sizing?

Accepted Answer

Higher ambient temperatures reduce cable current-carrying capacity. BS 7671 Table 4B1 provides correction factors (Ca). At 30°C (standard), Ca = 1.0. At 40°C, Ca = 0.87 for PVC cables. At 50°C, Ca = 0.71. Divide your required current by Ca to find the derated capacity needed.

Question 7

What is the grouping correction factor (Cg)?

Accepted Answer

When cables are grouped together, they share heat and each cable's capacity is reduced. Table 4C1 provides Cg values. Two circuits touching: Cg = 0.80. Three circuits: Cg = 0.70. Six circuits: Cg = 0.57. Apply Cg when cables run together for more than 500mm.

Question 8

When do I apply the thermal insulation factor (Ci)?

Accepted Answer

Apply Ci when cables pass through or are surrounded by thermal insulation. If surrounded for up to 100mm, Ci = 0.89. For 200mm, Ci = 0.81. For 400mm+, Ci = 0.68. If totally surrounded (e.g., in a loft), Ci = 0.5. This is one of the most commonly missed factors.

Question 9

What size cable for an electric shower?

Accepted Answer

Electric showers require dedicated circuits: 7.5kW needs 6mm² (33A), 8.5kW needs 6mm² (37A), 9.5kW needs 10mm² (41A), 10.5kW needs 10mm² (46A). Always verify voltage drop for longer runs and consider using 10mm² for future-proofing if replacing an older shower.

Question 10

What size cable for an EV charger?

Accepted Answer

A 7kW home charger draws 32A continuously and typically requires 6mm² for runs up to 15m, or 10mm² for longer runs. The continuous load rating means the cable operates at full capacity for hours, so voltage drop and thermal considerations are critical. Many installers specify 10mm² as standard.

Question 11

What size cable for a cooker circuit?

Accepted Answer

Most domestic cookers are 10-13kW but use diversity. A 6mm² cable on a 32A MCB suits most cookers with diversity applied. For large range cookers (15kW+) or where cooker and hob are separate circuits, 10mm² on a 45A MCB may be required. Check manufacturer specifications.

Question 12

What size cable for a submain to an outbuilding?

Accepted Answer

Submain sizing depends on total load and distance. For a garage/workshop with 32A capacity at 20m, 6mm² SWA is typical. For 40m+, consider 10mm² to manage voltage drop. For a garden office with 63A supply at 30m, 16mm² or 25mm² SWA is common. Always calculate voltage drop.

Question 13

How do I check voltage drop for cable sizing?

Accepted Answer

After selecting a cable for current capacity, verify voltage drop using Table 4D1B (mV/A/m values). Formula: VD = (mV/A/m × Ib × L) ÷ 1000. BS 7671 limits: 3% for lighting (6.9V), 5% for power (11.5V). If exceeded, increase cable size to get a lower mV/A/m value.

Question 14

What's the difference between T&E and SWA cable?

Accepted Answer

Twin & Earth (T&E) is for internal use only - in walls, ceilings, under floors. Steel Wire Armoured (SWA) has mechanical protection for external, underground, or exposed runs. SWA can be buried directly or surface-mounted outdoors. Use SWA for outbuilding supplies, external circuits, and anywhere cables need physical protection.

Question 15

Can I use these calculators for three-phase circuits?

Accepted Answer

Yes, our three-phase calculators apply the √3 factor (1.732) for line voltage and adjust current calculations accordingly. For three-phase motors, we also consider power factor and starting current. Three-phase voltage drop uses 0.866 multiplier. Select the three-phase option in the relevant calculator.

Cable Size	Method A (enclosed)	Method B (conduit)	Method C (clipped)	Typical Use
1.0mm²	11A	13A	15A	Lighting (6A MCB)
1.5mm²	14A	17A	20A	Lighting (10A MCB)
2.5mm²	18A	23A	27A	Ring final, radial 20A
4mm²	24A	30A	37A	Immersion, radial 32A
6mm²	31A	38A	47A	Cooker, shower, EV
10mm²	42A	52A	64A	Large shower, small submain
16mm²	56A	69A	85A	Submain, large loads
25mm²	73A	90A	112A	Main supply, large submain

Cable Size Calculator UK

What is Cable Sizing and Why Does It Matter?

Which Cable Sizing Calculator Do I Need?

💡 "I know the load current"

🚿 "Shower, cooker, or appliance"

🔌 "EV charger installation"

🏠 "Submain to outbuilding"

☀️ "Solar PV or heat pump"

📏 "Check voltage drop"

Domestic Circuit Cable Sizing

Electric Shower

Cooker Circuit

Immersion Heater

Ring Main Circuit

Storage Heaters

Underfloor Heating

Outdoor Lighting

Air Conditioning

Outbuildings & Submains

🏠 Garden Office

Garage / Workshop

Shed / Summer House

Annex / Granny Flat

BBQ / Outdoor Kitchen

Electric Gates

EV Charging & Renewables

⚡ EV Charger (Domestic)

EV Charger (Commercial)

☀️ Solar PV & Battery

Air Source Heat Pump

Ground Source Heat Pump

Battery Storage

Special Locations (BS 7671 Part 7)

Hot Tub / Spa

Swimming Pool

Sauna

Caravan / Marina

Commercial & Industrial

Commercial Kitchen

Shop / Retail Unit

Server Room

Farm / Agricultural

CCTV / Security

BS 7671 Table 4D1A - Current-Carrying Capacity

BS 7671 Correction Factors Explained

Ca - Ambient Temperature

Cg - Grouping Factor

Ci - Thermal Insulation

Frequently Asked Questions

Cable Sizing Glossary

⚡ Important Notes

Need a Custom Calculator?

Complete Your Electrical Design