Question 1

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

Accepted Answer

For a 32A circuit, you typically need 4mm² copper cable for runs up to 23 meters (clipped direct installation). However, the exact size depends on the installation method, cable length, and voltage drop requirements. Use the calculator above to get precise recommendations for your specific installation.

Question 2

How do I calculate voltage drop in a cable?

Accepted Answer

Voltage drop = (mV/A/m × Current × Length) ÷ 1000. The mV/A/m value is found in BS 7671 tables and varies by cable size and type. For example, 2.5mm² copper has 18 mV/A/m. Our calculator automatically performs this calculation and checks BS 7671 compliance (≤3% for lighting, ≤5% for other circuits).

Question 3

What is the difference between copper and aluminium cables?

Accepted Answer

Copper has lower resistance (better conductivity), is more flexible, and easier to terminate. Aluminium is lighter and cheaper but requires larger cross-sectional areas for the same current. Aluminium is typically used for large installations (>50mm²) where weight and cost savings matter. For domestic installations, copper is standard.

Question 4

What installation method should I choose?

Accepted Answer

Common installation methods:Clipped Direct (Method C): Cable clipped directly to wall/ceiling - best heat dissipationIn Conduit (Method B): Cable in plastic/metal conduit - reduced current capacityIn Insulation (Method 103): Cable touching thermal insulation - significant derating requiredUnderground (Method D): Direct burial or in ducts - good heat dissipation

Question 5

Why is my recommended cable size larger than expected?

Accepted Answer

Several factors increase cable size requirements: long cable runs (voltage drop), installation in conduit/trunking (reduced cooling), cables touching insulation (derating), high ambient temperatures, or multiple circuits grouped together. BS 7671 requires conservative sizing to ensure safety and compliance.

Question 6

Can I use this calculator for 3-phase installations?

Accepted Answer

Yes! Select "400V Three-Phase" in the voltage dropdown. For balanced 3-phase loads, the calculator uses √3 (1.732) in voltage drop calculations. For single-phase loads on a 3-phase supply (230V phase-to-neutral), select "230V Single-Phase".

Question 7

What does the recommended MCB rating mean?

Accepted Answer

The MCB (Miniature Circuit Breaker) rating is the maximum protective device size suitable for the cable. The MCB must protect the cable from overload, so its rating cannot exceed the cable's current-carrying capacity. For example, 2.5mm² copper (clipped direct) carries 27A, so the maximum MCB is 25A or 32A depending on installation factors.

Question 8

Do I need to consider diversity when sizing cables?

Accepted Answer

This calculator sizes cables for the full load current. Diversity (BS 7671 Appendix 1) is applied at the distribution board level to size the main incomer. Individual final circuits should be sized for their maximum demand without diversity, unless you're sizing a sub-main supplying multiple loads.

Cable Size Calculator - BS 7671 Compliant

Calculate Cable Size

How to Use the Cable Size Calculator

Understanding the Results

BS 7671 Compliance Information

Regulation 525 - Voltage Drop

Current-Carrying Capacity Considerations

Frequently Asked Questions