What Size Cable Do You Need for a 40 Amp Circuit?

For a 40 amp circuit, use 6mm² copper cable (BS 7671, IEC, AS/NZS) or 8 AWG (NEC). This is common for electric cookers and sub-main feeds. For cable runs exceeding 20m, consider 10mm² to keep voltage drop within limits.

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Quick Reference: Cable Size for 40 Amp Circuit by Standard

Standard	Cable Size	Conditions	Reference
BS 7671	6mm²	Method C (clipped direct), PVC insulation	Table 4D1A
NEC	8 AWG	THWN-2, 40A circuit	Table 310.16
AS/NZS 3008	6mm²	Enclosed in conduit, 30°C ambient	Table 13
IEC 60364	6mm²	Reference method C, PVC 70°C	Table B.52.4

Values shown are for copper conductors in typical domestic/commercial conditions. Actual size depends on installation method, ambient temperature, and cable length. Use our free cable sizing calculator for your exact scenario.

Typical Applications for a 40 Amp Circuit

Electric cooker
Large water heater
Workshop machinery
Sub-main feed

Calculate Your Exact Cable Size

These are general guidelines. Your specific installation needs exact calculation accounting for cable route length, installation method, ambient temperature, and cable grouping.

Cable Sizing Calculator Voltage Drop Calculator

Frequently Asked Questions

Is 6mm² enough for 40 amps?▼

Yes, 6mm² PVC copper cable clipped direct is rated at 47A under BS 7671 Table 4D1A. However, derating for conduit installation (Method B) reduces this to 36A — in which case, use 10mm².

What size cable for an electric cooker?▼

Most electric cookers draw 30-40A at 230V. A 6mm² cable on a 40A circuit is standard for cookers up to 9kW. For double ovens or higher-rated cookers, use 10mm² on a 50A circuit.

Can I use 4mm² for 40 amps?▼

No. 4mm² cable is rated at 36A clipped direct (Method C) — insufficient for a 40A circuit. You must use 6mm² minimum.

What is 8 AWG equivalent in mm²?▼

8 AWG has a cross-sectional area of 8.37mm², which is between 6mm² and 10mm² in metric sizing. Its ampacity at 75°C is 50A per NEC Table 310.16.

How far can I run 6mm² at 40 amps?▼

At 230V with 5% voltage drop limit, 6mm² can run approximately 20m at 40A. For longer runs, use 10mm² which extends the maximum to about 33m.