What Size Cable Do You Need for a 30 Amp Circuit?
For a 30 amp circuit, use 4.0mm² copper cable under BS 7671 Table 4D5A, rated at 36A clipped direct. Under NEC, 10 AWG on a 30A circuit handles the load per Table 310.16. For outbuilding sub-mains with SWA cable, 6mm² is recommended to account for longer runs and voltage drop.
Quick Reference: Cable Size for 30 Amp Sub-Main Circuit by Standard
| Standard | Cable Size | Conditions | Reference |
|---|---|---|---|
| BS 7671 | 4.0mm² | Method C (clipped direct), PVC insulation | Table 4D5A |
| NEC | 10 AWG | 30A circuit, THWN-2, 75°C column | Table 310.16 |
| AS/NZS 3008 | 4.0mm² | Enclosed in conduit, max 30°C ambient | Table 13 |
| IEC 60364 | 4.0mm² | Reference method B or C, PVC | 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 30 Amp Sub-Main Circuit
- Small sub-main feed
- Outbuilding supply
- Garage distribution board
- Annex supply
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.
Frequently Asked Questions
Is 4mm² enough for a 30 amp sub-main?▼
Yes, for short runs. 4mm² clipped direct is rated at 36A, above the 30A requirement. For SWA underground runs typical of outbuilding supplies, 6mm² is recommended due to slightly lower SWA ratings and longer cable lengths.
What size SWA cable for a 30 amp garage supply?▼
Use 6mm² 3-core SWA cable for a 30A garage supply. While 4mm² SWA is technically adequate at 32A, 6mm² provides better voltage drop performance for typical 15-30m garden runs and future-proofs for additional loads.
Can I use 2.5mm² for 30 amps?▼
No. 2.5mm² cable clipped direct is rated at 27A — below the 30A requirement even in ideal conditions. You must use 4.0mm² minimum.
What MCB size for a 30 amp circuit?▼
Use a 32A MCB (the nearest standard BS EN 60898 rating above 30A). A 30A MCB is not a standard size in the UK. The 32A MCB with 4mm² cable is a well-matched combination under BS 7671.
How far can I run cable to an outbuilding at 30 amps?▼
On 6mm² SWA at 30A (230V, 5% VD limit), maximum run is approximately 28m. On 10mm² SWA, this extends to about 47m. For very long runs to distant outbuildings, voltage drop is often the governing factor.
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