What Size Cable Do You Need for a 25 Amp Circuit?
For a 25 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 is standard per Table 310.16. The 4mm² cable provides ample margin for derating factors. For cable runs exceeding 25m, check voltage drop calculations.
Quick Reference: Cable Size for 25 Amp 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 25 Amp Circuit
- Water heater circuit
- Small electric cooker
- Single-phase motor
- Workshop power 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.
Frequently Asked Questions
Is 2.5mm² enough for a 25 amp circuit?▼
No. 2.5mm² cable clipped direct is rated at only 27A — too close to 25A after applying any derating factors for grouping, temperature, or insulation. Use 4.0mm² which provides a comfortable 36A capacity.
What MCB size for a 25 amp load?▼
In the UK, standard MCB ratings jump from 20A to 25A to 32A. A 25A Type B MCB is available but less common. Many electricians use a 32A MCB with 4mm² cable, which also future-proofs the circuit.
What appliances draw 25 amps?▼
Common 25A loads include storage water heaters (4.5-5.5kW), small electric cookers, single-phase industrial motors, and workshop equipment like large bench saws or air compressors.
Can I use 4mm² in conduit for 25 amps?▼
Yes. 4mm² in conduit (Method B) is rated at 30A under BS 7671, still above 25A. However, with grouping derating for multiple circuits in the same conduit, verify the effective capacity remains above your design current.
How far can I run 4mm² cable at 25 amps?▼
At 230V with a 5% voltage drop limit, 4mm² copper cable can run approximately 27m at 25A. For a 3% limit, maximum is about 16m. For longer runs, use 6mm² which extends the maximum to about 43m.
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