What Size Cable Do You Need for a 20 Amp Circuit?
For a 20 amp circuit, use 2.5mm² copper cable (BS 7671, IEC, AS/NZS) or 12 AWG (NEC). This provides adequate current carrying capacity with margin for derating. For runs exceeding 25 metres, check voltage drop — you may need to upsize to 4mm².
Quick Reference: Cable Size for 20 Amp Circuit by Standard
| Standard | Cable Size | Conditions | Reference |
|---|---|---|---|
| BS 7671 | 2.5mm² | Method C (clipped direct), PVC insulation | Table 4D1A |
| NEC | 12 AWG | THWN-2, 20A circuit | Table 310.16 |
| AS/NZS 3008 | 2.5mm² | Enclosed in conduit, 30°C ambient | Table 13 |
| IEC 60364 | 2.5mm² | Reference method B or 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 20 Amp Circuit
- Dedicated appliance circuit
- Water heater
- Small workshop
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 cable enough for 20 amps?▼
Yes. Under BS 7671 Table 4D1A, 2.5mm² PVC copper cable clipped direct is rated at 27A — well above 20A. Derating applies for grouping, high ambient temperature, or thermal insulation contact.
What is the NEC equivalent of 2.5mm²?▼
2.5mm² is closest to 12 AWG in the NEC system. 12 AWG has a cross-sectional area of 3.31mm² and is rated for 20A at 60°C or 25A at 75°C per Table 310.16.
Can I run 20 amps on 1.5mm² cable?▼
Not recommended. 1.5mm² is rated at only 20A clipped direct in ideal conditions. Any derating (grouping, temperature, insulation) will reduce this below 20A, creating a potential overload hazard.
What breaker size for a 20 amp load?▼
Use a 20A MCB or circuit breaker. The protective device must be rated at or above the design current (In >= Ib). A 20A breaker matches a 20A design current exactly.
How far can I run 2.5mm² cable at 20 amps?▼
At 230V single-phase with 5% max voltage drop, 2.5mm² cable can run approximately 27m at 20A. For 3% (lighting), the maximum is about 16m. Use a voltage drop calculator for exact results.
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