What Size Cable Do You Need for a 100 Amp Supply?
For a 100 amp supply, use 25mm² copper cable with PVC insulation (BS 7671, IEC, AS/NZS) or 3 AWG (NEC). With XLPE insulation, 16mm² is sufficient as its 90°C rating provides 100A capacity. Always check voltage drop on longer runs.
Quick Reference: Cable Size for 100 Amp Circuit by Standard
| Standard | Cable Size | Conditions | Reference |
|---|---|---|---|
| BS 7671 | 25mm² | Method C or E, PVC; or 16mm² XLPE | Table 4D1A/4E1A |
| NEC | 3 AWG or 2 AWG | THWN-2, 75°C column | Table 310.16 |
| AS/NZS 3008 | 25mm² | Clipped direct, PVC | Table 13 |
| IEC 60364 | 25mm² | Reference method 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 100 Amp Circuit
- Main incoming supply
- Large sub-main
- Industrial distribution
- Commercial building 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 25mm² enough for 100 amps?▼
Yes. 25mm² PVC copper cable clipped direct (Method C) is rated at 101A under BS 7671. However, in conduit (Method B) it drops to 89A — use 35mm² for conduit installations at 100A.
Can I use 16mm² XLPE for 100 amps?▼
Yes, 16mm² XLPE (90°C) cable clipped direct is rated at 100A under BS 7671 Table 4E1A. XLPE's higher temperature rating allows smaller conductor sizes compared to PVC.
What size cable for a 100 amp main fuse?▼
For a 100A main supply from the meter to the consumer unit, 25mm² meter tails (PVC) are standard in the UK. The supply cable from the DNO is typically 25mm² concentric.
What is 3 AWG cable in mm²?▼
3 AWG has a cross-sectional area of 26.67mm², which is closest to 25mm² in metric sizing. Its NEC ampacity at 75°C is 100A, matching the 100A circuit requirement.
What size earth cable for 100 amps?▼
Per BS 7671 Table 54.7, the minimum CPC for a 25mm² phase conductor is 16mm². For an XLPE 16mm² phase conductor, the minimum CPC is also 16mm².
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