What Size Cable Do You Need for a 125 Amp Circuit?
For a 125 amp circuit, use 25mm² copper cable with PVC insulation under BS 7671 Table 4D5A, rated at 134A clipped direct. Under NEC, 1 AWG at 75°C provides 130A per Table 310.16. With XLPE insulation, 16mm² provides 127A capacity, offering a more cost-effective solution for shorter runs.
Quick Reference: Cable Size for 125 Amp Industrial Sub-Main by Standard
| Standard | Cable Size | Conditions | Reference |
|---|---|---|---|
| BS 7671 | 25mm² | Method C or E, PVC or XLPE insulation | Table 4D5A |
| NEC | 1 AWG | THWN-2, 75°C column | Table 310.16 |
| AS/NZS 3008 | 25mm² | Clipped direct or cable tray | Table 13 |
| IEC 60364 | 25mm² | Reference method C or E, 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 125 Amp Industrial Sub-Main
- Industrial sub-main distribution
- Large commercial panel
- Data centre feed
- Manufacturing floor 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 25mm² enough for 125 amps?▼
Yes, when clipped direct. 25mm² PVC cable rated at 134A under BS 7671 covers 125A with margin. In conduit (Method B), the rating drops to 114A — use 35mm² for conduit installations at 125A.
Can I use 16mm² XLPE for 125 amps?▼
Yes. 16mm² XLPE (90°C rated) cable clipped direct provides 127A under BS 7671 Table 4E1A — just above 125A. However, any derating for grouping or temperature would reduce this below 125A. Use 25mm² XLPE for safety margin.
What size three-phase cable for 125 amps?▼
For a three-phase 125A supply, use 25mm² 4-core PVC cable or 16mm² 4-core XLPE cable. Three-phase conductor ratings are similar to single-phase for trefoil arrangements.
What is 1 AWG cable in mm²?▼
1 AWG has a cross-sectional area of 42.4mm², closest to 35mm² in metric sizing. Its NEC ampacity at 75°C is 130A per Table 310.16, suitable for 125A circuits.
What size busbar for 125 amps?▼
A 125A distribution board typically uses copper busbars rated to the panel's overall capacity. Standard 125A rated panels are widely available from major manufacturers for three-phase industrial distribution.
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