Wire size calculator
System, load, voltage, length and maximum drop: you get the minimum copper size by comparing voltage drop with ampacity, plus AWG equivalents.
Minimum wire size
4 mm²
- Minimum copper cross-section4 mm²
- Current25 A
- Size by voltage drop4 mm²
- Size by ampacity4 mm²
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How it's calculated (visible formula)
A_drop = k × L × I × PF ÷ (γ × VD_max); size = max(drop size; ampacity size)
- Net geometric measurement, no waste.
- Adjustable waste % for cuts and breakage.
- Purchase rounded up to sellable units (boxes, bags, buckets, boards).
How to calculate it, step by step
- Work out circuit current from the load.
- Measure the run length.
- Set the maximum voltage drop (3% typical).
- Size by voltage drop and by ampacity.
- Take the larger and check the breaker protects it.
Metric sizes and AWG equivalents
| mm² | Ampacity (1-ph / 3-ph, PVC in conduit) | ≈ AWG |
|---|---|---|
| 1.5 | 15 / 13.5 A | 15–14 |
| 2.5 | 21 / 18.5 A | 13–12 |
| 4 | 27 / 24 A | 11–10 |
| 6 | 36 / 32 A | 9–8 |
| 10 | 50 / 44 A | 7–6 |
| 16 | 66 / 59 A | 5–4 |
| 25 | 84 / 77 A | 3–2 |
| 35 | 104 / 96 A | 1 |
Worked example: 5,750 W at 230 V, 66 ft, 3% max drop
- I = 25 A
- By drop: 3.0 mm² → 4 mm²
- By ampacity: 25 A → 4 mm²
→ 4 mm² copper (≈ 10–11 AWG; use 10 AWG).
Common mistakes
- Sizing by ampacity alone on long runs.
- Ignoring derating for bundled circuits.
- Forgetting the breaker protects the wire, not the appliance.
Pro tips
- Apply derating for more than three current-carrying conductors in a raceway.
- Rooftop conduits in sun reduce ampacity.
- Use standard sizes only.
- Size the equipment ground per NEC 250.122.
From calculation to quote
Add labor, overhead and your margin, and generate the professional PDF.
FAQs
- What size wire for 20 amps?
- 12 AWG copper for a typical 20 A branch circuit.
- What size wire for 30 amps?
- 10 AWG copper, upsized for long runs.
- Why metric sizes?
- This tool uses IEC sizes and tables; the table shows the nearest AWG. Confirm with NEC 310.16.
- What wire for a 50 A electric range?
- 6 AWG copper on a 50 A breaker.
- What wire for lighting?
- 14 AWG on 15 A or 12 AWG on 20 A.
- Can 12 AWG go on a 30 A breaker?
- No — 12 AWG copper is generally limited to 20 A protection.
Sources and standards
Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.