Breaker Sizing Calculator — NEC 125% Rule (Free Tool)
Portlandia Electric Supply · Free contractor tools — no signup
Breaker & Wire Size Calculator
NEC 240.6(A) standard sizes · 125% continuous-load rule (210.19/215.2) · 310.16 75°C ampacity
Design load (NEC-adjusted)
Breaker size (NEC 240.6(A))
Minimum wire (75°C)
Ampacity margin
Next standard breaker size
🧰 What you'd need
- Breaker in your calculated size → Circuit breakers
- Wire — THHN/THWN-2 in your gauge → Cerro wire & cable
- Disconnect / safety switch for the circuit → Eaton breakers & disconnects
Estimates for guidance only — NEC 240.4(D) small-conductor limits applied; ambient correction, conduit fill, and local amendments not included. Confirm final design with a licensed electrician and your AHJ.
📖 Read the full guide: How to Size Circuit Breakers: NEC 125% Rule — charts, code references, and worked examples.
🧰 More free tools → — sizing, conversion, and payback calculators, all free.
© Portlandia Electric Supply — wholesale solar & electrical supply.