🔌 Circuit Breaker Sizing Calculator
NEC-compliant breaker sizing with continuous load calculations
💡 Expert Tips
🎯 The 125% Continuous Load Rule
NEC 210.20(A): Continuous loads (3+ hours) require breaker sized at 125% of continuous load. Why? Heat accumulation. A 20A load running 10 minutes won't heat breaker. Same load running 5 hours WILL. The 125% factor (or 80% utilization) prevents thermal overload. Exception: 100% rated assemblies (panel + breaker both listed) can carry 100% continuous – but RARE in residential. Cost trap: Oversizing to "be safe." 15A continuous needs 15 × 1.25 = 18.75A → 20A breaker minimum. Using 30A "just in case" removes overcurrent protection from your load.
📏 Standard Breaker Sizes (NEC 240.6)
Standard sizes: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100A. Can't buy an 18A breaker! Rule: Round UP to next standard size. 18.75A calculated? Use 20A. 52A? Use 60A. Exception per 240.4(B): "Next size up" rule allows using next standard size if conductor ampacity doesn't match – BUT can't exceed 800A and has restrictions. Don't confuse this with continuous load sizing. Real failure: 32A continuous load → 32 × 1.25 = 40A required → used 35A breaker because "close enough" = code violation + nuisance trips.
⚡ Small Wire Protection (240.4(D))
NEC 240.4(D) overrides calculations for small wire: #14 copper = 15A max, #12 = 20A max, #10 = 30A max. Even if #14 wire carries 25A @ 90°C rating, you can't use 25A breaker! Example error: 18A continuous load → need 18 × 1.25 = 22.5A → round to 25A breaker. If using #12 wire, LIMITED to 20A breaker per 240.4(D). Must use #10 wire for 25A breaker, or split to two 15A circuits. Seen countless fails where calculations said "25A breaker OK" but wire gauge violated 240.4(D) limits.
❓ FAQ
What size breaker for 20 amp continuous load?
Answer: 20A continuous × 1.25 = 25A minimum breaker per NEC 210.20(A). Use 25A or 30A standard breaker. Wire must also be rated ≥25A (typically #10 copper @ 75°C = 35A). If using #12 wire, it's limited to 20A breaker per NEC 240.4(D) – insufficient for 20A continuous load. Must upsize wire to #10.
How do I know if my load is continuous?
Answer: NEC defines continuous as "maximum current expected for 3+ hours." Common continuous: HVAC, water heaters, commercial lighting, refrigerators, data centers. NOT continuous: vacuum cleaners, toasters, most power tools (intermittent). If unsure, treat as continuous for safety. Residential lighting often considered non-continuous unless commercial/industrial with 8+ hour operation.
What's a 100% rated breaker?
Answer: Breaker + panel assembly tested and LISTED for 100% continuous operation (no 125% derating). Rare in residential, common in industrial (expensive units). Allows 100A continuous on 100A breaker. Standard breaker: 100A rated → max 80A continuous. Don't assume 100% rated unless explicitly marked. Cost: 100% rated panel $800-2000 vs standard $150-400.
