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Breaker Size - NEC-Compliant Circuit Protection

Circuit breakers protect wiring and equipment from overload and short circuits. NEC 2023 requires continuous loads to use 125% of ampacity; breaker must not exceed wire rating. Proper sizing ensures safety and code compliance.

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15A breaker typically pairs with 14 AWG; 20A with 12 AWG Continuous load = 3+ hours; multiply ampacity by 1.25 Motor circuits: breaker sized for locked-rotor, not FLC Ambient >30°C and multiple conductors require derating

Key quantities
I = P/(V×PF)
Load Current
Key relation
×1.25 NEC
Continuous
Key relation
≥ Load
Breaker
Key relation
Ampacity ≥ Breaker
Wire
Key relation

Ready to run the numbers?

Why: Oversized breakers allow wire overheating and fire risk; undersized breakers nuisance-trip. NEC 2023 and local codes govern residential, commercial, and industrial installations.

How: Single-phase: I = P/(V×PF). Three-phase: I = P/(√3×V×PF). Continuous loads multiply by 1.25. Motor loads use 250% FLC for inverse-time breakers. Wire ampacity derates with ambient and conductor count.

15A breaker typically pairs with 14 AWG; 20A with 12 AWGContinuous load = 3+ hours; multiply ampacity by 1.25
Sources:NEC 2023 - NFPA 70IEEE Std 141 - Power Distribution

Run the calculator when you are ready.

Calculate Breaker SizeEnter power, voltage, power factor, and load type for NEC-compliant breaker sizing.

Load Parameters

Wire Parameters

For educational and informational purposes only. Verify with a qualified professional.

🔬 Physics Facts

⚡

NEC 210.20(A): Overcurrent device rating must not be less than noncontinuous load plus 125% of continuous load

— NEC 2023

🔌

Standard breaker sizes: 15, 20, 30, 40, 50, 60, 70, 80, 90, 100, 125, 150, 175, 200 A and up

— UL 489

📏

60°C column for NM cable; 75°C for THHN in most applications; 90°C for derating only

— NEC Table 310.15(B)(16)

🛡️

Breaker protects wire; wire must carry breaker rating. Never upsize breaker without up-sizing wire

— ESFI

📋 Key Takeaways

  • • Continuous loads (operating 3+ hours) require 125% breaker sizing per NEC 210.20(A)
  • • Motor circuits require breakers sized at 250% of full load current per NEC 430.52
  • • Wire ampacity must exceed breaker rating to prevent wire overheating
  • • Ambient temperature and conductor bundling require derating factors per NEC Tables 310.15(B)(1) and (B)(3)(a)

💡 Did You Know?

⚡The National Electrical Code (NEC) is updated every 3 years. The 2023 edition includes new requirements for AFCI and GFCI protectionSource: NFPA 70
🔌Standard breaker sizes follow a geometric progression: 15, 20, 30, 40, 50, 60, 70, 80, 100, 125, 150, 200ASource: NEMA AB 1
🌡️Wire ampacity decreases by 41% at 55°C ambient temperature compared to 30°C, requiring larger wire gaugesSource: NEC Table 310.15(B)(1)
🔧Motor starting currents can be 6-10x full load current, requiring special breaker sizing to prevent nuisance trippingSource: IEEE Std 141
🏠Residential circuits typically use 15A or 20A breakers with 14 AWG or 12 AWG wire respectivelySource: NEC 210.20
⚙️Industrial motor circuits often require 250% breaker sizing to handle starting currents while providing overload protectionSource: NEC 430.52
📏Wire bundling reduces ampacity: 4-6 conductors = 80%, 7-9 = 70%, 10-20 = 50% of base ampacitySource: NEC Table 310.15(B)(3)(a)

📖 How Breaker Sizing Works

Proper breaker sizing ensures electrical safety by protecting circuits from overloads and short circuits. The breaker must be large enough to handle normal operating currents but small enough to trip quickly during faults.

Step 1: Calculate Load Current

For single-phase: I = P / (V × PF)

For three-phase: I = P / (√3 × V × PF)

Convert power to watts if needed (kW × 1000, hp × 745.7)

Step 2: Apply Load Multipliers

  • • Continuous Load: Multiply by 1.25 (125%) per NEC 210.20(A)
  • • Motor Circuit: Use 250% of full load current per NEC 430.52
  • • Non-Continuous: Use calculated current directly

Step 3: Select Standard Breaker Size

Round up to next standard size: 15, 20, 30, 40, 50, 60, 70, 80, 100, 125, 150, 200A, etc.

Breaker must be ≥ calculated current but wire ampacity must exceed breaker rating.

Step 4: Verify Wire Coordination

Wire ampacity (after derating) must exceed breaker rating. Apply temperature and bundling derating factors per NEC Tables 310.15(B)(1) and (B)(3)(a).

🎯 Expert Tips

💡 Always Size Wire First

Select wire gauge based on load current, then choose breaker to protect that wire. Never size breaker larger than wire ampacity.

💡 Consider Future Expansion

Add 20-25% safety margin for future loads. This prevents needing to upgrade circuits later.

💡 Motor Circuits Need Special Care

Motor starting currents are 6-10x FLC. Use 250% breaker sizing per NEC 430.52 to prevent nuisance tripping while providing protection.

💡 Don't Forget Derating

High ambient temperatures and conductor bundling significantly reduce wire ampacity. Always apply derating factors before selecting breaker size.

⚖️ Breaker Sizing Methods Comparison

MethodAccuracyNEC ComplianceWire CoordinationDerating Factors
This Calculator✅ High✅ Full✅ Yes✅ Yes
Rule of Thumb⚠️ Low❌ Partial❌ No❌ No
Manual Calculation✅ High✅ Full⚠️ Manual⚠️ Manual

❓ Frequently Asked Questions

What is the 125% rule for continuous loads?

NEC 210.20(A) requires breakers for continuous loads (operating 3+ hours) to be sized at 125% of load current. This prevents nuisance tripping from sustained operation.

How do I size a breaker for a motor circuit?

Motor circuits require breakers sized at 250% of full load current (FLC) per NEC 430.52. This allows motors to start (6-10x FLC) while providing overload protection.

What is wire coordination and why is it important?

Wire coordination ensures wire ampacity exceeds breaker rating. If breaker is larger than wire capacity, wire can overheat and cause fire before breaker trips.

How do temperature and bundling affect wire ampacity?

High ambient temperatures and conductor bundling reduce wire ampacity. NEC Tables 310.15(B)(1) and (B)(3)(a) provide derating factors that must be applied.

Can I use a larger breaker than calculated?

Yes, but wire ampacity must still exceed breaker rating. Larger breakers provide more headroom but reduce protection sensitivity.

What is the difference between continuous and non-continuous loads?

Continuous loads operate for 3+ hours continuously. Non-continuous loads operate intermittently. Only continuous loads require 125% breaker sizing.

How do I select wire gauge for a given breaker size?

Select wire gauge where derated ampacity exceeds breaker rating. Consider ambient temperature, conductor bundling, and wire temperature rating.

What happens if wire ampacity is less than breaker size?

This violates NEC requirements and creates a fire hazard. Wire can overheat and cause fire before breaker trips. Always ensure wire ampacity ≥ breaker rating.

📊 Electrical Safety by the Numbers

125%
Continuous Load Multiplier
250%
Motor Circuit Multiplier
41%
Ampacity Reduction at 55°C
50%
Ampacity Reduction (10-20 wires)

⚠️ Disclaimer: This calculator provides estimates based on NEC 2023 requirements. Actual breaker sizing must comply with local electrical codes and may require professional engineering review. Always verify wire ampacity, derating factors, and breaker ratings with manufacturer specifications. Not a substitute for professional electrical design.

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