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Per NEC 210.20(A) continuous load rule and NEC 240.6(A) standard ampere ratings. Wire hint per NEC Table 310.15(B)(16), 75°C. Always verify against current code and local amendments.
How to Use This Breaker Size Calculator
Enter your load current in amps and select whether the load is continuous. A continuous load is one that operates at a steady state for 3 hours or more — such as HVAC systems, commercial lighting, or electric vehicle chargers. The calculator applies the NEC 210.20 125% factor for continuous loads, then rounds up to the next standard breaker size per NEC 240.6(A).
The wire size suggestion is a starting point based on the NEC Table 310.15(B)(16) 75°C column. Always confirm with the actual circuit length, voltage drop, conduit fill, and terminal temperature ratings before installation.
Why Proper Breaker Sizing Matters
An undersized breaker trips too easily under normal operating current, causing nuisance trips. An oversized breaker fails to protect the downstream conductor — allowing current to flow that could overheat insulation and cause a fire before the breaker ever trips.
The continuous load rule exists because prolonged current causes more heat buildup in conductors and equipment than intermittent loads at the same level. That extra thermal stress is what NEC 210.20 compensates for by requiring a 25% safety margin on the breaker rating.
What Affects Breaker Size
The primary factor is the load current. Whether the load is continuous or non-continuous determines whether the 125% factor applies. The conductor material affects the suggested wire size but not the breaker size itself. Equipment manufacturer specifications may also require a specific breaker rating that overrides the calculated minimum — always check nameplate data.
Common Breaker Sizing Scenarios
- Sizing a 20-amp or 30-amp breaker for a general-purpose branch circuit
- Sizing a breaker for a continuous commercial lighting or HVAC circuit (125% rule)
- Selecting the breaker for a 40-amp dryer, 50-amp range, or 60-amp EV charger circuit
- Sizing a main breaker for a residential subpanel (100A, 125A, 150A, 200A)
- Calculating breaker size for a motor circuit with continuous duty
Frequently Asked Questions
A circuit breaker must protect the conductor and meet the load. For non-continuous loads, the breaker rating must be at least equal to the load current. For continuous loads, NEC 210.20(A) requires a 125% multiplier before selecting the next standard size.
NEC 240.6(A) standard sizes include: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, and 600 amps.
Per NEC 210.20(A), loads that operate for 3 or more continuous hours must have their breaker (or fuse) rated at 125% of the continuous load current. This accounts for the additional heat generated by sustained current flow.
For a non-continuous 20-amp load, a 20-amp breaker is the minimum standard size. For a continuous 20-amp load, 125% = 25 amps, so a 25-amp standard breaker is required per NEC 210.20(A).
A 20-amp breaker typically protects #12 AWG copper wire (20A rated at 75°C) per NEC Table 310.15(B)(16). Always confirm the wire size with actual circuit conditions, voltage drop, and conduit fill.
No. A breaker must be sized to protect the conductor. Using an oversized breaker that exceeds the conductor's ampacity is a NEC violation and a fire hazard. If more current is needed, both the breaker and the conductor must be upsized.