Circuit Breaker Size Calculator — Free Online Calculator
Calculate the correct circuit breaker size for your electrical load. NEC-compliant with continuous and non-continuous load factors.
How to Use This Calculator
Enter the continuous load (running more than 3 hours) and any non-continuous loads on the circuit.
The Formula Explained
NEC 210.20 requires breaker ≥ 125% of continuous load + 100% of non-continuous load. Then round up to the next standard breaker size.
Breaker Sizing: Protection and Safety
A circuit breaker serves two purposes: it protects the wire from overheating due to overload, and it protects equipment from fault currents. Getting the size right requires matching the breaker to the wire ampacity and the load characteristics. Oversize the breaker and you defeat wire protection, risking fires from overheated insulation. Undersize it and you get nuisance trips during normal operation. The NEC provides specific rules for sizing, including the critical 125% continuous load multiplier that catches many DIY projects.
The sizing process: calculate the load in amps, multiply continuous loads by 1.25 (non-continuous stays at 100%), sum the total, round up to the next standard breaker size from NEC 240.6, verify the wire ampacity meets or exceeds the breaker rating, and finally verify the breaker and wire match at the terminal temperature rating (usually 75°C for circuits over 100A and 60°C for smaller circuits). Miss any step and the installation may work but fails inspection or creates latent fire risk.
Worked Example: EV Charger Circuit
A Tesla Wall Connector set to draw 48 amps continuous at 240V. Continuous load (operates 4+ hours while charging): 48 × 1.25 = 60 amps.
Breaker: 60A (standard size). Conductor: must handle 60A at the terminal temperature rating. At 75°C: 6 AWG copper (65A). At 60°C terminal rating (for standard residential): 4 AWG copper (70A). Most residential terminals are 75°C rated, so 6 AWG copper THHN is correct.
Common mistake: using a 50A breaker because "the charger draws 48A which is below 50A". This violates the 125% continuous rule. 50A × 1.25 = 62.5A — but the load is 48A × 1.25 = 60A continuous. You need a 60A breaker to cover the continuous-load adjusted value. The 50A breaker would nuisance-trip during long charging sessions and is a code violation.
Worked Example: Mixed Commercial Load
A small office branch circuit serving: 8 receptacles at 180 VA each (NEC 220.14(I)) = 1,440 VA non-continuous. LED lighting 400 VA continuous. Laser printer 800 VA occasional (non-continuous).
Continuous load: 400 × 1.25 = 500 VA adjusted. Non-continuous loads: 1,440 + 800 = 2,240 VA at 100%. Total: 500 + 2,240 = 2,740 VA.
At 120V: 2,740 / 120 = 22.8 amps. Round up to 25A or 30A breaker. At 25A with 10 AWG copper, the circuit has adequate capacity and 25A is the next standard size above 22.8A. Actually 30A would be the next size up from 25A — wait, 25A is a standard size per NEC 240.6. Use 25A breaker with 10 AWG copper (30A ampacity). Or more commonly in residential/small commercial, use 30A breaker with 10 AWG for margin and to fit common equipment.
Five Breaker Sizing Mistakes
1. Forgetting the 125% continuous multiplier. The most common NEC violation in DIY work. Water heaters, HVAC, EV chargers, and commercial lighting all need the 1.25 factor.
2. Matching breaker to wire at 90°C rating. Most terminals are 75°C rated or lower. The wire ampacity at the terminal temperature governs, not the 90°C value in the wire insulation spec.
3. Using the wrong standard size. You must round UP to the next standard size, never down. A calculated 32A requires a 35A or 40A breaker, not a 30A.
4. Ignoring motor circuit rules. Motor circuits use NEC 430 which has specific multipliers for motor starting (instantaneous-trip breakers at 1100% of FLA, thermal-magnetic at 250% of FLA). Normal 125% does not apply to motor short-circuit protection.
5. Not derating for multi-conductor cable or high ambient. NEC Table 310.15(B) derates ampacity for more than 3 current-carrying conductors in a raceway, and NEC Table 310.15(B)(2)(a) derates for high ambient temperature. Attic conduits in Southern climates can easily hit 50°C ambient, derating 12 AWG copper from 20A to about 14A.
Common Breaker-Wire Pairings
Residential copper (75°C terminal): 15A — 14 AWG. 20A — 12 AWG. 25-30A — 10 AWG. 40-50A — 8 AWG. 60A — 6 AWG. 80-90A — 4 AWG. 100-110A — 3 AWG or 2 AWG. 125A — 2 AWG. 150A — 1/0 AWG. 200A — 2/0 AWG or 3/0 AWG (for service).
Aluminum sizing rule of thumb: Aluminum wire is about 2 gauge sizes larger than copper for the same ampacity. 200A service with copper = 2/0 AWG; with aluminum = 4/0 AWG. Aluminum is commonly used for service entrance and feeders due to cost; copper dominates branch circuits.
Breaker types: Standard thermal-magnetic breakers (residential). GFCI breakers (bathrooms, outdoors, kitchens near water). AFCI breakers (bedrooms, living rooms — arc fault protection). Dual-function AFCI/GFCI (newer installations). All breakers must be the correct type for the listed application per NEC 210.8 (GFCI) and 210.12 (AFCI).
NEC Code References
NEC 210.19(A)(1) — branch circuit conductor sizing (125% continuous). NEC 210.20(A) — branch circuit overcurrent device sizing (125% continuous). NEC 215.2(A)(1) — feeder conductor and overcurrent device (125% continuous). NEC 240.6 — standard breaker and fuse sizes. NEC 240.4(D) — small conductor rule (14 AWG max 15A, 12 AWG max 20A, 10 AWG max 30A in most applications).
NEC 240.4(B) — next-higher-size rule: for breakers above 800A, you must use the next SMALLER standard size, not larger. Below 800A you round UP. NEC 210.8 — GFCI requirements. NEC 210.12 — AFCI requirements. NEC 430.52 — motor short-circuit protection sizing (different from branch circuit rules).
Breaker size: NEC branch-circuit protection rules
Breaker sizing comes down to two rules. First, the breaker must protect the conductor (NEC 240.4): a 14 AWG conductor needs no more than a 15 A breaker, 12 AWG no more than 20 A. Second, for continuous loads (3 hours or more), the breaker must be sized at 125 percent of load (NEC 210.20). The calculator above applies both rules and gives you the next standard size.
The formula and what it does
Take the load in amps. If it runs 3+ hours continuously (EV charging, electric heat, commercial lighting), multiply by 1.25. Round up to the next standard breaker size (15, 20, 30, 40, 50, 60, 70, 80, 90, 100 A and so on). Verify the wire can handle this rating per NEC 310.16.
Worked example
Scenario: 32 A continuous Level 2 EV charger.
32 A x 1.25 = 40 A (continuous-load rule). Next standard breaker size: 40 A. Conductor: 8 AWG copper at 75 C handles 50 A, plenty of margin. NEC 625.42 specifically calls out the 125 percent rule for EV equipment. Final: 40 A two-pole breaker, 8 AWG THHN.
Counter-example: 18 A intermittent motor on a saw. Not continuous (use varies). Breaker can be sized at running current: 20 A breaker on 12 AWG copper. The motor LRA is handled by the inverse-time trip curve of a standard breaker.
Code references and standards
NEC 240.4 conductors must be protected at their ampacity per Table 310.16, with the small-conductor rule capping 14 AWG at 15 A, 12 AWG at 20 A, 10 AWG at 30 A.
NEC 210.20(A) branch-circuit OCPD for continuous loads at 125 percent.
NEC 215.3 same 125 percent rule applies to feeders.
NEC 240.6(A) standard fuse and breaker sizes (15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, ...).
Common mistakes to avoid
Skipping the 125 percent rule. 32 A EV charger on a 35 A breaker is non-compliant. Continuous load requires the next size up, 40 A.
Sizing the breaker to the wire instead of the load. A 20 A breaker on 14 AWG protects the wire correctly only on 15 A. Over-sizing the breaker on small wire is an NEC violation and a fire risk.
Round down to standard size. NEC 240.4(B) lets you round up to the next standard size when the calculated value falls between standard sizes, but only at 800 A and below and only when the resulting current does not exceed certain thresholds. For most residential, round up.
Frequently asked questions
What is a continuous load?
NEC defines a continuous load as one that operates for 3 hours or more. EV charging, electric heat, electric water heaters that hold setpoint, commercial lighting, store HVAC. Cyclical loads (fridge compressor, A/C compressor) are not continuous.
Can I put a 20 A breaker on 14 AWG?
No. NEC 240.4(D) caps 14 AWG at 15 A breaker, regardless of conductor ampacity. This is the "small-conductor" rule.
When can I round breaker up to next standard?
NEC 240.4(B). If the conductor ampacity does not correspond to a standard breaker size, you can round up to the next standard size, provided the load does not exceed conductor ampacity and the breaker is 800 A or less.
Why does GFCI protection sometimes require a specific breaker?
GFCI-protected branch circuits in kitchens, bathrooms, outdoor receptacles (NEC 210.8). Either GFCI breaker at panel or GFCI receptacle in the first device on the circuit. The breaker sizing rule does not change; just the protection type.
Are AFCI breakers required?
Most residential branch circuits in dwelling units per NEC 210.12: bedrooms, living rooms, kitchens, dining rooms, hallways, family rooms, dens, laundry. Combination AFCI/GFCI (dual function) breakers are common where both are required.
How does this work for motors?
Different rules. NEC 430.52 sizes motor branch-circuit OCPD at 175-300 percent of motor FLA depending on protection type (time-delay fuse, inverse-time breaker, instantaneous). Motor overload protection is separate (NEC 430.32).
What about three-phase loads?
Same rules. Three-pole breaker sized for the highest leg current. Continuous-load 125 percent rule applies the same way.