25
A
32,260
A
30
A
1,613
x
-32,230
A
4,800
VA
4.32
kW
25
A
32,260
A
30
A
1,613
x
-32,230
A
4,800
VA
4.32
kW
A circuit breaker is the critical safety gateway between your power source and your electrical load. Selecting the wrong size — too small and nuisance tripping interrupts operations; too large and the breaker fails to protect conductors and equipment during a fault. The Circuit Breaker Sizing Calculator applies the fundamental principles of the National Electrical Code (NEC) and IEC 60364 to determine the minimum rating, recommended standard size, and maximum permissible rating for your specific application.
The basic NEC sizing rule for non-motor branch circuits (NEC 210.20) is straightforward: the overcurrent protective device (OCPD) must be rated at no less than 125% of the continuous load current. A load is considered continuous if it is expected to remain on for three hours or more — lighting circuits, HVAC equipment, server room loads, and commercial refrigeration all qualify. For a 20 A continuous load, the minimum breaker rating is 25 A (20 × 1.25).
Motor loads follow a different set of rules under NEC Article 430. During start-up, motors draw inrush currents that can be six to ten times the full-load running current. To prevent nuisance tripping on this normal — but brief — inrush, NEC 430.52 permits oversizing the breaker up to 250% of the motor full-load current (FLC) for standard inverse-time circuit breakers. This calculator uses the 250% factor as the design value, which covers most squirrel-cage induction motors.
The upper boundary on breaker size is equally important. The breaker must never exceed the ampacity of the conductor it protects. NEC 240.4 requires that the OCPD rating not exceed the conductor's ampacity unless a specific exception applies. For example, a #12 AWG copper conductor rated at 20 A may not be protected by a 30 A breaker, even if the load calculation suggests otherwise.
Three-phase systems introduce the √3 multiplier into apparent power calculations. A three-phase, 480 V load drawing 100 A per phase represents 480 × 100 × 1.732 = 83,136 VA of apparent power — nearly 83 kVA. Each pole of the three-phase breaker still sees the line current (100 A), so the per-pole sizing calculation is identical to single-phase, but the aggregate power is substantially higher.
Beyond raw sizing, engineers must also consider the breaker's interrupt rating (kAIC — kiloamperes of interrupting capacity), which must exceed the available fault current at the point of installation. A standard residential breaker rated at 10 kAIC may be inadequate in a commercial panel where available fault current reaches 22 kAIC or more. This calculator addresses ampere rating; separate short-circuit analysis determines the required interrupting rating.
Standard breaker sizes follow a recognized sequence codified in NEC 240.6(A): 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 A. If your calculated minimum does not land on a standard size, you must round up to the next standard size to maintain protection.
Environmental derating — altitude above 2000 m, ambient temperatures above 40 °C, or bundled conductors — can reduce both conductor ampacity and breaker continuous-current ratings. Always apply manufacturer derating tables and NEC correction factors when conditions deviate from standard.
The calculator follows a three-step process aligned with NEC Article 210 and 430:
If the recommended breaker equals the minimum breaker, your load current lands precisely on a standard size boundary — ideal. If the recommended breaker is notably larger than the minimum, verify that it does not exceed conductor ampacity. A large gap between minimum and recommended suggests considering a higher-ampacity conductor to allow use of a smaller standard breaker. For motor loads, always verify that the selected breaker will coordinate with motor thermal protection (overload relay) to provide complete motor protection per NEC 430.32.
Inputs
Results
A 16 A continuous lighting load requires 16 × 1.25 = 20 A minimum. The #12 AWG conductor (20 A ampacity) limits the breaker to 20 A maximum — they coincide, so a 20 A breaker is the only valid choice.
Inputs
Results
NEC Table 430.248 lists the FLC of a 15 HP, 460 V motor at 21 A. At 250%, the breaker minimum is 52.5 A; the next standard size is 60 A, which falls within the 250% limit. The motor draws about 14.8 kW of real power.
The 125% factor accounts for the continuous-load rule: a conductor and breaker carrying 100% of their rated current indefinitely will overheat. NEC limits continuous loading to 80% of the OCPD and conductor ratings (equivalently, you need a device rated at 125% of the load). This margin also accommodates harmonic currents in modern electronic loads.
No. Installing a breaker smaller than the minimum calculated rating will cause nuisance tripping under normal operating conditions. For continuous loads, the NEC minimum of 125% is a safety floor — not a conservative suggestion. Persistent nuisance tripping often leads to the dangerous practice of bridging the breaker, which eliminates all protection.
The sizing rules are nearly identical. Both must exceed 125% of continuous load current and must not exceed conductor ampacity. The key difference is response speed: current-limiting fuses can interrupt fault current faster than standard breakers. For motor protection, fuse sizing follows NEC 430.52 Table values (which differ slightly from the 250% breaker rule) — for instance, dual-element time-delay fuses may be sized up to 175% of motor FLC.
kAIC (kiloamperes interrupting capacity) is the maximum fault current a breaker can safely interrupt without catastrophic failure. If available fault current at the panel exceeds the breaker's kAIC rating, the breaker may explode, arc flash, or fail to clear the fault. Residential circuits typically see 10 kAIC; commercial panels may require 22–65 kAIC. Always calculate available fault current and match breaker kAIC accordingly.
No. The calculator assumes standard conditions: 40 °C ambient, installation in free air, no bundling. For higher ambient temperatures or conductors bundled in conduit, apply NEC Table 310.15(B)(1) and (B)(3)(a) correction factors to the conductor ampacity before entering it into this calculator.
Generally no. Each branch circuit must have its own overcurrent protection at the point where the conductor receives its supply (NEC 240.21). Multi-wire branch circuits (shared neutral) are an exception, but each ungrounded conductor must still be individually protected, and certain loads require AFCI or GFCI protection that must be provided per circuit.
NEC 408.36 requires that the total load on a panelboard not exceed 80% of its busbar rating for continuous loads. This is the panel-level equivalent of the branch-circuit 125% rule. A 200 A panel should carry no more than 160 A of continuous load to prevent overheating of the bus and lugs.
Roboculator Team
The Roboculator Team explains calculations, planning tools, and practical formulas in clear language for real-life situations.
How helpful was this calculator?
Be the first to rate!
