This powerful calculator assists in sizing circuit breakers and determining their thermal and magnetic trip settings for a wide range of industrial applications, adhering to international electrical standards like IEC and NEC. It provides detailed calculations and recommendations for optimal system protection and coordination.
| Motor Circuit Breaker Results |
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| PCC/MCC Incomer Breaker Results |
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| Transformer Breaker Results |
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Properly sizing a circuit breaker is critical for both safety and reliability. A breaker that is too small will cause nuisance tripping, while one that is too large will fail to protect equipment and cables from dangerous overloads and short circuits. This calculator uses fundamental principles outlined in standards like the NEC (National Electrical Code) and IEC (International Electrotechnical Commission) to provide a comprehensive recommendation. Here are the key concepts this tool evaluates:
What it is: This is the rated current a motor or transformer draws while operating at its maximum rated power (HP or kW/kVA). It is the baseline value for all protection calculations. The calculator finds this using the power formula:
What it is: This function protects the circuit from currents that are slightly above the FLA but persist for a long time, which can cause cables and windings to overheat and melt. This is handled by the thermal trip (or long-time delay, `L`) setting on a breaker.
What it is: This function provides near-instantaneous protection against extremely high currents caused by a short circuit (e.g., a phase-to-ground or phase-to-phase fault). This is handled by the magnetic trip (or instantaneous, `I`) setting.
What it is: When a motor (especially a "squirrel-cage" type) starts "Direct Online (DOL)", it can momentarily draw 6 to 8 times its FLA. This is a normal part of its operation. The breaker's magnetic trip must be set *above* this inrush current to prevent tripping every time the motor starts. Starting methods like "Star-Delta" or "Soft Starter" are used specifically to reduce this inrush current, allowing for a more sensitive magnetic setting.
What it is: This is the *maximum* fault current a breaker can safely interrupt without destroying itself. The "Available Short Circuit Current" you input is the worst-case fault current that your power system can deliver to that point. The breaker you select *must* have a breaking capacity (e.g., 25kA, 50kA) equal to or greater than this value.
What it is: In a large system, you don't want a small fault on a single motor to trip the main incomer breaker for the entire building. Selective Coordination ensures that only the breaker *closest* to the fault trips, isolating the problem while keeping the rest of the system online. This is achieved by carefully setting the time-delay and instantaneous trip levels of breakers in series (e.g., the motor breaker is set to trip faster than the MCC incomer, which is set to trip faster than the main transformer breaker).