Calculadora de Disjuntor
- Created by
- Renato Passos, Eng. de Software
- Reviewed by
- Renato Passos, Eng. de Software
Last updated: Apr 18, 2026
Formula
I_proj = P/V × 1,25; disjuntor = próxima bitola padrão ≥ I_proj
About this calculator
The Circuit Breaker Calculator helps determine the appropriate breaker to protect a residential or commercial electrical circuit. It calculates the design current by applying a 25% safety factor over the nominal current, ensuring the breaker handles momentary surges without tripping unnecessarily. The result is the nearest standard breaker rating (in amperes) equal to or greater than the calculated current.
How it works: enter the total power of the equipment (in watts) and the circuit voltage (usually 127V or 220V). The calculator divides power by voltage to get the nominal current and multiplies by 1.25 for the safety margin. Then it selects the next higher standard breaker size (e.g., 10A, 16A, 20A, 25A, 32A, 40A, 50A, 63A).
Use this tool when designing a new electrical installation, remodeling a room, or adding a dedicated circuit for a shower, water heater, air conditioner, or machinery. It is also useful to verify if an existing breaker can handle the current load, preventing overloads and fire hazards. Remember, the breaker protects the wiring, not the appliances.
Important cautions: the 25% safety factor is a minimum; refer to local electrical codes (e.g., NEC) for specific requirements. Never use a breaker rated lower than the calculated current, as it will trip frequently. Also, ensure the wire gauge matches the breaker rating. For inductive loads (motors), a larger safety factor may be needed.
Frequently asked questions
Why is the calculated breaker rating higher than the nominal current?
The 25% safety factor is applied to prevent tripping due to momentary current surges, such as motor starts or simultaneous equipment activation. This ensures the breaker does not trip under normal operating conditions.
Can I use a breaker with a lower rating than calculated?
No. Using a lower-rated breaker may cause frequent tripping and will not adequately protect the circuit against overloads, potentially damaging wires and equipment.
What if the calculated current exactly matches a standard breaker size?
In that case, use that breaker size. The 25% margin is already included in the calculation, so the standard breaker meets the requirement.
Does this calculator work for three-phase circuits?
Currently, the calculator is for single-phase circuits. For three-phase, the formula is different (I = P / (V * √3 * PF)). Consult an electrical engineer for such cases.
Why is the correct voltage important?
Voltage directly affects the current calculation. Using 127V when the circuit is 220V (or vice versa) will result in an inappropriate breaker, causing oversizing or undersizing.