Wire size and breaker calculator (mm²)
Calculate the wire size (mm²) and breaker from the power, voltage and length of the circuit, considering current capacity and voltage drop under IEC-based rules (Brazilian NBR 5410).
Which wire and breaker to use
Estimate for PVC-insulated copper, 2 or 3 loaded conductors in embedded conduit (NBR 5410, method B1). Confirm with an electrician.
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Wire size and breaker calculator (mm²): what it is for
Enter the power of the appliance or circuit, the voltage and the distance to the panel. The calculator finds the current, picks the smallest wire size that carries it and keeps the voltage drop within the limit, following NBR 5410 minimums, and suggests a suitable standard breaker.
How to use
- Type the power, voltage and distance.
- Choose lighting or outlets.
- See the recommended wire and breaker.
When to use it
- Install an electric shower or water heater.
- Plan an air conditioner circuit.
- Check the wiring of a renovation.
Frequently asked questions
What is the minimum wire size?
Under the Brazilian NBR 5410 standard (based on IEC 60364), copper wiring in homes must be at least 1.5 mm² for lighting circuits and 2.5 mm² for outlets. Electric showers and high-power appliances usually need 4, 6 or 10 mm². In the US, wire is sized in AWG under the NEC.
Why does length change the wire size?
The longer the wire, the higher the resistance and the voltage drop. The standard limits the drop to 4% on branch circuits, so long circuits need thicker wire even with the same current.
How do I choose the breaker?
The breaker must be greater than or equal to the circuit current and less than or equal to the wire capacity, to protect the conductor. The calculator suggests the smallest standard breaker that meets both.
Can I rely on the calculator alone?
It gives an estimate for PVC-insulated copper wire in embedded conduit (method B1). Grouped circuits, temperature and other methods change the result: the design must be done by a licensed electrician.
Is it really free?
Yes, 100% free. No sign-up, no subscription and no usage limits. All Tools123 tools are free and run right in your browser.