Minimum Cable Size (NBR 5410)

Cable sizing by current capacity (NBR 5410)

Under the Brazilian NBR 5410, a cable has to clear three checks before you can call it sized. There's (1) current-carrying capacity (ampacity), which depends a lot on how the cable is run (reference methods A1 to G cover everything from buried conduit to open air). Then (2) voltage drop, and finally (3) short-circuit protection. For PVC-insulated copper with 2 loaded conductors in conduit at 30 °C ambient, the usual ampacities run 1.5 mm² → 14 A, 2.5 mm² → 18 A, 4 mm² → 25 A, 6 mm² → 32 A, 10 mm² → 43 A. Here's the key rule: the circuit breaker rating must be ≤ the cable ampacity, so the breaker trips before the wire ever overheats. Grounding (green-yellow) has been mandatory since NBR 5410:2004.

Applications: residential, industrial, solar and automotive

A typical residential setup looks like this: lighting and outlet circuits at 1.5–2.5 mm², air-conditioner splits at 2.5 or 4 mm², an electric shower at 6 or 10 mm² depending on its power, and stove or oven feeders at 6 mm². On conductor colors, NBR 5410 asks for phase in black, red or grey, neutral in light blue, and ground in green-yellow (or bare green). The same logic carries over to industrial machinery, photovoltaic systems (DC strings and AC feeders, usually UV-resistant cables) and automotive wiring (12 V DC, where the higher current for a given power means you need a fatter cross-section).

FAQ

Why does the same wire have different ampacities? It comes down to how well the heat escapes. A cable inside conduit dissipates poorly, the same cable in free air does much better, bundling it with other circuits makes things worse again, and burying it lands somewhere in the middle.

Can I use a 20 A breaker on a 2.5 mm² wire? No. In conduit, 2.5 mm² only handles about 18 A, so either drop to a 16 A breaker or step the wire up to 4 mm².

What about three-phase loads? Add a third loaded conductor and the ampacity slips a bit, roughly a 0.9 correction factor. The NBR 5410 tables spell out separate values for 2 and 3 loaded conductors.

Solid or stranded wire? Up to 6 mm² you can use solid wire in a home. Above 6 mm² stranded (flexible) wire is required, and it's the better choice for anything that moves around.