The two things that decide the gauge
1. The current. Each gauge carries a certain current without overheating: its ampacity, listed in Table 310-15(b)(16) of NOM-001-SEDE-2012, Mexico's electrical code (it follows the US NEC closely, and also uses AWG sizes). At home the 60 °C column is used, as the standard requires for terminals on circuits of 100 A or less. If the place is hot or many wires share a conduit, ampacity drops with the standard's correction factors.
2. The distance. All wire loses some voltage because of its resistance, and the longer and thinner, the more it loses. The NOM recommends keeping this voltage drop under 3 % on a branch circuit. On long runs, it's the distance that forces a thicker gauge.
The calculator checks both and recommends the thicker of the two results. Household circuits in Mexico are 127 V (220 V for large appliances like some A/C units).
The breaker and thin wires
The breaker protects the wire, not the appliance. That's why the NOM limits the breaker on small gauges: 15 A for 14 gauge, 20 A for 12 and 30 A for 10, even if the table says they carry more. A breaker larger than the wire allows leaves the wire unprotected: it can overheat without the breaker tripping.
Frequently asked questions
How many amps can 12 gauge wire carry?
According to Table 310-15(b)(16) of Mexico's NOM-001-SEDE-2012, 12 AWG copper carries 20 A in the 60 °C column, 25 A at 75 °C and 30 A at 90 °C. But the same standard (240-4(d)) limits its protection to a 20 A breaker.
Which gauge for 15, 20 and 30 amps?
With the NOM rule for small conductors: a 15 A breaker with 14 gauge, 20 A with 12 gauge and 30 A with 10 gauge. If the circuit is long, voltage drop may require a thicker gauge.
What is voltage drop and why does it matter?
It's the voltage lost in the wire because of its resistance: the longer and thinner, the more is lost. The NOM recommends no more than 3 % on a branch circuit so appliances work well. That's why a long circuit may need a thicker gauge than the current alone requires.
Why does the calculator use the 60 °C column?
Because the NOM (110-14(c)) requires the 60 °C column on circuits of 100 A or less, unless outlets, breakers and equipment are rated for 75 °C. Even if the wire is THW-LS rated 75 °C, the lowest terminal temperature rules.
What is a continuous load?
A load that runs 3 hours or more at its maximum current, like a heater, an air conditioner or a pump in some uses. For those, the NOM (210-19) requires the wire to carry 125 % of the current.
Source: NOM-001-SEDE-2012, Instalaciones Eléctricas (utilización), Diario Oficial de la Federación, November 29, 2012: Tables 310-15(b)(16), 310-15(b)(2)(a), 310-15(b)(3)(a), Chapter 10 Table 8, and sections 110-14(c), 210-19, 240-4(d) and 240-6.
Learn more: What wire gauge to use at home in Mexico