Its a basic rule I traced back to at least the 1937 NEC (section 2105) branch circuits in dwellings that supply lamp holders and small appliances were not permitted to be over 150V to ground, and had to use one grounded conductor, the wording changes over the years to 150V between conductors, but the general idea since Pre WW2 in the US is branch circuits for lighting and small appliances are 120V to ground. It pre dates GFCI and AFCI rules, this is the way its worded in the 2017, I dont think it changed in the 2020, 2023 or now 2026:
"210.6 Branch-Circuit Voltage Limitations. The nominal voltage
of branch circuits shall not exceed the values permitted by
210.6(A) through (E).
(A) Occupancy Limitation. In dwelling units and guest rooms
or guest suites of hotels, motels, and similar occupancies, the
voltage shall not exceed 120 volts, nominal, between conductors
that supply the terminals of the following:
(1)(2)Luminaires
Cord-and-plug-connected loads 1440 volt-amperes, nominal,
or less or less than 1⁄4 hp"

AC units that are small ones were under 1440 watts would be technically a violation, but now with advances in inverter driven heat pumps and AC its common to get a 240V window unit that is under the limit, then technically its not allowed. Also manufacturers of portable space heaters have cited the NEC as a reason for not making 240V portable heaters, which would be safer than overloading 120V 15A circuits.

The proposed 2029 wording is:
"210.6 Branch-Circuit Voltage Limitations. The nominal volt-
age of branch circuits shall not exceed the values permitted by
210.6(A) through (E).
(A) Occupancy Limitation. In dwelling units and guest rooms
or guest suites of hotels, motels, and similar occupancies, the
voltage shall not exceed 150 volts to ground
that supply the terminals of the following:
(1)(2)Luminaires
Cord-and-plug-connected loads 1440 volt-amperes, nominal,
or less or less than 1⁄4 hp"

That opens it up so you could have 240V outlets if you wanted, as long as its Line - Line 240 with each line being 120V to ground. Most people will stick to 120 but it wont be a hard and fast code blocker.
Since WW2 most of the world has converted to a 240V system, this would be the first nudge in that direction.

Last edited by tortuga; Yesterday at 07:57 PM.