interpretação do ecmag.com de 314.16 (B) (1) preenchimento do condutor:
There is an exception under the conductor-fill provision that pertains to luminaire (fixture) conductors. An equipment-grounding conductor or four or fewer luminaire (fixture) wires smaller than 14 AWG, or both, entering a box from a domed luminaire (fixture) or similar canopy and terminating within that box, can be omitted from box-fill calculations. Under certain conditions, up to four luminaire (fixture) conductors and one equipment-grounding conductor can be installed but not counted in the box-fill calculation. The conductors must be 16 AWG or smaller and must enter the box from a domed luminaire (fixture) or similar canopy.
Sempre me perguntei por que os fios das luzes são tão frágeis; é por isso que você não precisa contá-los em preenchimentos de caixa.