8 months ago, I installed a surface mounted incandescent ceiling light with 2 60w bulbs enclosed by the glass cover. I installed new NM-B 14g wire to supply the light. Now, I open the light outlet box to check it due to arc fault breaker tripping and I cannot find any short or problem. However, I notice that the white neutral wire in the outlet box has become slightly brown, obviously due to heat. Does the slightly brown color mean there is definitely an overheating problem or does that sometimes happen with normal heat levels?