Mike,

You forgot about the exceptions to 300.5(I) which allow isolated phase, neutral, and equipment grounding conduits where an underground installation uses nonmetallic conduit.

There is also 300.3(B)(3) which allows 2 or 3 nonmetallic sheathed cables between 2 boxes to provide 4 to 9 current carrying conductors.

Mike Cole mc5w at earthlink dot net

I forgot about 300.3(B)(1) exception as well.

Direct current circuits are theoretically allowed to be isolated phase, neutral, and equipment ground in aboveground applications. This used to be an explicit exception but would now fall under 90.4 alternative methods.

Advantages of isolated phase, neutral, and equipment ground for underground installations is that it is easier to make paralled conductors the same length and conductors do not have to cross each other at terminations. If say 500 KCM copper wires are crossing each other at terminations a they are allowed to press against each other, insulation can cold flow and create a short circuit. Some people do not pay attention to that when running say 3/0 through LB fittings leading to premature failure. Usually takes 5 to 40 years to happen.

The use of the word bonding can be misleading when refering to a ground method or type [250.118] that has not been bonded yet by using exothermic, crimp, or pressure clamp means of bonding. The NEC has purposely worded sections such as [250.134] with only the word grounding to be precise in the definition. Using grounding (bonding) can be missleading in the context of reference. Hope this helps.

This is not a comment, it is a question regarding wire size and load.

An electrician says 12/2 wire is the correct size for lighting with 12 lights on a circuit. The other electrician says 14/2 is the correct size of wire for a circuit with six ligths. Which one of them is correct? Please reply.

Thank you.