new bath vent/light in existing ceiling

[kwired]

I understand that however 300.3 states that the neutral has to be run in the same cable assembly as the conductors that are feeding the loads unlike a switch leg where you feed a switch with 2 wire (one conductor to switch and switch leg back). If you phase one conductor as a hot leg you have 2 conductors feeding 2 loads with no neutral in cable assembly to cancel out eddy currents or hysteresis. This is what I understand from the code.

What I was referring to was the fact that without a neutral in the cable with the current carrying conductors it builds up more heat. In conduit you get heat from hysteresis. From what I understand the magnetic fields will create heat if they aren't cancelled out. Albeit in this install you probably wont see anything appreciable but in larger loads this could be problematic.

I think you are on the right track, but I want to clarify a couple things.

300.3 does not require a neutral conductor, it says all conductors of the same circuit, but I think it may be easy to confuse the intent of this section and maybe a little wording change could help with that but AFAIK the concept here is to have cancellation of EMF's of the conductors in any particular raceway or cable run.

So one example where a neutral may not even be necessary is a simple switch loop with one "hot" to the switch and a "switched" return conductor. If you have both those conductors in same raceway or cable their magnetic effects are equal and opposite in magnitude and therefore they cancel one another and have low level of EMF effects away from the conductors. Running a neutral to that switch does nothing that will effect the EMF properties of that portion of the circuit, unless you have a pilot light or other "load" at the switch location, then you need a neutral conductor anyway and it would need to be run with the other circuit conductors.

If you have a straight 240 volt load, or a straight three phase load (no neutral required by either of those loads) kind of same thing, running a neutral conductor is meaningless, it does nothing and will not change the overall EMF effects of the raceway or cable if it is not carrying any current.

The other thing that is effected is the impedance of the conductors. Not so much a big deal under normal operation, but during a short circuit or ground fault, having the two conductors of the circuit that are carrying this high level current in close proximity will result in less inductive reactance in those conductors. The main benefit of this is faster response time of overcurrent protection. The lower the impedance of the conductors during a fault the higher the possible level of fault current meaning faster response time from the overcurrent device. This is one reason 300.3 mentions the equipment grounding conductor as well in those requirements.

I personally have no issue based on information I just mentioned running two 2 wire with ground NM cables between the switch location and the bath fan, light, night light - if both cables run between same two points and are routed in close proximity to one another even though it may be stretching the wording of 300.3 a little bit I think it still covers the intent, but you may want to run this by your AHJ before assuming he will see it the same way. Routing separate conductors of same circuit in multiple steel raceways or steel jacketed cables however will result in magnetizing the raceway or cable jacket and will introduce all the issues 300.3 is trying to prevent.

 

[FREEBALL]

Thanx Kwired The part about the switch loop was bugging me when reading 300.3 but I figured since it is the same load and the switch was just breaking the circuit it was meaningless to run a neutral with no load on it. Thanx for the clarification. I also don't see much of an issue with running the the circuit the way JDB3 and yourself said but 300.3 stuck in my mind and I have been reading that and 300.20 in both the NEC handbook and Mike holts understanding the 2014 code it all seems geared towards metal raceways in the wording and illustrations with just "cable" being mentioned with no examples. Most of my experience has been in commercial and some industrial in the past you know cutting, bending, threading and pulling wire, code was up to the engineers lol. I know myself I did the same thing way back when I first started out jerking romex but since getting back into it I have been trying to understand the code a bit more and the intent. Thanx again and be safe Have a safe and happy holiday.