Greetings Gents,
So I will comment on the "nonmetallic sheathed cable" inner conductors....lol
For starters, here is what UL 719 states about the subject.
4.4 Insulation
4.4.1 Both before and after assembly into finished cable, the insulation employed on the circuit
conductors shall comply with (a) or (b):
a) THHN Construction – Jacketed insulation for a Type NM or NMC cable shall have a nylon or
similar jacket and shall comply with the thickness and other applicable requirements for Type
THHN thermoplastic-insulated wire without (see 6.2.4) any surface marking of "THHN", "-B", or
any ampacity or temperature rating.
b) TW Construction – Insulation without a nylon jacket shall comply with the thickness
requirements for a Type TW thermoplastic-insulated wire. The insulation material shall comply
with (1) or (2):
1) For TYPE NM or Type NMC Cable – The insulation material shall be a dry-locations
PVC that complies with the requirements for Type THHN insulation without any surface
marking of the ampacity or temperature rating (the surface marking "-B" is optional on
the insulation). The tensile strength and elongation are to comply with the “Physical
properties of PVC insulation from Type TFN and TFFN fixture wires² Table in UL 1581.
The deformation test is to be conducted at 121.0 ±1.0°C (249.8±1.8°F) with a decrease
of not more than 50 percent in the thickness of the PVC insulation.
2) For TYPE NM only – Type NM is eligible to use a thermoplastic insulating material
other than PVC. The performance and ratings of the material as insulation shall be
determined by investigation and shall be:
i) Electrically comparable to the PVC insulation in Type THHN wire, and
ii) Mechanically comparable to the nylon or similarly jacketed PVC insulation of
Type THHN wire, and
iii) In accordance with “Physical properties of insulation of thermoplastic other
than PVC from Type NM cables” Table in UL 1581.
So as you can see it is not required to be marked. However, many manufacturers do mark the conductors and do use THHN/THWN-2 inners to avoid having to make something special for standard Type NM-B cable.
Now, just like many other cable assemblies (Type SE Cable for example in UL 854) may contain conductors that do not have any markings on them as well. However, due to the same cost practice they may indeed have the markings found on normal thermoplastic or thermoset insulated conductors for use in raceways and so on. It is commonly understood or accepted (doesn't make it right) that when a cable assembly is marked in accordance with the UL Standards that the inner conductors have to be also in compliance with that standard and the standard of their design, such as UL 83 and UL 44.
Type NM-B (and Type SE) are marked in accordance with their applicable standards and meet 310.120(A) and (B) most notable is Items (B) and (C) of 310.120(B)(1). Now, when a known product is being used as the cable is intended then no issues and it is common sense that folks can identify the fact an SE Cable or an NM-B is what it actually is at termination it is true that if and when someone choose to do a transition from a known cable assembly to another raceway method that in doing so the conductors are now being used as a single-conductor and thus the marking requirements of 310.120(A) and (B)(1)(1) do indeed come into play.
It is a good question but a bit too deep in the weeds to be concerned with in my opinion (yes only my opinion). The termination aspects as mentioned in a panel enclosure and so on to me would not be considered as a raceway and still part of the cable assembly simply being terminated and the identify of the fact it is a cable assembly is evident.
Now it is quite common to transition Type MC Cable as mentioned thus we and most all manufacturers use normal THHN/THWN-2 or XHHW-2 as inners simply because it is more cost effective and to quite honest you don't have as much leeway as you do with Type NM-B in UL 719. Plus, since we know it can potentially be used with a transition fitting it makes sense to use standard THHN/THWN-2 or XHHW-2 conductors which can transition with no problem.
Agree to disagree...that is our take on it.
