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I was at a big-box home improvement store recently and saw this wire type vs amperage rating chart:

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Up through 10ga all the copper wires have the same rating but then they start to diverge. I am particularly surprised that there is a different between NM-B cable and THHN wires... don't NM-B cables use THHN wires as the individual conductors?

According to this chart even alumnium wire has higher capacities than NM-B for most sizes.

And what do you use for a wire that has multiple ratings, like THHN/THWN? Do you use the higher value or the lower value?

Can anyone explain these numbers?

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    When you use THHN wire, for example, code requires you to derate the amperage based on how many conductors are in the raceway. With NM-B, there is a known amount of conductors, and so the derating is already built in to the spec. I'll let someone with more experience post an answer with references... – bitsmack Oct 26 '15 at 4:02
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The current carrying capacity of wire and cable is determined by (1) the heat produced due to conductor resistance, (2) the high temperature limit of the insulation used, and (3) the surroundings of the wire to the degree it traps heat in the insulator (everything such as insulation thickness, conduit, proximity to a hot roof, etc.). Items (1) and (2) are characteristics of the wire. Item (3) is a characteristic of both the wire and the installation.

Even though the wires within NM-B cable are THHN wire and have insulation rated for 90 degrees C, many versions of the code, including 2011 NEC Section 334-80, limit ampacity of non-metalic cables such as NM-B to no more than that permitted with an insulation rated for 60 degrees C (probably due to heat retention by the thick outer PVC jacket, although they do not explicitly say that is the reason):

334.80 Ampacity. The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The allowable ampacity shall not exceed that of a 60°C (140°F) rated conductor. The 90°C (194°F) rating shall be permitted to be used for ampacity adjustment and correction calculations, provided the final derated ampacity does not exceed that of a 60°C (140°F) rated conductor. The ampacity of Types NM, NMC, and NMS cable installed in cable tray shall be determined in accordance with 392.80(A).

The code goes on to explicitly describe how the cable must additionally be de-rated for the installation (multiple cables in one conduit, multiple adjacent cables in an insulated wall, etc, etc.)

The differences in ampacity are all about the effect of heat on the insulation. Regarding multiple ampacities for a single wire type, there is always an asterisk somewhere describing the particular instance where the wire is to be used and which ampacity is applicable in that case.

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    How do come to the determine that NM-B is THHN? If you were to strip back NM-B, you won't find the label THHN. – Kris Oct 26 '15 at 12:03
  • Manufacturer product information. From United Copper Industries, "Construction Type NM-B cable is manufactured as 2- or 3- conductor (Type THHN/THWN-2) cable, with a ground wire. Copper conductors are soft-annealed. Stranded conductors are compressed stranded. Phase conductors are insulated with tough, heat- and moisture-resistant, lead-free Polyvinyl Chloride (PVC) over which a nylon (polyamide) or UL-recognized equivalent jacket is applied. They are jacketed with moisture- and fungus-resistant lead-free PVC." – user39367 Oct 26 '15 at 14:27
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    But, unfortunately and inconveniently, the inner conductors are never (IME) marked, and thus cannot be used (stripped) where individual conductors would be more appropriate (such as in conduit.) – Ecnerwal Oct 26 '15 at 15:34

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