What type of wire can you run inside Wiremold?

Wiremold is a brand name for surface metal raceway, which the NEC covers in Article 386. Nonmetallic surface raceway is Article 388. Inside either one you run individual insulated conductors of a type recognized in Table 310.4(A) and suitable for the location, which in practice means THHN or THWN-2 for almost every job. There is no special "Wiremold wire." The raceway is a wiring method, and the conductors inside it follow the same rules as conductors in EMT.

What Article 386 actually limits

Section 386.21 says the size of conductors shall not be larger than that for which the raceway is designed. Section 386.22 says the number of conductors shall not be greater than the number for which the raceway is designed, and the raceway is not required to meet the 40 percent fill and 31 conductor limits in Chapter 9 as long as the internal cross sectional area and conductor count marked by the manufacturer are respected. That is the part electricians miss: fill is set by the listing, not by Chapter 9 Table 1. A 500 series base and cover has a published conductor capacity per size, and that published number governs.

Where it is allowed

Section 386.10 permits surface metal raceway in dry locations, and 386.12 prohibits it where subject to severe physical damage, where the voltage is 300 volts or more between conductors unless the metal is at least 1.0 mm thick, in hoistways, in hazardous locations except as permitted, and where concealed except as permitted for the extension of a raceway through a wall or floor. Nonmetallic surface raceway under 388.12 has similar limits plus temperature restrictions, since the plastic is not listed for ambient temperatures above its listing or for locations where it will be subject to a temperature outside its rating.

Cables versus conductors

Individual conductors are the normal fill. NM cable inside surface metal raceway comes up constantly, and it is not universally accepted: Article 386 speaks in terms of conductors, and while some inspectors permit a cable to pass through a piece of raceway used as sleeving, running NM cable the length of a surface raceway is not the intent of the article and should be cleared with the AHJ before it is installed. Pulling the jacket and using the THHN conductors is not an option either, since the conductors inside NM cable are not individually listed as building wire in most constructions. The clean answer is to install THHN or THWN-2 from the box into the raceway.

Grounding and derating

Surface metal raceway is permitted as an equipment grounding conductor under 250.118 when installed with listed fittings, but running a separate green or bare equipment grounding conductor inside is cheap insurance and is required when the raceway is nonmetallic. If more than three current carrying conductors share the raceway, the adjustment factors of 310.15(C)(1) apply the same as in any other raceway. Also remember 386.56: splices and taps are permitted in surface metal raceway with a removable cover that is accessible after installation, and the conductors including splices may not fill the raceway more than 75 percent of its area at that point.

Field takeaway

THHN or THWN-2 individual conductors, dry locations only, conductor count per the manufacturer listing, and a separate equipment grounding conductor whenever there is any doubt about the metal path.