In keeping with the emphasis of the metric system of measurements in the NEC, a Metric Designator has been introduced to provide an equivalency to the inch system of measurement for circular raceways
used for many years as shown in table below.

You may have noticed the NEC uses the term “trade size” rather than simply “size” or “size in
inches” to indicate the size of circular raceways. There is a simple explanation for this designation.
Trade size 2 conduit is not two inches!

You should know that the circular raceway with the largest internal size in trade sizes 1⁄2 through 2 (metric designator 16 through 53) size is IMC and from trade size 21⁄2 through 4 (metric designator 63 through 103) is EMT. Thus, these raceways allow a larger number of conductors in some cases.

It will be useful to review some of the terms used in the NEC concerning raceways:

• A ferrous conduit is made of iron or steel; a nonferrous conduit is made of a metal other than iron or steel.

• Common nonferrous raceways include aluminum, brass and stainless steel.

• Metal or metallic would include both ferrous and nonferrous.

• Nonmetallic raceways would include PVC, ENT, and fiberglass.

• Couplings are used to connect sections of a raceway.

• Locknuts, metal bushings, or connectors are used to connect raceways to boxes or fittings.

• Integral couplings are formed into some raceways and cannot be removed.

• Associated fittings such as couplings and connectors are separate items.

• A running thread is a longer-than-standard length thread cut on one conduit and is sometimes referred to as “continuous-thread” or “all-thread.” Connection to another conduit is achieved by screwing a coupling on the running thread, butting the conduits together, and then backing the coupling onto the second conduit.

As shown in Figure 6-1, running threads are not permitted for connecting conduits together with a coupling as a tight connection is not assured, NEC 344.42(B). Running thread can be used to connect two enclosures by cutting the conduit to length, installing the outer locknuts on the running thread, and sliding the conduit into one enclosure, then into the other.

Caution: If running thread is created by cutting threads in the field, no galvanizing will remain on the conduit,
and protection against rusting should be provided. An option to installing running thread between enclosures is to connect two conduit nipples together with a split coupling.

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