What percentage can a wireway be filled?

NEC 376.22(A) limits conductors to 20 percent of the interior cross sectional area of the wireway at any cross section. Nonmetallic wireway under Article 378 uses the same 20 percent limit.

How many square inches of wire fit in a 4x4 wireway?

A 4 by 4 inch wireway has 16 square inches of interior area, and 20 percent of that is 3.2 square inches of total conductor cross sectional area, calculated using Chapter 9 Table 5.

When do you have to derate conductors in a wireway?

NEC 376.22(B) requires the adjustment factors of 310.15(C)(1) once more than 30 current carrying conductors are present at any cross section. Control, signaling and non current carrying neutral conductors are not counted.

What is the allowed fill percentage for a 4 x 4 inch wireway?

Twenty percent. NEC 2023 Section 376.22(A) states that the sum of the cross sectional areas of all contained conductors at any cross section of a metal wireway shall not exceed 20 percent of the interior cross sectional area of the wireway. A 4 inch by 4 inch wireway has 16 square inches of interior area, so the conductors may occupy 3.2 square inches. Article 378 applies the same 20 percent number to nonmetallic wireway.

Running the numbers

Use Chapter 9 Table 5 for the approximate area of each insulated conductor and add them up. Working with 3.2 square inches of usable area, 500 kcmil THHN at 0.7073 square inches each gives four conductors with room left over. 4/0 THHN at 0.3237 square inches gives nine conductors. 12 AWG THHN at 0.0133 square inches gives 240 conductors, which is far more than you would ever want in a wireway for other reasons. The fill calculation is easy. The conductor count is what bites.

The derating trap that costs more than fill

Section 376.22(B) says the adjustment factors in 310.15(C)(1) apply where the number of current carrying conductors at any cross section exceeds 30. Conductors used only for control or signaling, and neutrals that carry only the unbalanced current from other conductors, do not count as current carrying under 310.15(E). Thirty is a generous allowance compared to a raceway, but once you pass it the penalty is steep: 31 to 40 current carrying conductors gives a 40 percent factor, and 41 or more gives 35 percent. A 12 AWG THHN conductor at 30 amps in the 90 degree C column drops to 10.5 amps of allowable ampacity at 35 percent, which will not support a 20 amp circuit.

Splices and taps

Section 376.56(A) permits splices and taps in a wireway where the wireway is accessible and has a removable cover, and limits the conductors including splices and taps to 75 percent of the wireway area at that point. That is a local allowance at the splice, not a general increase in fill. The 20 percent limit still applies everywhere else along the run.

Other wireway rules worth remembering

Section 376.23(A) requires that where conductors are deflected within a wireway, the wireway be sized per Table 312.6(A) using the wire bending space at terminals. Section 376.23(B) applies the 6 times and 8 times conduit diameter pull box rules of 314.28(A)(1) and (A)(2) where a raceway enters a wireway and conductors are deflected. Section 376.30 requires horizontal support at intervals not exceeding 5 feet unless listed for a greater distance, and vertical runs supported at intervals not exceeding 15 feet with no more than one joint between supports. Section 376.21 limits conductors to 1000 kcmil.

Field takeaway

Twenty percent fill everywhere, 75 percent at accessible splice points, and start applying 310.15(C)(1) adjustment once you pass 30 current carrying conductors. On a 4 by 4 wireway that is 3.2 square inches of conductor area from Chapter 9 Table 5.