How do you calculate wireway fill?

Add the NEC Chapter 9 Table 5 cross-sectional area of each conductor, including insulation, times its quantity. Multiply the wireway interior cross-sectional area (width times height) by 0.20 to get the allowable fill area. The conductor total must not exceed that allowable area. For a 4 by 4 inch wireway, 16 square inches times 0.20 is 3.2 square inches of allowable conductor fill.

When do the 310.15(C)(1) adjustment factors apply in a wireway?

NEC 376.22(B) and 378.22 apply the Table 310.15(C)(1) ampacity adjustment factors only where the number of current-carrying conductors exceeds 30 at a cross section of the wireway. Neutral conductors classified as current-carrying under 310.15(E) count toward the 30. Signaling-circuit conductors and motor-starter control conductors used only for starting duty are not counted. At 30 or fewer current-carrying conductors, no adjustment is required, which is more permissive than the more-than-3-conductor rule that applies inside conduit.

Is wireway fill the same as auxiliary gutter fill?

The 20 percent area limit is the same. NEC 366.22(A) for auxiliary gutters uses the identical 20 percent of interior cross-sectional area rule and the same more-than-30-current-carrying-conductor adjustment trigger as wireways. The difference is application: an auxiliary gutter supplements wiring space at equipment such as meter stacks and switchboards, while a wireway is a general raceway. This calculator applies the 20 percent rule that governs both.

What interior area should I use for a standard wireway?

Standard lay-in wireways are square: 2-1/2, 4, 6, 8, 10, and 12 inches. The nominal interior area is the trade size squared, so a 4 by 4 inch wireway is treated as 16 square inches. Listed interior dimensions are slightly smaller than nominal because of wall thickness, so nominal is marginally less conservative. For a fill near the limit, use the calculator custom-size input with the interior dimensions from the manufacturer cut sheet.

Wireway Fill Calculator

Check conductor fill in a metal or nonmetallic wireway against the NEC 20% area limit. Enter each conductor by insulation, size, and quantity; the tool sums the Chapter 9 Table 5 areas, returns the minimum standard wireway size, and flags the more-than-30-current-carrying-conductor ampacity adjustment. Default NEC 2020; 376.22(A) and 378.22 are unchanged across the 2017 through 2023 editions.

Conductors

Add each conductor spec. Leave "Current-carrying" checked for phase and current-carrying neutral conductors; uncheck it for equipment grounding conductors and control conductors that do not count toward the more-than-30 adjustment (NEC 376.22(B)).

Insulation

Size (AWG/kcmil)

Qty

Current-carrying

Wireway

Wireway size

Standard sizes use the nominal square interior (width x height); the 20% limit applies to that area. For an exact result, choose "Custom interior size" and enter the listed interior dimensions from the product cut sheet.

How to use this calculator

Add each conductor spec in the run: insulation (THHN/THWN-2 or XHHW), size, and quantity.
Leave "Current-carrying" checked for phase and current-carrying neutral conductors; uncheck it for equipment grounding and starting-duty control conductors so they do not count toward the more-than-30 adjustment.
Pick a wireway size to test a specific size, choose "Custom interior size" to enter listed interior dimensions, or leave it on Auto for the minimum standard size that passes.
Read the total conductor area, the minimum wireway size, the 20% allowable fill, the actual fill percentage, and the pass/fail result.
If the adjustment flag appears, apply the Table 310.15(C)(1) factor when sizing the conductor ampacity separately.

NEC reference

Fill is computed per NEC 2020 376.22(A) for metal wireways and 378.22 for nonmetallic wireways: the sum of the cross-sectional areas of all contained conductors must not exceed 20% of the interior cross-sectional area of the wireway at any cross section. Conductor areas are the NEC Chapter 9 Table 5 approximate areas, including insulation. The Table 310.15(C)(1) ampacity adjustment factors apply only where the number of current-carrying conductors exceeds 30 (376.22(B), 378.22), counting neutrals classified as current-carrying under 310.15(E). These provisions are unchanged across the 2017, 2020, and 2023 editions.

Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.

Standard wireway sizes and 20% allowable fill

Standard lay-in wireways are square. The nominal interior area is the trade size squared, and the NEC 376.22(A) allowable conductor fill is 20% of that area.

Wireway size (in)	Interior area (in²)	20% allowable fill (in²)
2-1/2 x 2-1/2	6.25	1.25
4 x 4	16	3.20
6 x 6	36	7.20
8 x 8	64	12.80
10 x 10	100	20.00
12 x 12	144	28.80

Interior area is taken as the nominal width times height. Listed interior dimensions are slightly smaller than nominal; for a fill near the limit, enter the cut-sheet interior dimensions with the custom-size option.

Worked example: #12 THHN in a 4 x 4 wireway

A common question is how many 12 AWG THHN conductors fit in a 4 by 4 inch wireway.

Interior area: 4 × 4 = 16 in².
Allowable fill (20%): 16 × 0.20 = 3.2 in².
#12 THHN area (Table 5): 0.0133 in² each.
Maximum count: 3.2 ÷ 0.0133 = 240.6, so 240 conductors.

240 conductors of 12 THHN fill 240 × 0.0133 = 3.192 in², just under the 3.2 in² limit. Because 240 current-carrying conductors far exceed 30, NEC 376.22(B) requires the Table 310.15(C)(1) adjustment factor when sizing those conductors for ampacity, which is a separate calculation from fill.

Wireway fill vs. conduit fill

Conduit fill (NEC Chapter 9, Table 1) steps the allowable percentage with the conductor count: 53% for one conductor, 31% for two, and 40% for three or more. A wireway is simpler and more restrictive at low counts: it is a flat 20% of the interior area regardless of how many conductors are present. The adjustment trigger also differs. Inside conduit, the Table 310.15(C)(1) factors kick in above three current-carrying conductors; in a wireway they do not apply until the count exceeds 30, so a wireway can carry many more conductors before derating begins.

Scope and limitations

Version 1 computes the 20% conductor-area fill for metal (376.22(A)) and nonmetallic (378.22) wireways using NEC Chapter 9 Table 5 insulated-conductor areas for THHN/THWN-2 and XHHW. The same 20% rule governs auxiliary gutters (366.22(A)); enter your gutter interior dimensions with the custom-size option to apply it. The tool reports the more-than-30-current-carrying-conductor adjustment flag but does not perform the ampacity derating itself; use the Wire Ampacity Calculator for that. Splices and taps within a wireway (limited to 75% of the cross-sectional area at that point per 376.56) and busbar sizing are out of scope.

Frequently asked questions

What is the maximum fill for a wireway?

The sum of the conductor cross-sectional areas must not exceed 20% of the wireway interior cross-sectional area (NEC 376.22(A) for metal, 378.22 for nonmetallic). It is a flat 20% regardless of conductor count.

Does the 20% limit change with the number of conductors?

No. Unlike conduit fill, the wireway limit stays at 20% whether the wireway holds one conductor or one hundred. What can change above 30 current-carrying conductors is the ampacity adjustment factor, not the fill percentage.

Which conductor area does this use?

The NEC Chapter 9 Table 5 approximate area, including insulation, the same source the conduit fill tools use. THHN/THWN-2 and XHHW are supported in version 1.

Can I use this for an auxiliary gutter?

Yes. NEC 366.22(A) applies the same 20% of interior area limit and the same more-than-30-conductor adjustment. Enter the gutter interior dimensions with the custom-size option.

Related calculators

Conduit Fill Calculator: conductor fill for EMT, RMC, IMC, and PVC per NEC Chapter 9, Table 1.
Cable Tray Fill Calculator: multiconductor cable fill for ladder and ventilated trough trays per NEC 392.22(A).
Wire Ampacity Calculator: derated ampacity with the Table 310.15(C)(1) adjustment factor.
Conduit Size Calculator: minimum conduit trade size for a conductor bundle.

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