Short answer: Three articles, three different jobs. A metal wireway (Article 376) is a trough with a removable cover for running and splicing conductors: the conductors may fill no more than 20 percent of its cross section (NEC 376.22(A)), splices and taps may bring a cross section up to 75 percent (NEC 376.56(A)), and ampacity adjustment factors apply only when more than 30 current-carrying conductors are at one cross section (NEC 376.22(B)). An auxiliary gutter (Article 366) supplements the wiring space at switchboards and panels. A busway (Article 368) is a factory-made enclosure of bus bars, supported at intervals of not more than 5 feet unless designed and marked otherwise (NEC 368.30).
PSI lists raceways in the Wiring Methods and Materials subject area of the Texas Journeyman exam: 10 scored NEC Knowledge questions and 2 scored Calculations questions. PSI does not say how many cover wireways, gutters or busways. The Texas Journeyman Practice Pack has 1,000 original questions and 10 timed mock exams in the real two-part format, with step-by-step solutions for every calculation. Try free practice questions without signing up. Get the full pack for US$39 with one payment.
Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the ideas in our own words and do not reproduce NEC text. Check every rule and table value in your own 2026 NEC, and follow your supervisor and the local inspector.
The numbers side by side
| Rule | Metal wireway, 376 | Auxiliary gutter, 366 | Busway, 368 |
|---|---|---|---|
| Conductor fill | 20 percent of the cross section (NEC 376.22(A)) | Check 366.22 in your NEC | Not a fill question: the bars are factory installed |
| Splices and taps | Up to 75 percent at the splice point (NEC 376.56(A)) | Check 366.56 in your NEC | Not applicable |
| When adjustment factors apply | More than 30 current-carrying conductors at a cross section (NEC 376.22(B)) | More than 30 current-carrying conductors at a cross section, for sheet metal gutters (NEC 366.23(A)(1)) | Not applicable |
| Support | Horizontal runs: each end and not over 5 feet, unless listed for other intervals, never over 10 feet between supports (NEC 376.30(A)) | Check 366.30 in your NEC | Not over 5 feet unless designed and marked otherwise (NEC 368.30) |
Read the count at one cross section: 30 current-carrying conductors there means no adjustment under these rules, 31 means you adjust.
Worked example 1: how many conductors fit in a wireway
A metal wireway has an interior cross section of 6 × 6 inches. Assume each insulated conductor takes 0.40 square inches. This is a practice value; on the exam, use the conductor in the question and find its area in Chapter 9, Table 5.
- Cross-section area. 6 × 6 = 36 square inches.
- 20 percent. 36 × 0.20 = 7.2 square inches.
- Divide by each conductor’s area. 7.2 ÷ 0.40 = 18 conductors.
- Check adjustment. 18 is not more than 30, so no adjustment factor applies for the wireway.
Worked example 2: room for splices
In the same 36-square-inch wireway, at a splice point:
- 75 percent limit. 36 × 0.75 = 27 square inches for conductors, splices and taps together at that cross section.
- Compare. If the conductors already take 7.2 square inches there, the splices and taps can use up to 27 − 7.2 = 19.8 square inches at that point.
Worked example 3: copper bars in an auxiliary gutter
Bare copper bars in a sheet metal auxiliary gutter are rated at 1,000 amperes per square inch of cross section (NEC 366.23(A)(2)).
- Bar area. A 1/4 × 2 inch bar is 0.25 × 2 = 0.5 square inches.
- Current. 0.5 × 1,000 = 500 A.
- Turned around. To carry 1,500 A: 1,500 ÷ 1,000 = 1.5 square inches. With 0.5-square-inch bars, that is 3 bars, a combined 1.5 square inches. This is the simplified calculation only; a real bar arrangement must also meet its design, connection and equipment requirements.
Busways: two rules that look like exceptions
- Reducing the size without a new breaker. Where a busway is reduced in ampacity, overcurrent protection is required at the reduction. In industrial establishments only, it can be omitted when the smaller busway is not over 50 feet long, has an ampacity at least one-third of the overcurrent device next back on the line, and is free from contact with combustible material (NEC 368.17(B) Exception).
- Cord drops from a busway. Cord or cable branches are attached to the building by approved means, run not over 6 feet from the plug-in device to a tension take-up support, are installed as a vertical riser, and have strain-relief grips at both ends (NEC 368.56(B)).
Check yourself
- A metal wireway has 31 current-carrying conductors at one cross section. Do adjustment factors apply? Yes, because it is more than 30.
- A metal wireway has an interior cross section of 4 × 4 inches. Each insulated conductor takes 0.50 square inches. Under the 20 percent limit, how many fit? 16 × 0.20 = 3.2; 3.2 ÷ 0.50 = 6.4, so 6.
- How far apart may busway supports be, unless designed and marked otherwise? Not over 5 feet.
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Find Articles 376, 366 and 368 faster on exam day
NFPA 70, National Electrical Code, 2026 Edition (softcover). Highlight “20 percent,” “75 percent” and “more than 30” in Articles 376 and 366 in the same color, so the three numbers this page uses stand out together. The Texas exam is open book. PSI allows a soft-bound NEC with highlighting, underlining and notes made in advance. Loose-leaf, spiral-bound and ring-bound copies are not allowed, and only permanent index tabs made or supplied by the NEC publisher are permitted; homemade and third-party tabs are not.
FAQ
What is the fill limit for a metal wireway?
The conductors may fill no more than 20 percent of the interior cross-sectional area. At splice points, conductors with splices and taps may reach 75 percent.
When do adjustment factors apply in a wireway?
Only when more than 30 current-carrying conductors are at any cross section.
How far apart are busway supports?
Not more than 5 feet, unless the busway is designed and marked for a different interval.
What is the difference between a wireway and an auxiliary gutter?
A wireway is a raceway for running conductors from point to point. An auxiliary gutter supplements the wiring space at switchboards, panels and similar equipment.
How do these articles relate to the Texas Journeyman exam?
PSI lists raceways under Wiring Methods and Materials, an area with 10 scored NEC Knowledge questions and 2 scored Calculations questions. PSI does not say how many cover Articles 376, 366 or 368.
Sources checked October 2026
- PSI Candidate Information Bulletin, Texas electrician examinations (updated September 15, 2026): https://test-takers.psiexams.com/api/content/bulletin/1682
- NFPA 70, National Electrical Code, 2026 Edition, Articles 366, 368 and 376, and Chapter 9, Table 5
