+---
+title: Wireways and Auxiliary Gutters
+category: Electrical
+toc_depth: 3
+description: >
+ When to use: Selection, sizing, and installation of sheet-metal and nonmetallic
+ enclosed raceway troughs (wireways per NEC Articles 376 and 378) and supplemental
+ conductor enclosures adjoining switchgear, switchboards, panelboards, meter centers,
+ and distribution centers (auxiliary gutters per NEC Article 366). Covers lay-in and
+ pull-through configurations, hinged- and lift-off-cover styles, NEMA 1/3R/4/4X/12
+ enclosure types, fittings (elbows, tees, crosses, reducers, end caps, splice plates),
+ support hardware, and conductor fill and derating calculations for commercial,
+ industrial, institutional, and heavy-industrial work.
+ Not intended for: conduit raceways (use [[sync/raceways-and-conduit]]); cable tray
+ (use [[sync/cable-tray]]); busway and plug-in bus duct (use [[sync/busway]]); the
+ conductors and cables pulled through the trough (use [[sync/conductors-and-cables]]);
+ grounding and bonding conductors within the enclosure (use [[sync/grounding-and-bonding]]);
+ panelboard, switchboard, and switchgear enclosure interiors (use
+ [[sync/low-voltage-panelboards]], [[sync/low-voltage-switchboards]],
+ [[sync/low-voltage-switchgear]]); and wiring troughs integral to an MCC lineup
+ (use [[sync/motor-control-centers]]).
+---
+
+# Scope {toc}
+
+## This standard covers enclosed metal and nonmetallic troughs used to group, route, and access insulated conductors at and between electrical equipment, comprising both wireways (a raceway) and auxiliary gutters (a supplemental enclosure). {note}
+
+## The defining distinction in this standard is between a wireway and an auxiliary gutter, because each is governed by a different NEC article with different fill limits and application restrictions. A wireway is a sheet-metal or nonmetallic trough with a hinged or removable cover used as a raceway to carry conductors from point to point; it is governed by NEC Article 376 (metal) or Article 378 (nonmetallic) and has no length restriction. An auxiliary gutter is a supplemental enclosure that extends from and supplements the wiring space of equipment such as switchboards and distribution centers; it is governed by NEC Article 366 and is not a raceway. {note}
+
+## The two assemblies frequently use identical hardware and finishes, so the governing article must be established per segment rather than per product; a single physical trough can change classification along its length. {note}
+
+## Each trough segment shall be classified on the drawings or in the conductor fill schedule as either a wireway (Article 376 or 378) or an auxiliary gutter (Article 366).
+
+## Conductor fill, support spacing, ampacity derating, and length limits shall be calculated using the NEC article that governs the segment's classification.
+
+## Wireways and auxiliary gutters shall be UL 870 listed assemblies unless a field-fabricated auxiliary gutter is expressly permitted and detailed in accordance with NEC 366.6.
+
+```datasheet
+label: Primary assembly classification
+type: radio
+options:
+ - Wireway, metal (NEC Article 376 raceway)
+ - Wireway, nonmetallic (NEC Article 378 raceway)
+ - Auxiliary gutter (NEC Article 366 supplemental enclosure)
+drawing_ref: "wireway and gutter classification schedule"
+default: deferred
+```
+
+# Referenced Standards {toc}
+
+## Equipment, materials, and installation shall comply with the latest adopted edition of each of the following unless a specific edition is cited.
+
+## Where referenced standards conflict, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
+
+## The edition of NFPA 70 (NEC) that applies shall be the edition adopted by the authority having jurisdiction for the project. {note}
+
+| Standard | Title |
+|----------|-------|
+| NFPA 70 (NEC) Article 100 | Definitions (Auxiliary Gutter; Wireway; Raceway) |
+| NFPA 70 (NEC) Article 366 | Auxiliary Gutters |
+| NFPA 70 (NEC) Article 376 | Metal Wireways |
+| NFPA 70 (NEC) Article 378 | Nonmetallic Wireways |
+| NFPA 70 (NEC) Table 310.15(C)(1) | Adjustment Factors for More Than Three Current-Carrying Conductors |
+| UL 870 | Wireways, Auxiliary Gutters, and Associated Fittings |
+| NEMA 250 | Enclosures for Electrical Equipment (1000 Volts Maximum) |
+| ANSI/NEMA 70 | National Electrical Code (as adopted) |
+
+# Submittals {toc}
+
+## Action Submittals {toc}
+
+### The Contractor shall submit the following action submittals for review prior to fabrication or ordering:
+
+- Product data for each wireway and auxiliary gutter type, including cross-sectional dimensions, material, finish, NEMA enclosure type, voltage rating, and UL 870 listing.
+- Product data for all fittings, including elbows, tees, crosses, reducers, end caps, splice plates, couplings, and expansion fittings.
+- Conductor fill calculations for each trough segment, identifying segment classification (wireway or auxiliary gutter), interior cross-sectional area, conductor count and size, computed percent fill, and applied ampacity adjustment factor.
+- Shop drawings showing routing, dimensions, support locations and types, section joints, and coordination with adjacent equipment and other trades.
+- Support and seismic anchorage details where required by the project structural criteria.
+
+```datasheet
+label: Action submittals required
+type: checkbox
+options:
+ - Product data (troughs)
+ - Product data (fittings)
+ - Conductor fill calculations
+ - Shop drawings (routing and supports)
+ - Support and anchorage details
+default:
+ - Product data (troughs)
+ - Product data (fittings)
+ - Conductor fill calculations
+ - Shop drawings (routing and supports)
+```
+
+## Informational Submittals {toc}
+
+### The Contractor shall submit the following informational submittals:
+
+- Manufacturer installation instructions for the specified troughs and fittings.
+- UL 870 listing documentation, including the listed enclosure type and corrosion-protection classification.
+- Where field-fabricated auxiliary gutters are permitted, evidence that the sheet-metal and construction comply with NEC 366.6.
+
+```datasheet
+label: Informational submittals required
+type: checkbox
+options:
+ - Manufacturer installation instructions
+ - UL 870 listing documentation
+ - Field-fabrication compliance evidence (if applicable)
+default:
+ - Manufacturer installation instructions
+ - UL 870 listing documentation
+```
+
+## Closeout Submittals {toc}
+
+### The Contractor shall submit the following closeout submittals:
+
+- Record drawings showing as-installed routing, segment classifications, and fitting locations.
+- Field test reports for equipment grounding conductor continuity across section joints.
+
+```datasheet
+label: Closeout submittals required
+type: checkbox
+options:
+ - Record drawings (as-installed)
+ - Grounding continuity test reports
+default:
+ - Record drawings (as-installed)
+ - Grounding continuity test reports
+```
+
+# Quality Assurance {toc}
+
+## Wireways, auxiliary gutters, and associated fittings shall be products of a single manufacturer to the greatest extent practicable so that fittings, covers, and finishes mate without field modification. {note}
+
+## All troughs and fittings shall be UL 870 listed and bear the listing mark, except field-fabricated auxiliary gutters permitted under NEC 366.6.
+
+## The installer shall be experienced in the installation of enclosed raceway and supplemental enclosure systems of the type and size specified.
+
+## Conductor fill and ampacity derating calculations shall be prepared and stamped by, or under the direction of, the Engineer of Record where required by the AHJ. {note}
+
+## UL 870 (9th Edition, 2016, reaffirmed as an ANSI standard in 2023 with no technical changes) is the single product safety standard covering both wireways and auxiliary gutters and their fittings. {note}
+
+```datasheet
+label: Product listing requirement
+type: radio
+options:
+ - UL 870 listed (factory assembly)
+ - UL 870 listed troughs with field-fabricated gutter permitted (NEC 366.6)
+default: UL 870 listed (factory assembly)
+```
+
+# Environmental and Service Conditions {toc}
+
+## Enclosure selection shall match the environment of each installed location, because the NEMA enclosure type controls protection against water, dust, and corrosion and a mismatch is a common field deficiency discovered after rough-in. {note}
+
+## NEMA type definitions for wireway and auxiliary gutter selection: Type 1 is general-purpose indoor, suitable for clean dry electrical rooms only (not wet, damp, or outdoor); Type 3R is rain-tight for outdoor use; Type 4 is watertight; Type 4X is watertight with corrosion resistance for washdown and corrosive atmospheres; Type 12 is dust-tight and drip-tight for industrial interiors. {note}
+
+## Wireways and auxiliary gutters installed in wet or damp locations shall be NEMA Type 3R minimum.
+
+## Wireways and auxiliary gutters installed in corrosive atmospheres shall be NEMA Type 4X stainless steel or fiberglass.
+
+## Troughs installed in mechanical rooms or in proximity to plumbing subject to leaks or condensation shall not be NEMA Type 1.
+
+```datasheet
+label: NEMA enclosure type
+type: radio
+options:
+ - Type 1 (indoor, general purpose)
+ - Type 3R (outdoor, rain-tight)
+ - Type 4 (watertight)
+ - Type 4X (watertight, corrosion-resistant)
+ - Type 12 (dust-tight industrial)
+default: Type 1 (indoor, general purpose)
+```
+
+```datasheet
+label: Installed-location ambient
+type: radio
+options:
+ - Indoor, dry electrical/equipment room
+ - Indoor, damp or industrial process area
+ - Outdoor, exposed to weather
+ - Corrosive or washdown environment
+default: Indoor, dry electrical/equipment room
+```
+
+# Construction and Materials {toc}
+
+## Construction material is the primary durability decision and is driven by the installed environment and by mechanical exposure, not by cost alone. {note}
+
+## Steel construction (painted or galvanized) is the standard for indoor and most outdoor work; nonmetallic PVC suits mildly corrosive or nonconductive-requirement areas at smaller sizes; fiberglass suits aggressive corrosive environments where metal would degrade. {note}
+
+## Metal wireways and gutters shall be fabricated from sheet steel with a corrosion-resistant finish, either factory paint over phosphate pretreatment or hot-dip galvanizing.
+
+## Nonmetallic wireways shall be rigid PVC or fiberglass reinforced polyester listed under NEC Article 378.
+
+## Nonmetallic wireways shall include a separate equipment grounding conductor, because the nonmetallic enclosure cannot serve as the equipment grounding path.
+
+## Interior surfaces and all edges over which conductors pass shall be free of burrs and sharp edges that could damage conductor insulation.
+
+```datasheet
+label: Construction material
+type: radio
+options:
+ - Steel, ANSI 61 gray painted
+ - Steel, hot-dip galvanized
+ - Stainless steel
+ - PVC (nonmetallic)
+ - Fiberglass (FRP, nonmetallic)
+default: Steel, ANSI 61 gray painted
+```
+
+```datasheet
+label: Finish / coating
+type: select
+options:
+ - ANSI 61 gray paint
+ - Powder coat
+ - Hot-dip galvanized
+ - Stainless (no coating)
+ - Fiberglass (integral color)
+default: ANSI 61 gray paint
+```
+
+## Voltage Rating {toc}
+
+### Standard wireways and auxiliary gutters are rated 600 V; products listed for 1000 V are available for 1000 V distribution systems and shall be specified where the system voltage exceeds 600 V. {note}
+
+### The trough voltage rating shall equal or exceed the nominal system voltage of the conductors it encloses.
+
+```datasheet
+label: Voltage rating
+type: radio
+options:
+ - 600 V
+ - 1000 V
+default: 600 V
+unit: V
+```
+
+## Cross-Sectional Size {toc}
+
+### The interior cross-section shall be sized so that the conductor fill does not exceed the limit set by the governing NEC article for the segment classification. {note}
+
+### Specifying a cross-section without also specifying section length and a fittings bill of material leads to quantity RFIs; the schedule shall give width, height, lengths, and fittings together. {note}
+
+### The trough cross-section shall be selected so that the design conductor fill does not exceed 20% in a wireway, or 20% conductors-only (75% with splices and taps) in an auxiliary gutter.
+
+### Cross-section size shall be coordinated with the BIM/coordination model, because wide low-profile troughs commonly conflict with ductwork, piping, and cable tray at shared ceiling elevations.
+
+```datasheet
+label: Cross-sectional size (W x H)
+type: select
+options:
+ - 2 x 2 in.
+ - 4 x 4 in.
+ - 4 x 6 in.
+ - 4 x 8 in.
+ - 6 x 6 in.
+ - 6 x 8 in.
+ - 6 x 10 in.
+ - 8 x 8 in.
+ - 8 x 10 in.
+ - 8 x 12 in.
+ - 10 x 10 in.
+ - 12 x 12 in.
+default: 6 x 6 in.
+```
+
+```datasheet
+label: Section length
+type: select
+options:
+ - 24 in.
+ - 36 in.
+ - 48 in.
+ - 60 in.
+ - 72 in.
+default: 36 in.
+unit: in.
+```
+
+## Cover Type {toc}
+
+### Cover style governs how conductors are installed and how readily splices and taps can be reached for inspection and future work. {note}
+
+### Lay-in (hinged) covers allow conductors to be laid into the trough from the side and are preferred for equipment lineups with frequent additions; screw covers and lift-off covers suit fixed runs and locations requiring tamper resistance. {note}
+
+### Covers at splice and tap points shall be removable or hinged to provide the access required by NEC 376.56 (wireways) or 366.56 (auxiliary gutters).
+
+```datasheet
+label: Cover type
+type: radio
+options:
+ - Hinged (lay-in)
+ - Screw cover
+ - Lift-off cover
+default: Hinged (lay-in)
+```
+
+# Conductor Fill and Ampacity {toc}
+
+## Conductor fill and ampacity derating are the two calculations most often misapplied on wireway and gutter work, and they differ by classification; getting the classification right is therefore a precondition to getting the fill right. {note}
+
+## In a wireway (Article 376/378), the sum of the cross-sectional areas of all contained conductors shall not exceed 20% of the interior cross-sectional area of the wireway.
+
+## In an auxiliary gutter (Article 366), conductors alone shall not exceed 20% of the interior cross-sectional area, and conductors together with splices and taps shall not exceed 75%.
+
+## The design fill target should not exceed 15% for wireways and 50% for auxiliary gutters with splices, to preserve margin for future conductor additions.
+
+## Where more than 30 current-carrying conductors occupy any cross-section of a wireway or auxiliary gutter, the ampacity adjustment factors of NEC Table 310.15(C)(1) shall be applied.
+
+## At 40 current-carrying conductors in a single cross-section, the Table 310.15(C)(1) adjustment factor is 50%, which halves allowable ampacity; this derating is frequently missed in long equipment lineups carrying many grouped branch and control circuits. {note}
+
+## Conductors that pass through partitions or that leave the wireway or gutter shall be protected by smooth rounded bushings or grommets in accordance with NEC 376.56 / 366.56.
+
+```datasheet
+label: Design conductor fill target
+type: range
+unit: "%"
+min: 10
+max: 75
+step: 5
+default: 15
+```
+
+```datasheet
+label: Current-carrying conductor count (per cross-section)
+type: range
+unit: conductors
+min: 3
+max: 60
+step: 1
+default: 20
+```
+
+```datasheet
+label: Ampacity adjustment applied (Table 310.15(C)(1))
+type: radio
+options:
+ - None required (3 CCC or fewer)
+ - 80% (4-6 CCC)
+ - 70% (7-9 CCC)
+ - 50% (10-20 CCC)
+ - 45% (21-30 CCC)
+ - 40% (31-40 CCC)
+ - 50% (>40 CCC)
+default: None required (3 CCC or fewer)
+```
+
+# Fittings {toc}
+
+## Fittings complete the routing geometry of a wireway or gutter system and shall be listed components of the same product family as the straight sections to ensure dimensional and finish compatibility. {note}
+
+## Direction changes, intersections, size transitions, and terminations shall be made with manufactured fittings matching the trough material, size, and NEMA type.
+
+## Fittings shall include, as required by the layout, elbows (30°, 45°, 90°), tee fittings, cross fittings, size reducers, end caps, flanged couplings, and expansion fittings.
+
+## Field-cut openings shall not be used in place of manufactured reducers or fittings where a listed fitting is available for the transition.
+
+```datasheet
+label: Fittings required (select all that apply)
+type: checkbox
+options:
+ - 90° elbow
+ - 45° elbow
+ - 30° elbow
+ - Tee
+ - Cross
+ - Reducer
+ - End cap / closure
+ - Flanged coupling
+ - Expansion fitting
+default:
+ - 90° elbow
+ - Tee
+ - End cap / closure
+ - Flanged coupling
+```
+
+## Expansion Fittings {toc}
+
+### Nonmetallic wireways expand and contract significantly more than steel and require more frequent expansion fittings to prevent buckling or joint failure. {note}
+
+### Long straight runs subject to thermal cycling shall be provided with expansion fittings.
+
+### Expansion fittings shall be provided in nonmetallic wireway runs at intervals not exceeding the manufacturer's recommendation, approximately every 10 ft (3 m) where not otherwise specified.
+
+```datasheet
+label: Expansion fittings provided
+type: radio
+options:
+ - Not required (short metal run)
+ - Provided per thermal calculation (metal)
+ - Provided per manufacturer interval (nonmetallic)
+default: Not required (short metal run)
+```
+
+# Grounding and Bonding {toc}
+
+## Metal wireways and auxiliary gutters serve as part of the equipment grounding path only if the metal-to-metal continuity across every joint is maintained, which paint and plating at section joints can interrupt. {note}
+
+## Section joints shall be bonded so that the assembly provides a continuous, low-impedance equipment grounding path in accordance with NEC 376.70 and the project [[sync/grounding-and-bonding]] requirements.
+
+## Bonding jumpers shall be installed across joints where paint, plating, or gasketing would otherwise interrupt metal continuity.
+
+## Nonmetallic wireways shall contain a separate equipment grounding conductor sized per NEC 250.122 for the largest circuit within the trough.
+
+## The equipment grounding path of the installed assembly shall be field-verified for continuity in accordance with NEC 250.96.
+
+```datasheet
+label: Equipment grounding method
+type: radio
+options:
+ - Metal enclosure with bonded joints (EGC path)
+ - Metal enclosure plus supplemental EGC
+ - Nonmetallic enclosure with separate EGC
+default: Metal enclosure with bonded joints (EGC path)
+```
+
+# Testing {toc}
+
+## Factory and field testing confirm that the assembly is electrically safe and that the grounding path is continuous before energization. {note}
+
+## Listed assemblies shall have passed the UL 870 factory tests, including dielectric voltage-withstand, grounding path continuity, cover retention, and corrosion resistance.
+
+## The installed equipment grounding conductor and bonded metal path shall be field-tested for continuity in accordance with NEC 250.96 prior to energization.
+
+## Any joint failing the continuity test shall be re-bonded and re-tested until continuity is confirmed.
+
+```datasheet
+label: Field tests required
+type: checkbox
+options:
+ - EGC / bonded-path continuity (NEC 250.96)
+ - Visual inspection of fill and access
+default:
+ - EGC / bonded-path continuity (NEC 250.96)
+ - Visual inspection of fill and access
+```
+
+# Installation {toc}
+
+## Support spacing differs by classification and material and is a code requirement, not a convenience; under-supporting a run is a common inspection failure. {note}
+
+## Metal wireways shall be supported horizontally at intervals not exceeding 5 ft (10 ft maximum where the wireway is designed and listed for the greater spacing), and vertically at intervals not exceeding 15 ft (NEC 376.30).
+
+## Nonmetallic wireways shall be supported horizontally at intervals not exceeding 3 ft (10 ft maximum where listed), and vertically at intervals not exceeding 4 ft (NEC 378.30).
+
+## Auxiliary gutters shall be supported at intervals not exceeding 5 ft throughout their entire length (NEC 366.30).
+
+## An auxiliary gutter shall not extend more than 30 ft (9 m) beyond the equipment it supplements; runs exceeding 30 ft shall be reclassified and installed as a wireway raceway (NEC 366.12).
+
+## Wireways and gutters shall be installed so that covers remain accessible for the entire length to permit conductor inspection and future additions.
+
+## Penetrations and openings not used shall be closed with listed closure plates to maintain the NEMA enclosure rating.
+
+## Where a trough enters or adjoins switchgear, switchboards, panelboards, or motor control centers, the connection shall be coordinated with the equipment manufacturer's wiring space and listed couplings used; see [[sync/low-voltage-switchboards]] and [[sync/motor-control-centers]].
+
+```datasheet
+label: Support method
+type: radio
+options:
+ - Ceiling trapeze hanger
+ - Wall bracket
+ - Floor stand / support
+ - Equipment-mounted (adjoining lineup)
+default: Ceiling trapeze hanger
+```
+
+```datasheet
+label: Maximum horizontal support spacing
+type: select
+options:
+ - 3 ft (nonmetallic wireway)
+ - 5 ft (metal wireway / auxiliary gutter)
+ - 10 ft (listed for greater spacing)
+default: 5 ft (metal wireway / auxiliary gutter)
+unit: ft
+```
+
+## Routing Coordination {toc}
+
+### Routing, extents, and arrangement of wireway and gutter runs are location-specific and shall be taken from the drawings, not fixed by this standard. {note}
+
+### Wireway and auxiliary gutter routing shall follow the contract drawings [[drawing: wireway routing plan]].
+
+### Equipment lineup adjacencies and gutter extension limits shall be coordinated with the equipment elevations [[drawing: equipment lineup elevation]].
+
+# Delivery, Storage, and Handling {toc}
+
+## Wireways, gutters, and fittings shall be delivered in the manufacturer's original packaging with listing labels intact.
+
+## Materials shall be stored indoors or under cover, protected from weather, dust, and physical damage until installation.
+
+## Nonmetallic and fiberglass sections shall be protected from prolonged ultraviolet exposure during storage unless rated for outdoor use.
+
+# Warranty {toc}
+
+## The manufacturer shall warrant the wireways, auxiliary gutters, and fittings against defects in materials and workmanship for the manufacturer's standard period from the date of substantial completion.
+
+## The Contractor shall correct defects in installation, including failed joint bonding and inadequate support, for the project warranty period at no cost to the Owner.
+
+```datasheet
+label: Warranty period
+type: radio
+options:
+ - 1 year
+ - 2 years
+ - 5 years
+default: 1 year
+```
+
+# Spare Parts {toc}
+
+## Spare covers, fittings, and closure plates allow minor field changes and repairs without a new procurement cycle. {note}
+
+## The Contractor shall furnish spare end caps, splice plates, and cover sections as required by the project, of the same material and NEMA type as installed.
+
+```datasheet
+label: Spare parts furnished
+type: checkbox
+options:
+ - Spare cover sections
+ - Spare end caps / closures
+ - Spare splice plates
+ - Spare couplings
+default:
+ - Spare end caps / closures
+ - Spare splice plates
+```