1 Scope
NOTE This standard covers the materials, ratings, construction, testing, and installation of busway systems rated 600 V or less and 6000 A or less for power distribution. (1.1)
NOTE Busway is a prefabricated assembly of bus bars, supports, and an enclosing housing, manufactured in standard straight lengths and fittings that bolt together to form a continuous distribution run. Compared with cable in raceway, busway delivers high current in a compact cross-section, dissipates heat efficiently, and allows load tap-off at intervals without cutting conductors, which makes it the preferred method for risers and high-density feeders. (1.2)
1.3 Busway shall comply with UL 857 and the applicable provisions of NEMA BU 1 for the listed type, voltage, and ampere rating specified.
NOTE This standard applies to feeder busway, plug-in busway, and the associated fittings, elbows, tees, expansion fittings, end closures, and plug-in tap units that complete the system. (1.4)
NOTE This standard does not cover the following, which are specified elsewhere: (1.5)
- Medium-voltage metal-enclosed bus above 600 V, which uses a different listing, dielectric design, and short-circuit basis (governed by IEEE C37.23)
- Low-ampere lighting busway (trolley duct), whose construction, ratings, and tap method differ fundamentally from power busway
- The bus connections internal to switchboards, panelboards, transformers, or generators where busway terminates, which are governed by the standard for that terminating equipment (see Low Voltage Switchboards, Low Voltage Panelboards)
2 Referenced Standards
NOTE The following standards are referenced in this document; the edition adopted by the Authority Having Jurisdiction governs where an edition conflict exists. (2.1)
| Standard |
Title |
| UL 857 |
Standard for Busway |
| NEMA BU 1 |
Busways, 600 V or Less |
| NEMA BU 1.1 |
Handling, Installation, Operation, and Maintenance of Busway |
| NFPA 70 |
National Electrical Code (Article 368 — Busways; Article 300 — Wiring Methods) |
| NFPA 70E |
Standard for Electrical Safety in the Workplace |
| IEEE C37.23 |
Guide for Metal-Enclosed Bus |
| IBC |
International Building Code |
| ASCE 7 |
Minimum Design Loads and Associated Criteria for Buildings and Other Structures |
3 Submittals
3.1 Action Submittals
3.1.1 The Contractor shall submit the following action submittals for review before fabrication:
- Product data for each busway type, including ampere rating, voltage rating, conductor material, and listing
- Short-circuit current rating (SCCR) and bracing certification for the busway and all fittings
- Shop drawings showing the complete routing, fitting schedule, dimensions, and support locations
- Plug-in unit schedule identifying each unit by ampere rating, device type, and pole configuration
- Coordination drawings reconciling busway routing with structure, piping, and ductwork
- Manufacturer's installation instructions, including joint torque values and re-torque schedule
- Seismic anchorage and bracing calculations where required by the seismic design category
☑ Product data (rating, voltage, material, listing)
☑ SCCR and bracing certification
☑ Shop drawings (routing, fittings, supports)
☑ Plug-in unit schedule
☐ Coordination drawings
☑ Installation instructions with torque values
☐ Seismic anchorage and bracing calculations
3.1.2 Shop drawings shall be coordinated against busway riser and plan routing before fabrication so that field-measured lengths and fitting locations are confirmed. 3.2 Closeout Submittals
3.2.1 The Contractor shall submit the following closeout submittals before final acceptance:
- Record drawings showing as-installed routing, fitting locations, and plug-in unit positions
- Field test reports for insulation resistance and joint torque verification
- Manufacturer's operation and maintenance manuals
- Re-torque verification record after the first thermal cycle
- Warranty documentation
☑ Record drawings (as-installed)
☑ Insulation resistance test reports
☑ Joint torque verification reports
☑ Operation and maintenance manuals
☑ Re-torque verification after first thermal cycle
☑ Warranty documentation
3.3.1 The Contractor shall submit the following informational submittals:
- Manufacturer's certification of compliance with UL 857 and NEMA BU 1
- Short-circuit study correlating available fault current with the specified SCCR
- Qualification statements for the installing personnel
- Field quality-control test plan
☑ Certification of compliance (UL 857, NEMA BU 1)
☑ Short-circuit study correlation
☐ Installer qualification statements
☑ Field quality-control test plan
4 Quality Assurance
4.1 Busway and all associated fittings shall be products of a single manufacturer and a single product series, so that joint geometry, bracing, and plug-in interfaces are mutually compatible.
NOTE Plug-in units are proprietary to each manufacturer's busway series and are not interchangeable between brands or product lines; specifying an "equal" without identifying the series produces field incompatibility. (4.2)
4.3 The busway and its fittings shall be listed and labeled by a Nationally Recognized Testing Laboratory under UL 857.
4.4 Each straight section, elbow, tee, and end closure shall carry the same listing as the run it serves; an outdoor run requires every fitting to share the outdoor listing, not only the straight sections.
NOTE Outdoor-rated runs are a common failure point because elbows, tees, and end closures are sometimes left at an indoor rating; verify that the weatherproof listing covers the entire assembly. (4.5)
4.6 The installing personnel shall be trained by the busway manufacturer or its authorized representative in joint assembly and torque verification for the specified product series.
4.7 Manufacturer qualification: the manufacturer shall have produced UL 857-listed busway of the specified type and rating for not less than five years.
5 Environmental and Service Conditions
NOTE Busway ratings are established at a defined ambient temperature; the specified rating must be verified against the actual installed environment, because high ambient temperature derates current-carrying capacity. (5.1)
5.2 Standard busway is rated for operation in a maximum ambient air temperature of 40 °C unless a higher-ambient rating is specified.
5.3 Where busway is routed through boiler rooms, rooftop runs in direct sun, or other elevated-ambient locations, the ampacity shall be derated or a high-ambient-rated product specified.
5.4 Indoor busway shall be installed only in dry, ventilated locations free from corrosive vapors, conductive dust, and dripping liquids unless a higher enclosure rating is specified.
5.5 Outdoor and wet-location busway shall use a weatherproof enclosure listed for the location, with drain provisions and gasketed joints.
NOTE Select the enclosure type to match the installed environment; an indoor feeder housing relocated outdoors will admit water at the joints and fail. (5.6)
Indoor, dry, ventilated
Indoor, high-ambient (>40 °C)
Outdoor, weatherproof (NEMA 3R)
Corrosive or washdown area
Ventilated indoor feeder
Ventilated indoor plug-in
Totally enclosed (non-ventilated)
Weatherproof NEMA 3R
6 Ratings and Electrical Characteristics
6.1 Voltage and System Configuration
6.1.1 The busway voltage rating shall equal or exceed the nominal system voltage at the point of installation.
NOTE Standard low-voltage busway is rated 600 V or less; systems above 600 V require medium-voltage bus outside this scope. (6.1.2)
6.1.3 The number of poles and the neutral configuration shall match the system and the load characteristics served.
208Y/120 V, 3Φ, 4W
480Y/277 V, 3Φ, 4W
480 V, 3Φ, 3W
240 V, 3Φ, 3W
600 V, 3Φ, 3W
3-pole, 3-wire (no neutral)
4-pole, 100% neutral
4-pole, 200% (double) neutral
6.1.4 Where the served load is harmonic-rich, such as data center, UPS, or VFD-heavy installations, a 200% (double) neutral shall be specified to carry triplen-harmonic currents.
NOTE Specifying a standard 100% neutral for harmonic-heavy loads is a common error; third-harmonic currents add arithmetically in the neutral and can overload a standard neutral bar. (6.1.5)
6.2 Ampere Rating
6.2.1 The continuous ampere rating shall be selected from the manufacturer's standard catalog ratings and shall equal or exceed the calculated feeder load.
NOTE Copper bus offers higher conductivity in a smaller cross-section but is heavier and more costly; aluminum bus is lighter and lower cost but requires a larger cross-section and antioxidant compound at terminations. (6.2.2)
6.2.3 The conductor material shall be specified, because it governs cross-section, weight, support spacing, and termination preparation.
225
400
600
800
1000
1200
1350
1600
2000
2500
3000
4000
5000
6.2.4 Aluminum busway terminations shall be prepared with the manufacturer's specified antioxidant compound and termination hardware; aluminum is not a plug-and-play substitute for copper.
NOTE Treating aluminum terminations as identical to copper leads to high-resistance joints and overheating; the oxide film must be abraded and inhibited at every connection. (6.2.5)
6.3 Short-Circuit Current Rating
6.3.1 The short-circuit current rating (SCCR) of the busway and its bracing shall equal or exceed the available fault current at every point of the run.
6.3.2 A system short-circuit study shall be obtained before the SCCR is selected, so that the busway bracing is coordinated with the actual available fault current.
NOTE Specifying an SCCR below the available fault current is among the most common and most dangerous errors in busway design; an under-braced bus can deform or rupture during a fault. (6.3.3)
6.3.4 The SCCR shall be expressed in symmetrical RMS amperes and shall be referenced to the fault-clearing time of the upstream overcurrent device.
NOTE Where current-limiting fuses or breakers reduce the let-through current, the reduced rating may be applied only when the protective device and its coordination are documented. (6.3.5)
7 Construction
7.1 Bus Bars
7.1.1 Bus bars shall be of the specified conductor material, of rectangular cross-section, and sized for the continuous rating at the rated temperature rise.
7.1.2 Bus bars shall be plated at all joint and plug-in contact surfaces with tin or silver to ensure a stable, low-resistance contact over the life of the system.
NOTE Joint plating prevents the contact resistance from rising as the bare base metal oxidizes; an unplated joint runs hot and loosens over time. (7.1.3)
7.1.4 Bus bars shall be supported and insulated within the housing so that they withstand the mechanical forces of the rated short-circuit current without permanent deformation.
7.1.5 Bus bar insulation shall be a flame-retardant material with a temperature rating not less than the maximum bus operating temperature.
7.2 Housing
7.2.1 The housing shall be a totally enclosing metallic enclosure of steel or aluminum that provides mechanical protection and serves as the equipment grounding path where so listed.
NOTE A ventilated housing permits convective cooling and supports a higher continuous rating; a non-ventilated housing is required where airborne contaminants or wet locations are present. (7.2.2)
7.2.3 The housing finish shall be a corrosion-resistant coating suitable for the installed environment.
7.2.4 Plug-in housing shall provide plug-in openings on one or both sides at the specified interval, each with a protective cover that maintains the enclosure rating when no unit is installed.
7.3 Grounding
7.3.1 The busway shall provide an equipment grounding path of the rated capacity, either through the housing itself where listed or through an internal ground bus bar.
7.3.2 Where the served load is sensitive electronic equipment, a separate insulated (isolated) ground bus bar shall be provided in addition to the housing ground.
NOTE An isolated ground bus keeps the clean ground reference independent of the housing, which carries circulating and fault currents; this matters for data and instrumentation loads. (7.3.3)
7.3.4 Grounding and bonding of the busway shall comply with Grounding And Bonding and NFPA 70 Article 250. Housing as equipment ground
Internal ground bus bar
Internal ground bus plus isolated ground bus
7.4 Fittings
7.4.1 Elbows, tees, offsets, and transformer or switchboard taps shall be factory-fabricated fittings of the same series, rating, and listing as the straight busway.
7.4.2 Field-fabricated fittings are prohibited; the bracing and joint geometry of a busway run are validated only as a manufactured assembly.
7.4.3 End closures (dead ends) shall be installed at every unused open end of the busway to maintain the enclosure and isolate the live bus.
7.4.4 Phase transposition or center-tapped sections shall be provided on long plug-in runs where load is consistently tapped from one side, to equalize phase loading.
NOTE Without transposition on a long one-sided plug-in run, the phase nearest the loads carries more current than the others, creating measurable phase unbalance and voltage drop differences. (7.4.5)
Not required (balanced tapping)
Required (one-sided plug-in run)
7.5 Expansion Fittings
NOTE Busway expands and contracts with temperature and with building movement; without an accommodation the joints carry the resulting stress and eventually loosen or overheat. (7.5.1)
7.5.2 Expansion fittings shall be provided on long straight runs at the manufacturer's maximum spacing for the expected temperature differential.
7.5.3 Expansion fittings shall be provided where a busway run crosses a building expansion joint, in accordance with NFPA 70 Section 300.7(B).
NOTE Omitting expansion fittings on long runs is a frequent defect that transfers thermal stress into the bolted joints; it is corrected only by re-engineering the run. (7.5.4)
☑ Required where run crosses a building expansion joint
8 Plug-In Units
NOTE Plug-in units provide overcurrent-protected load tap-off at the busway plug-in openings and shall match the busway series and the branch-circuit requirements. (8.1)
8.2 Each plug-in unit shall be furnished with a disconnect device, either a fusible switch or a molded-case circuit breaker, as specified for the served load.
8.3 Fusible plug-in units shall use the specified fuse class and ampere rating; molded-case units shall be rated for the available fault current at the plug-in point.
8.4 Plug-in unit overcurrent protection shall be sized within the limits of NFPA 70 Article 368.17 and Article 240 for the busway ampacity and the branch circuit served.
NOTE Oversizing the plug-in overcurrent device is a code violation; Article 368.17 limits the device rating relative to the busway, and the branch circuit must comply with Article 240. (8.5)
8.6 Plug-in units shall include an interlock that prevents insertion or removal while the unit is in the ON position.
NOTE The on/off interlock protects personnel during energized insertion and removal; busway is frequently live when units are added, so this interlock is a core safety feature. (8.7)
Fusible switch
Molded-case circuit breaker
Class J
Class RK1
Class RK5
30
60
100
200
400
600
800
1200
1600
9 Testing
9.1 Factory Testing
9.1.1 Each busway section shall be production-tested at the factory in accordance with UL 857, including dielectric withstand and continuity verification.
9.1.2 Certified factory test reports shall be furnished for each production run on request.
9.2 Field Testing
9.2.1 After installation and before energization, the complete busway run shall be tested for insulation resistance phase-to-phase and phase-to-ground.
9.2.2 Insulation resistance shall meet or exceed the manufacturer's minimum acceptance value for the rating and length installed.
9.2.3 All bolted joints shall be verified to the manufacturer's specified torque value before energization, and the verification recorded.
NOTE Loose joint hardware is the leading cause of busway failures; a thermal hot spot at an under-torqued joint cascades into insulation failure, so torque verification is mandatory, not optional. (9.2.4)
9.2.5 Joints shall be re-torqued after the first full thermal load cycle and the re-torque recorded, because initial heating relaxes the joint hardware.
☑ Phase-to-phase
☑ Phase-to-ground
☑ Neutral-to-ground
10 Installation
10.1 General
10.1.1 Busway shall be installed in accordance with NFPA 70 Article 368, NEMA BU 1.1, and the manufacturer's installation instructions.
10.1.2 Busway shall be installed with the housing in the orientation for which the run is rated; some products are derated when mounted on edge or flat.
10.1.3 Busway shall not be installed in hoistways, in ducts used for environmental air handling except as permitted by Article 368, or where subject to physical damage.
10.2 Support and Suspension
10.2.1 Busway shall be supported at intervals not exceeding the manufacturer's maximum spacing and not exceeding the spacing required by NFPA 70 Article 368.30.
NOTE Horizontal runs and vertical risers have different support requirements; vertical risers require spring-cushioned hangers at each floor to carry the dead weight and absorb thermal movement. (10.2.2)
10.2.3 Vertical riser busway shall be supported at each floor penetration with the manufacturer's spring-type hanger assembly.
NOTE Inadequate support spacing and missing seismic bracing are frequently missed on ceiling-mounted runs; verify hanger spacing and seismic restraint before energization. (10.2.4)
10.2.5 Where the seismic design category requires it, busway supports shall be braced and anchored in accordance with the IBC and ASCE 7.
Horizontal, flat
Horizontal, edgewise
Vertical riser
Not required
Required per IBC / ASCE 7
10.3 Penetrations
10.3.1 Where busway passes through a fire-rated floor or wall, a UL-listed fire-stop system shall be installed in accordance with NFPA 70 Section 300.21 and the IBC.
NOTE Failing to specify and install the fire-stop method at rated penetrations is a routine cause of AHJ rejection and costly field remediation; the system, not just a sleeve, must be listed for the assembly. (10.3.2)
10.3.3 Through-floor penetrations shall include a curb or barrier at least 100 mm (4 in.) high to prevent liquids from entering the floor opening, as required by NFPA 70 Article 368.10.
10.3.4 The fire-stop method shall be coordinated with the AHJ for the specific floor or wall assembly penetrated.
Listed sleeve and fire-stop system
Poke-through firestop device
Intumescent wrap system
10.4 Labeling
10.4.1 Each busway run shall be labeled at its origin with the ampere rating, voltage, phase configuration, and source identification, in accordance with Equipment Labeling. 10.4.2 Plug-in units shall be labeled with the served load identification and the source busway run.
10.4.3 Arc-flash and shock-hazard warning labels shall be applied in accordance with NFPA 70E.
11 Delivery, Storage, and Handling
11.1 Busway sections shall be delivered in the manufacturer's original protective packaging with the listing label and section identification intact.
11.2 Busway shall be stored indoors in a clean, dry, heated space and shall be protected from moisture, dust, and physical damage until installed.
NOTE Busway insulation absorbs moisture in storage; energizing a damp run can fail the dielectric, so moisture control before energization is essential. (11.3)
11.4 Busway that has been exposed to moisture shall be dried and shall pass an insulation resistance test before energization.
11.5 Busway shall be handled and rigged in accordance with NEMA BU 1.1 and the manufacturer's instructions, using only the designated lifting points.
12 Warranty
12.1 The manufacturer shall warrant the busway against defects in materials and workmanship for the specified warranty period from the date of substantial completion.
12.2 The warranty shall cover repair or replacement of defective sections, fittings, and plug-in units, including the bus bars, insulation, and joint hardware.
13 Spare Parts
NOTE The Contractor shall furnish spare plug-in units and joint hardware so that future load additions and joint maintenance do not require a long procurement lead time. (13.1)
13.2 The Contractor shall furnish spare plug-in opening covers, joint hardware kits, and antioxidant compound where aluminum bus is specified.
13.3 The Contractor shall furnish spare plug-in units of the specified ratings where future load growth is anticipated.
☑ Plug-in opening covers
☑ Joint hardware kits
☐ Antioxidant compound (aluminum bus)
☐ Spare plug-in units