Busway
Corpus sync: neutrality remakes, note hygiene, datasheet relocation, transformer-split cross-refs
Showing changes from Rev 2
to Rev 3
in Busway.
---
title: Busway
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- 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 [[sync/low-voltage-switchboards]], [[sync/low-voltage-panelboards]])
+- The bus connections internal to switchboards, panelboards, transformers, or generators where busway terminates, which are governed by the standard for that terminating equipment (see [[sync/low-voltage-switchboards]], [[sync/panelboards]])
−## Related distribution and protection requirements are addressed in [[sync/low-voltage-switchboards]], [[sync/low-voltage-panelboards]], [[sync/conductors-and-cables]], [[sync/grounding-and-bonding]], and [[sync/equipment-labeling]]. {note}
+## Related distribution and protection requirements are addressed in [[sync/low-voltage-switchboards]], [[sync/panelboards]], [[sync/conductors-and-cables]], [[sync/grounding-and-bonding]], and [[sync/equipment-labeling]]. {note}
# Referenced Standards {toc}
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# Environmental and Service Conditions {toc}
−## 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. {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.
−## Standard busway is rated for operation in a maximum ambient air temperature of 40 °C unless a higher-ambient rating is specified.
−
−## 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.
−
−## 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.
−
−## Outdoor and wet-location busway shall use a weatherproof enclosure listed for the location, with drain provisions and gasketed joints.
−
−## Select the enclosure type to match the installed environment; an indoor feeder housing relocated outdoors will admit water at the joints and fail. {note}
−
```datasheet
label: Installation Environment
…7 unchanged lines
```
+## Standard busway is rated for operation in a maximum ambient air temperature of 40 °C unless a higher-ambient rating is specified.
+
```datasheet
label: Maximum Ambient Air Temperature
…6 unchanged lines
```
+## 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.
+
+## 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.
+
+## Outdoor and wet-location busway shall use a weatherproof enclosure listed for the location, with drain provisions and gasketed joints.
+
```datasheet
label: Enclosure Type
…7 unchanged lines
```
+## Select the enclosure type to match the installed environment; an indoor feeder housing relocated outdoors will admit water at the joints and fail. {note}
+
# Ratings and Electrical Characteristics {toc}
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### 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.
−### 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. {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.
## Short-Circuit Current Rating {toc}
### 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.
−### 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.
−
−### 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. {note}
−
−### The SCCR shall be expressed in symmetrical RMS amperes and shall be referenced to the fault-clearing time of the upstream overcurrent device.
−
−### 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. {note}
−
```datasheet
label: Short-Circuit Current Rating (SCCR)
…8 unchanged lines
```
+### 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.
+
+### 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. {note}
+
+### The SCCR shall be expressed in symmetrical RMS amperes and shall be referenced to the fault-clearing time of the upstream overcurrent device.
+
```datasheet
label: Bracing Basis (Fault-Clearing Time)
…5 unchanged lines
```
+### 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. {note}
+
### The available fault current at the busway origin shall be confirmed against [[drawing: one-line diagram fault values]] before the SCCR is finalized.
−# Construction {toc}
+# Bus Bars {toc}
−## Bus Bars {toc}
+## Bus bars shall be of the specified conductor material, of rectangular cross-section, and sized for the continuous rating at the rated temperature rise.
−### Bus bars shall be of the specified conductor material, of rectangular cross-section, and sized for the continuous rating at the rated temperature rise.
+## 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.
−### 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.
+## Joint plating prevents the contact resistance from rising as the bare base metal oxidizes; an unplated joint runs hot and loosens over time. {note}
−### Joint plating prevents the contact resistance from rising as the bare base metal oxidizes; an unplated joint runs hot and loosens over time. {note}
+## 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.
−### 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.
+## Bus bar insulation shall be a flame-retardant material with a temperature rating not less than the maximum bus operating temperature.
−### Bus bar insulation shall be a flame-retardant material with a temperature rating not less than the maximum bus operating temperature.
−
```datasheet
label: Joint Contact Plating
…15 unchanged lines
```
−## Housing {toc}
+# Housing {toc}
−### 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.
+## 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.
−### 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. {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. {note}
−### The housing finish shall be a corrosion-resistant coating suitable for the installed environment.
+## The housing finish shall be a corrosion-resistant coating suitable for the installed environment.
−### 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.
+## 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.
```datasheet
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```
−## Grounding {toc}
+# Grounding {toc}
−### 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.
+## 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.
−### Where the served load is sensitive electronic equipment, a separate insulated (isolated) ground bus bar shall be provided in addition to the housing ground.
+## Where the served load is sensitive electronic equipment, a separate insulated (isolated) ground bus bar shall be provided in addition to the housing ground.
−### 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. {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. {note}
−### Grounding and bonding of the busway shall comply with [[sync/grounding-and-bonding]] and NFPA 70 Article 250.
+## Grounding and bonding of the busway shall comply with [[sync/grounding-and-bonding]] and NFPA 70 Article 250.
```datasheet
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```
−## Fittings {toc}
+# Fittings {toc}
−### Elbows, tees, offsets, and transformer or switchboard taps shall be factory-fabricated fittings of the same series, rating, and listing as the straight busway.
+## Elbows, tees, offsets, and transformer or switchboard taps shall be factory-fabricated fittings of the same series, rating, and listing as the straight busway.
−### Field-fabricated fittings are prohibited; the bracing and joint geometry of a busway run are validated only as a manufactured assembly.
+## Field-fabricated fittings are prohibited; the bracing and joint geometry of a busway run are validated only as a manufactured assembly.
−### End closures (dead ends) shall be installed at every unused open end of the busway to maintain the enclosure and isolate the live bus.
+## End closures (dead ends) shall be installed at every unused open end of the busway to maintain the enclosure and isolate the live bus.
−### 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.
+## 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.
−### 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. {note}
−
```datasheet
label: Phase Transposition Section
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```
−## Expansion Fittings {toc}
+## 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. {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. {note}
+# Expansion Fittings {toc}
−### Expansion fittings shall be provided on long straight runs at the manufacturer's maximum spacing for the expected temperature differential.
+## Busway expands and contracts with temperature and with building movement; without an accommodation the joints carry the resulting stress and eventually loosen or overheat. {note}
−### Expansion fittings shall be provided where a busway run crosses a building expansion joint, in accordance with NFPA 70 Section 300.7(B).
+## Expansion fittings shall be provided on long straight runs at the manufacturer's maximum spacing for the expected temperature differential.
−### 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. {note}
−
```datasheet
label: Expansion Fitting Spacing (long straight runs)
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```
+## Expansion fittings shall be provided where a busway run crosses a building expansion joint, in accordance with NFPA 70 Section 300.7(B).
+
```datasheet
label: Expansion Fitting at Building Expansion Joint
…5 unchanged lines
```
+## 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. {note}
+
# Plug-In Units {toc}
−## 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. {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.
## 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.
−## 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.
−
−## 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.
−
−## 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. {note}
−
−## Plug-in units shall include an interlock that prevents insertion or removal while the unit is in the ON position.
−
−## 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. {note}
−
```datasheet
label: Plug-In Unit Disconnect Type
…5 unchanged lines
```
+## 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.
+
```datasheet
label: Fusible Unit Fuse Class
…6 unchanged lines
```
+## 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.
+
```datasheet
label: Plug-In Unit Ampere Rating
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```
+## 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. {note}
+
+## Plug-in units shall include an interlock that prevents insertion or removal while the unit is in the ON position.
+
+## 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. {note}
+
+## The number of poles for each plug-in unit shall be specified to match the phase configuration of the load it serves.
+
```datasheet
label: Plug-In Unit Poles
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# Installation {toc}
−## General {toc}
+## Busway shall be installed in accordance with NFPA 70 Article 368, NEMA BU 1.1, and the manufacturer's installation instructions.
−### Busway shall be installed in accordance with NFPA 70 Article 368, NEMA BU 1.1, and the manufacturer's installation instructions.
+## 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.
−### 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.
+## 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.
−### 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.
+## The complete routing, mounting elevations, and fitting arrangement shall follow [[drawing: busway routing and elevations]].
−### The complete routing, mounting elevations, and fitting arrangement shall follow [[drawing: busway routing and elevations]].
−
## Support and Suspension {toc}
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### 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.
−### 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. {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.
### 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.
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# Spare Parts {toc}
−## 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. {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.
## The Contractor shall furnish spare plug-in opening covers, joint hardware kits, and antioxidant compound where aluminum bus is specified.
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- Joint hardware kits
```