Corpus sync: neutrality remakes, note hygiene, datasheet relocation, transformer-split cross-refs
---
title: Secondary Unit Substations
−category: Power Distribution Equipment
+category: Electrical / Power Distribution Equipment
toc_depth: 3
description: >
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industrial, campus, water/wastewater, data-center, and healthcare sites where load
concentration justifies transforming close to the load.
− Not intended for: stand-alone distribution transformers (use [[sync/transformers]]);
+ Not intended for: stand-alone distribution transformers (use [[sync/low-voltage-dry-type-transformers]] or [[sync/medium-voltage-transformers]]);
stand-alone MV switchgear not integrated in a package (use [[sync/medium-voltage-switchgear]]);
600 V service entrance from the utility (use [[sync/electrical-service-entrance]]);
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## The transformer section, because it is mechanically integrated into the substation, is governed by IEEE C57.12.55 (dry-type) or IEEE C57.12.36 (liquid-immersed) rather than the general standalone transformer standards. {note}
−## This is the key distinction from [[sync/transformers]], which covers standalone pad-mounted and dry-type units. {note}
+## This is the key distinction from [[sync/low-voltage-dry-type-transformers]] and [[sync/medium-voltage-transformers]], which cover standalone dry-type and pad-mounted units. {note}
## The transformer section shall not be specified to IEEE C57.12.00 or IEEE C57.12.01 alone; the integration requirements of IEEE C57.12.55 (dry-type) or IEEE C57.12.36 (liquid-immersed) shall also apply.
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## The assembly shall be the product of a single manufacturer; the primary section, transformer, and secondary section shall not be sourced from separate vendors and field-married.
−## The complete assembly shall carry type-tested short-circuit withstand ratings established by tests on the integrated lineup, not by component ratings combined on paper. {note}
+## The complete assembly shall carry type-tested short-circuit withstand ratings established by tests on the integrated lineup, not by component ratings combined on paper.
## A transformer rated to withstand a through-fault and a switchgear bus rated for a fault current do not, when bolted together, automatically produce a type-tested assembly. The single-source requirement exists precisely so that one manufacturer certifies the integrated withstand and the arc-flash labels. {note}
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## The assembly shall be rated for continuous operation at the site altitude, ambient temperature range, and humidity without derating below the specified ratings.
−## Where site altitude exceeds 1000 m, transformer and switchgear ratings shall be derated per the applicable IEEE standard.
−
−## Outdoor enclosures shall be rated for the design wind, ice, and solar loading.
−
−## Outdoor enclosures shall include space heaters with humidistat or thermostat control to prevent internal condensation.
−
−## The assembly shall be seismically qualified for the site Risk Category and the site Ss and S1 spectral acceleration values per ASCE 7-22 Chapter 13 and certified per ICC-ES AC156. {note}
−
−## A unit substation weighs between roughly 5,000 lb and 50,000 lb. Seismic anchorage, the housekeeping pad, and the anchor-bolt pattern must be coordinated with the structural engineer early; omitting this coordination routinely produces RFIs and delays at delivery. {note}
−
```datasheet
label: Ambient temperature range (design)
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max: 50
step: 5
−setpoints: [-30, 40]
+setpoints: [-30, 40, 50]
```
+## Where site altitude exceeds 1000 m, transformer and switchgear ratings shall be derated per the applicable IEEE standard.
+
```datasheet
label: Site altitude above which derating applies
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```
+## Outdoor enclosures shall be rated for the design wind, ice, and solar loading.
+
+## Outdoor enclosures shall include space heaters with humidistat or thermostat control to prevent internal condensation.
+
+## The assembly shall be seismically qualified for the site Risk Category and the site Ss and S1 spectral acceleration values per ASCE 7-22 Chapter 13 and certified per ICC-ES AC156. {note}
+
+## A unit substation weighs between roughly 5,000 lb and 50,000 lb. Seismic anchorage, the housekeeping pad, and the anchor-bolt pattern must be coordinated with the structural engineer early; omitting this coordination routinely produces RFIs and delays at delivery. {note}
+
+## Seismic qualification of the unit substation shall be specified, and its anchorage and housekeeping pad shall be coordinated with the structural engineer for the assembly weight and site seismic demand.
+
```datasheet
label: Seismic qualification (ICC-ES AC156)
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## The primary section shall be rated for a maximum system voltage and a momentary and interrupting capability equal to or greater than the available fault current at the point of connection.
−## A fused interrupter primary shall use current-limiting power fuses coordinated with the transformer inrush and the downstream protection.
−
−## A metal-clad primary shall use drawout vacuum circuit breakers with multifunction protective relaying and provision for remote racking where arc-flash incident energy warrants.
−
−## Where required by the Owner or the facility insurer, the primary section shall be of arc-resistant construction tested to IEEE C37.20.7 accessibility type. {note}
−
−## Arc-resistant construction redirects the products of an internal arcing fault away from personnel. On medium-voltage sections the incident energy is high enough that arc-resistant equipment, remote racking, or both are frequently the only practical way to bring the labeled PPE category to a workable level. {note}
−
```datasheet
−label: Primary section type
−type: radio
−options:
− - Fused metal-enclosed interrupter switch
− - Metal-clad drawout vacuum circuit breaker
−default: Fused metal-enclosed interrupter switch
−```
−
−```datasheet
label: Primary voltage class
type: radio
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```
+## A fused interrupter primary shall use current-limiting power fuses coordinated with the transformer inrush and the downstream protection.
+
+## A metal-clad primary shall use drawout vacuum circuit breakers with multifunction protective relaying and provision for remote racking where arc-flash incident energy warrants.
+
+## Where required by the Owner or the facility insurer, the primary section shall be of arc-resistant construction tested to IEEE C37.20.7 accessibility type. {note}
+
+## Arc-resistant construction redirects the products of an internal arcing fault away from personnel. On medium-voltage sections the incident energy is high enough that arc-resistant equipment, remote racking, or both are frequently the only practical way to bring the labeled PPE category to a workable level. {note}
+
+## The primary section type shall be specified, selected to achieve the interrupting capability and arc-flash incident energy reduction the application requires.
+
```datasheet
+label: Primary section type
+type: radio
+options:
+ - Fused metal-enclosed interrupter switch
+ - Metal-clad drawout vacuum circuit breaker
+default: Fused metal-enclosed interrupter switch
+```
+
+## Arc-resistant primary construction shall be specified where needed to bring the labeled arc-flash incident energy to a workable personal-protective-equipment category.
+
+```datasheet
label: Arc-resistant primary construction (IEEE C37.20.7)
type: radio
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## The secondary section short-circuit current rating shall equal or exceed the available symmetrical fault current at the transformer secondary terminals as established by the fault-current study.
−## The secondary bus shall be rated for the full forced-cooled output of the transformer plus the specified design margin; the bus ampacity cannot be increased later without replacing the section. {note}
+## The secondary bus shall be rated for the full forced-cooled output of the transformer plus the specified design margin; the bus ampacity cannot be increased later without replacing the section.
## A transformer can be uprated by adding fans, but a 2000 A secondary bus cannot be field-converted to 3000 A. Size the bus for the ultimate transformer rating, not merely for today's load. {note}
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# Metering and Instrumentation {toc}
−## Metering type and current-transformer ratios shall be specified in the contract documents; leaving the choice to the contractor is not permitted because the metering compartment is sized for the selected configuration and cannot be changed in the field. {note}
+## Metering type and current-transformer ratios shall be specified in the contract documents; leaving the choice to the contractor is not permitted because the metering compartment is sized for the selected configuration and cannot be changed in the field.
## Revenue metering, sub-metering, and power-quality monitoring each demand different CT ratios and meter form factors. An omitted metering specification is discovered only when the meter will not fit the compartment. {note}
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# Installation {toc}
−## The assembly shall be set on a level concrete housekeeping pad sized and reinforced for the assembly weight, with the anchor-bolt pattern matching the approved template and the seismic anchorage detail. {note}
+## The assembly shall be set on a level concrete housekeeping pad sized and reinforced for the assembly weight, with the anchor-bolt pattern matching the approved template and the seismic anchorage detail.
## The civil and structural coordination - pad height, anchor pattern, and seismic anchorage for an assembly weighing up to 50,000 lb - must be settled before the housekeeping pad is poured. This is among the most frequent sources of field RFIs on this equipment. {note}
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# Warranty {toc}
−## The manufacturer shall warrant the complete assembly - all three sections - against defects in material and workmanship for not less than the specified period from energization or beneficial use. {note}
+## The manufacturer shall warrant the complete assembly - all three sections - against defects in material and workmanship for not less than the specified period from energization or beneficial use.
## A single-source warranty on the integrated assembly is the practical payoff of the single-manufacturer requirement: there is one responsible party for the whole substation, with no finger-pointing between a transformer vendor and a switchgear vendor when a fault reveals a coordination gap. {note}
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default: [One set of primary power fuses per rating (fused primary), Spare indicating lamps and control fuses, Touch-up finish paint]
```