NOTE A secondary unit substation is a single, factory-coordinated assembly comprising three close-coupled sections: a primary medium-voltage incoming section, a step-down transformer, and a secondary low-voltage distribution section, all furnished and warranted as one package by a single manufacturer. (1.1)

NOTE The defining characteristic is integration. Buying the three sections separately and bolting them together in the field defeats the purpose of the assembly standard and is the most common and most expensive mistake in this equipment class. (1.2)

NOTE This standard governs assemblies within the rating envelope of IEEE C37.121: primary voltage 601 V through 38 kV, transformer 112.5 kVA through 10,000 kVA three-phase, and secondary 208 V through 600 V. (1.3)

NOTE Equipment outside this envelope - a 40,000 kVA transmission substation, or a utility-owned primary substation - is not a secondary unit substation and is outside this standard. (1.4)

NOTE 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. (1.7)

NOTE This is the key distinction from Transformers, which covers standalone pad-mounted and dry-type units. (1.8)

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. (4.3)

NOTE 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. (4.4)

NOTE 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. (5.5)

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. (5.6)

NOTE The most consequential single selection in a unit substation is the primary section type: a fused metal-enclosed interrupter switch, or a metal-clad drawout circuit breaker. (6.1)

NOTE A fused load-interrupter switch (IEEE C37.20.3) is the most common, lowest-cost primary for radial systems; it clears faults with current-limiting fuses and is replaced, not reset, after a fault. A metal-clad drawout vacuum breaker (IEEE C37.20.2) provides full, resettable fault interruption, relay protection, and drawout maintenance, and is selected for looped, utility-interconnected, or critical systems. The choice drives interrupting capability, maintenance access, and cost more than any other decision. (6.2)

NOTE 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. (6.6)

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. (6.7)

NOTE The transformer type drives the fire-protection and vault requirements for the entire installation more than any other single attribute. (7.1)

NOTE A liquid-immersed transformer using flammable mineral oil generally requires a fire-rated vault indoors per NEC 450. A less-flammable dielectric fluid (natural ester or silicone) listed for indoor installation per NEC 450.23 may be installed indoors without a vault. A dry-type unit eliminates fluid entirely but is larger, louder, and limited in kVA per frame. Select the type against the fire-protection strategy, not against first cost alone. (7.2)

NOTE The transformer shall be selected at not less than 125 percent of the present connected load to provide a load-growth margin, and the impedance and kVA shall be coordinated with the secondary short-circuit current rating before the rating is fixed. (7.6)

NOTE Impedance and kVA together set the available fault current at the secondary terminals. Reducing impedance from 5.75 percent to 4 percent raises that fault current by roughly 30 percent and can force a more expensive secondary bus rating. (7.7)

NOTE A 2000 kVA dry-type transformer produces roughly 67-70 dBA at 1 m. In an occupied building, neglecting sound level and vibration isolation makes the substation room - and the spaces adjacent to it - acoustically unacceptable. (7.14)

NOTE The single most dangerous latent defect in a unit substation is a secondary section whose short-circuit current rating is below the available fault current at the transformer secondary terminals. (8.1)

NOTE This defect does not appear on a nameplate inspection: a bus stamped 65 kA looks identical to one stamped 42 kA. Only the fault study confirms adequacy. Because %Z and kVA set the available fault current, the secondary SCCR must be checked against the as-built transformer rating, not against a generic catalog figure. (8.2)

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. (8.4)

NOTE 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. (8.5)

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. (9.1)

NOTE 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. (9.2)

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. (11.1)

NOTE 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. (11.2)

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. (13.1)

NOTE 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. (13.2)