VTI UDS Series - Unitized Distribution Substation

Standalone Power Distribution Unit

The VTI Unitized Distribution Substation (UDS) is a versatile, proven and unique solution to electrical power distribution problems in industrial plants and buildings with requirements that cannot be met by the local power grid. The UDS is designed to provide electrical power distribution for industrial plants, commercial buildings, instrument loads, outdoor lighting or any other general purpose power distribution application. It is also used in typical Power Distribution Unit (PDU) applications such as communication facilities and data centers.

One unit provides all the components necessary for distribution of electrical power in an efficiently designed standalone enclosure, including a power distribution transformer, a current limiting disconnect device in the transformer secondary, and a panelboard for individual branch circuit breakers.

VTI's UDS's are engineered, fabricated, assembled, factory wired, quality controlled and tested on-site at VTI and shipped to the customer's site for easy installation. They are all UL or CUL labeled.

Limitless variations in power inputs and outputs are possible by selecting among the many available options for the UDS, which include primary disconnect devices, lighting contactors, weatherproof enclosures and K-rated transformers, among many others.

An integral part of the design criteria is to provide the appropriate ventilation for the transformer heat, in order for the transformer to be mounted internally in the UDS. This is accomplished by the rear chimney design which allows the transformer heat to bypass the distribution equipment and safely dissipate through the top of the cabinet.

All compartment doors are mechanically interlocked with the secondary disconnect switch handle so that the internal access is restricted until power is disconnected or "screwdriver override" is activated. The panelboard wiring compartment is especially spacious to allow easy top access for wire pulls and convenient connection of field wiring to the branch circuit breakers. The net result is an easy to order, easy to install, all in one, UL Labeled power distribution system meeting OSHA and NEC electrical standards.

Dual-voltage UDS

Please note, our Dual-Voltage UDS model (dvUDS) is also available. The dvUDS model is similar to the standard UDS but provides power at both 480/277 Volts and 208/120 Volts or 240/120 Volts in the same enclosure. Find out more...

UDS Advantages

The UDS offers the following advantages in electrical power distribution applications:
  • Cabinet includes everything from one vendor
  • Easy to install; connects to a 480V source
  • Easy to wire; large spacious wire ways for load connections
  • Completely factory wired and tested
  • Interlocked compartment doors for safety
  • Meets NEC requirements
  • Field labor installation savings due to reduced installation time
  • Additional savings acheived in reduced design, purchasing, and material handling
  • Electrical and assembly drawings can be provided by electronic file

Please see our UDS Cost Comparison for a detailed cost/benefit analysis of a UDS installation.


The UDS is designed with the following standard safety and convenience features:

  • Free standing, totally enclosed, unitized structure
  • Welded, heavy gauge steel construction
  • Internally mounted transformer
  • Separate compartmentalized secondary main breaker or fused disconnect switch with external lockable operator handle
  • Rear chimney design allows the transformer heat to ventilate through the top of the cabinet, bypassing the distribution components
  • 42 circuit, 3 or 4 wire, 225 Amp Panelboard for dead-front mounted plug-on or bolt-on circuit breakers
  • 480 Volt primary wires isolated through factory installed internal conduit
  • Separate power compartments isolated from one another (transformer, current limiting disconnects and circuit breaker panelboard)
  • Extra large field wiring compartment
  • Mechanically interlocked compartment doors with "bypass" mechanism to prevent entry by unauthorized personnel
  • Key locked door for access to branch circuit breakers
  • Recessed panelboard to allow individual circuit breaker lockout while remaining branch circuits are operating with panelboard door closed
  • Cirrus grey enamel finish
  • Lifting eyes for easy handling
  • Factory wired, quality controlled and tested
  • Meets all OSHA and NEC safety and electrical standards and is UL Listed and CUL (Canadian UL) Approved as a complete system.


Units can be configured to meet specific customer requirements from the following options:

Transformer sizes:
1 Phase: 15, 25, 37.5 KVA
3 Phase: 15,30,45,75 KVA
Transformer Voltages:
1 Phase - 480 or 600V to 240/120
3 Phase - 480 or 600V Delta to 208V/120V Wye
3 Phase - 480 or 600V Delta to 480V/277V Wye
3 phase - 415/50Hz Delta to 415/240V Wye
Primary Disconnect:
Fused Disconnect Switch or
Main Breaker; Lockable Operator Handle
Secondary Disconnect:
Fused Disconnect Switch or
Main Breaker; Lockable Operator Handle
Branch Circuit Breakers up to 42 positions,
selectable 1, 2, or 3 poles from 10 to 100A.
Enclosure: [NEMA Enclosure definitions]
NEMA 1 (General Purpose, Indoors) - 88.5"H x 27"W x 20"D
NEMA 12 (Dust tight) - 88.5"H x 27"W x 20"D
NEMA 3R (Weather tight) - 84"H x 27"W x 26"D
NEMA 4 (Water tight) * - 84"H x 27"W x 26"D
* The transformer compartment is vented for cooling
Single Phase Transformer Rating Primary Protection Device Size Secondary Protection Device Size Contactor Rating ( * )
15KVA 40A - 480V
35A - 600V
80A - 240/120V 70A
37.5KVA 100A - 480V
80A - 600V
200A - 240/120V 175A
* Applicable only with contactor option. Rating is for Contactor when switching total secondary current to panelboard.
Three Phase Transformer Rating Primary Protection Device Size Secondary Protection Device Size Contactor Rating ( * )
15KVA 25A - 480V
20A - 600V
50A (208Y/120V)
25A (480Y/277V)
30KVA 45A - 480V
40A - 600V
100A (208Y/120V)
45A (480Y/277V)
45KVA 70A - 480V
60A - 600V
150A (208Y/120V)
75A (480Y/277V)
75KVA 125A - 480V
100A - 600V
250A (208Y/120V)
125A (480Y/277V)
(Main Breaker only)
* Applicable only with contactor option.

Other Options

Many additional options are available to meet electrical engineering specifications and customer requirements:

  • A current limiting disconnect device (circuit breaker or fused disconnect switch) on the transformer primary side.
  • An electrostatically shielded transformer for instrument loads.
  • 80°C rise transformer
  • Transient Suppression Network
  • A NEMA 12, Modified 3R or 4X enclosure
  • A Circuit Breaker Lockout Kit for padlocking Branch Circuit breakers in the "off" position.
  • A lighting contactor with HOA switch for photocell or timer-controlled night lights.
  • Isolated Copper or Aluminum Ground Bar
  • All-copper lugs, neurtal and ground bus
  • GFI or EPD style circuit breakers
  • Flush mounting trim
  • Bottom entry for customer load connections
  • Dual 42 position panelboards, providing potential for up to 84 circuits per unit.
  • IEC Panelboard with branch circuit breakers

Transformer Options include:

  • Copper or Aluminum Windings
  • K-rating
  • 80°C and 115°C rise

VTI can accomodate many more options than are listed here. See the Ordering Information below, or contact us for inquiries or more information about UDS options.


Power Dissipation

The following power dissipation rates are based on a UDS with the following configuration:

  • Standard (non K-rated) HEVI-DUTY NEMA Type TP-1 Energy Star compliant 3-phase transformer rated for 480V Primary and 208/120V Secondary with a 150°C Rise, fully loaded
  • SquareD NQOD Panelboard populated with 42 1-pole, 20A Type QO circuit breakers
KVA Rating Phase Power Dissipation
15 KVA 3 606
30 KVA 3 921
45 KVA 3 1203
75 KVA 3 1647


UDS doors closed
UDS doors closed
42 Circuit Panelboard with Circuit Breaker Lockout Kit installed (CBL option)
42 Circuit Panelboard with Circuit Breaker Lockout Kit installed (CBL option)
NEMA 3R UDS front view
NEMA 3R UDS front view
NEMA 3R UDS w 2 12pole lighting contactors
NEMA 3R UDS w 2 12pole lighting contactors
30KVA xformer in NEMA 3R UDS
30KVA xformer in NEMA 3R UDS
42 circuit panelboard,trim panel removed
42 circuit panelboard,trim panel removed
UDS NEMA 3R Transient Suppression Network - closed door
UDS NEMA 3R Transient Suppression Network - closed door
Main Breaker with External Door Interlocks
Main Breaker with External Door Interlocks
UDS with a Top Mounted Terminal Box (TB option)
UDS with a Top Mounted Terminal Box (TB option)
Top Mounted Terminal Box (TB option)
Top Mounted Terminal Box (TB option)
Stainless Steel UDS (NEMA 4X option)
Stainless Steel UDS (NEMA 4X option)
Stainless Steel UDS (NEMA 4X option) - Closeup Full View from left
Stainless Steel UDS (NEMA 4X option) - Closeup Full View from left


The following materials are available:

Ordering Information

VTI can modify the UDS in many ways according to a customer's needs. We recommend our online UDS Design Guide, which will allow you to configure UDS units to your own specifications, and will automatically generate a written quote, including pricing. Registration for this service is easy and free, and includes access to Design Guides for our other products as well.

You are also welcome to contact VTI directly for questions or to discuss ordering if you prefer.

UDS Model Number Construction Guide

VTI uses a systematic method to construct descriptive Model Numbers for the UDS. These model numbers include complete specifications on any unit and are suitable for inclusion in your Request for Quote, Project Plan or other documents. To build a model number for a UDS, please use the following guide:

UDS - [disconnect] [KVA] [CU] - [phase] [voltage] - 42 / [option list]

where the components of the model number include:
disconnect FD (Fused Disconnect)
MB (Main Breaker)
KVA 1Ø : 15, 25 or 37 KVA
3Ø : 15, 30, 45 or 75 KVA
CU enter CU if copper transformer only.
phase 1 Phase or 3 Phase
voltage 1Ø : 240 or 480 Volt
3Ø : 480, 600 or 480/277 Volt
option list enter comma-separated list of option codes (see below)

add multiple suffixes as required to the end of the Model Number:
Option Suffixes
PD Primary Non-Fused Disconnect K4 K4 Transformer for Typical Non-Linear Load
PB Primary Breaker K13 K13 Transformer for more severe Non-Linear Load
PFD Primary Fused Disconnect 80C 80°C rise transformer
RF Rejection Fuses IGBCU Isolated Ground Bar, Copper
ST Shielded Transformer IGB Isolated Ground Bar
LCT Lighting Contactor (must specify Amps and # of poles) ACU All Copper Lugs, Neutral and Panelboard Bus
HOA Hand-off Auto switch for Contactor 4X Modified NEMA 4 Enclosure (must specify corrosion type: SST, AL or Corrosion Resistive paint)
CP Copper Panelboard bus only 3R NEMA Type 1 NEMA 3R Constructed Enclosure
BE Bottom Entry GBCU Ground Bar, Copper
FMT Flush-Mounting Trim panel GB Ground Bar (aluminum)
2X42 Dual 42 Circuit Panelboards (34" Enclosure Only)
a 3 phase UDS with a Main Breaker (MB), 30KVA Transformer (30), a 480V secondary (480), an Isolated Ground Bar (IGB), and an enclosure conforming to NEMA 3R (3R) would have the model number:

Note: all UDS's come with a 42-circuit panelboard. Please specify the quantity and size of branch circuit breakers required when placing an order. You may download a sample Panel Schedule format from this site.

Please note that our online UDS Design Guide will automatically generate these model numbers for you, as well as provide a written Quotation for any number of UDS units, including pricing. You are welcome to Register for VTI Technical Services now to use this service. You will then have access to the UDS Design Guide as well as Design Guides for other products.

VTI · P.O. Box 9086 · 24 McMillan Way · Newark, DE 19714 · USA
Phone: (302) 738 0500 · Fax: (302) 738 6594

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