UFC 3-550-01 Exterior Electrical Power Distribution (1 September 2016, Change 3, 01 November 2019)

Overview

This document belongs to the Unified Facilities Criteria (UFC) series prepared by NAVFAC, USACE and AFCEC and sets out how exterior electrical distribution is to be designed at military installations. It is read together with NFPA 70 and IEEE C2: it does not replace them, it adds DoD-specific requirements on top. The 2016 edition supersedes the 3 February 2010 version (including Change 1 of July 2012). The text described here is updated through Change 3 (1 November 2019), which added environmental severity classification, humidity design requirements and corrosion-prevention rules.

Chapter 1 covers purpose and exclusions and Chapter 2 covers power requirements. Chapter 2 rests on two principles: primary voltage is selected from NEMA C84.1, and primary distribution equipment is designed with future periodic maintenance as a principal consideration. Looped and alternate feeds are described as essential so that equipment can be taken out of service while limiting outage time for affected facilities. New primary systems are four-wire, multi-grounded and wye-connected at the source transformer.

Chapter 3 proceeds equipment by equipment: primary and secondary unit substations, pad-mounted distribution transformers, medium-voltage switchgear, pad-mounted switches, capacitors, overhead lines and underground systems. The underground section is the most detailed, defining ductbank sizes, burial depths, spare-duct ratios, manhole dimensions, equipment prohibited in manholes and the circumstances in which directional boring may be used. Appendix B treats directional boring and Appendix C gives best practices for general electrical power requirements.

The limits are clear. The document is not data centre specific, it assumes military-installation infrastructure (airfields, ordnance facilities, piers) and it lists separate exceptions for the Air Force and the Navy. The Navy, for example, requires SF6 or high-fire-point liquid insulated switchgear and EPR-insulated MV cable, while the Air Force bans T and Y splices on all medium-voltage systems. A data centre designer should treat the figures as a numeric reference point and not as a substitute for the utility operator's requirements or local standards.

Key points

1. Pad-mounted transformers are not used above 3,000 A secondary current; units above 1,000 kVA at 208Y/120 V or above 2,500 kVA at 480Y/277 V must be built as secondary unit substations.
2. Three-phase pad-mounted transformers must be loop-feed capable with 6 bushings; three switches are provided if a loop is possible and a single on-off switch for the end of a radial.
3. If the 208Y/120 V dry-type load exceeds 40 percent of the 480 V service transformer capability, two liquid-filled transformers replace one 480Y/277 V service; double transformations are to be minimised and equipment connected at the highest available voltage.
4. Elbow-type arresters go at the ends of all radials and at normally open loop points, for all voltages above 5 kV.
5. Primary conduits are at least 5 in (127 mm), secondary (LV) conduits at least 4 in (103 mm) for conductors of 500 kcmil and larger; concrete encasement is at least 3 in (75 mm), burial at least 18 in (450 mm), and 24 in (600 mm) under roads and for 22-40 kV.
6. On primary circuits at least one third of the ductbank must be empty spare conduits (and at least one conduit), and all spares get pull wires.
7. Power manholes are at least 6.5 ft (2 m) deep and at least 9 ft by 12 ft (2.8 m by 3.7 m) inside for circuits above 15 kV; covers are designed for H20 highway loading. Transformers, switches, load junctions and separable (bolt-T) splices are prohibited inside manholes.
8. Directional boring is permitted only for named crossings (roads, parking lots, water, approved sensitive areas), must not be chosen on cost alone, and is prohibited above 34.5 kV; medium voltage is defined as 1,000 V to 34.5 kV.
9. Ground rods are at least 10 ft (3.0 m) long and 3/4 in (17 mm) in diameter, with below-grade connections by exothermic weld or hydraulic compression; systems not classified as multi-grounded need a 4/0 bare copper ring with at least four rods.

Using it in design

• Settle the loop or alternate-feed arrangement and the maintenance isolation points for the MV run from utility entry to the building early; the text treats shortening maintenance outages as a direct design driver.
• Check transformer selection against the 3,000 A secondary limit and the 1,000 kVA (208Y/120 V) and 2,500 kVA (480Y/277 V) thresholds; for larger data hall loads consider secondary unit substations or several transformers.
• In ductbank design verify the spare-conduit share (at least one third), the 3 in (75 mm) spacing between conduits, concrete cover and burial depth together with the corresponding ampacity derating.
• Place manholes at every splice and direction change and within 100 ft (30 m) of each riser pole, pad-mounted transformer or unit substation, keeping straight-run spacing to 400 ft (120 m), or 600 ft (180 m) where justified.
• For site grounding, add to the checklist the 10 ft (3 m) clear working space in front of pad-mounted gear, bollards where vehicles can strike equipment, and bonding of metal bollards within 8 ft to the ground ring.
• Note that oil-filled transformer siting points to UFC 3-600-01 and corrosive or humid locations point to UFC 3-501-01; read those dependent documents separately.

Who should read it

Electrical engineers and project managers designing medium-voltage site distribution, transformers and ductbanks for a data centre.

Document contents

1. Chapter 1 General
2. Chapter 2 Electrical Power Requirements: selection of primary voltage, design for maintenance
3. 3-1 Main and Utilization Electric Supply Stations
4. 3-2 General Electrical Requirements
5. 3-3 / 3-4 Primary and Secondary Unit Substations
6. 3-5 Pad-Mounted Distribution Transformers
7. 3-6 / 3-7 Medium Voltage Switchgear and Pad-Mounted Switchgear
8. 3-9 Capacitors
9. 3-10 Overhead Power Distribution
10. 3-11 Underground Electrical Systems: ductbanks, directional boring, manholes, MV cable
11. 3-14 Distribution System Grounding
12. 3-18 / 3-19 Environmental and Fire Protection Considerations
13. Appendix A References
14. Appendix B Directional Boring
15. Appendix C Best Practices: General Electrical Power Requirements
16. Appendix D Glossary

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Publication details

Authors

Naval Facilities Engineering Command (NAVFAC); U.S. Army Corps of Engineers (USACE); Air Force Civil Engineer Center (AFCEC)

Publisher

U.S. DoD (UFC)

Year

2019

Document language

English

Source type

Government / public

Access

Open licence — downloadable

Licence

U.S. public domain — U.S. Government work; free to reuse

Licence evidence

www.copyright.gov

Citation