UFC 3-520-01, Interior Electrical Systems (16 January 2025)

Overview

UFC 3-520-01 supersedes the 06 October 2015 edition and is mandatory for planning, design, construction, sustainment, restoration and modernisation of DoD-owned facilities under all delivery methods. Its scope extends about 5 feet beyond the facility envelope; exterior systems fall under UFC 3-550-01. It applies together with NFPA 70, and facilities outside the United States must also meet host nation standards. It points to UFC 3-560-01 for electrical safety and energised work and to UFC 4-010-06 for control system cybersecurity. For projects designated Critical Operations Power Systems (COPS), stricter UFC requirements prevail over NFPA 70; this UFC does not address COPS. It is not data-centre specific and defers to NFPA 70 and other UFCs on many topics.

For voltage, 240/120 V is used for single-phase-only facilities, three-phase four-wire 208Y/120 V for demand loads under 150 kVA, and 480Y/277 V for demand loads above 150 kVA unless 208Y/120 V is shown to be more cost-effective. Distribution frequency is 60 Hz, using the local frequency where it differs as far as practical. Dry-type transformers of 15 kVA and larger are specified per NEMA ST 20 with a 220 C insulation system and 115 C rise, able to carry 115% of nameplate continuously at 40 C ambient; where extra overload capacity is needed, an 80 C rise unit carrying 130% is specified. Altitude and ambient temperature derating follow the manufacturer and NEMA ST 20.

For service entrance and distribution equipment, circuit breaker interrupting ratings follow available short-circuit current but never below 10 kA symmetrical at 240 V and below or 14 kA at 480 V; series-rated breaker combinations are prohibited. All switchgear, switchboards, panelboards and load centers get at least 15% empty space and 15% spare ampacity, panelboards get one 3/4 in spare conduit per three spare breakers or empty poles, and spare capacity is not used in demand calculations. For breakers with trip settings of 1,200 A or higher, an arc-energy reduction method must be selected. Metal-enclosed switchgear follows IEEE C37.20.1 and UL 1558, with arc-resistant gear, infrared windows, remote racking from at least 20 ft and drawout shutters to be evaluated, plus MV gas-insulated gear above 99.9% availability.

The SPD section sets, for 480/277 V service entrances, a voltage protection rating of 1,200 V in L-N, L-G and N-G modes and 2,000 V L-L, a surge current of at least 40 kA per mode (80 kA L-L) and a surge life of over 5,000 IEEE C62.41 C3 surges. Conductors of #6 AWG and smaller must be copper, feeder and branch conductors are never smaller than #12 AWG, sizing uses the 60 C ampacity for circuits of 100 A or less and 75 C above, and ambient correction assumes no less than 25 C. Conductors upsized for voltage drop may be reduced at terminations only for up to 10 ft or 10% of circuit length, whichever is less.

For stationary batteries, vented lead-acid is preferred for switchgear control power and UPS; VRLA is not used where vented will do, since it has shorter life and can fail without warning (small-footprint or leakage-sensitive locations are exceptions). Nickel-cadmium is considered for extreme temperatures or engine starting. Lithium-ion, lithium metal polymer and other lithium-based batteries must not be used for stationary applications in occupied buildings. For life-cycle costing, service lives are 3 years (small VRLA), 7 (large VRLA), 10 (small vented lead-acid), 15 (large vented lead-acid) and 15 (NiCd). Battery areas follow UFC 3-520-05, UPS selection follows TSEWG TP-19 and generators and transfer switches UFC 3-540-01. Appendices cover adjustable speed drives, power quality (unbalance, harmonics, transformer derating, neutral sizing) and a glossary.

Key points

1. Use 208Y/120 V three-phase four-wire for demand loads under 150 kVA and 480Y/277 V above 150 kVA unless 208Y/120 V is shown to be more cost-effective.
2. Dry-type transformers of 15 kVA and larger: 220 C insulation, 115 C rise, 115% of nameplate continuous at 40 C ambient; 80 C rise units carry 130%; derate above 3,300 ft (1,000 m).
3. Minimum circuit breaker interrupting ratings are 10 kA symmetrical at 240 V and below and 14 kA at 480 V; series-rated combinations are prohibited.
4. Provide at least 15% empty space and 15% spare ampacity in all switchgear, switchboards, panelboards and load centers, plus one 3/4 in spare conduit per three spare breakers or empty poles.
5. Service entrance SPDs on 480/277 V systems: voltage protection rating 1,200 V (L-N, L-G, N-G) and 2,000 V L-L; surge current at least 40 kA per mode (80 kA L-L); surge life over 5,000 IEEE C62.41 C3 surges.
6. For breakers with trip settings of 1,200 A or higher, select an arc-energy reduction method such as maintenance switching or an active arc flash mitigation system.
7. Conductors upsized for voltage drop may be reduced at terminations only for up to 10 ft or 10% of circuit length, whichever is less.
8. Lithium-based batteries must not be used for stationary applications in occupied buildings.
9. Life-cycle battery service lives: small VRLA 3 years, large VRLA 7, small vented lead-acid 10, large vented lead-acid 15, NiCd 15.
10. The document is not data-centre specific, defers to NFPA 70 and other UFCs for many requirements and does not address COPS.

Using it in design

• Select supply voltage by demand load (208Y/120 V below 150 kVA, 480Y/277 V above) and attach a life-cycle comparison to the design analysis if 208Y/120 V is proposed for a larger data hall.
• Select breaker interrupting ratings from available short-circuit current, apply the 10 kA and 14 kA floors, avoid series ratings, and state the arc-energy reduction method for breakers of 1,200 A and above.
• Leave 15% empty space and 15% spare ampacity in every switchgear, switchboard and panelboard, write the spare conduits into the specification, and do not count that margin again in the demand calculation.
• Specify service entrance SPDs to 1,200 V (L-N, L-G, N-G) and 2,000 V (L-L) voltage protection rating, 40 kA and 80 kA surge current and 5,000-surge life.
• In conductor sizing, do not assume ambient below 25 C, check the 10 ft or 10% limit on reducing voltage-drop-upsized conductors at terminations, and apply Appendix B for transformer derating and neutral sizing with harmonic loads.
• For backup energy storage, do not use lithium-based batteries in occupied buildings; prefer vented lead-acid, justify any VRLA use against the exceptions, and use the 3-15 year service lives in life-cycle comparisons.

Who should read it

Electrical engineers designing interior electrical systems for federal and military facilities, and designers using these criteria as a benchmark in data centre design.

Document contents

1. Introduction
2. Purpose and Scope
3. General Building Requirements
4. Cybersecurity
5. Design Standards
6. General Power System Criteria
7. Voltage
8. Frequency
9. Power Distribution and Utilization
10. Transformers
11. Service Entrance and Distribution Equipment
12. Spare Capacity
13. Switchgear
14. Switchboards
15. Panelboards
16. Motor Control Centers (MCCs)
17. Power for Fire Protection Systems
18. Interrupting Ratings
19. Motors and Motor Control Circuits
20. Surge Protective Devices (SPDs)
21. Metering
22. Raceway and Wiring
23. Lighting
24. Generators
25. Automatic Transfer Equipment
26. Uninterruptible Power Supply
27. Stationary Batteries and Battery Chargers
28. Grounding, Bonding, and Static Protection
29. Lightning Protection Systems
30. Power Factor Correction
31. Power Quality
32. ASHRAE Compliance
33. Electric Vehicle Supply Equipment
34. Appendix A Adjustable Speed Drives
35. Appendix B Power Quality
36. Appendix C Glossary
37. Appendix D References

Read the document

Publication details

Authors

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

Publisher

U.S. DoD (UFC)

Year

2025

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