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7.3 Transformer Secondary Conductors — 240.21(C)

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Alaska Journeyman Electrician Study Guide Exam

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1 Chapter 1: Alaska Certificates of Fitness, Law & Exam Logistics

1.1 The Alaska Certificate of Fitness System 1.2 Journeyman Eligibility: 8,000 Hours and How Alaska Counts Them 1.3 Trainee, Residential Wireman, Lineman, Provisional & Reciprocal Certificates 1.4 The Journeyman Examination: Format, Scheduling, Open-Book Rules & Retakes 1.5 Adopted Codes, Fees, Renewal, Inspection & Enforcement

2 Chapter 2: Using the 2020 NEC

2.1 Article 90: Purpose, Scope, Arrangement & Enforcement 2.2 Navigating the Code Book: Structure, Index & Tabbing Strategy 2.3 Article 100 Definitions You Must Know Cold 2.4 Electrical Theory for the Exam: Ohm's Law, Power & Three-Phase

3 Chapter 3: General Requirements — Article 110

3.1 Approval, Listing, Interrupting Rating & Terminations 3.2 Temperature Limitations of Terminations — 110.14(C) 3.3 Working Space, Access, Egress & Illumination — 110.26(A) through (D) 3.4 Dedicated Equipment Space, Guarding & Over-1000-Volt Installations

4 Chapter 4: Conductors and Ampacity — Article 310

4.1 Conductor Types, Insulation Markings & Table 310.16 4.2 Ambient Temperature Correction and Alaska's Cold 4.3 Adjustment for More Than Three Current-Carrying Conductors 4.4 Putting It Together: Full Conductor-Sizing Problems 4.5 Parallel Conductors, Dwelling Service Sizing & Conditions of Use

5 Chapter 5: Wiring Methods and Materials — NEC Chapter 3

5.1 Article 300 General Requirements and Underground Cover 5.2 Thermal Movement: Expansion Fittings and Table 352.44 5.3 Boxes and Enclosures: Box Fill and Pull Box Sizing 5.4 Cable Assemblies: NM, MC, AC, SE, UF and TC 5.5 Raceways: RMC, IMC, EMT, FMC, LFMC, PVC and ENT 5.6 Conduit Fill: Chapter 9 Tables and Annex C 5.7 Cable Trays, Wireways, Busways, Gutters and Surface Raceways

6 Chapter 6: Branch Circuits and Feeders

6.1 Branch-Circuit Ratings, Conductor Sizing & Multiwire Circuits 6.2 Required Outlets and Receptacle Spacing in Dwellings 6.3 GFCI Protection — Article 210.8 6.4 AFCI Protection and Receptacle Requirements 6.5 Feeders and Outside Branch Circuits — Articles 215 and 225 6.6 Grounded Conductor Identification, Switches & Panelboards

7 Chapter 7: Overcurrent Protection — Articles 240 and 242

7.1 Article 240 Fundamentals: Ratings, Locations & Conductor Protection 7.2 Feeder Tap Rules — 240.21(B) 7.3 Transformer Secondary Conductors — 240.21(C) 7.4 Fuses, Circuit Breakers, Series Ratings & Article 242 Surge Protection

8 Chapter 8: Services and Load Calculations

8.1 Service Conductors, Clearances and Service Equipment 8.2 Service Disconnects, the 2020 Rule Change, SPDs and Service GFPE 8.3 Article 220 Part II: General Lighting, Receptacles and Demand Factors 8.4 The Standard Dwelling Calculation — Article 220 Part III 8.5 Optional Dwelling and Multifamily Calculations 8.6 Commercial and Industrial Load Calculations 8.7 Neutral Load Calculations and Harmonics — 220.61

9 Chapter 9: Grounding and Bonding — Article 250

9.1 System Grounding, 250.24 and the Main Bonding Jumper 9.2 The Grounding Electrode System — 250.50 through 250.53 9.3 Grounding Electrode Conductor Sizing and Installation 9.4 Bonding: Services, Metal Piping, Structural Metal and Other Systems 9.5 Equipment Grounding Conductors: Types, Sizing and Continuity 9.6 Separately Derived Systems and Separate Buildings 9.7 Ground-Fault Protection of Equipment and Grounding of Specific Equipment

10 Chapter 10: Equipment for General Use — NEC Chapter 4

10.1 Flexible Cords, Cables and Fixture Wires 10.2 Switches, Receptacles and Industrial Control Panels 10.3 Luminaires, Lampholders and Recessed Installations 10.4 Appliances and Fixed Electric Space Heating 10.5 Deicing, Snow Melting, Pipeline and Vessel Heating 10.6 Motors Part I: Full-Load Current, Conductors and Overload Protection 10.7 Motors Part II: Short-Circuit Protection, Controllers, Disconnects and HVAC

11 Chapter 11: Transformers, Generators and Special Occupancies

11.1 Transformers — Article 450 11.2 Generators, Storage Batteries and Energy Storage Systems 11.3 Hazardous (Classified) Locations — Articles 500 through 506 11.4 Garages, Hangars, Fuel Dispensing, Bulk Storage and Spray Application 11.5 Health Care Facilities — Article 517 11.6 Agricultural Buildings, Manufactured Homes, RV Parks and Marinas 11.7 Assembly, Theaters, Carnivals, Temporary Installations and Pools

12 Chapter 12: Special Equipment, Special Conditions and Exam Strategy

12.1 Signs, Cranes, Elevators, Electric Vehicle Charging and Welders 12.2 Solar Photovoltaic, Interconnection and Stand-Alone Systems 12.3 Wind Electric Systems and Fire Pumps 12.4 Emergency, Legally Required Standby, Optional Standby and COPS 12.5 Class 1, 2 and 3 Circuits, Fire Alarm, Optical Fiber and Communications 12.6 Cold-Climate Practice in Alaska: Code Basis and Engineering Judgment 12.7 Exam-Day Strategy: Navigation, Calculation Workflow and Time Management

7.3 Transformer Secondary Conductors — 240.21(C)

Key Takeaways Explain This Topic AI

NEC 240.21(C)(1) permits primary-side protection of secondary conductors only for a single-phase transformer with a two-wire secondary or a delta-delta transformer with a three-wire secondary, using the secondary-to-primary voltage ratio.

The 10-foot secondary rule in 240.21(C)(2) requires the secondary conductor ampacity to be at least one tenth of the primary overcurrent device rating multiplied by the primary-to-secondary voltage ratio.

The 25-foot secondary rule in 240.21(C)(6) requires the secondary conductor ampacity to be at least one third of the primary overcurrent device rating multiplied by the primary-to-secondary voltage ratio.

NEC 240.4(F) states that other than the two arrangements in 240.21(C)(1), transformer secondary conductors are not considered protected by the primary overcurrent device.

A transformer's own overcurrent protection under Table 450.3(B) is a separate requirement from the protection of its secondary conductors under 240.21(C).

Last updated: August 2026

Three Separate Questions

When a transformer appears in an exam question, three different rules may be in play and each must be answered separately:

  • Question Governing rule
  • How is the transformer protected? Table 450.3(B)
  • How are the primary conductors protected? 240.4 and 450.3, or a feeder tap rule in 240.21(B)(3)
  • How are the secondary conductors protected? 240.21(C)

Answering the transformer-protection question when the exam asked about secondary conductors is the most common error on this topic.

Why Secondary Conductors Are Special

A primary overcurrent device sees current on the primary side. A fault on the secondary reflects back to the primary transformed by the turns ratio, so a 200-ampere secondary fault on a 480-to-240 V transformer appears as only 100 amperes on the primary. Unless the geometry is simple, the primary device cannot reliably protect the secondary conductors.

240.4(F) states this directly:

Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors shall not be considered to be protected by the primary overcurrent protective device .

The Two Cases Where Primary Protection Works — 240.21(C)(1)

Only two transformer configurations have a fixed, predictable relationship between primary and secondary current in every fault case:

A single-phase transformer with a two-wire (single-voltage) secondary ;

A three-phase delta-delta connected transformer with a three-wire (single-voltage) secondary .

For these, 240.21(C)(1) permits the secondary conductors to be considered protected by the primary overcurrent device, provided that protection is in accordance with 450.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary voltage ratio .

Worked Example — Two-Wire Secondary

A single-phase 480 V to 240 V transformer has a two-wire secondary. The secondary conductors have an ampacity of 100 A. What is the maximum primary overcurrent device that can be said to protect them? Secondary-to-primary voltage ratio = 240 / 480 = 0.5

Maximum primary OCPD = 100 A x 0.5 = 50 A

A primary device of 50 A or less protects those secondary conductors. Anything larger requires secondary-side protection.

Note that a 208Y/120 V wye secondary never qualifies for 240.21(C)(1), because it is a four-wire, two-voltage secondary. Almost every commercial dry-type transformer you will meet falls outside the exception and needs secondary protection.

The 10-Foot Secondary Conductor Rule — 240.21(C)(2)

Where the length of secondary conductor does not exceed 10 feet (3.0 m) and complies with all of the following:

The ampacity of the secondary conductors is:

not less than the combined calculated loads on the circuits supplied, and

not less than the rating of the device supplied by the secondary conductors or not less than the rating of the overcurrent protective device at the termination of the secondary conductors;

The secondary conductors do not extend beyond the switchboard, switchgear, panelboard, disconnecting means, or control devices they supply;

The secondary conductors are enclosed in a raceway , extending from the transformer to the enclosure of the equipment supplied;

For field installations where the secondary conductors leave the enclosure or vault in which the supply connection is made, the rating of the overcurrent device protecting the primary of the transformer, multiplied by the primary-to-secondary transformer voltage ratio , shall not exceed 10 times the ampacity of the secondary conductor .

Worked Example — 10-Foot Secondary Tap

A 480 V to 240 V single-phase transformer has a 100 A primary overcurrent device. A 9-foot secondary run in a raceway supplies a panelboard. What is the minimum secondary conductor ampacity under condition 4? Primary-to-secondary voltage ratio = 480 / 240 = 2

  • Primary OCPD x ratio = 100 A x 2 = 200 A
  • Minimum secondary conductor ampacity = 200 / 10 = 20 A

20 amperes , subject also to condition 1 — the conductors must still carry the calculated load and match the rating of the device they terminate in.

The 25-Foot Secondary Conductor Rule — 240.21(C)(6)

For industrial installations only , where the length of the secondary conductors does not exceed 25 feet and:

The ampacity of the secondary conductors is not less than the secondary current rating of the transformer , and the sum of the ratings of the overcurrent devices does not exceed the ampacity of the secondary conductors;

All overcurrent devices are grouped;

The secondary conductors are protected from physical damage by being enclosed in an approved raceway or by other approved means.

A more commonly applied 25-foot rule appears in 240.21(C)(6) in the general form used by many jurisdictions: the secondary conductors have an ampacity not less than one third of the rating of the overcurrent device protecting the primary of the transformer, multiplied by the primary-to-secondary voltage ratio ; terminate in a single circuit breaker or set of fuses limiting the load to that ampacity; and are protected from physical damage.

Because the subsection numbering and conditions for the 25-foot secondary rule have shifted between editions, look this one up in the 2020 book rather than reciting it from memory . Tab 240.21(C) and read the subsections in order on exam day.

Worked Example — 25-Foot Secondary

A 480 V to 208Y/120 V transformer has a 100 A primary device. A 20-foot secondary run supplies a single main breaker panel. Using the one-third rule with the voltage ratio: Primary-to-secondary voltage ratio = 480 / 208 = 2.31

  • Primary OCPD x ratio = 100 A x 2.31 = 231 A
  • Minimum secondary conductor ampacity = 231 / 3 = 77 A

77 amperes.

The Other Subsections of 240.21(C)

240.21(C)(3) Industrial Installation Secondary Conductors — not over 25 feet, for industrial installations only, where conditions of maintenance and supervision ensure only qualified persons service the installation.

240.21(C)(4) Outside Secondary Conductors — no length limit, mirroring 240.21(B)(5): outside the building except at the point of termination, protected from physical damage, terminating in a single overcurrent device, with the device an integral part of or immediately adjacent to a disconnect at a readily accessible location outside or nearest the point of entrance.

240.21(C)(5) Secondary Conductors from a Feeder Tapped Transformer — where the primary conductors are themselves a tap under 240.21(B)(3).

Transformer Overcurrent Protection — Table 450.3(B) Preview

The transformer itself is protected under Table 450.3(B) for transformers 1000 volts nominal or less:

Protection method Primary current Maximum rating

Primary only 9 A or more 125 percent of primary current; where 125 percent does not correspond to a standard rating, the next higher standard rating is permitted

  • Primary only Less than 9 A 167 percent
  • Primary only Less than 2 A 300 percent
  • Primary and secondary 9 A or more (secondary) Secondary at 125 percent; primary permitted up to 250 percent
  • Worked Example — Primary-Only Protection

A 45 kVA, 480 V three-phase to 208Y/120 V transformer has a primary full-load current of 54.1 A. Using primary-only protection under Table 450.3(B): 54.1 A x 1.25 = 67.6 A

67.6 A is not a standard rating; next higher standard from 240.6(A) = 70 A

A 70-ampere primary overcurrent device. Note that this device protects the transformer . Whether it also protects the secondary conductors is answered separately by 240.21(C) — and for a 208Y/120 V four-wire secondary the answer is no, so a secondary overcurrent device is required.

Putting It Together

A complete transformer installation with a 480 V primary and 208Y/120 V secondary typically requires:

A primary overcurrent device sized per Table 450.3(B) — up to 250 percent of primary current where secondary protection is also provided;

Primary conductors sized for the primary current and protected by that device or by a feeder tap rule;

A secondary overcurrent device — usually the main breaker of the panel the transformer feeds — sized at not more than 125 percent of the secondary current;

Secondary conductors sized per 240.21(C), most often the 10-foot rule if the panel is close-coupled;

A system bonding jumper and grounding electrode conductor for the separately derived system per 250.30, covered in Chapter 9 of this guide.