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4.4 Putting It Together: Full Conductor-Sizing Problems

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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

4.4 Putting It Together: Full Conductor-Sizing Problems

Key Takeaways Explain This Topic AI

The complete sizing sequence is: determine the load, apply the 125 percent continuous-load multiplier, select a conductor, correct and adjust its ampacity, cap it at the termination column, then verify the overcurrent device against 240.4.

NEC 240.4(D) limits overcurrent protection after correction and adjustment to 15 amperes for 14 AWG copper, 20 amperes for 12 AWG copper, and 30 amperes for 10 AWG copper.

NEC 240.4(B) permits the next higher standard overcurrent device above the conductor ampacity where the device is 800 amperes or less, the conductors do not supply multioutlet receptacle branch circuits, and the ampacity does not match a standard size.

NEC 240.4(C) requires the conductor ampacity to equal or exceed the device rating where the overcurrent device exceeds 800 amperes.

Conductor ampacity and overcurrent device rating are separate determinations, and a question asking for one is not answered by the other.

Last updated: August 2026

The Complete Sequence

Every conductor-sizing question, no matter how it is dressed up, follows the same six steps: 1. LOAD Determine the load in amperes

  • 2. CONTINUOUS Multiply continuous portion by 125% (210.19(A)(1), 215.2(A)(1))
  • 3. SELECT Pick a conductor size
  • 4. DERATE Apply correction (310.15(B)(1)) x adjustment (310.15(C)(1))
  • 5. TERMINATE Cap at the 110.14(C) column ampacity; take the LOWER value
  • 6. PROTECT Size the OCPD per 240.4, including 240.4(B) and 240.4(D)

Steps 5 and 6 are where marks are lost. Step 5 produces the conductor's allowable ampacity . Step 6 produces the overcurrent device rating . These are different numbers answering different questions.

The Continuous-Load Multiplier

210.19(A)(1) for branch circuits and 215.2(A)(1) for feeders both require the conductor to have an allowable ampacity of not less than the noncontinuous load plus 125 percent of the continuous load . 210.20(A) and 215.3 impose the same requirement on the overcurrent device.

A continuous load (Article 100) is one where the maximum current is expected to continue for three hours or more . Store lighting, most commercial HVAC, and sign circuits are continuous. A dwelling range is not.

Worked Problem 1 — Commercial Lighting Feeder

A feeder supplies 64 A of continuous lighting load and 22 A of noncontinuous receptacle load. THWN-2 copper in EMT, three current-carrying conductors, 30 degrees C ambient. Equipment terminations are marked 75 degrees C. Size the conductor and the overcurrent device.

Step 1–2, required ampacity: (64 x 1.25) + 22 = 80 + 22 = 102 A

Step 3–4, select and derate: three CCC at 30 degrees C means no correction and no adjustment.

Step 5, termination cap: 75 degrees C column, copper. 2 AWG = 115 A; 3 AWG = 100 A. 100 A is less than 102 A, so 3 AWG fails. 2 AWG copper at 115 A is the answer.

Step 6, overcurrent device: 210.20(A)/215.3 require the device to be at least 102 A. The next standard size from 240.6(A) is 110 A . Check 240.4: the conductor ampacity is 115 A, so a 110 A device protects it. 2 AWG copper on a 110 A device.

Worked Problem 2 — Derating Governs

Six current-carrying 4 AWG THHN copper conductors in one raceway, ambient 40 degrees C, terminations 75 degrees C. What is the allowable ampacity and the largest standard overcurrent device? Base, 90 C column, 4 AWG Cu ................ 95 A

  • Correction, 36-40 C, 90 C column ........... x 0.91 = 86.45 A
  • Adjustment, 4-6 CCC ........................ x 0.80 = 69.16 A
  • Termination cap, 75 C column, 4 AWG Cu ..... 85 A
  • Final allowable ampacity ................... 69.16 A (the lower)

Overcurrent device: 69.16 A does not correspond to a standard size. Under 240.4(B) the next higher standard device is permitted if the device is 800 A or less, the conductors do not supply multioutlet branch circuits for cord-and-plug-connected portable loads, and the ampacity does not correspond to a standard rating. All three conditions are met, so 70 A is permitted.

Worked Problem 3 — Small Conductors and 240.4(D)

A 12 AWG copper THHN conductor, six current-carrying in a raceway, 30 degrees C. What is the maximum overcurrent device? Base, 90 C column, 12 AWG Cu ............... 30 A

  • Adjustment, 4-6 CCC ........................ x 0.80 = 24 A
  • Termination cap, 75 C column ............... 25 A
  • Allowable ampacity ......................... 24 A

Now apply 240.4(D)(5) : unless specifically permitted in 240.4(E) or (G), the overcurrent protection for 12 AWG copper shall not exceed 20 A after any correction factors for ambient temperature and number of conductors have been applied . The adjusted ampacity of 24 A exceeds 20 A, so 20 A is the maximum device.

Had the adjustment produced 18 A, the device would be limited to 15 A — the next standard size below , because 240.4(D) is a hard cap and 240.4(B)'s round-up is not available for small conductors that exceed their 240.4(D) limit.

240.4(D) Small-Conductor Limits

  • Conductor Maximum overcurrent device
  • 18 AWG copper 7 A
  • 16 AWG copper 10 A
  • 14 AWG copper 15 A
  • 12 AWG copper 20 A
  • 10 AWG copper 30 A
  • 12 AWG aluminum / copper-clad aluminum 15 A
  • 10 AWG aluminum / copper-clad aluminum 25 A

240.4(E) lists tap conductors governed instead by 210.19(A)(3) and (A)(4), 240.5(B)(2), 240.21, 368.17(B) and (C), and 430.53(D). 240.4(G) lists specific conductor applications — motor circuits under Article 430, air-conditioning under 440, fire alarm under 760, and others — that follow their own article's protection rules rather than 240.4(B) through (D).

The Round-Up Rule — 240.4(B) and (C)

240.4(B) Overcurrent Devices Rated 800 A or Less. The next higher standard overcurrent device rating above the ampacity of the conductors being protected shall be permitted , provided all of the following are met:

The conductors being protected are not part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads;

The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or circuit breaker in 240.6 without overload adjustments; and

The next higher standard rating does not exceed 800 A .

240.4(C) Overcurrent Devices Rated Over 800 A. Where the overcurrent device exceeds 800 A, the conductor ampacity shall be equal to or greater than the device rating. No rounding up above 800 A.

240.6(A) Standard Ampere Ratings: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, 6000. Additional standard ratings for fuses are 1, 3, 6, 10, and 601.

Worked Problem 4 — Over 800 Amperes

A service feeder's calculated conductor ampacity works out to 1150 A after paralleling. May a 1200 A overcurrent device be used?

No. 240.4(C) applies above 800 A: the conductor ampacity must equal or exceed the device rating. With 1150 A of conductor you may install a 1000 A device, or you must increase the conductor set until its ampacity reaches at least 1200 A.

The One-Sentence Summary

Ampacity answers how much current the conductor may carry . The overcurrent device answers how much current the circuit is allowed to draw before it opens . Below 800 A and outside the small-conductor range, the device may round up past the ampacity; for 14, 12, and 10 AWG it may not exceed the 240.4(D) values; and above 800 A the conductor must always be at least as large as the device.