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4.3 Adjustment for More Than Three Current-Carrying Conductors

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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

4.3 Adjustment for More Than Three Current-Carrying Conductors

Key Takeaways Explain This Topic AI

Table 310.15(C)(1) reduces ampacity to 80 percent for 4 to 6 conductors, 70 percent for 7 to 9, 50 percent for 10 to 20, 45 percent for 21 to 30, 40 percent for 31 to 40, and 35 percent for 41 and above.

Adjustment applies where more than three current-carrying conductors share a raceway or cable, or where conductors are bundled or stacked without maintaining spacing for a distance longer than 24 inches.

The neutral of a three-wire single-phase circuit is not a current-carrying conductor, but the neutral of a four-wire three-phase wye circuit carrying a major portion of nonlinear load is counted.

Equipment grounding and bonding conductors are never counted as current-carrying conductors under 310.15(F).

Chapter 9 Table 1 Note 4 exempts nipples not exceeding 24 inches in length from the adjustment factors and allows 60 percent conduit fill.

Last updated: August 2026

The Physics and the Rule

Each current-carrying conductor in a raceway is a heat source, and each one also insulates its neighbors from the surrounding air. Beyond three, the mutual heating becomes significant enough that the Code reduces the allowable ampacity of every conductor in the group.

310.15(C)(1) states that where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 24 inches and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(C)(1).

  • Number of current-carrying conductors Percent of value in Table 310.16
  • 4–6 80 percent
  • 7–9 70 percent
  • 10–20 50 percent
  • 21–30 45 percent
  • 31–40 40 percent
  • 41 and above 35 percent

Note how steeply the factor drops between 9 and 10 conductors — from 70 percent to 50 percent. Splitting a 10-conductor pull into two raceways of five is often cheaper than upsizing every conductor.

Counting Current-Carrying Conductors

This is where most errors happen. Work through the rules in 310.15(E) and 310.15(F) :

Always counted:

Every ungrounded (phase) conductor .

Never counted (310.15(F)):

Equipment grounding conductors and bonding conductors , whether insulated, covered, or bare.

Neutral conductors — it depends (310.15(E)):

Circuit Is the neutral a CCC?

Two-wire circuit (one hot, one neutral) Yes — the neutral carries the same current as the hot

Three-wire circuit from a single-phase three-wire system (120/240 V multiwire) No — it carries only the unbalanced current

  • Four-wire three-phase wye where the load is balanced and linear No — 310.15(E)(2)
  • Four-wire three-phase wye where the major portion of the load is nonlinear Yes — 310.15(E)(3)

Three-wire circuit made of two phase conductors and the neutral of a four-wire three-phase wye Yes — 310.15(E)(2)

The nonlinear-load rule exists because triplen harmonics — the third, ninth, and fifteenth — do not cancel in the neutral of a wye system. They add arithmetically, so the neutral of a panel feeding LED drivers, computer power supplies, or variable-frequency drives can carry as much current as the phases, or more.

Counting Practice

  • Raceway contents Current-carrying conductors
  • Three 120 V two-wire circuits (3 hots, 3 neutrals) + 1 EGC 6 — every neutral in a two-wire circuit counts

One 120/240 V multiwire branch circuit (2 hots, 1 neutral) + 1 EGC 2 — the neutral of a three-wire single-phase circuit does not count

  • One 208Y/120 V three-phase four-wire feeder, linear load + 1 EGC 3
  • The same feeder with a major portion nonlinear load 4 — the neutral counts, triggering the 80 percent factor
  • Four 208Y/120 V three-phase four-wire circuits, linear, + EGCs 12 — 50 percent factor
  • Exemptions from Adjustment

Several situations escape the adjustment factors entirely:

Nipples 24 inches or less. Chapter 9, Table 1, Note 4 : where conduit or tubing nipples having a maximum length not exceeding 24 inches are installed between boxes, cabinets, and similar enclosures, the nipples shall be permitted to be filled to 60 percent of their total cross-sectional area, and the adjustment factors of 310.15(C)(1) need not apply to this condition.

Spacing maintained. 310.15(C)(1) applies to conductors bundled or stacked without maintaining spacing for more than 24 inches. Cable tray installations that maintain spacing per Article 392 are handled by that article's own ampacity rules.

310.15(C)(1) Exception conditions — the Code contains specific exceptions, including for certain underground conductors entering or leaving an outdoor trench, where physical protection is provided for not more than 10 feet and the number of conductors does not exceed four.

Type AC and Type MC cable exceptions in 310.15(B) and 320.80/330.80 address particular assemblies.

Worked Example — Both Factors Together

Nine current-carrying 6 AWG THHN copper conductors run in a single EMT through an unheated Anchorage warehouse where the design ambient is 15 degrees C. Terminations are rated 75 degrees C.

Base, 90 degrees C column: 6 AWG copper = 75 A .

Correction , Table 310.15(B)(1), 11–15 degrees C, 90 degrees C column = 1.12 . 75 x 1.12 = 84 A.

Adjustment , Table 310.15(C)(1), 7–9 conductors = 70 percent . 84 x 0.70 = 58.8 A .

Termination cap , 110.14(C): 6 AWG copper at 75 degrees C = 65 A .

Final allowable ampacity: the lower of 58.8 A and 65 A = 58.8 A .

Here the cold ambient genuinely helped. Without the 1.12 correction the result would have been 75 x 0.70 = 52.5 A. The cold bought about 6 amperes of usable capacity because the adjustment factor, not the termination, was the binding constraint.

Order of Operations

Correction and adjustment are multiplied together against the base ampacity; neither is applied to the other's result in a special sequence, since multiplication commutes. What matters is that you:

  • start from the column matching the conductor insulation ;
  • multiply by the correction factor ;
  • multiply by the adjustment factor ;
  • compare against the termination column ampacity from 110.14(C) and take the lower;

then check 240.4 for the overcurrent device.

Steps 4 and 5 are separate checks, not further multiplications.