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4.1 Conductor Types, Insulation Markings & Table 310.16

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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

4.1 Conductor Types, Insulation Markings & Table 310.16

Key Takeaways Explain This Topic AI

Table 310.16 in the 2020 NEC gives allowable ampacities for insulated conductors rated up to 2000 volts, with not more than three current-carrying conductors in a raceway, cable, or earth, based on a 30 degrees C ambient.

Insulation letters decode systematically: T is thermoplastic, H is heat resistant to 75 degrees C, HH is 90 degrees C, W is wet-location rated, N is nylon jacketed, and the suffix -2 means 90 degrees C wet and dry.

Type THHN is dry and damp only at 90 degrees C, while THWN-2 and XHHW-2 are rated 90 degrees C in both wet and dry locations, which is why most building wire is dual-marked THHN/THWN-2.

NEC 310.10(C) requires conductors in wet locations — including raceways underground or in concrete slabs in direct contact with the earth — to be listed for use in wet locations.

NEC 310.14(A)(2) requires the lowest applicable ampacity to be used where more than one ampacity applies along a circuit, unless the reduced-ampacity portion does not exceed the lesser of 10 feet or 10 percent of the higher-ampacity length.

Last updated: August 2026

What Table 310.16 Actually Says

The full title matters, because everything in the table is conditional on it:

Table 310.16 Ampacities of Insulated Conductors with Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30 degrees C (86 degrees F)

Three assumptions are baked in:

Not more than three current-carrying conductors — a fourth triggers the adjustment factors of Table 310.15(C)(1).

30 degrees C (86 degrees F) ambient — anything else triggers the correction factors of Table 310.15(B)(1).

In raceway, cable, or directly buried earth — conductors in free air use the much higher values of Table 310.17 .

Note also the 2020 renumbering. What was Table 310.15(B)(16) in the 2014 and 2017 editions is Table 310.16 in the 2020 NEC. Ambient correction is Table 310.15(B)(1) in degrees C and Table 310.15(B)(2) in degrees F. Conductor-count adjustment is Table 310.15(C)(1) . Older prep books using the 2017 numbering will send you to the wrong page.

Reading the Insulation Letters

Conductor type designations are built from a small alphabet. Decode them rather than memorizing the whole of Table 310.4(A):

  • Letter Meaning
  • T Thermoplastic insulation
  • H Heat resistant — 75 degrees C
  • HH Highly heat resistant — 90 degrees C
  • W Suitable for wet locations
  • N Nylon or thermoplastic outer jacket, resistant to oil and gasoline
  • X Cross-linked synthetic polymer (thermoset, XLPE)
  • U Underground
  • 2 Rated 90 degrees C in both wet and dry locations

Working through the common types:

  • Type Dry rating Wet rating Notes
  • TW 60 C 60 C Legacy thermoplastic
  • THW 75 C 75 C
  • THHN 90 C Not wet rated Dry and damp locations only
  • THWN 75 C 75 C
  • THWN-2 90 C 90 C The workhorse
  • XHHW 90 C 75 C Thermoset
  • XHHW-2 90 C 90 C Thermoset, excellent cold handling

USE / USE-2 75 C / 90 C 75 C / 90 C Underground service entrance; USE is not flame retardant and is not permitted inside buildings

RHH / RHW-2 90 C 90 C (RHW-2) Thermoset rubber

Most building wire on the shelf is dual-marked THHN/THWN-2 , which means it carries the 90 degrees C rating in both wet and dry locations. Be careful with a conductor marked THHN only : it is not permitted in a wet location, and by 310.10(C) the interior of a raceway installed underground or in a slab in direct contact with earth is a wet location .

Locations — 310.10

310.10(A) Dry Locations. Any insulation specified for the purpose in Table 310.4(A).

310.10(B) Dry and Damp Locations. Types listed in Table 310.4(A) for those locations.

310.10(C) Wet Locations. Insulated conductors in wet locations shall be (1) moisture-impervious metal-sheathed, (2) types MTW, RHW, RHW-2, TW, THW, THW-2, THHW, THWN, THWN-2, XHHW, XHHW-2, XHWN, XHWN-2 or ZW, or (3) of a type listed for use in wet locations.

Article 100 defines a wet location to include installations underground or in concrete slabs in direct contact with the earth, locations subject to saturation with water, and unprotected locations exposed to weather. A damp location is protected from weather and not subject to saturation — the underside of a canopy or a cold-storage warehouse.

310.10(D) Exposure to Sunlight. Insulated conductors used where exposed to direct rays of the sun shall be listed and marked sunlight resistant , or covered with insulating material listed as sunlight resistant.

310.10(F) Corrosive Conditions. Conductors exposed to oils, greases, vapors, gases, fumes, liquids, or other substances having a deleterious effect shall be of a type approved for the application, or shall be protected by suitable sheaths.

310.10(H) Direct-Burial Conductors. Conductors used for direct burial applications shall be of a type identified for such use.

Conductor Constructions — 310.4 and 310.6

310.4(A) contains the master table of conductor applications and insulations — type letters, maximum operating temperature, application provisions, insulation, thickness, and outer covering.

310.6 Conductor Identification. Grounded conductors comply with 200.6; equipment grounding conductors comply with 250.119; ungrounded conductors comply with 310.6(C), which requires that where a premises has more than one nominal voltage system, each ungrounded conductor be identified by phase or line and system at all termination, connection, and splice points, with the method documented and readily available.

310.3(B) Conductor Minimum Size. The minimum size of conductors for 0 to 2000 volts is 14 AWG copper and 12 AWG aluminum or copper-clad aluminum, except as permitted elsewhere in the Code — fixture wires, flexible cords, Class 2 circuits, and similar applications have their own smaller minimums.

Multiple Ampacities Along One Circuit — 310.14(A)(2)

Where more than one ampacity applies for a given circuit length, the lowest value shall be used.

Exception: Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 10 feet or 10 percent of the circuit length figured at the higher ampacity, whichever is less .

This is the "10-foot / 10-percent" rule. A feeder that passes through a short high-ambient zone — over a boiler, through a mechanical chase — does not have to be sized for that zone if the hot section is short enough to satisfy the exception. Note that the allowance is the lesser of the two figures, so on a 60-foot run the allowance is 6 feet, not 10.

Temperature Limitation — 310.14(A)(3)

No conductor shall be used in such a manner that its operating temperature exceeds that designated for the type of insulated conductor involved.

This is the general prohibition that 110.14(C) implements at terminations, and it is also the reason ampacity correction and adjustment exist at all.

Ampacities to Have at Your Fingertips

On an open-book exam you will look these up, but recognizing them speeds sanity checks. Copper, Table 310.16:

  • Size 60 C 75 C 90 C
  • 14 AWG 15 20 25
  • 12 AWG 20 25 30
  • 10 AWG 30 35 40
  • 8 AWG 40 50 55
  • 6 AWG 55 65 75
  • 4 AWG 70 85 95
  • 3 AWG 85 100 115
  • 2 AWG 95 115 130
  • 1 AWG 110 130 145
  • 1/0 AWG 125 150 170
  • 2/0 AWG 145 175 195
  • 3/0 AWG 165 200 225
  • 4/0 AWG 195 230 260
  • 250 kcmil 215 255 290

Notice that 14, 12, and 10 AWG copper show 15, 20, and 30 A in the 60 degrees C column — the same numbers as the 240.4(D) overcurrent limits, which is a coincidence of the table, not the reason for the rule. Their 75 degrees C values are 20, 25, and 35 A, which are higher than the permitted overcurrent devices. Section 4.4 works through why.