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4.2 Ambient Temperature Correction and Alaska's Cold

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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

4.2 Ambient Temperature Correction and Alaska's Cold

Key Takeaways Explain This Topic AI

Table 310.15(B)(1) corrects ampacity from the 30 degrees C base; at an ambient of 10 degrees C or less the factors are 1.29 for 60 degrees C conductors, 1.20 for 75 degrees C, and 1.15 for 90 degrees C.

Correction factors above 1.00 do increase a conductor's allowable ampacity, but the result is still capped by the termination temperature limits of 110.14(C) and by the overcurrent rules of 240.4.

Correction is applied to the ampacity column matching the conductor's own insulation rating, not to the termination column.

The controlling ambient for a run is the highest ambient any portion of the conductor experiences, which for an Alaska service riser is usually the heated interior, not the outdoor air.

Table 310.15(B)(2) gives the same factors indexed in degrees Fahrenheit, which is often faster when a question states the ambient in Fahrenheit.

Last updated: August 2026

Why Ampacity Depends on Ambient

A conductor's ampacity is a thermal limit. Current through resistance produces heat at a rate of I squared times R; the conductor rises above ambient until it sheds that heat as fast as it makes it. The insulation's temperature rating sets the maximum allowable conductor temperature, so the amount of current permitted depends on how much temperature rise is available — which depends on where you start.

Table 310.16 assumes a 30 degrees C (86 degrees F) starting point. A 90 degrees C conductor in that ambient has 60 degrees C of headroom. Move it into a 10 degrees C garage and it has 80 degrees C of headroom, so it can carry more current. Move it into a 50 degrees C boiler room and it has only 40 degrees C, so it must carry less.

Table 310.15(B)(1)

  • Ambient (degrees C) 60 C column 75 C column 90 C column
  • 10 or less 1.29 1.20 1.15
  • 11–15 1.22 1.15 1.12
  • 16–20 1.15 1.11 1.08
  • 21–25 1.08 1.05 1.04
  • 26–30 1.00 1.00 1.00
  • 31–35 0.91 0.94 0.96
  • 36–40 0.82 0.88 0.91
  • 41–45 0.71 0.82 0.87
  • 46–50 0.58 0.75 0.82
  • 51–55 0.41 0.67 0.76
  • 56–60 — 0.58 0.71

Table 310.15(B)(2) presents the identical factors indexed in degrees Fahrenheit — 50 degrees F or less for the top row, 51–59, 60–68, and so on. If a question gives you Fahrenheit, use the Fahrenheit table rather than converting; converting is where arithmetic errors creep in.

Two structural points about the table:

The correction is applied to the column matching the conductor's insulation rating , not the termination rating. A THHN conductor in a 40 degrees C ambient is corrected by 0.91 , the 90 degrees C figure — even if it lands on 75 degrees C lugs. The termination limit is applied afterward, as a separate cap.

Lower-rated insulations are more sensitive to ambient. At 46–50 degrees C the 60 degrees C column loses 42 percent of its ampacity while the 90 degrees C column loses only 18 percent. That is the practical reason 90 degrees C conductors are specified in hot environments even when the terminations are 75 degrees C.

Worked Example — Cold Ambient

A 10 AWG THHN copper conductor runs through an unheated Fairbanks equipment enclosure where the design ambient is 10 degrees C.

Base ampacity, 90 degrees C column: 10 AWG copper = 40 A .

Correction factor at 10 degrees C or less, 90 degrees C column = 1.15 .

Corrected ampacity: 40 x 1.15 = 46 A .

So the corrected ampacity is 46 A. But look at the two limits that follow:

110.14(C): if the terminations are 75 degrees C, 10 AWG copper is 35 A in that column. The final allowable ampacity is 35 A .

240.4(D)(7): for 10 AWG copper, the overcurrent device may not exceed 30 A regardless.

The cold ambient bought nothing in this case. That is the general pattern for small building-wire conductors, and it is exactly what an exam question about "Alaska's cold climate" is likely to test: cold air raises the corrected ampacity, but terminations and 240.4 usually govern anyway .

Where Cold Correction Actually Pays

The benefit shows up in the situations where neither of those two caps binds:

Large feeders on 90 degrees C conductors with 75 degrees C terminations that are not the binding constraint — for example, where several adjustment factors have already pushed the calculated value below the 75 degrees C column, cold correction claws part of it back.

Conductors in free air or on messenger using Table 310.17, where no termination is involved in the middle of the run.

Heavily bundled runs , where the adjustment factor is the governing reduction and the correction factor offsets it.

Choosing the Correct Ambient — the Real Alaska Trap

The ambient you correct for is the highest ambient the conductor experiences over its run , because that is where it will run hottest. An Alaska service conductor that leaves a meter socket at minus 30 degrees C outside and terminates in a heated mechanical room does not get a 1.29 correction factor. The heated interior — often 25 to 30 degrees C, warmer inside a closed panel — is the controlling ambient.

This is where honest cold-weather design differs from the shortcut some prep material teaches:

  • Situation Controlling ambient
  • Outdoor riser into a heated building The heated interior
  • Conductor entirely in an unheated crawl space or arctic entry The design low for that space

Conductor above a boiler and then through a cold chase The boiler-room ambient (subject to the 10-ft/10-percent exception in 310.14(A)(2))

Rooftop raceway exposed to sun Historically the rooftop adder; see 310.15(B)(3) in the 2020 Code for the current treatment of raceways exposed to sunlight on rooftops

Also remember that a design ambient is not a record low . Sizing to the coldest hour Fairbanks has ever recorded is not a defensible engineering basis; the ambient used should be one the installation actually operates in continuously.

What Cold Does Not Change

A cold ambient does not :

  • permit an overcurrent device larger than 240.4 allows;
  • permit exceeding the termination temperature limit of 110.14(C);
  • eliminate the adjustment factors of Table 310.15(C)(1) for bundled conductors;

change conductor voltage drop , which is a resistance-and-length problem, not a thermal one. Cold copper is actually slightly less resistive, which marginally helps voltage drop, but no Code calculation credits it.

The Rooftop Note

Earlier editions of the Code contained a rooftop ambient adder table for raceways exposed to sunlight on or above rooftops. The 2020 NEC addresses raceways and cables exposed to sunlight on rooftops in 310.15(B)(3) , which states that the ambient temperature adjustment for circular raceways exposed to sunlight on or above rooftops shall be 33 degrees C (60 degrees F) added to the outdoor ambient where the raceway is less than 7/8 inch above the roof . Look this up rather than trusting memory of an older edition — the rule has moved and changed more than once.