Short answer: Read the conductor’s ampacity from Table 310.16 in its insulation’s temperature column. Multiply by the ambient temperature correction factor, then apply the adjustment factor where the conductor-count rules require it, after checking the exceptions for your wiring method. Then compare the result with the ampacity the equipment terminations allow, and check the overcurrent-protection and load requirements, including the small-conductor rules. Write every factor down.
Ampacity questions punish one skipped factor. The Texas Journeyman Practice Pack has step-by-step solutions for every calculation question. Each one shows the rule used, the given numbers, every step, the rounding where it matters and why each wrong option is wrong. US$39, one payment.
Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the method in our own words and quote only the few table values each example needs. Look up every value in your own 2026 NEC. Texas uses the 2026 NEC for installations and exams from September 1, 2026 (TDLR rule adoption notice, TDLR exam page).
Six core ampacity checks for the worked examples
| Step | What you do | Where to look (2026 NEC) |
|---|---|---|
| 1 | Find the insulation’s temperature rating (THHN is 90°C in dry locations) | Table 310.4(1) |
| 2 | Read the ampacity in that temperature column | Table 310.16 |
| 3 | Correct for ambient temperature other than 30°C (86°F) | Section 310.15(B); confirm the table number in your 2026 book |
| 4 | Adjust for more than three current-carrying conductors together | Section 310.15(C); confirm the table number in your 2026 book |
| 5 | Determine the allowable ampacity at the actual equipment termination temperature, including the equipment rating, conductor size, and listing conditions; do not assume all equipment uses the same temperature column | 110.14(C)(1) |
| 6 | Check the overcurrent device limits | 240.4(B), 240.4(D), standard sizes in 240.6(A) |
Two additional issues to check are rooftop sunlight exposure and whether a conductor counts as current-carrying for adjustment purposes. The precise 2026 provisions, including exceptions and any rooftop temperature-adder requirements, must be confirmed in the applicable code text before these rules are used in an installation.
Why start at 90°C? For THHN conductors used under conditions that permit the 90°C insulation rating, that ampacity may be used as the starting point for the correction and adjustment calculations. Separately verify wet-location suitability, termination temperature limits, and all other conditions of use. The adjusted result must also be compared with the ampacity the equipment terminations allow, which is why step 5 exists. The insulation rating alone does not set the terminal temperature.
Worked example 1: nine conductors in one raceway
Nine current-carrying 12 AWG THHN copper conductors share one EMT run at a normal 30°C (86°F) ambient. Terminations are rated 75°C.
- Table 310.16, 12 AWG copper, 90°C column: 30 amperes.
- Ambient is 30°C, so the correction factor is 1.00.
- Seven to nine current-carrying conductors: adjustment factor 70%. 30 × 0.70 = 21 amperes.
- Terminal check: the 75°C column for 12 AWG copper is 25 amperes. 21 is lower, so 21 amperes governs.
- Overcurrent limit: 240.4(D) limits 12 AWG copper to 20 amperes.
- The adjusted ampacity is 21 amperes, and a 20-ampere overcurrent device passes the checks shown in this example. Actual installations must also meet the applicable load, equipment, and 2026 NEC requirements.
Check: if you started at the 75°C value by mistake, 25 × 0.70 = 17.5 amperes, and you would wrongly reject 20-ampere circuits.
Worked example 2: a hot attic
Three current-carrying 4 AWG THHN copper conductors run through an attic that reaches 113°F (45°C). Terminations are rated 75°C.
- Table 310.16, 4 AWG copper, 90°C column: 95 amperes.
- Correction for 41°C to 45°C in the 90°C column: 0.87. 95 × 0.87 = 82.65 amperes.
- Three current-carrying conductors: no adjustment.
- Terminal check: the 75°C column for 4 AWG copper is 85 amperes. 82.65 is lower, so 82.65 amperes governs.
- Overcurrent device: 80 amperes is a standard size and does not exceed 82.65. A 90-ampere overcurrent device must not be selected from this calculation alone. The next-higher-standard-rating provision in Section 240.4(B), if applicable, requires separate verification of all conditions, including equipment limitations and load requirements.
Check: 95 × 0.87 = 82.65, and 82.65 ÷ 0.87 = 95. The arithmetic holds both ways.
Worked example 3: what counts as current-carrying?
One conduit holds two 120/240-volt, 3-wire multiwire branch circuits (each with two ungrounded conductors and a shared neutral) and one equipment grounding conductor, all 10 AWG THHN copper, at 30°C.
- Ungrounded conductors: 2 circuits × 2 = 4. These count.
- Neutrals: The two shared neutrals are excluded from the adjustment count in this example, assuming each circuit qualifies under the applicable 2026 NEC neutral-counting rule for a single-phase, 120/240-volt, 3-wire multiwire branch circuit.
- Equipment grounding conductor: never counted.
- Count: 4 current-carrying conductors, so the adjustment factor is 80%.
- Ampacity: 10 AWG copper at 90°C is 40 amperes. 40 × 0.80 = 32 amperes. The 75°C column value is 35 amperes, so 32 governs. 240.4(D) limits 10 AWG copper to a 30-ampere overcurrent device.
Factors used on this page
| Factor | Value | Used in |
|---|---|---|
| 4 to 6 current-carrying conductors | 80% | Example 3 |
| 7 to 9 current-carrying conductors | 70% | Example 1 |
| Ambient 41°C to 45°C, 90°C column | 0.87 | Example 2 |
| Ambient 36°C to 40°C, 90°C column | 0.91 | Check yourself, question 2 |
For any other condition, use the full tables in your code book. These worked examples are for code-study practice only. They do not replace a complete conductor-sizing calculation, equipment instructions or review by your supervising electrician and the authority having jurisdiction.
Mistakes that cost points
- Starting a correction or adjustment calculation from the 60°C or 75°C ampacity when the conductor has a usable 90°C insulation rating, without separately checking the termination-temperature limitation.
- Forgetting the termination check, so the final answer exceeds what the lugs allow.
- Counting the equipment grounding conductor as current-carrying.
- Applying only one factor when both a hot ambient and a crowded raceway apply.
- Picking a 25-ampere breaker for 12 AWG copper because the ampacity came out at 25. The small-conductor rule in 240.4(D) still caps it at 20.
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A 2026 NEC reference for the ampacity tables
NFPA 70, National Electrical Code (NEC) 2026, softbound. The tables and sections in this article, including Table 310.16, are in Article 310 of the 2026 edition. Read each one in your own copy before you rely on it.
Check yourself
Our own practice questions. Answers below each.
- Five current-carrying 10 AWG THHN copper conductors, 30°C, 75°C terminations. What is the ampacity, and the largest overcurrent device under the small-conductor rule? Answer: 40 × 0.80 = 32 amperes (below the 35-ampere 75°C value). The device is limited to 30 amperes.
- Three current-carrying 6 AWG THHN copper conductors at 104°F (40°C), 75°C terminations. What governs? Answer: 75 × 0.91 = 68.25 amperes, but the 75°C column value is 65 amperes, so 65 amperes governs.
- Six conductors share a raceway: four ungrounded conductors, one neutral that carries only the unbalanced current of a single-phase 120/240-volt, 3-wire circuit, and one equipment grounding conductor. Under the applicable neutral-counting exception, how many conductors count for ampacity adjustment? Answer: 4. Leave out the neutral and the grounding conductor, and confirm the neutral-counting exception in the 2026 NEC text.
FAQ
What is ampacity?
The maximum current, in amperes, a conductor can carry continuously under its conditions of use without exceeding its insulation temperature rating. Conditions of use include ambient temperature and how many current-carrying conductors are grouped together.
Do you derate from the 90°C or the 75°C column?
When the conductor’s insulation rating and conditions of use permit the 90°C ampacity to serve as the starting point, apply the applicable correction and adjustment factors to that value. Then check the equipment termination limits, overcurrent protection, and all other applicable requirements. The insulation rating does not by itself permit a 90°C termination ampacity.
Does the ground wire count as a current-carrying conductor?
No. Equipment grounding conductors are not counted when you apply the adjustment factors for more than three current-carrying conductors.
What is the maximum breaker for 12 AWG copper?
Under the small-conductor rule in 240.4(D), 20 amperes for 12 AWG copper, 15 amperes for 14 AWG and 30 amperes for 10 AWG, unless a specific code rule allows otherwise.
Sources checked October 2026
- TDLR rule adoption notice, 2026 NEC: https://www.tdlr.texas.gov/news/rulemaking/2026/09/01/commission-adopts-rules-12/
- TDLR exam page: https://www.tdlr.texas.gov/electricians/elecexam.htm
- PSI Candidate Information Bulletin for TDLR (updated 9/15/2026): https://test-takers.psiexams.com/api/content/bulletin/1682
- NFPA 70 (NEC), 2026 edition, Article 310 and Table 310.16, read in your own copy: https://www.nfpa.org/codes-and-standards/nfpa-70-standard-development/70
