Use Table 310.16 for its specified raceway, cable and direct-burial conditions. Table 310.17 covers single insulated conductors in free air. Both tables appear in 2020 NEC Article 310. Choose the table that matches the installation before choosing a wire size or temperature column.
Watch JCR explain conductor and terminal temperatures
The installation table is only one part of sizing a conductor. Victor’s lesson on 110.14(C) explains why the equipment’s temperature rating still limits your choice.
Watch JCR explain conductor and terminal temperaturesWatch the existing JCR lesson on YouTubeRead the table heading first
| Check | Table 310.16 | Table 310.17 |
|---|---|---|
| Installation | Not more than three current-carrying conductors in raceway, cable or earth | Single insulated conductors in free air |
| Base ambient temperature | 30°C / 86°F | 30°C / 86°F |
| What still needs checking | Insulation, terminals, ambient temperature correction (310.15(B)), adjustment and applicable equipment rules | Insulation, terminals, ambient temperature correction (310.15(B)) and actual free-air installation conditions |
A cable installed outdoors is not automatically a free-air installation. Neither is a conductor simply because part of its route is visible. Use the conditions stated in the Code.
Why are free-air values higher?
Conductors generate heat as they carry current. A single conductor in free air can shed heat differently from conductors grouped inside a raceway or cable. The table values account for those different conditions.
Putting the same conductor in conduit changes the installation. You cannot keep a free-air ampacity just because it is a more convenient number.
Practice ampacity tables with JCR
Turn lessons like this into test-day skills. Find the rule, work the calculation and practice with JCR’s mock exam questions.
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See CE courses and class datesDo terminal ratings still limit free-air conductors?
Yes. Under 110.14(C)(1), equipment termination provisions are generally based on Table 310.16 unless the equipment is listed and marked otherwise. A higher value in Table 310.17 does not automatically allow that current through the connected equipment.
The UL panelboard guide explains how equipment markings and conductor temperature ratings work together. A 90°C insulation marking does not make a 75°C terminal a 90°C terminal. Use our ampacity correction and adjustment guide to work through the remaining checks.
A useful order for exam questions
- Identify the conductor material, insulation and installation method.
- Choose the applicable table and read its assumptions.
- Apply required correction and adjustment factors.
- Check the equipment termination limit and the cable or equipment article.
- Finish the load-sizing and overcurrent-protection checks.
Check the special rules for the question. Dwelling service and feeder sizing under 310.12 has specific conditions. Voltage drop is another check. It is not an ampacity table.
Practice question
Three current-carrying copper conductors are in a raceway at 30°C. May you use Table 310.17 because its values are higher?
No. That installation fits Table 310.16’s stated conditions. Select the proper insulation and termination temperature, then check any other applicable requirements.
Common questions
Is Table 310.17 only for utilities?
No. Its use is determined by its installation conditions. Whether utility work is outside the NEC’s scope is a separate question under 90.2. A table number alone does not decide that.
Can I use an online ampacity chart?
Use one that identifies its code edition and table. For example, HELU’s published chart is labeled for the 2023 NEC. For a 2020 exam, work from the approved 2020 reference instead of assuming every chart is interchangeable.
Keep practicing
Try our most-missed NEC mock exam questions and 2020 NEC practice questions. Find the rule for each answer, work the calculation, and repeat the questions you miss.
Prepared October 25, 2026, for the 2020 NEC. Use the edition required by your licensing exam. On the job, use the locally adopted code and amendments.
