NEC Article 310 Practice Questions: Conductors & Ampacity

Table 310.16, temperature correction, and the more-than-three-conductors adjustment — the ampacity workflow behind nearly every conductor-sizing question on the exam.

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83 Article 310 questions in the bankWritten in the PSI computer-based exam formatEvery answer cites the NEC section

What Article 310 Covers on the Exam

Article 310 is where conductor sizing lives: the base ampacities of Table 310.16, temperature correction factors (Table 310.15(B)(1)), and the adjustment factors for more than three current-carrying conductors in a raceway (Table 310.15(C)(1)). Ampacity questions are rarely one-step on the exam — the standard pattern is base ampacity × temperature correction × fill adjustment, then check against the termination rating from 110.14(C).

Tested subtopics from Article 310

Base ampacities of insulated conductors — Table 310.16
Ambient temperature correction factors — Table 310.15(B)(1)
Adjustment factors for more than 3 current-carrying conductors — Table 310.15(C)(1)
When the neutral counts as a current-carrying conductor — 310.15(E)
Conductor insulation types and temperature ratings — Table 310.4

Drill a different article

83 questions in our bank cite Article 310. Practice the other high-yield articles too:

Art. 110 — General Requirements Art. 210 — Branch Circuits Art. 220 — Load Calculations Art. 230 — Services Art. 240 — Overcurrent Protection Art. 250 — Grounding and Bonding Art. 300 — Wiring Methods Art. 314 — Boxes & Box Fill Art. 430 — Motors Art. 450 — Transformers Art. 680 — Swimming Pools & Spas

How to Study Article 310

Practice the full derating chain until it is mechanical: start from the conductor insulation column in Table 310.16 (THHN = 90°C column for derating), multiply by the temperature correction, multiply by the fill adjustment, then compare against the 60°C/75°C termination limit — the final ampacity is the lowest of these. Exam questions are built to punish skipping the termination check.

Then put it under exam conditions: the free diagnostic scores you across every category and shows whether conductors & ampacity is actually a weak area — or whether your study time belongs somewhere else.

Sample Questions (From Our Real Question Bank)

Here are 4 real questions from our question bank. The free diagnostic quiz has 10 more.

NEC NEC Table 310.16Calculations - Conductor Sizing/Ampacity

1. Using NEC Table 310.16, what is the ampacity of a 6 AWG THWN-2 copper conductor in a raceway with no more than 3 current-carrying conductors at an ambient temperature of 30°C?

A55 amperes
B65 amperes
C75 amperes
D85 amperes

Show explanation

THWN-2 is a 90°C rated insulation type per NEC Table 310.16 column headings. Per Table 310.16, a 6 AWG copper conductor in the 90°C column has an ampacity of 75 amperes at 30°C ambient with no more than three current-carrying conductors. The 65 A value applies to THWN (75°C column). THWN-2 — note the '-2' suffix — is always the 90°C column.

NEC NEC Table 310.16, Table 310.15(B)(1)(1)Calculations - Conductor Sizing/Ampacity

2. A 3 AWG THHN copper conductor has an ampacity of 115A per Table 310.16 at 30°C. If the ambient temperature is 40°C, what is the corrected ampacity? (Correction factor for 36-40°C for 90°C rated conductor = 0.91)

A95.7 amperes
B100.1 amperes
C104.7 amperes
D110.0 amperes

Show explanation

Per Table 310.16, 3 AWG THHN (90°C) = 115A. Per Table 310.15(B)(1)(1), correction factor for 36-40°C ambient at 90°C = 0.91. Corrected ampacity = 115A × 0.91 = 104.65A ≈ 104.7A.

NEC NEC Table 310.15(C)(1)Calculations - Conductor Sizing/Ampacity

3. Six current-carrying conductors are installed in the same raceway. Per NEC Table 310.15(C)(1), what is the adjustment factor that must be applied to the conductor ampacity?

A100%
B80%
C70%
D50%

Show explanation

NEC Table 310.15(C)(1) requires 80% adjustment for 4-6 current-carrying conductors in the same raceway.

NEC NEC Table 310.15(C)(1)Calculations - Conductor Sizing/Ampacity

4. A raceway contains 9 current-carrying THHN copper conductors in an ambient temperature of 35°C. Per NEC Table 310.15(C)(1), what adjustment factor applies for 9 conductors?

A80%
B70%
C50%
D35%

Show explanation

NEC Table 310.15(C)(1) requires a 70% adjustment factor for 7-9 current-carrying conductors in the same raceway.

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Frequently Asked Questions

What is Table 310.16 and what does it give me?

Table 310.16 lists the allowable ampacities of insulated conductors in raceway, cable, or earth, by size and temperature rating — for example, 8 AWG copper THHN is 40 A at 60°C, 50 A at 75°C, and 55 A at 90°C. It assumes an ambient of 30°C and no more than three current-carrying conductors; anything else requires correction or adjustment.

Can I use the 90°C column for THHN conductors?

Only as the starting point for derating. You may apply temperature correction and adjustment factors from the 90°C column, but the final ampacity cannot exceed what the termination allows under 110.14(C) — usually the 75°C column (or 60°C for small circuits with unmarked terminations).

When do conductors in a conduit need to be derated?

When there are more than three current-carrying conductors in a raceway or cable, Table 310.15(C)(1) applies: 4–6 conductors derate to 80%, 7–9 to 70%, 10–20 to 50%, and so on. Neutrals that carry only unbalanced current generally do not count, but a neutral on a two-wire circuit or one serving nonlinear loads does.