What Chapter 3 Does

Chapter 3 explains how conductors, cables, raceways, boxes, cabinets, and supporting systems must be selected and installed. A safe installation must use a wiring method permitted for the location, protect conductors from physical and environmental damage, stay within conductor ampacity, and provide enough space for conductors and splices.

General Requirements for Wiring Methods

Article 300 — General Requirements for Wiring Methods

Article 300 applies broadly unless another article modifies it. High-use requirements include:

• 300.4: Protect conductors and cables from nails, screws, sharp edges, and physical damage.
• 300.5: Underground installation requirements, including cover, emerging raceways, splices, backfill, and sealing.
• 300.6: Protect metal equipment and wiring methods from corrosion.
• 300.9: The inside of raceways installed in wet locations above grade is a wet location.
• 300.15: Use a box, conduit body, or fitting where required at conductor splices, terminations, and transitions.
• 300.21: Firestop penetrations so the required fire-resistance rating is maintained

Basic Table 300.5(A) cover values

Wiring method or qualifying circuit	Minimum cover
Direct-buried cable or conductors	24 in.
RMC or IMC	6 in.
Nonmetallic raceway listed for direct burial	18 in.
Residential branch circuit, 120 V or less, GFCI protected, maximum 20 A	12 in.
Qualifying irrigation or landscape-lighting circuit, 30 V or less	6 in.

Table process

1. Select the row for the actual surface: ordinary location, slab, street, driveway, or another listed condition.
2. Select the column for the actual wiring method or qualifying circuit.
3. Read cover, measured from the top of the wiring method to finished grade.
4. Apply the notes and the rest of 300.5.

Practice problem

A listed PVC raceway is buried in an ordinary landscaped area that is not covered by another row. What cover is required?

Process: Table 300.5(A) → “all locations not specified” row → nonmetallic raceway column.

**Key answer: 18 in. minimum cover.

Article 310 — Conductors for General Wiring

Article 310 covers conductor construction, identification, insulation, uses, and ampacity. Table 310.16 is a common starting point, but it is not the last step.

Common copper ampacities from Table 310.16

AWG	60°C	75°C	90°C
14	15 A	20 A	25 A
12	20 A	25 A	30 A
10	30 A	35 A	40 A
8	40 A	50 A	55 A
6	55 A	65 A	75 A
4	70 A	85 A	95 A
3	85 A	100 A	115 A
2	95 A	115 A	130 A
1	110 A	130 A	145 A
1/0	125 A	150 A	170 A

Table 310.16 assumes no more than three current-carrying conductors and 30°C (86°F) ambient temperature.

Table 310.15(C)(1) adjustment factors

Current-carrying conductors	Adjustment factor
4–6	80%
7–9	70%
10–20	50%
21–30	45%
31–40	40%
41 or more	35%

Complete ampacity process

1. Calculate the load under the applicable article.
2. Identify conductor material, insulation, wiring method, and location.
3. Use 110.14(C) and equipment markings to determine the permitted terminal-temperature column.
4. Read the base ampacity from Table 310.16.
5. Apply ambient correction from Table 310.15(B)(1), when required.
6. Apply the current-carrying-conductor factor from Table 310.15(C)(1), when required.
7. Verify the result against terminal limits and overcurrent protection, including 240.4(D) for small conductors.

Practice problem

Nine 12 AWG copper THHN current-carrying conductors share a raceway at 30°C. What is the adjusted ampacity before applying terminal and small-conductor limits?

Process: Table 310.16, 90°C column = 30 A. Table 310.15(C)(1), 7–9 conductors = 70%.

30 A × 0.70 = 21 A

Key answer: 21 A adjusted ampacity; final branch-circuit overcurrent protection is typically limited to 20 A by 240.4(D).

How to use Table 310.16

How to use Table 310.16

1. Identify conductor material, size, insulation type, and installation method.
2. Determine the permitted temperature column using 110.14(C), equipment markings, and insulation rating.
3. Read the base ampacity from Table 310.16.
4. Apply ambient-temperature correction from Table 310.15(B)(1), when required.
5. Apply the current-carrying-conductor adjustment from Table 310.15(C)(1), when required.
6. Check the final value against the load, terminal limitations, and overcurrent-protection rules—including 240.4(D) for small conductors.

A cable assembly contains conductors. A raceway provides a pathway for conductors or cables. Cable tray is a support system; it is not interchangeable with conduit.

What Should You Check Before Installing?

Begin with the location: Is it dry, damp, wet, corrosive, or exposed to physical damage? Then confirm that the wiring method and its fittings are permitted for those conditions. Check how the system must be secured and supported, whether bends or expansion need special attention, and how grounding and bonding will be maintained.

Next, check conductor space and ampacity. Raceway fill, box fill, and conductor ampacity are separate calculations. They often require you to move between the wiring-method article, Articles 300, 310, and 314, Article 250, and Chapter 9.

For journeyman preparation, practice finding the rule that matches the exact conditions. Useful question types include selecting an appropriate wiring method, finding support spacing, checking bends between pull points, sizing raceways, and identifying unsuitable fittings. Use the edition specified by your exam; this section follows the 2023 NEC.