Okay, let's break down these cable tray sizing problems. The key here is understanding the NEC rules, specifically for cable tray fill. Without knowing the chapter of the code, this is what is referenced.

Key NEC Sections (Likely Relevant - check your Prep Book's reference)



NEC 392.22 (A) (1) (b)

Single-Conductor Cables. This is the likely section we're dealing with since THWN-2 conductors are usually run as single conductors in cable tray. This section outlines different fill requirements based on cable size and tray width.

NEC 392.22 (A)(3)

Any Mixture of Cables This section can be used where a tray consists of the same cable sizes.

NEC 392.22(A)(5)

Solid Bottom Cable Tray. This section should be taken into consideration depending on the application.

Understanding the Fill Rules

The core concept is that NEC limits the total cross-sectional area of the conductors in the cable tray based on the inside width of the tray.

Assumptions



Ladder or Ventilated Trough Cable Tray

Since no tray type is specified, we will assume a ladder or ventilated trough type. This is a reasonable assumption as it's the most common type.

Single-Conductor Cables in a Single Layer

We assume the conductors will be laid out in a single layer, which is typical for single conductor installations.

THWN-2 is a common insulation type, and we'll use NEC Chapter 9, Table 5 to find conductor diameters.Step-by-Step Calculation - Question 1 (Ten 2/0 THWN-2 Conductors)


Find Conductor Diameter

Look up the diameter of 2/0 THWN-2 in NEC Chapter 9, Table 5. It's listed as

0.467 inches.2. Calculate Total Conductor Diameter

Multiply the diameter of one conductor by the number of conductors: 0.467 inches/conductor 10 conductors = 4.67 inches.
3.

Apply the NEC Fill Rule (392.22 (A) (1) (b) )

This rule says that the sum of the diameters of all single-conductor cables in a cable tray cannot exceed the inside width of the cable tray. Therefore, you need a cable tray with an inside width greater than 4.67 inches.
4.

Closest Standard Size

Cable trays come in standard widths. The next standard size larger than 4.67 inches is

6 inches. However, 6-inch tray fill is limited to 5.5 in total conductor diameter. Thus, the next size up is 12-inch.

Therefore, 12 inches is the most likely answer.Step-by-Step Calculation - Question 2 (Ten 1/0 THWN-2 Conductors)


Find Conductor Diameter

Look up the diameter of 1/0 THWN-2 in NEC Chapter 9, Table 5. It's listed as

0.414 inches.2. Calculate Total Conductor Diameter

Multiply the diameter of one conductor by the number of conductors: 0.414 inches/conductor 10 conductors = 4.14 inches.
3.

Apply the NEC Fill Rule (392.22 (A) (1) (b) )

This rule says that the sum of the diameters of all single-conductor cables in a cable tray cannot exceed the inside width of the cable tray. Therefore, you need a cable tray with an inside width greater than 4.14 inches.
4.

Closest Standard Size

Cable trays come in standard widths. The next standard size larger than 4.14 inches is

6 inches. However, 6-inch tray fill is limited to 5.5 in total conductor diameter. Thus, the next size up is 8-inch.

Therefore, 8 inches is the most likely answer.Important Considerations and Potential Discrepancies



Prep Book Errors

It's possible the prep book has errors. These things happen.

Local Amendments

Always be aware of local electrical code amendments, as they can supersede the NEC.

Specific Tray Manufacturer

The exact dimensions of cable trays can vary slightly by manufacturer. While the NEC focuses on inside width, the physical space available can be a factor.

Recommendation


Double-Check your NEC Edition

Make sure your NEC edition matches the one the prep book is based on. There might be minor changes.2.

Check the Prep Book's Rationale

If the prep book provides a detailed explanation of why* the answer is what it is, review it closely. They might be using a different section of the code or making a different assumption.3.

Consider Reaching Out to the Publisher

If you're still convinced the answer is wrong, consider contacting the publisher of the prep book. They may have an errata sheet or be able to provide clarification.4.

Focus on Understanding the Principles: Ultimately, the most important thing is to understand the NEC rules and how to apply them. If you understand the principles, you'll be able to solve similar problems even if the prep book has an error.Good luck with your exam prep! Let me know if you have any other questions. Flag for review