Guide to Box Fill / Sizing Questions on NEC Electrical Exams

NEC exam questions often focus on determining box sizes based on their contents (wires or equipment). Proper box sizing is crucial for safety, preventing overcrowding and potential hazards. These questions are frequent on journeyman, master, and contractor exams in the US. A typical box fill / box sizing question might look like this:

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Example Box Fill Questions on NEC Electrical Exams

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The smallest size box for four No. 10 AWG XHHW and two No. 14 AWG XHHW conductors is ___________.

Determine the minimum cubic inch capacity for a square junction box accommodating three 16 AWG conductors, two 14 AWG conductors, one 14 AWG equipment grounding conductor, two internal cable clamps, a single-gang device yoke and a pigtail. The required capacity is ________ cubic inches.

A 4 11/16-inch (119 mm) square box with a depth of 2 1/8 inches (54 mm) contains six 14 AWG conductors, three 12 AWG conductors, and two 10 AWG equipment grounding conductors. The number of additional 14 AWG conductors that can be added to this box without exceeding its capacity is ________.

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How to Identify a Box Fill / Sizing Question on NEC Electrical Exams

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Key phrases to look out for in the box fill question:

• Box capacity, size, or measurement
• Number of conductors
• Multiple conductors including grounding conductors
• Mention of clamps, yokes, support fittings, or devices

When you spot these elements:

1. Confirm it's a box fill question (not conduit fill)
2. Refer to NEC section 314.16

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Box Fill Articles: NEC 314.16

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To correctly apply NEC Article 314.16, focus on three main elements:

• Conductor volume: Refer to Table 314.16(B)(1) to determine the volume allowance for each conductor based on its size.
• Box dimensions: Use Table 314.16(A) to find the total volume of different box sizes and shapes.
• Box fill calculations: Follow the guidelines in 314.16 to count the number of items (conductors, devices, clamps, etc.) allowed in a box.

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To summarize, here are the rules:

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Most questions will be focused on conductors, support fittings, devices, and equipment grounding conductors. Based on our student’s feedback, we rarely see coiled conductors or isolated equipment grounding conductors in questions but the rules are above nonetheless.

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Walkthrough for a NEC Electrical Exam Box Fill Question

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Question:

Determine the minimum cubic inch capacity for a square junction box accommodating three 16 AWG conductors, two 14 AWG conductors, three 14 AWG equipment grounding conductor, two internal cable clamps, a single-gang device yoke connected to 14 AWG conductors and a 14 AWG pigtail. The required capacity is ________ cubic inches.

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Step 1: Identify all items inside of the questions, so for this question:

• 3 x 16 AWG conductor
• 2 x 14 AWG conductor
• 3 x 14 AWG equipment grounding conductor
• 2 x internal cable clamps
• 1 x device yoke connected to 14 AWG
• 1 x 14 AWG pigtail

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Step 2: Determine the count towards box fill for each item

• 3 x 16 AWG conductor -> 3 x 16 AWG
• 2 x 14 AWG conductor -> 2 x 14 AWG
• 3 x 14 AWG equipment grounding conductor -> 1 x 14 AWG (counts as one & not above 4 conductors)
• 2 x internal cable clamps -> 1 x 14 AWG (counts as one)
• 1 x device yoke connected to 14 AWG -> 2 x 14 AWG (counts as two)
• 1 x 14 AWG pigtail -> Not applicable as it originates and does not leave the box

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Step 3: Sum up the different conductors and multiply by the volume in Table 314.16(B)(1)

• 3 x 1.75 in³ (16 AWG volume allowance) = 5.25 in³
• 6 x 2.00 in³ (14 AWG volume allowance) = 12.00 in³ 

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Step 4: Max box size calculation

• 5.25 in³ + 12.00 in³ = 17.25 in³

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