Master Electrician Exam Prep: How to Spot a Wrong Answer in a Box Fill Question

Always Question the Source — Even the Experts

In a recent Master the NEC session, instructor Paul Abernathy — a 40-year veteran who serves on Code-Making Panels 5 and 17 — made a point that every exam candidate should take to heart: question the source, even when it’s a trusted one.

“You need to always question the source, even us… we all make mistakes and sometimes things that get transposed or moved around from cycle to cycle can be a little off.”

He then walked through a real box fill question whose answer key was simply wrong. Not because the question was bad — it was a good question — but because the “correct” answer didn’t match the code. Working it through is a perfect lesson in both box fill calculations and critical thinking under exam pressure.

The Question

Here’s the scenario, straight from a 2026 NEC practice database:

What is the minimum volume in cubic inches required for a box containing:

• Two 10 AWG THHN conductors passing through the box
• Four 16 AWG conductors spliced in the box
• Two 12 AWG THHN conductors terminating on a single 20-amp receptacle
• One 12 AWG bonding jumper

The answer choices were 18, 20, 21.75 and 22 cubic inches, and the database flagged 22 as correct.

Read Only What’s on the Page

Before touching the math, Abernathy makes the most important point of the whole exercise: don’t read anything into the question that isn’t there.

“We only want to answer the question based on what we’re given… Remember, don’t read anything into the question that’s not there. It’s not a trick.”

That discipline matters here in three places:

• The two 10 AWG conductors simply “pass through.” Nothing says they loop or coil, so we don’t treat them as doubled. Under 314.16(B), a loop or coil only counts as two conductors when it’s an unbroken length of at least twice the free-conductor minimum — roughly 12 inches. No length was given, so we count each one once.
• The four 16 AWG conductors are not described as fixture wire. The omission allowance in 314.16(A) lets you leave out an equipment grounding conductor or up to four fixture wires smaller than 14 AWG — but only if the question tells you that’s what they are. It doesn’t, so we count them as ordinary conductors.
• The single 12 AWG bonding jumper originates inside the box and runs to the device. It’s not an equipment grounding conductor entering from a raceway, and there’s only one. A bonding jumper that starts and ends inside the box isn’t counted toward fill.

Working the Math with Table 314.16(B)(4)

The per-conductor volume allowances come from Table 314.16(B)(4):

Conductor	Volume each
16 AWG	1.75 cu in
12 AWG	2.25 cu in
10 AWG	2.50 cu in

Now count each item:

1. Two 10 AWG passing through → 2 × 2.50 = 5.00 cu in
2. Four 16 AWG spliced → 4 × 1.75 = 7.00 cu in
3. Two 12 AWG terminating on the receptacle → 2 × 2.25 = 4.50 cu in
4. The receptacle (device fill) → a yoke or strap gets a double volume allowance based on the largest conductor connected to it. The largest here is 12 AWG, so 2 × 2.25 = 4.50 cu in

Add them up:

5.00 + 7.00 + 4.50 + 4.50 = 21.00 cubic inches

The box must provide at least 21.00 cubic inches. Of the answer choices, the smallest one that satisfies that minimum is 21.75 — not 22.

“If we need the minimum volume, then it would be 21.75… but it says it needs to be 22 cubic inches. We just did the math, and the math was 21 cubic inches.”

The answer key was wrong.

What Would Have Made 22 Correct?

Abernathy presses the point further, because a good electrician asks how an answer could be justified. The only way to add more fill would be if that bonding jumper were actually an equipment grounding conductor entering the box. Under the grounding fill rules in 314.16(B), up to four equipment grounding conductors count as a single conductor — based on the largest one present.

With a 10 AWG circuit in the box, that grounding conductor would likely be 10 AWG (2.50 cu in):

21.00 + 2.50 = 23.50 cubic inches

But that only proves the point harder — at 23.50 cubic inches, neither 21.75 nor 22 would work. So no matter how you read the question, 22 cannot be the right answer.

“So at the end of the day — good question, bad answer.”

The Real Lesson: Challenge It, Then Learn From It

The takeaway isn’t that practice databases are untrustworthy. It’s that every question is a chance to learn if you slow down and dissect it.

• Treat each question as a navigation exercise, not a speed test — break it into pieces and tie each piece back to a specific code section.
• Take the wording at face value. Don’t import how you’d wire it in the field into an exam answer.
• When the math and the answer key disagree, trust your work, then go verify against the code.

“There is no bad question if it makes you learn. But if you challenge a question, make sure you have the ability for somebody to answer that for you.”

That last part is the practical warning. Challenging an answer is only useful if you can confirm who’s right. Studying from a static stack of questions with no explanations leaves you stuck the moment one is wrong. The fix is a study tool that shows its work.

How NEC Mastery Fits Into This

Paul’s box fill walkthrough lands on two skills: knowing exactly which part of 314.16 governs each conductor, and being able to verify a questionable answer instead of just trusting it. That’s precisely what NEC Mastery is built to support.

• 8,000+ exam-style questions give you the repetition to internalise calculations like box fill — work them slowly first, dissecting each one the way Paul demonstrates, then switch to timed runs near exam day.
• Detailed explanations that reference specific NEC articles mean every answer points you back to the governing section — Table 314.16(B)(4), the device fill rule, the 314.16(A) omission allowance — so when something looks off, you can check the reasoning instead of guessing.
• Timed mock exams weighted to your exam type let you practise reading carefully under pressure, so you stop importing real-world assumptions and start answering only what’s actually on the page.

You don’t need a magic technique to pass the master electrician exam. You need to dissect questions, cite the code, and verify your answers — and have a tool that lets you do all three.