Maryland box fill practice questions
Box fill is one of those rules the open-book format rewards every time, because the answer lives in one table and one paragraph of the code. Learn how the count actually works, run one clean example, then drill it until the lookup is automatic under the clock.
Last reviewed June 2026
The rule, in plain terms
Box fill is the minimum cubic-inch volume a box needs for everything you are stuffing into it. The code does not make you guess. NEC 314.16(B) tells you what counts and how much each item is worth, and Table 314.16(B) gives the volume per conductor size. Get the count right and the math is just multiplication and addition.
- Conductors: each current-carrying conductor that enters the box and stays counts once.
- Device yoke: a receptacle or switch yoke counts as two, based on the largest conductor connected to it.
- Equipment grounds: all of them together count as one, based on the largest ground in the box.
That is the whole shape of it. The traps are forgetting the yoke counts twice and double-counting the grounds. Get those two right and box fill becomes free points.
Worked example
Here is the exact kind of setup the PSI exam asks. Work it the same way every time so the count is muscle memory by test day.
A device box holds six 12 AWG conductors, one duplex receptacle, and the equipment grounds. What is the minimum box volume?
Step 1: count the conductors
Six 12 AWG conductors count as six. The duplex receptacle yoke counts as two. All the equipment grounds together count as one. That gives 6 + 2 + 1 = 9 conductor equivalents.
Step 2: pull the volume from the table
Each 12 AWG conductor is 2.25 cubic inches per Table 314.16(B). Every one of those 9 equivalents is sized off 12 AWG here, so each is worth 2.25 cubic inches.
Step 3: multiply and add
9 × 2.25 = 20.25 cubic inches minimum. Any box you pick has to be at least that volume, so the box has to be 20.25 cubic inches or larger.
Notice what the open-book format is really testing. You did not have to memorize 2.25 cubic inches. You had to know the count rule lives in 314.16(B), flip to it fast, and read 2.25 off the table without second-guessing the yoke and the grounds. That is the skill the Maryland exam pays for.
Why this topic costs points
Box fill looks easy on paper, which is exactly why it bleeds points on the real exam. Three things go wrong under the clock.
- The yoke trap: people count the receptacle as one instead of two, and the whole answer is off.
- Double-counting grounds: three grounds in a box are still one count, not three. Miss this and the box looks bigger than the code requires.
- The slow lookup: open book does not help if you cannot land on Table 314.16(B) in seconds. Every slow flip is a question you do not finish on a 70-question, time-boxed exam.
None of that is hard once you have run the count a dozen times. It is hard the first time you see it under pressure with the clock running. That is the whole argument for drilling it now.
Practice box fill inside the Maryland bank
The fastest way to make this automatic is not reading about it. It is working original 314.16 questions on the 2020 NEC until the count and the lookup are reflex. Start with the free diagnostic to see whether box and conduit sizing is one of the article families costing you points, then put your reps there.
Ready to see where box fill ranks against your other weak spots? Take the free Maryland diagnostic.
Where these exam facts come from: Maryland State Board of Electricians — License Requirements, PSI Maryland Master & Journeyperson Candidate Bulletin, Maryland Electricians Act (SB 762, 2021), COMAR 09.09.02.01 (continuing education). Box fill rule and volumes paraphrased from NEC 314.16(B) and Table 314.16(B). Last reviewed June 2026.
Drill box fill until it is reflex
Box fill is repeatable points once the count is muscle memory. The free diagnostic shows whether box and conduit sizing is dragging your score, so you drill the right article instead of everything.