MI box fill practice questions

What box fill questions actually ask

A box fill question on the Michigan journeyman exam hands you a box with a known set of conductors, maybe a receptacle or a switch, and some grounds, then asks for the minimum cubic inches the box must hold. Or it flips it: here is a box of a given volume, will this set of conductors fit. Either way the work is the same. You count by the rule in NEC 314.16(B), pull the per-conductor volume from Table 314.16(B), and do the arithmetic. There is no judgment call. The answer is a number, and it is either right or it is not.

That is exactly why box fill is worth drilling. Calculation and grounding questions can swing on a setup choice, but box fill rewards a clean process every single time. Get the count right and the box fits or it does not, with no gray area for the exam to hide a trap in.

The counting rule, in plain terms

NEC 314.16(B) sets out how each thing in the box counts. Memorize the three lines that trip people up and you have most of it.

• Each current-carrying conductor counts once. A wire that enters the box and lands on a device or splice counts as one. A wire that passes through unbroken still counts as one.
• A device yoke counts as two. A receptacle or switch on its strap counts as two conductors, based on the largest conductor connected to that yoke. Two strapped devices, four.
• All equipment grounds together count as one. No matter how many grounding conductors land in the box, the whole group counts as a single conductor for fill.

Then you convert counts to volume. Table 314.16(B) gives the cubic inches for each wire size: a 12 AWG conductor is 2.25 cubic inches, a 14 AWG is 2.0. Multiply your count for each size by that figure, add the sizes together, and that total is the minimum box volume the rule demands. We will not reprint the NEC table here, but those two figures are the ones most Michigan questions lean on.

Worked example

Here is a box you will see a version of on the exam. A device box holds six 12 AWG conductors, one duplex receptacle, and the equipment grounds. Find the minimum box volume in cubic inches. Work it in two stages: count first, then convert to volume.

Step 1: count the conductors

1. Six 12 AWG conductors. Each counts once, so that is 6.
2. One duplex receptacle. The device yoke counts as two, so add 2.
3. The equipment grounds. All of them together count as one, so add 1.
4. Total count: 6 + 2 + 1 = 9 conductors.

Step 2: convert the count to volume

Everything in this box is sized off 12 AWG, which is 2.25 cubic inches per conductor from Table 314.16(B). Multiply the count by that figure:

So the box has to provide at least 20.25 cubic inches. On the exam, the trap answers come from miscounting: people forget the receptacle yoke is two, or they count each ground separately, or they leave the yoke out entirely. The rule is fixed. Each current-carrying conductor counts once, a device yoke counts as two, and all grounds together count as one. Run it that way every time and the number falls out.

Why this topic costs points

Box fill looks easy on paper, and that is the problem. Candidates skim past it in prep because the rule fits on a notecard, then they lose the points on test day for two reasons. First, the count is where the mistake lives, not the multiplication. Forget that the receptacle yoke is two, or count four grounds as four instead of one, and your minimum volume is wrong before you ever touch the table.

Second is the clock. The Michigan exam is 80 questions in 2 hours 30 minutes, which is under two minutes a question, and you need 75% to pass. A box fill question you have drilled takes 30 seconds. One you are working out cold, flipping to Article 314 to reread the counting rule, can burn three or four minutes and still come out wrong. It is open book, but page-flipping for a rule you should know cold is how the time runs out before question 60. These are the give-me points. Leaving them on the table is what turns a near miss into a retake.

How to practice box fill for the Michigan exam

Drill the count until it is automatic, then practice it under a timer in your own tabbed 2023 NEC, the same book you will carry into the PSI test center. We will not claim a fixed number of box fill questions in the Michigan bank, because we have not counted them per topic and we do not invent numbers. What we will tell you is where it sits in your prep.

• Start with the free 15-minute diagnostic. It shows whether box fill and the rest of Chapter 3 are actually costing you points, or whether your time is better spent on calculations or grounding.
• Drill box fill inside the Michigan bank, which trains the LARA journeyman exam on the 2023 NEC, not the master or contractor exam, and cites the NEC article behind every answer.
• Rehearse the lookup. Tab Article 314 and Table 314.16(B) in your own book so a box fill question is a 30-second count, not a four-minute page hunt.