Box fill on the 2026 NEC, step by step
Box fill questions look easy and still cost points, because one item is counted wrong: the device, the clamps or the grounding conductors. The method is two steps. Count the items in volume allowances, then multiply by the volume for the conductor size. The rules are in 314.16 of the 2026 NEC. We found no 2026 change to the counting method or the volumes, but subdivision numbers moved between editions, so find each rule in your own book before exam day.
Step 1: count the allowances
Connectors fitted in a knockout from outside the box are not internal clamps and count 0. Terminal blocks have their own counting rule in 314.16; check it in your book if a question shows one.
| Item in the box | Counts as | Sized by |
|---|---|---|
| Each insulated conductor that comes from outside and ends or is spliced in the box | 1 each | That conductor |
| Conductor that passes through without a splice | 1 | That conductor |
| Unbroken conductor looped in the box, 12 in or longer | 2 | That conductor |
| Pigtail or jumper that never leaves the box | 0 | - |
| Internal cable clamps (one or more) | 1 in total | Largest conductor in the box |
| Luminaire stud or hickey | 1 for each type | Largest conductor in the box |
| Device or equipment on one yoke or strap | 2 for each yoke | Largest conductor connected to that device |
| Equipment grounding conductors, up to four | 1 in total | Largest grounding conductor |
| Each grounding conductor beyond four | 1/4 more | Largest grounding conductor |
Step 2: multiply by the volume per conductor
The volume for each conductor size is in a table in 314.16. The three sizes you will use most: 14 AWG = 2.00 in³, 12 AWG = 2.25 in³, 10 AWG = 2.50 in³.
Box volume comes from Table 314.16(A) for standard boxes, or from the volume marked on the box. Add the volume of a plaster ring or extension only if it is marked on it.
Worked example 1: how many conductors fit
A 4 in square box, 1-1/2 in deep, has 21.0 in³ in Table 314.16(A). How many 12 AWG conductors may it hold if nothing else is in it?
21.0 ÷ 2.25 = 9.33. You may not exceed the volume, so round down: 9 conductors.
Common wrong answer: 10 (rounded up).
Worked example 2: a receptacle box
Two 12/2 NM cables enter a box with internal clamps. One receptacle is installed. Each cable has a bare grounding conductor.
Count: 4 insulated conductors + 1 for the clamps + 2 for the receptacle yoke + 1 for both grounding conductors = 8. Minimum volume: 8 × 2.25 = 18.00 in³.
Common wrong answers: 15.75 in³ (forgot the clamps, or counted the yoke as 1) and 20.25 in³ (counted each grounding conductor separately).
Worked example 3: two conductor sizes
A box holds a 20 A circuit (two 12 AWG conductors) and a 15 A circuit (four 14 AWG conductors). A switch on the yoke is connected only to the 14 AWG conductors. Grounding conductors: one 12 AWG and one 14 AWG. No clamps.
12 AWG: 2 × 2.25 = 4.50 in³. 14 AWG: 4 × 2.00 = 8.00 in³. Switch yoke, sized by the largest conductor connected to it (14 AWG): 2 × 2.00 = 4.00 in³. Grounding conductors, sized by the largest (12 AWG): 2.25 in³.
Total: 4.50 + 8.00 + 4.00 + 2.25 = 18.75 in³.
Common wrong answers: 19.25 in³ (sized the yoke by the largest conductor in the box) and 20.25 in³ (used 2.25 in³ for everything).
Worked example 4: more than four grounding conductors
A box has six 12 AWG equipment grounding conductors.
The first four count as 1. The other two add 1/4 each: 1 + 0.25 + 0.25 = 1.5 allowances. 1.5 × 2.25 = 3.375 in³.
Common wrong answer: 2.25 in³ (the extra 1/4 allowances were left out, which was the rule in older books).
Worked example 5: pass-through, loop and pigtails
All 14 AWG. Two conductors pass through without a splice. One more conductor is not cut but is looped in the box with 14 in of length. Four conductors come in and are spliced, and two short pigtails are added to the splices without leaving the box. Ignore devices and grounding conductors here.
Count: 2 pass-through + 2 for the loop + 4 spliced + 0 for the pigtails = 8. 8 × 2.00 = 16.00 in³.
Common wrong answers: 14.00 in³ (loop counted as 1) and 20.00 in³ (pigtails counted).
Habits that save points
- List every item in the box before you calculate: conductors, clamps, studs, yokes, grounding conductors.
- A yoke is sized by what is connected to it. Clamps and studs are sized by the largest conductor in the box.
- All grounding conductors together start at one allowance. Add 1/4 only for each one beyond four.
- Round down when the question asks how many conductors fit. Never round the required volume down.
- Use the volume marked on the box when the question gives it, and Table 314.16(A) only for standard boxes.
Check yourself in 30 minutes
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- Electrical License Renewal: 2026 NEC 314.16(B) box fill (2026 NEC changes, updated 19 Jan 2026)
- Tsai Electro: NEC 314 box fill for apprentices, counting rules and worked examples (2026 NEC)
- Zing: NEC Table 314.16(A) metal box fill (NEC 2026)
- Box Fill Calculator: NEC 314.16 complete guide (volumes per conductor, counting rules)
- EC&M: NEC requirements for sizing boxes for conductor fill (looped conductors)
- PSI Candidate Information Bulletin, Texas electrician examinations (updated 15 Sep 2026)
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