TDLR box fill practice
Box fill adds up the cubic inches every conductor, device, clamp, and ground takes up, then checks it against the box. This is a TDLR Calculations-part question type, now scored on its own. Here is the pattern, one worked example, and an original question to try.
Last reviewed June 2026
One worked example
A 4 by 4 by 1.5 inch square box has four 12 AWG conductors, one device, the grounds, and an internal clamp. Does it have room?
- 1Name the problem
This is a box fill. Every conductor, device, clamp, and ground takes up cubic inches. We add them all up and compare to what the box holds.
- 2Count the conductors
Each wire that enters and leaves counts once, sized by its own AWG (Table 314.16(B)).
4 × 12 AWG = 9.00 cu in
Subtotal: 9.00 cu in
- 3Add devices, grounds, and clamps
All the grounds together count as one, sized by the largest. Each device yoke counts as two of the largest conductor. An internal clamp counts as one.
Yokes: 1 × 2 × 2.25 = 4.50 cu in
Grounds: 1 × 2.25 (12 AWG) = 2.25 cu in
Clamp: 1 × 2.25 = 2.25 cu in
- 4Add it up
That total is the volume the box has to give you.
Required = 9.00 + 4.50 + 2.25 + 2.25
Required = 18.00 cu in
- 5Compare to the box
The 4 in. square × 1-1/2 in. holds 21.00 cu in. You need 18.00. The box has room. You are clear.
Box 21.00 cu in vs needed 18.00 cu in
Try an original question
A device box holds six 12 AWG conductors, one duplex receptacle, and the equipment grounds. Each 12 AWG counts as 2.25 cubic inches.
What is the minimum box volume required?
- A15.75 cubic inches
- B18.00 cubic inches
- 20.25 cubic inches
- D22.50 cubic inches
Answer C. Count six conductors, plus 2 for the duplex receptacle yoke, plus 1 for all the grounds together, which is 9. 9 x 2.25 = 20.25 cubic inches (Table 314.16(B)).
- NEC 2023 Table 314.16(B)
- NEC 2023 314.16(B)(1) through (B)(5)
Why the other answers tempt you
- A: 15.75 forgets the device. A duplex receptacle on a yoke counts as two conductors.
- B: 18.00 counts the device as one instead of two, or skips the grounds.
- D: 22.50 counts the grounds as two. All the equipment grounds together count as one conductor.
Train it, one step at a time
Check every domain in 15 minutes.
Drill this topic, then run the diagnostic to see if anything else is leaking points on the TDLR Journeyman exam. Free, no signup.
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