Volume allowance per conductor
| Conductor size | Volume each (in³) |
|---|---|
| 18 AWG | 1.50 |
| 16 AWG | 1.75 |
| 14 AWG | 2.00 |
| 12 AWG | 2.25 |
| 10 AWG | 2.50 |
| 8 AWG | 3.00 |
| 6 AWG | 5.00 |
What counts, and how much
| Item | Counts as | Based on |
|---|---|---|
| Each conductor entering the box | 1 | Its own size |
| Conductor passing through unbroken | 1 (2 if a loop of 12 in or more) | Its own size |
| Internal cable clamps (any number) | 1 | Largest conductor in the box |
| Luminaire studs or hickeys | 1 per type | Largest conductor in the box |
| Each device yoke (switch, receptacle) | 2 | Largest conductor connected to the device |
| Equipment grounding conductors | 1 for up to four, plus ¼ each after | Largest grounding conductor |
| Pigtails, wire nuts, locknuts | 0 | Not counted |
Worked example
A 3 × 2 × 3½ inch device box holds a single receptacle fed by two 12/2 NM cables, and the box has internal clamps.
- Four 12 AWG insulated conductors (two blacks, two whites): 4 × 2.25 = 9.00 in³.
- The clamps: 1 × 2.25 = 2.25 in³.
- The receptacle on one yoke: 2 × 2.25 = 4.50 in³.
- Two bare grounds, counted as one: 2.25 in³.
- Total 18.00 in³. The box is 18.0 in³, so it fits exactly. Add a third cable and it no longer does.
Standard metal boxes
| Box (inches) | Volume (in³) |
|---|---|
| Device box 3 × 2 × 1½ | 7.5 |
| Device box 3 × 2 × 2 | 10.0 |
| Device box 3 × 2 × 2¼ | 10.5 |
| Device box 3 × 2 × 2½ | 12.5 |
| Device box 3 × 2 × 2¾ | 14.0 |
| Device box 3 × 2 × 3½ | 18.0 |
| Round or octagonal 4 × 1¼ | 12.5 |
| Round or octagonal 4 × 1½ | 15.5 |
| Round or octagonal 4 × 2⅛ | 21.5 |
| Square 4 × 1¼ | 18.0 |
| Square 4 × 1½ | 21.0 |
| Square 4 × 2⅛ | 30.3 |
| Square 4-11/16 × 1¼ | 25.5 |
| Square 4-11/16 × 1½ | 29.5 |
| Square 4-11/16 × 2⅛ | 42.0 |
Try 5 electrician exam practice questions
During rough-in inspection, an inspector checks a box holding six 12 AWG insulated conductors, two 12 AWG pigtails that stay inside the box, internal cable clamps and one receptacle. What is the minimum box volume required?
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C. 20.25 in³ Under NEC 314.16(B), the six 12 AWG conductors count 6 × 2.25 = 13.5 in³, the internal clamps count once at the largest conductor (2.25 in³), and the receptacle's yoke counts twice (4.5 in³). A conductor no part of which leaves the box, such as a pigtail, is not counted. Total: 20.25 in³. Counting the pigtails gives the wrong 24.75 in³.
A 40-amp, 240-volt branch circuit feeds a wall oven. What is the minimum conductor size for copper conductors with 75°C insulation?
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A. 8 AWG Table 310.16: 8 AWG copper is rated 50 amps at 75°C, which is sufficient for a 40-amp circuit; 10 AWG (35 A at 75°C) is too small. 6 AWG would work but is not the minimum. The 60°C column (40 A for 8 AWG) only governs where terminations are not rated 75°C.
A new residential service is being installed. What is the minimum size copper grounding electrode conductor required for a 200-amp service?
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C. 4 AWG Table 250.66 sizes the grounding electrode conductor from the largest ungrounded service conductor. A 200-amp residential service typically uses 2/0 or 3/0 AWG copper, and for service conductors of 2/0 through 3/0 AWG the table requires a 4 AWG copper GEC. 6 AWG is only permitted for 1/0 AWG or smaller service conductors.
An electrician is running wiring through a commercial hallway ceiling with exposed pendant lighting. Which raceway is the most practical choice for this exposed application?
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C. EMT (Electrical Metallic Tubing) EMT is lightweight, easy to bend, and approved for exposed work in commercial buildings. NM-B is limited to dry, concealed locations in residential wood-frame construction.
A dwelling unit has a 240-volt, 12-kW electric range. Using the standard calculation method, what is the demand load for the range?
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C. 8.0 kW Table 220.55, Column C: for one household range rated not over 12 kW, the standard-method demand load is 8 kW. The 12 kW nameplate is not used directly, and Note 1 (5% per kW over 12 kW) does not apply to a 12 kW range.
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Common questions
Add up the volume allowances from NEC 314.16(B): each insulated conductor that enters the box counts once at its size; all internal cable clamps together count once; each type of luminaire stud or hickey counts once; each device yoke counts twice; and up to four equipment grounding conductors count once, with a quarter more for each one after four. Clamps, fittings and grounds use the largest conductor in the box, devices use the largest conductor connected to them. Compare the total with the box's volume.
Divide the box volume by 2.25 cubic inches, then subtract what the devices, clamps and grounds take. An 18 cubic inch device box holds eight 12 AWG allowances in total, for example two 12/2 cables (four conductors), clamps (one), one receptacle (two) and the grounds (one).
Yes, but as a group. Up to four equipment grounding conductors count as a single allowance based on the largest one. Each additional ground beyond four adds a quarter allowance.
No. A conductor that starts and ends inside the box, such as a pigtail, is not counted.
No. Wire connectors, small fittings such as locknuts and bushings, and conductors that do not leave the box are not counted.
Nonmetallic boxes are not in Table 314.16(A). Use the volume molded or stamped inside the box, which is what 314.16(A)(2) requires to be marked.
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Sources, checked 2026-10-29: NFPA 70, National Electrical Code (2023), 314.16 and Tables 314.16(A) and 314.16(B); California applies the NEC through the California Electrical Code (Title 24, Part 3). Check the edition your jurisdiction has adopted and any local amendments. QuizCram is not affiliated with the agencies or standards bodies named here.
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