เครื่องคำนวณปริมาตรกล่องไฟ
// SIZE OUTLET, SWITCH, AND JUNCTION BOXES WITH NEC 314.16 VOLUME ALLOWANCES FOR CONDUCTORS, DEVICE YOKES, GROUNDING CONDUCTORS, CLAMPS, AND FITTINGS. //
2.25 cu in spare capacity remains.
NEC 314.16(B) assigns a cubic-inch volume allowance by conductor size. For example, 14 AWG uses 2.00 cu in, 12 AWG uses 2.25 cu in, and 10 AWG uses 2.50 cu in per allowance.
Count every conductor that enters the box and is spliced, terminated, or passes through. Short pigtails that start and end inside the same box are not counted separately. Each device yoke counts as two allowances, all equipment grounding conductors together count as one allowance, and internal clamps count as one allowance.
Use this calculator for planning, then verify the installation against the adopted local electrical code and the box manufacturer markings.
NFPA 70 National Electrical CodeIEC overviewNEC references are for U.S. installations. IEC projects should follow the local implementation of IEC rules and product standards.
| Scenario | Conductor | What is counted | Box volume | Result |
|---|---|---|---|---|
| 20 A receptacle box | 12 AWG | 4 insulated, 1 yoke, grounds, internal clamp | 18.0 cu in | Pass, 15.75 cu in required |
| 15 A switch loop | 14 AWG | 4 insulated, 1 yoke, grounds | 18.0 cu in | Pass, 14.0 cu in required |
| GFCI with feed-through | 12 AWG | 6 insulated, 1 yoke, grounds, clamp | 22.5 cu in | Pass, 22.5 cu in required |
| Three-way switch box | 14 AWG | 7 insulated, 1 yoke, grounds, clamp | 22.5 cu in | Pass, 22.0 cu in required |
| Crowded old work box | 12 AWG | 6 insulated, 2 yokes, grounds, clamp | 18.0 cu in | Fail, 27.0 cu in required |
- Use the cubic-inch volume stamped inside the box, not the catalog name.
- Count a conductor once if it passes through without splice, and once if it terminates or is spliced in the box.
- A duplex receptacle on one yoke counts as two conductor allowances, not four.
- For multi-gang boxes, add every device yoke and every cable entering the box before checking fill.
- When the calculator is close to 100%, choose the next deeper box for easier splicing and device installation.
IEC practice usually starts from product standards such as IEC 60670 for boxes and enclosures plus national wiring rules based on IEC 60364. Those systems often specify usable space, terminal capacity, and heat limits differently from NEC cubic-inch box fill.
For international projects, convert the physical box volume and conductor cross-section carefully. A 2.5 mm² conductor is close to 13 AWG by area, but local rules may not use AWG allowances.
Do not use NEC box fill as a substitute for local inspection requirements outside NEC jurisdictions.
How do I calculate electrical box fill under NEC 314.16?
Count conductor allowances, multiply by the cubic-inch allowance for the largest conductor, and compare with the box volume stamped on the box. For 12 AWG, each allowance is 2.25 cu in.
Does a pigtail count in box fill?
A short pigtail that originates and terminates inside the same box does not count separately. A conductor entering the box and spliced to that pigtail still counts once under NEC 314.16(B)(1).
How are ground wires counted for box fill?
All equipment grounding conductors together count as one conductor allowance based on the largest equipment grounding conductor in the box.
How much volume does a receptacle add?
One device yoke, including a duplex receptacle, counts as two conductor allowances. With 12 AWG conductors, that adds 4.50 cu in.
What happens if my box is overfilled?
An overfilled box can damage insulation, make terminations unreliable, and fail inspection. If the calculator shows a 1.00 cu in shortage, select a larger box or reduce the conductor count.
Can I use this for IEC installations?
Use it only as a planning reference. IEC-based installations should follow local IEC 60364 rules and the box standard markings, not NEC 314.16 cubic-inch allowances.
Use the result with the wire gauge calculator, conduit fill calculator, and local code review before installing or modifying electrical wiring.