NEC Code for Junction Box Sizing: 314.16 and 314.28 Rules (2026)
The NEC code for junction box sizing uses two different methods, and picking the wrong one is the most common mistake. Boxes with conductors 6 AWG or smaller are sized by volume under section 314.16. Boxes with conductors 4 AWG or larger are sized by dimensions under section 314.28. This guide explains both methods, gives the formulas, standard box sizes and worked examples, and answers the usual questions about clearance, attics, outdoor boxes and labeling.
Quick answer: Under the NEC code for junction box sizing, for 6 AWG and smaller, add up the volume allowances and make sure the box volume is equal or larger (314.16). For 4 AWG and larger, a straight pull needs a box at least 8 times the trade size of the largest raceway long, and an angle or U pull needs at least 6 times the largest raceway plus the other entries on the same wall (314.28).
Table of Contents
Table of Contents
What Is the NEC Code for Junction Boxes?
The NEC rules for junction boxes are in Article 314, Outlet, Device, Pull, and Junction Boxes; Conduit Bodies; Fittings; and Handhole Enclosures. Article 314 covers box sizing, where boxes may go, how they are supported and covered, and what they must be listed for. Where in the NEC are the rules for box sizing? Two sections: 314.16 for volume and 314.28 for pull and junction boxes with large conductors. Over 1,000 volts, Part IV of the article applies, with larger multipliers.
NEC Code for Junction Box Sizing: Which Method Applies?
The conductor size picks the method:
| Conductors in the box | Method | Section |
|---|---|---|
| 6 AWG or smaller | Volume (cubic inches) | 314.16 |
| 4 AWG or larger | Dimensions (inches) | 314.28 |
Method 1: Volume Under 314.16
Under the NEC code for junction box sizing, this is the box fill method. Each conductor, clamp, device and ground gets a volume allowance from Table 314.16(B) (for example 2.00 cu in. for 14 AWG and 2.25 cu in. for 12 AWG). Add them up and compare with the box volume from Table 314.16(A) or the volume marked on the box. We explain the counting rules in our guide to the NEC article for box fill.
The junction box sizing formula for this method is:
Required volume = sum of all allowances ≤ box volume
Standard sizes help you pick a starting point:
| Box | Volume | Max 14 AWG | Max 12 AWG |
|---|---|---|---|
| 4 in. round or octagonal × 1-1/2 in. | 15.5 cu in. | 7 | 6 |
| 4 in. square × 1-1/2 in. | 21.0 cu in. | 10 | 9 |
| 4 in. square × 2-1/8 in. | 30.3 cu in. | 15 | 13 |
| 4-11/16 in. square × 1-1/2 in. | 29.5 cu in. | 14 | 13 |
| 4-11/16 in. square × 2-1/8 in. | 42.0 cu in. | 21 | 18 |
Those maximums only apply to same-size conductors with nothing else in the box. For help, try our electrical junction box size calculator.
Method 2: Dimensions Under 314.28
When conductors are 4 AWG or larger, size the box by trade size of the raceways, not by volume. NEC code for junction box sizing under 314.28(A) has three main rules:
- Straight pull: the box length is at least 8 times the trade size of the largest raceway.
- Angle or U pull: the distance from each raceway entry to the opposite wall is at least 6 times the trade size of the largest raceway in that row, plus the sum of the other entries on the same wall.
- Spacing: the distance between entries for the same conductors is at least 6 times the trade size of the larger raceway.
Trade size means the nominal size, such as 2 in., not the measured outside diameter. Listed boxes may be smaller in some cases if they are marked for the number and size of conductors.
Worked examples
- Straight pull, one 2 in. conduit: 8 × 2 = 16 in. minimum box length.
- Angle pull, 3 in., 2 in. and 1-1/2 in. conduits on one wall: 6 × 3 + 2 + 1.5 = 21.5 in. to the opposite wall.
- Spacing, two 3 in. conduits carrying the same conductors: 6 × 3 = 18 in. between nearest edges.
For more on these rules, see our NEC 314.28 pull box sizing requirements guide, our straight pull vs angle pull comparison, the pull box sizing chart and the pull box sizing calculator.
How to Calculate the Size of a Junction Box
To apply the NEC code for junction box sizing to a real job, follow these steps:
- Find the largest conductor. At 6 AWG or smaller use the volume method. At 4 AWG or larger use the dimension method.
- For volume, add the allowances and choose a box with equal or greater volume.
- For dimensions, identify each pull as straight, angle or U, and apply the 8x or 6x rule.
- Check conduit fill separately with our conduit fill calculator and the NEC article for conduit fill.
- Check that the conductors can be bent safely inside. Use our cable bending radius calculator for large cable.
Standard Sizes for Pull and Junction Boxes
Common NEMA-rated pull box sizes in the US include 6 × 6 × 4 in., 8 × 8 × 4 in., 12 × 12 × 4 in., 12 × 12 × 6 in., 16 × 16 × 6 in. and 24 × 24 × 8 in. Pick the next size up from your calculation, not the one below.
NEC Code for Junction Box Clearance and Access
Does the NEC code for junction box sizing say anything about clearance? No: sizing and clearance are different rules. The rule that matters is accessibility under 314.29: a junction box must be reachable without removing part of the building.
Working space under 110.26 applies to equipment that needs examination while energized, so it usually does not apply to a plain junction box. Surface gaps around a box must not exceed 1/8 in. (314.21), and boxes in combustible walls must be flush with the surface or project from it (314.20).
NEC Code for Junction Boxes in an Attic
Junction boxes in an attic follow the NEC code for junction box sizing like any other box, and they are allowed if they stay accessible. Do not bury them under insulation or flooring. Keep them near the attic opening, fit covers, and protect cable as the wiring method article requires. Our NEC code for outlet orientation guide covers receptacle boxes in more detail.
NEC Code for Outdoor Junction Boxes
The NEC code for junction box sizing is the same outdoors, but boxes in damp or wet locations must be listed for the location and installed so moisture cannot enter or collect inside (314.15). Use a weatherproof box such as NEMA 3R, or 4X where corrosion is a concern, with sealed fittings. For buried runs, check burial depths in our NEC code for underground conduit depth guide, and ground metal boxes as covered in our NEC article for grounding and bonding.
NEC Code for Labeling Junction Boxes
Labeling is not part of the NEC code for junction box sizing, and the NEC has no general rule that every junction box be labeled. It does have specific marking rules for certain systems, such as emergency circuits (700.10) and solar PV circuits (690.31), and many inspectors and employers expect circuit identification. Check the section for your system and the local rules. A label maker keeps it neat.
Free PDF: Junction Box Sizing and Code Requirements
If you search for the NEC code for junction box sizing PDF, remember that the NEC is copyrighted and available to read free on NFPA’s free access page. For outside the US, see our IEC standard for junction box guide.
Tools for Junction Box Work
Once the NEC code for junction box sizing is settled, these help. As an Amazon Associate I earn from qualifying purchases, at no extra cost to you. See our affiliate disclosure.
- Weatherproof pull boxes: NEMA 3R boxes in the sizes above.
- Deep and extension boxes: more volume for crowded boxes.
- Lever connectors: see our WAGO lever nut guide.
- Label maker: to mark circuits. See our label maker reviews.
- Testers and strippers: see our best wire strippers.
Frequently Asked Questions
What is the NEC code for junction box sizing?
Volume under 314.16 for 6 AWG and smaller, and dimensions under 314.28 for 4 AWG and larger.
How do I calculate the size of a junction box?
Add the volume allowances for small conductors, or apply the 8x straight pull and 6x angle pull rules for large ones.
What are standard sizes for electrical junction boxes?
Common ones are 4 in. square and 4-11/16 in. square boxes for small wiring, and 6 x 6 x 4 in. up to 24 x 24 x 8 in. pull boxes for large conductors.
Conclusion
The NEC code for junction box sizing comes down to the conductor size: volume at 6 AWG and smaller, dimensions at 4 AWG and larger. Check your adopted edition, confirm access and weatherproofing, and use our junction box size calculator and pull box sizing calculator to check your answer.
Sources: NFPA 70, National Electrical Code (Article 314 including 314.15, 314.16, 314.20, 314.21, 314.28, 314.29; 110.26; 690.31; 700.10).
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