Руководство по заполнению коробок для нейтралей и пигтейлов
Правильно считайте нейтрали, общие нейтрали, пигтейлы, устройства и заземляющие проводники перед выбором коробки.
Руководство по заполнению коробок для нейтралей и пигтейлов
Правильно считайте нейтрали, общие нейтрали, пигтейлы, устройства и заземляющие проводники перед выбором коробки. NEC 314.16(B)(1) counts each insulated conductor that enters a box and is spliced, terminated, or passes through. A neutral from a cable is usually counted like an ungrounded conductor.
Internal pigtails normally do not add a separate box-fill allowance, but they still need real bending and termination space. Use the numbers below before choosing the smallest box.
Five rules for neutral and pigtail counts
Count every outside neutral that enters the box
If a 12 AWG neutral enters the box from a cable or raceway and is spliced, terminated, or looped through, count it once at 2.25 cu.in. under NEC Table 314.16(B).
Internal pigtails usually do not add box-fill volume
A pigtail that originates inside the box and connects to a device or splice in the same box normally does not count as another conductor allowance, even though it still uses working space.
Shared neutrals still count as real conductors
A multiwire branch circuit neutral is counted once when it enters and is spliced or terminated in the box. Also verify simultaneous disconnecting rules and conductor grouping for the circuit.
Device yokes are separate from pigtail counts
Each device yoke adds two conductor allowances under NEC 314.16(B)(4), based on the largest conductor connected to that yoke. The yoke count applies even if the device is fed by pigtails.
Grounding conductors are counted as one allowance total
All equipment grounding conductors and bonding jumpers together count as one allowance under NEC 314.16(B)(5), based on the largest grounding conductor in the box.
Common neutral and pigtail scenarios
These examples use NEC Table 314.16(B) values: 14 AWG = 2.00 cu.in. and 12 AWG = 2.25 cu.in. The required volume is the code minimum, not a workmanship reserve.
| Scenario | Conductor equivalents | Required volume | Practical box choice | Field note |
|---|---|---|---|---|
| 20 A receptacle with one 12/2 feed, one 12/2 load, neutral pigtail, device yoke, and grounds | 7 equivalents at 12 AWG | 15.75 cu.in. | 18 cu.in. or deeper device box | Four insulated outside conductors + one ground allowance + two for the yoke = 7. The neutral pigtail does not add another allowance, but the device still needs room. |
| Multiwire branch-circuit splice with two ungrounded conductors, one shared neutral, grounds, and one internal clamp | 7 equivalents at 12 AWG | 15.75 cu.in. | 18 cu.in. minimum; 21 cu.in. is easier to inspect | Five insulated conductors plus one ground allowance and one clamp allowance = 7 x 2.25 = 15.75 cu.in. The shared neutral is counted because it enters the box. |
| Smart switch added to a 12 AWG switch box with line, load, neutral bundle, grounds, clamp, and one yoke | 8 equivalents at 12 AWG | 18.00 cu.in. | 21 cu.in. deep box | Smart devices often need a neutral pigtail. The pigtail may not count, but the neutral bundle, yoke, and clamp bring the legal fill to 18.00 cu.in. |
| Two-gang box with two 12 AWG switches, shared neutral splice, grounds, internal clamp, and two device yokes | 11 equivalents at 12 AWG | 24.75 cu.in. | 30 cu.in. class two-gang box | Two yokes alone add four allowances. With line/load conductors, neutral, grounds, and clamp, the minimum reaches 24.75 cu.in. |
| 14 AWG lighting junction with three 14/2 cables, neutral splice, grounding bundle, and no device yoke | 8 equivalents at 14 AWG | 16.00 cu.in. | 18 cu.in. junction box | Six insulated conductors plus one grounding allowance and one clamp allowance require 16.00 cu.in. Pigtails to the luminaire do not erase the outside conductor count. |
Worked examples with specific numbers
Example 1: 20 A receptacle with a neutral pigtail
A 12/2 feed and a 12/2 load create four insulated 12 AWG conductors entering the box. Add one allowance for all equipment grounds and two allowances for the receptacle yoke. Total = 7 allowances. At 2.25 cu.in. each, the box needs 15.75 cu.in. The short neutral pigtail to the receptacle normally does not add a separate allowance, but an 18 cu.in. box is still a better field choice than a shallow exact-limit box.
Example 2: Shared neutral in a multiwire branch circuit
Assume a 12 AWG multiwire branch-circuit splice with two ungrounded conductors entering, two ungrounded conductors leaving, one shared neutral splice, one grounding allowance, and one internal clamp. That is seven 12 AWG allowances, so the required volume is 15.75 cu.in. NEC 314.16 controls the box volume, while NEC 210.4-style multiwire branch-circuit rules still need to be checked for handle ties, disconnecting, and grouping.
Example 3: Smart switch neutral bundle
A smart switch often turns an old switch box into a crowded neutral-splice box. Count the 12 AWG line, load, neutral conductors that enter from outside, all grounds as one allowance, one internal clamp allowance, and two allowances for the switch yoke. Eight 12 AWG allowances require 18.00 cu.in. under NEC 314.16. The smart-switch lead and neutral pigtail may be internal, but the device body and connectors make a 21 cu.in. box the practical choice.
NEC and IEC references worth checking
Box-fill math is an NEC method, but the design idea applies broadly: count the conductors that enter the enclosure, leave space for terminations, and avoid forcing devices into boxes that are only legal on paper.
- National Electrical Code overview: Use Article 314.16 for box fill and verify the exact adopted NEC edition with the authority having jurisdiction.
- Multiwire branch circuit overview: Helpful context for shared neutrals, simultaneous disconnecting, and why the neutral still matters in the box count.
- American wire gauge reference: Useful when the same conductor count changes from 14 AWG at 2.00 cu.in. to 12 AWG at 2.25 cu.in. per allowance.
- IEC 60364 overview: IEC installations do not use NEC cubic-inch arithmetic, but enclosure space, separation, and maintenance access still matter.
Neutral and pigtail box-fill FAQ
Do neutral wires count in box fill?
Yes. An insulated neutral that enters the box and is spliced, terminated, or passes through normally counts once under NEC 314.16(B)(1). For 12 AWG, that allowance is 2.25 cu.in.; for 14 AWG, it is 2.00 cu.in.
Does a neutral pigtail count as another conductor?
Usually no, when the pigtail originates and terminates inside the same box. It still takes physical room, so a legal 15.75 cu.in. calculation may still deserve an 18 cu.in. or 21 cu.in. box.
How do shared neutrals count in a multiwire branch circuit?
The shared neutral counts once when it enters the box and is spliced or terminated. A 12 AWG shared neutral adds 2.25 cu.in. to the fill calculation, and the circuit still needs the separate multiwire branch-circuit checks.
Do device pigtails remove the yoke allowance?
No. A receptacle, switch, dimmer, or smart device yoke still adds two conductor allowances under NEC 314.16(B)(4), based on the largest conductor connected to that yoke.
How much volume should I reserve for smart switches?
The code count may be 18.00 cu.in. for a common 12 AWG smart-switch layout, but a 21 cu.in. or larger box is usually easier because the device body, wirenuts, and neutral pigtails need real working space.
Run the count before adding another pigtail
Use the calculator after you list every outside conductor, yoke, clamp, and grounding allowance. It will show whether the neutral bundle is code-compliant before the device is folded into the box.
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