Húzódoboz méretezése és dobozkitöltés útmutató
Akkor használja, amikor a doboz már nem egyszerű szerelvény- vagy kötődoboz, hanem NEC 314.28 szerinti húzási geometriát igényel.
Miért más számítás a húzódoboz mérete
Box fill asks whether the enclosure has enough free volume for conductors, grounds, device yokes, clamps, fittings, and splices. Pull-box sizing asks whether larger conductors can be pulled, bent, and serviced without abusive geometry.
The practical mistake is using one result to answer both questions. A 4 in. square box may pass a small 12 AWG splice under NEC 314.16, while a feeder pull point with 4 AWG or larger conductors may need dimensions driven by NEC 314.28 instead.
Külön kezelendő fogalmak
Box fill is the NEC 314.16 volume calculation that assigns cubic inches to countable conductors, equipment grounding conductors, devices, clamps, and fittings.
A pull box is an enclosure used to pull, route, or splice conductors through raceways; for 4 AWG and larger conductors, NEC 314.28 can control length and spacing.
A junction box is an enclosure for splices and taps. Small-conductor junction boxes usually live under NEC 314.16, while large-conductor pull or junction boxes may move into NEC 314.28.
Öt szabály a doboz kiválasztása előtt
Identify the largest conductor first
If every counted conductor is 6 AWG or smaller, ordinary NEC 314.16 cubic-inch fill often applies. When 4 AWG or larger conductors enter, check NEC 314.28.
Separate volume from pulling geometry
A cubic-inch pass does not prove that a straight pull, angle pull, or U-pull has enough length and raceway spacing.
Device boxes still need yoke allowances
A receptacle, switch, GFCI, or disconnect yoke adds two allowances under NEC 314.16(B)(4), based on the largest conductor on that yoke.
Grounding conductors still count in small boxes
All equipment grounding conductors together count as one allowance under NEC 314.16(B)(5), based on the largest grounding conductor present.
IEC work needs the same design discipline
IEC 60364 projects use different national rules, but bending radius, terminal space, heat, access, and maintainability still need separate review.
NEC 314.16 és NEC 314.28 összehasonlítás
These examples use familiar branch-circuit counts and larger-conductor pull-box geometry. They show why passing one check does not prove the other check.
| Scenario | Sizing method | Specific number | Practical decision |
|---|---|---|---|
| 12 AWG receptacle with line, load, GFCI yoke, grounds, and internal clamp | NEC 314.16 box fill | 8 allowances x 2.25 cu.in. = 18.00 cu.in. | Use a deep 1-gang or 4 in. square box with listed cover rather than a shallow device box. |
| 10 AWG appliance splice with four insulated conductors and equipment grounds | NEC 314.16 box fill | 5 allowances x 2.50 cu.in. = 12.50 cu.in.; add clamp or yoke if present | The box may pass by volume, but larger conductors need extra bend room. |
| 6 AWG range receptacle box with three insulated conductors, grounds, and device yoke | NEC 314.16 box fill | 6 allowances x 5.00 cu.in. = 30.00 cu.in. | A 4-11/16 in. square box or large listed enclosure is usually the practical minimum. |
| 4 AWG feeder conductors passing through a pull point without devices | NEC 314.28 pull-box geometry | Straight pull length is commonly 8 x raceway trade size | Do not size this like a small device box; check straight-pull dimension and raceway entries. |
| 2 AWG conductors entering one wall and leaving an adjacent wall | NEC 314.28 angle-pull and U-pull rules | Use 6 x largest raceway plus the sum of other raceways in the same row | Raceway spacing and removable covers matter as much as enclosure volume. |
| IEC-style control enclosure with 16 mm2 conductors and terminal blocks | Local IEC/national enclosure rules | Check bend radius, terminal torque access, heat, and service clearance | Do not import NEC cubic-inch values directly; use the workflow to avoid cramped terminations. |
Példák konkrét számokkal
Example 1: 12 AWG GFCI device box
A 20 A receptacle box has line and load conductors, one equipment-grounding group, one internal clamp, and one GFCI yoke. The count is 8 allowances. At 2.25 cu.in. per 12 AWG allowance, the minimum is 18.00 cu.in. This is a box-fill problem, not a pull-box problem.
Example 2: 6 AWG range receptacle
A range receptacle with three insulated 6 AWG conductors, one grounding allowance, and a yoke reaches 6 allowances. NEC Table 314.16(B) gives 5.00 cu.in. per 6 AWG allowance, so the required volume is 30.00 cu.in. Device depth and conductor stiffness make extra space valuable.
Example 3: 4 AWG feeder pull point
Once 4 AWG or larger conductors are pulled through a raceway box, NEC 314.28 can control the enclosure dimensions. A straight pull is not solved by adding conductor allowances; the box length must support the raceway layout and pulling geometry.
Ellenőrizendő kódhivatkozások
Use these public references for terminology, then verify the exact adopted NEC edition, manufacturer listing, and local inspection requirements.
- National Electrical Code: Background for NEC Articles 314.16 and 314.28; verify the adopted edition with the authority having jurisdiction.
- Junction box: General enclosure terminology for splices, taps, pull points, and service access.
- American wire gauge: Useful for understanding why 6 AWG, 4 AWG, and 2 AWG conductors change enclosure decisions.
- IEC 60364: International readers should compare local enclosure-space and conductor-routing requirements.
Húzódoboz méretezési GYIK
When does NEC 314.28 apply instead of NEC 314.16?
NEC 314.28 applies to pull and junction boxes for conductors 4 AWG and larger. Ordinary outlet, device, and smaller splice boxes are typically checked with NEC 314.16 cubic-inch allowances.
Can a box need both checks?
Yes. A mixed enclosure can contain small conductors, devices, grounding conductors, and larger feeder conductors. In that case, verify the applicable volume counts and the pull-box geometry instead of assuming one rule replaces the other.
How big is a 12 AWG GFCI box by box-fill math?
A common line/load GFCI with grounds and an internal clamp can reach 8 allowances. At 2.25 cu.in. per 12 AWG allowance, that is 18.00 cu.in. before adding practical working margin.
Why is 4 AWG the trigger point electricians remember?
NEC 314.28 is written for boxes and conduit bodies containing conductors 4 AWG or larger, because pulling and bending geometry becomes a primary safety and workmanship issue.
Do IEC installations use the same pull-box dimensions?
No. IEC projects follow different national rules and product standards, but engineers should still check conductor bending radius, terminal space, heat, accessibility, and maintenance clearance.
Should I choose the exact calculated minimum?
For small boxes, exact volume leaves little room for devices and splices. For pull boxes, exact minimum geometry can still be awkward if raceways are crowded. Extra space usually reduces rework.
Műszaki megjegyzés
Hommer Zhao reviews box-fill pages from the perspective of conductor packaging, termination access, and field rework risk. This guide is educational; the adopted code, product listing, and local inspector control the final installation.
Először térfogat, aztán geometria
Számolja ki a NEC 314.16 allowance értékeket, majd ellenőrizze, kell-e NEC 314.28 szerinti méret.
Kapcsolódó kalkulátorok
Box Fill Calculator · Box Fill Chart · Conduit Fill Calculator · NEC Code Reference