プルボックス寸法とボックスフィルのガイド

通常の器具箱や接続箱ではなく NEC 314.28 のプルボックス形状確認が必要になる場面で、NEC 314.16 の体積計算と切り分けます。

プルボックス寸法が同じ計算ではない理由

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.

電気工事で分けるべき定義

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.

箱を選ぶ前の5つの規則

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 と NEC 314.28 の比較表

小導体の箱内容積と大導体の引き込み形状を分けて、片方の合格がもう片方の合格ではないことを示します。

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.

具体的な数値例

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.

確認すべきコード参照

用語の確認には公開資料を使い、最終判断は採用 NEC 版、製品表示、地域検査要件で確認してください。

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.

プルボックス寸法 FAQ

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.

技術メモ

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.

まず体積、次に形状を確認

NEC 314.16 の導体許容量を計算し、必要なら NEC 314.28 のプルボックス寸法も確認します。