Short answer: treat a junction box as a point and a wireway as a run. A box takes fill volume — 2.5 cu in per #10 conductor — and no ampacity derating, even at 126 conductors. A wireway is a raceway, so it takes the 20% area fill limit and derates once more than 30 current-carrying conductors share the run. The 24 in figure people quote belongs to raceway length, never to conductor length inside a box. A 36 × 24 × 6 in box holding 126 #10 THHN conductors uses 315 of its 5184 cu in, and every conductor keeps its full rating.
A box is a point, a wireway is a run
The NEC ampacity adjustment exists for bundled conductors inside a raceway: conduit, tubing, cable or a wireway. Heat has nowhere to spread when many conductors share one narrow channel, so each conductor gives up capacity. A box plays no part in that count. Under the NEC wiring rules, a box is where conductors land, pass through or splice; it is a termination point with a cover, and the conductors inside it never enter the adjustment.
A wireway sits on the other side of the line. It is a raceway: conductors lie in it along a length as part of the run, so both raceway rules attach — area fill and the conductor-count line.
Decide what the enclosure does before you size it. If conductors enter from one raceway and leave through another at a point, box rules govern. If the enclosure carries conductors along its length as part of the circuit run, wireway rules govern. With no splices the box is called a pull box; add splices and it becomes a junction box; both follow the same volume rules, as the guide to splicing branch circuits in a junction box explains.
The box side: fill by volume
The NEC box fill table assigns a volume allowance to each conductor and fitting:
| Item | Allowance |
|---|---|
| #14 conductor | 2.00 cu in |
| #12 conductor | 2.25 cu in |
| #10 conductor | 2.50 cu in |
| #8 conductor | 3.00 cu in |
| #6 conductor | 5.00 cu in |
| Each clamp or support fitting | one allowance at the largest conductor size |
| The grounding set | one allowance at the largest ground |
A conductor that passes through counts once, even though it enters one face and leaves another. Splices and devices add their own allowances; the NEC box fill chart page walks the counting method in detail.
Worked example, with no splices, devices or clamps: a 36 × 24 × 6 in box passes 126 #10 THHN conductors. Fill: 126 × 2.50 = 315 cu in. Box volume: 36 × 24 × 6 = 5184 cu in. Used: 315 ÷ 5184 = 6.1%. The volume floor alone is 315 cu in; even a 12 × 12 × 4 in box at 576 cu in clears it. Raceway entries, bending space and pull direction size the real box long before volume does.
Ampacity: none of this derates a conductor. The adjustment counts conductors in raceways, and the box is not one. For straight pulls with #4 and larger conductors, the NEC sizes the box at eight times the largest raceway outside diameter. Along the 36 in side, that allows raceways up to 4.5 in outside diameter. With #10 conductors, volume fill governs, and 315 cu in is your floor. Large-conductor pulls follow their own sizing, covered in the pull box sizing article for 350 kcmil conductors and the enclosure sizing guide.
The wireway side: 20% fill and the 30-conductor line
Two limits govern a metal wireway. First, at any cross section, conductors take up at most 20% of the interior area. Second, up to 30 current-carrying conductors ride with no ampacity adjustment; past 30, the NEC adjustment factors for bundled conductors apply:
| Current-carrying conductors | Factor |
|---|---|
| 4–6 | 80% |
| 7–9 | 70% |
| 10–20 | 50% |
| 21–30 | 45% |
| 31–40 | 40% |
| 41–60 | 35% |
| 61 or more | 30% |
Splices are permitted in an accessible metal wireway, which is why gutters double as splice troughs.
Run the area math on a 4 × 4 in wireway: 16 in² of cross-section, so 20% gives 3.2 in² of conductor. A #10 THHN conductor occupies 0.0172 in² in the standard table area, so area alone would hold 3.2 ÷ 0.0172, about 186 conductors. The same 126 conductors from the example occupy 126 × 0.0172 = 2.17 in², about 13.5% of that wireway. Fill passes; the count does not. With small conductors, the 30-conductor line binds long before the 20% area limit does. The wireway conductor derating article carries the adjustment math further, and the lay-in vs pull-through wireway comparison covers the routing choice.
Where the 24 in rule applies
The 24 in figure is a raceway length, not a conductor length. A raceway up to 24 in (600 mm) long between enclosures needs no ampacity adjustment, whatever the conductor count inside it. That is why short nipples and interconnecting stubs stay undearated.
Inside a box there is no length trigger at all. A 26 in conductor passing through a box between two short nipples is decided by box fill alone — 2.50 cu in for #10 — because the box adds no raceway length. Each nipple is its own raceway, checked separately. A close nipple is a short, fully threaded conduit connector; the usual fill limit for a complete run is 40% of the conduit area, and the NEC raises it to 60% for a nipple up to 24 in between boxes. Count the conductors in the nipples and in any upstream conduit on their own; the box count never adds to them.
What derating costs in each layout
Take the same 63 #10 THHN conductors and place them three ways:
- In a box between raceways: no adjustment. Ampacity is set by the terminations — 35 A on 75 °C lugs, the ampacity the NEC tables give #10 copper at 75 °C.
- In one wireway carrying all 63: the 30% factor gives 40 × 0.30 = 12 A. The 12 A governs, well below the 35 A termination value.
- In three wireways at 21 conductors each: every run stays at or below 30, so no adjustment applies and each conductor keeps the full 35 A termination rating.
That spread — 35 A against 12 A — is the price of carrying a raceway run through one channel instead of three, or of calling a gutter a pull box on the drawings. For the full 126-conductor example the box needs nothing extra; a single wireway would land in the 30% band at 12 A per conductor.
One safety note: lock the feeder out and verify absence of voltage before you open a panel packed with energized lugs.
Panel duct is the third case
Slotted wiring duct inside an enclosure follows different rules again. The fill tables used across the panel industry cap duct at 50% of the interior cross-section, citing NFPA 79-2012 section 13.5.2. Panel wiring in duct runs between terminal blocks and devices within one enclosure; the raceway adjustment rule does not follow it there. So the three figures sit side by side: 50% in panel duct, 20% in a field wireway, and pure volume in a box. The backplate and DIN rail layout guide puts the duct rule in panel context.
Sizing and specifying the box
Once the rules point to a box, the enclosure itself is the easy part. For a panel room or washdown area, 304 stainless covers most sites; salt air, brine and chlorides call for 316L. Our stainless junction and pull boxes come rated NEMA 4X / IP66 by the manufacturer; not UL listed. Order both entry faces cut to your nipple pattern on the drawing — the custom stainless enclosures with factory cutouts service builds a 36 × 24 × 6 in body from one piece, delivered in 10 business days from your approval of the drawing.
If the box lives where temperature swings, plan condensation control before you order: a drain or breather rated below Type 4X sets the whole assembly rating, so pick accessories documented for the service, as the junction box breather and drain sizing article lays out. Whether a pull box carries the enclosure rating at all is settled in the pull box enclosure article.
The decision
| The layout | The rules that apply | The numbers |
|---|---|---|
| Box between two raceways, any count | Volume fill | 2.50 cu in per #10; 126 conductors = 315 cu in; no derating |
| Wireway carrying the run | 20% area fill, 30-conductor line | 3.2 in² allowed in a 4 × 4; derate above 30 |
| Nipple up to 24 in between boxes | No adjustment | 60% fill instead of 40% |
| Panel wiring duct | 50% fill per NFPA 79 | Routing rule; no adjustment |
| Raceway leg over 24 in | Full adjustment factors | 80% at 4 conductors down to 30% at 61 |
Call the enclosure what it does on the drawings: a point gets box rules and a volume calculation; a run gets wireway rules and the 20%-and-30 line. When several circuits land in one box, the multiple motor circuits in one junction box article adds the matching NEC and NFPA 79 details. Then send your cutout layout with the entry pattern to the custom enclosure builder, and the sizing question answers itself.
Catalog dimensions to compare
Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 6 × 6 × 4 in | 304 stainless | 6 x 6 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 8 × 6 × 4 in | 304 stainless | 8 x 6 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 10 × 8 × 4 in | 304 stainless | 10 x 8 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Does a junction box ever require ampacity derating?
No. The ampacity adjustment counts conductors inside a raceway, and a box is a termination point, not a raceway. Fill it by volume instead: 2.5 cu in per #10 conductor, so 126 conductors take 315 cu in before clamps, splices or grounding conductors add their allowances. The raceways feeding the box keep their own fill and adjustment rules, including the 60% nipple allowance for short entries.
How many conductors fit a wireway before derating starts?
Thirty current-carrying conductors. A metal wireway holds up to 30 with no ampacity adjustment, provided fill stays at or below 20% of the interior cross-section. Above 30 the NEC factors apply: a 63-conductor run lands in the 30% band, taking #10 THHN from 40 A down to 12 A. Split the same 63 conductors into three wireways of 21 and each run keeps the 35 A rating set by 75 °C lugs.
Do conductors over 24 in inside a box trigger derating?
No. The 24 in exemption belongs to the raceway length: a conduit or nipple up to 24 in (600 mm) long between boxes needs no adjustment, and a box adds no raceway length at all. A 26 in conductor passing through a box between two short nipples is decided by box fill alone, at 2.5 cu in for #10, and each nipple keeps the usual 40% fill or 60% for nipples up to 24 in.
What volume does the box fill table assign per #10 conductor?
2.5 cu in [41 cm³] each. The neighboring rows are 2.0 cu in for #14, 2.25 for #12, 3.0 for #8 and 5.0 for #6. A conductor that passes through counts once even though it enters and leaves, and the whole grounding set takes a single allowance at the largest conductor size present.
How big a box do 126 #10 conductors need?
Volume gives the floor: 126 × 2.5 cu in = 315 cu in, and even a 12 × 12 × 4 in box at 576 cu in clears it. Entries and bending space usually size the real box first. For a straight pull with #4 and larger conductors, the NEC sizes the box at eight times the largest raceway outside diameter; along the 36 in side of the example box that allows raceways up to 4.5 in OD.
