No. A pull box routes conductors, and a 480 V fused disconnect belongs in its own switch enclosure, with its handle operable from a readily accessible place. The fuse question carries a second trap: a let-through chart only earns its keep when a tested combination backs it. The worked example below takes a 200 A feeder, a 10 kAIC panelboard with no main and 25 kA of available fault current after a transformer upgrade. It shows where a let-through value of about 5 kA counts, and what it takes to make it count.
Can you mount a fused disconnect inside a pull box?
No: a pull box exists to route and protect conductors, so a disconnect at this voltage goes in its own switch enclosure, and the 48 x 48 x 72 in [1,220 x 1,220 x 1,830 mm] box keeps doing the two 350 kcmil feeder pulls it was sized for.
Pull boxes are sized from the raceways they serve. For conductors 4 AWG and larger, the NEC's dimension rules set box length at eight times the largest raceway on a straight pull. Two 350 kcmil feeders usually travel in 2½ in conduit, so the rule asks 8 x 2.5 in = 20 in [500 mm] of straight length; the 48 in [1,220 mm] box passes easily. That arithmetic gives the box no hinged door, no handle, no door interlock and no fuse access.
A fusible safety switch is listed to UL 98 as a switch-and-enclosure assembly. Panel-mount rotary disconnects carry the maker's own conditions for enclosure size, fuse holders and interlock, and a pull box supplies none of them. A device dropped into a box it was never listed with repeats the mistake in putting contactors inside a breaker enclosure; the line between the two box types is drawn in whether a pull box counts as an enclosure.
What does readily accessible mean for this switch?
The NEC requires a switch to sit where it can be operated quickly, without climbing, ladders or tools; a bolted pull-box cover fails that test outright, and NEC 240.24(A) caps the handle grip at 2.0 m [6 ft 7 in] above the floor for an overcurrent device.
"Readily accessible" is the code's own phrase: reachable quickly for operation without removing obstacles or reaching for tools. A wall-mounted safety switch at chest height, handle around 48 to 60 in [1,220 to 1,520 mm], passes without thought. A switch behind a screwed cover cannot be operated at all, so it fails before any height check starts. Disconnect placement and door rules at this voltage get a longer treatment in the 480 V cabinet disconnect and door rules.
How much working clearance does the disconnect need?
Keep 4 ft [1.2 m] of clear depth in front of it — 277 V to ground sits above the 3 ft [900 mm] low-voltage band — with 30 in [750 mm] of width and room for the door to open 90 degrees.
NEC 110.26(A) requires clear working space in front of equipment likely to be examined, adjusted, serviced or maintained while energized. Condition 1 means grounded metal, an indoor space and no conductive surfaces nearby. This system's line-to-line voltage gives 277 V to ground, which is why the depth in Table 110.26(A)(1) moves past the 3 ft [900 mm] figure that serves at 0-150 V.
| Dimension | Condition 1, at this voltage |
|---|---|
| Depth in front | 4 ft [1.2 m] |
| Width | 30 in [750 mm] or the equipment width |
| Headroom | 6.5 ft [2.0 m] or the equipment height |
| Door swing | 90 degrees |
Mount the switch enclosure beside the pull box rather than tight against it, and keep the full depth clear in front of both. The overhead version of this check continues in junction box working clearance under NEC 110.26.
Does a current-limiting fuse lower a panelboard's fault rating?
A let-through chart cannot rate the panel on its own: the NEC credits a reduced fault rating only for a tested, marked combination or an engineered design, so the fault level available at the line side stays the number to satisfy until one of those exists.
Worked example — treat the inputs as illustrative and replace them with the stamped ratings and the fault study on the job:
| Quantity | Example value |
|---|---|
| Available fault current at the panel line side after the transformer upgrade | 25 kA |
| Panelboard short-circuit current rating, no main | 10 kA |
| Overstress without added protection | 25 ÷ 10 = 2.5 times the rating |
| Peak let-through, 200 A Class J fuse at the full available current | about 5 kA (chart assumption) |
| Let-through as a share of prospective peak | 5 ÷ 25 = 20% |
| Heating at equal clearing time | (5/25)² = 1/25 of prospective |
| Clearing time at high fault current | under half a cycle, about 8 ms at 60 Hz |
Heating follows the square of current, so a 5 kA peak out of the prospective 25 kA passes 1/25 of the thermal load at equal clearing time; the fuse's I²t chart shows a smaller figure still. The arithmetic looks like a pass, since the let-through sits at half the panel's 10 kAIC rating. On its own it earns nothing.
Fuse class decides first whether the surplus switch can connect at all. Class J and Class R fuses carry 200 kA interrupting ratings; Class H stops at 10 kA, under the fault level available. A Class J time-delay fuse still follows its time-current curve. It carries 5 times its rating — 1,000 A on a 200 A fuse — for at least 10 s under UL 248-8. Current limiting appears only at high fault currents. What sets a panelboard's tested limit is covered in panelboard SCCR and series ratings. Equipment carries these numbers from its listing, the same way a transfer switch's kAIC comes from its listed voltage ratings.
What makes a fault-current combination valid?
Two routes make the combination valid: a tested series rating published by the equipment makers and marked at the panel, or, on an existing installation, an engineered combination stamped by a licensed professional engineer — either way the combination rating must cover the 25 kA available.
The makers publish tested series combinations, and the NEC requires the combination to be marked legibly at the equipment where it is applied. Pick a pair whose rating meets or beats the available fault current, then mark it. Where no tested pair exists, the NEC allows an engineered combination of current-limiting protection and downstream equipment on an existing installation when a licensed professional engineer supervises the selection and stamps the documents. A third route removes the question entirely: replace the panelboard with one rated at or above the available 25 kAIC.
Rated versus listed is its own decision, laid out in listed versus manufacturer-rated equipment. A field-built assembly around a listed panel raises the question handled in an unlisted enclosure for a listed control panel.
What should you install and order instead?
Order a 200 A, 480 V three-pole fusible safety switch in its own UL 98 enclosure, mount it on the wall beside the pull box with the handle within easy reach and 4 ft [1.2 m] clear in front, and leave the pull box to its two feeder pulls.
- Switch: 200 A, 600 V class, three-pole fusible safety switch listed to UL 98, with door interlock.
- Fuses: Class J or Class R, 200 kA interrupting rating, sized to the feeder; Class H at 10 kA is out at this location.
- Location: readily accessible, handle grip no higher than 2.0 m [6 ft 7 in], no tools needed to operate.
- Clearance: 4 ft [1.2 m] deep, 30 in [750 mm] wide, 6.5 ft [2.0 m] headroom, door opening 90 degrees.
- Rating proof: a marked series combination at the panelboard, or the PE-stamped engineered study.
- Pull box: conductors only, with the 350 kcmil pulls and the eight-times-raceway length intact.
De-energize, lock out and test for absence of voltage before opening the pull box or setting the new switch. Entry direction has its own rules; feeding a disconnect switch from the bottom covers the line-side case.
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
Can the fused switch sit right next to the pull box?
Yes. Wall-mount the switch enclosure beside the 48 x 48 x 72 in pull box and feed it with short conduit, keeping 4 ft [1.2 m] of clear depth in front of both. Adjacency costs nothing; each enclosure keeps its own cover, handle and listing, and the switch stays operable without tools.
Does the 200 A feeder breaker protect the 10 kAIC panelboard on its own?
No. A trip setting controls load current, not fault stress, so the panel still faces the full 25 kA available. The breaker counts only when it and the panelboard form a tested series rating marked on the equipment, or when a licensed professional engineer documents an engineered combination for the existing installation.
How fast does a current-limiting fuse open?
At high fault currents, in under half a cycle — about 8 ms at 60 Hz. In the example, the fuse passes about 5 kA of peak let-through out of 25 kA prospective, so the heating falls to (5/25)² = 1/25 of what an unprotected path would pass. Speed limits damage; a rating still comes from the tested combination.
What if the surplus switch only accepts Class H fuses?
Refit or replace. Class H fuses interrupt 10 kA, below the 25 kA available, so they cannot be connected there. Class J or Class R fuses carry 200 kA interrupting ratings; the switch itself must be listed for the fuse class, which usually means the replacement is a new switch, not the surplus unit.
