Separating 480V power from controls in one enclosure

Give the 480 V motor hardware its own closed column and the control side its own air. In a 30 x 36 in [762 x 914 mm] enclosure, that means a grounded steel partition 12 in [305 mm] in from the left wall, running floor to ceiling, with a bolted cover over the power compartment and its own arc flash label on that cover. A barrier earns a lower PPE level only where it completely encloses the three-phase section; the lug shield sold for disconnects does a different, smaller job. Feed the power column from glands in its own bottom wall, keep one voltage per DIN rail on the open side, and cross the partition once. Two enclosures beat one when you have the wall; the layout below carries you when the footprint is fixed.

Does a partition inside the enclosure actually cut arc flash?

Yes, but only when it fully encloses the three-phase section — walls, top, bottom and a bolted cover — because an arc vents plasma and molten metal through every gap you leave; a partial plate keeps the full PPE inside that compartment. NFPA 70E sets the PPE from the incident energy at your working distance, and the study behind it, IEEE 1584, calculates that energy from bolted fault current, arcing current, clearing time, enclosure size and distance. The arc flash boundary sits where the energy falls to 1.2 cal/cm2 [5 J/cm2]. A barrier moves the number only when it stands between the arc source and you for the whole task: five sides plus a bolted cover qualifies. That is also why arc-resistant switchgear carries an internal-arc classification tested to IEC 62271-200; the maker proved the joints stay closed under a tested arc.

Shorten the fault too. A current-limiting fuse clears a high-current bolted fault in under half a cycle, and one cycle at 60 Hz is 16.7 ms, so clearing collapses from tens of cycles to single-digit milliseconds. The partition contains the arc; the fuses shorten it. For the full treatment of driving the category down, read enclosing equipment to reduce arc flash to category 2, and how enclosure depth changes the arc flash number. The strongest protection is still the lockout: de-energize before your hands go in, and treat everything here as margin for the work that must happen live.

Can the motor starters and transformer go under a disconnect shield?

No; a disconnect shield is a lug shroud, a small formed plate that covers the line terminals of one switch — the terminals that stay live after you open it. That shield does one job: keeping your wrench off energized metal while the handle is off, which is why the line side stays live in a separate disconnect enclosure. Starters and transformers want three things a lug plate never gives: bracing for bolted fault current, wire bending space at the terminals per NEC 312.6, and airflow. A dry-type transformer loses 1-3% of its rating as heat — 7.5 to 22.5 W on a 750 VA unit — and a shroud traps exactly that heat.

Put all three devices in the column instead: the disconnect at the top with its handle through the cover, the starters below it, the transformer at the bottom where its heat and its bottom-entry glands both belong.

How do you lay out a 30 x 36 in enclosure with 480 V and 24 VDC?

Split a 30 x 36 in [762 x 914 mm] backplane into 12 in and 18 in: a full-height power column on the left, and a control side with one voltage per DIN rail, 24 VDC above the relays. Treat the device sizes below as typical for the class and swap in your datasheets before anything is drilled. You need a 14 or 16 ga [1.5-1.9 mm] steel or stainless partition with mounting feet, a bolted cover with captive screws, snap bushings, a 10 AWG green bonding jumper, and the labels from your arc flash study.

1. Set the partition 12 in [305 mm] from the left wall, back to door and floor to ceiling, so the column is closed on five sides. Bond it as described below before any line-voltage conductor lands.
2. Stack the column top-down. From the top: 1.0 in margin; the 60 A fusible disconnect, about 9 in tall, handle extended through the cover; 1.5 in gap; Size 1 starter, 6.5 in; 1.5 in gap; second starter, 6.5 in; 2.0 in of clear air; the 750 VA transformer, 5.5 in; 2.5 in to the bottom wall. Sum: 1 + 9 + 1.5 + 6.5 + 1.5 + 6.5 + 2 + 5.5 + 2.5 = 36.0 in, so the column closes with every device clear of its neighbor.
3. Land the power glands under the column. The incoming feed and both motor leads enter the bottom wall inside that 12 in, so no power conductor ever crosses the control side.
4. Rail the control side one voltage per rail. Top rail: 24 VDC, carrying the PLC and DC relays. Duct. Next: the 120 V relays and the 120-to-24 VDC supply, output feeding straight up. Duct. Bottom: terminal rows at 45-55 in above the floor, both groups divided and marked.
5. Cross the partition once, at the bushed hole described next.
6. Bolt the cover with screws about 6 in [152 mm] apart along the edges, and fix its label to the face.
7. Check the heat. Losses: transformer 22.5 W at 3%, the 24 VDC supply about 7 W at your load, coils and relays about 10 W — 40 W total. The body sheds it through stainless at k = 3.7 W/m2K over about 2.24 m2 of surface: 2 x (0.762 x 0.914) + 2 x (0.762 x 0.254) + 2 x (0.914 x 0.254). Rise = 40 / (3.7 x 2.24) = 4.8 K, so a 35 C plant puts the interior at 39.8 C, the top of the 40 C range most electronics carry. Every extra 10 W adds 1.2 K; at 50 W, order the 36 x 48 in body instead, which rises about 3.3 K over the same math.

Where do the control wires cross the partition?

At one bushed point near the top of the partition: the transformer secondaries and the two starter coil commands cross through a single 0.5 in [13 mm] hole with an insulating snap bushing, and nothing else crosses. Fuse both secondaries right at the transformer, before they cross, so a control-wiring fault clears at the secondary fuse instead of up at the primary. Run that group down its own duct to the relay rail and keep the 24 VDC duct separate for its whole run, so a meter lead on one rail never meets the other level. NFPA 79-2012 section 13.5.2 caps duct fill at 50% of the cross-section, and round cable makes 50% look full, so budget the crossing group its own channel once field wires arrive. Use flame-retardant nonmetallic duct, the IEC 60332-1 test NFPA 79-2015 section 13.3.1 calls for inside enclosures. Mark the bushing on both faces: 120 V SEC + COILS. The backplate and DIN rail layout guide carries the rail-spacing math.

How do you ground and label the power compartment?

Bond the partition to the ground bus with a 10 AWG green jumper — the copper size NEC Table 250.122 gives for a 60 A feed — then label the column's cover with its own arc flash values from the study. NEC 250.96(A) wants metal parts that could carry a fault bonded so they take the current, and your partition sits inches from the line lugs. Scrape the coating where its feet and screws land: NEC 250.12 requires paint and lacquer off the contact surfaces at grounding connections. Land grounds on the ground bus and the transformer neutral on its own isolated terminal, which is how NEC 250.6 keeps neutral current off the metal. At 6 AWG and smaller, the grounding conductor is green or green with a yellow stripe under NEC 250.119. The enclosure grounding and bonding guide lays out the full scheme.

Two labels, two compartments. The outer door carries the control side's values: a 750 VA secondary at 120 V delivers 6.25 A rated and on the order of 100 A into a bolted fault, and IEEE 1584's equations start at 208 V, so studies usually put that section below the 1.2 cal/cm2 [5 J/cm2] boundary. The column cover carries the full line-voltage energy, boundary and PPE from the same study. NEC 409.110 puts the enclosure type number on the panel nameplate. Where the plant's program calls for listed equipment, OSHA 1910.303(a) wants approved gear and 1910.399 counts NRTL listing or labeling as acceptable, with UL 508A the listing standard for an industrial control panel as NEC 409.2 defines it — order that panel from a 508A shop. Our bodies carry a manufacturer rating, written NEMA 4X / IP66 by the manufacturer, not UL listed, and NEMA does not test or certify enclosures.

How much working space does the power side need?

At least 3.5 ft [1.1 m] deep in front: 480 V puts 277 V to ground, which lands you in the 151-750 V row of NEC Table 110.26(A)(1), Condition 1, and the space runs at least 30 in wide with a 90-degree door swing. Height comes to 6.5 ft or the equipment height, both under NEC 110.26(A). Mount the body with its bottom at 42 in: the top lands at 78 in and the disconnect handle at about 72 in, inside the 6 ft 7 in [2.0 m] ceiling NEC 240.24(A) sets for the highest operating handle. Keep that front clear for good; pallets and shelving in front of the door are how the 3.5 ft disappears. The enclosure sizing guide works the same math for other voltages.

One enclosure or two — which should you build?

Two, whenever the wall allows: the power box holds the disconnect, starters and transformer, the control box holds the relays and 24 VDC behind its own door, and each label tells the truth about what is behind it. Maintenance wins the argument — read mixed voltage control panel safety for what the control door is worth when the arc flash energy lives elsewhere, and the main-enclosure-or-remote-box question for drives for the same split on a bigger load.

Stack them — power below, controls at standing height — and you keep one working front and one gland run between the boxes. Ordering from us: draw the partition, the column cover and the gland zones into your layout, send it for a quote, and the body is delivered in 10 business days counted from your approval of the drawing, through the custom stainless enclosures and cutouts page — 304 for indoor washdown, 316L where chlorides or salt air reach the box. Check seals, mounting and rear clearance on the approval drawing before anything is cut, and use the enclosure selector to resize the body when your device stack differs from the example.

What checks prove the separation works?

Walk these six checks with a tape and a meter: they prove the column is closed, bonded, labeled and cool, and they take about ten minutes.

1. Continuity: partition-to-ground-bus reads the same fraction of an ohm as backplate-to-ground-bus.
2. Tape measure: 3.5 ft of clear front, 30 in wide, door swinging a full 90 degrees; highest handle at or below 79 in.
3. Cover: every screw home, and the disconnect operates with the cover bolted on.
4. Labels: two arc flash labels with study values on them, plus the nameplate with its type number.
5. Heat: after two hours at load the interior sits within about 5 K of room per the math above; more means your watt count ran low, so recount it.
6. Crossing: one bushed hole, secondary fuses fitted, duct fill at or under 50% on the tightest run.

The separation fails four ways, all seen in the field: a column left open at the top, power leads routed across the control side to reach a gland, a partition bolted through paint with no jumper, and one label doing duty for two compartments. Ten minutes of checks catches all four before the door closes on commissioning day.

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.

Questions