Short answer: keep the independent feed only if it gets its own lockable disconnect, because the fuses on that circuit are never an off point. NFPA 79 hands you the marking scheme: orange for the ungrounded conductors that stay live when the main disconnect opens, voltage labels naming both sources at the disconnect area, and a finger-safe barrier over the section that stays hot. Both isolation points go on the lockout placard. What follows puts each piece where it belongs in your cabinet.
Can you keep two power sources in one control cabinet?
Yes, when every source entering the cabinet gets its own lockable isolation point, and NFPA 79 builds exactly that scheme: orange conductors, warning labels, and a finger-safe barrier for the circuit that stays live with the main disconnect off.
The hazard in your cabinet is an assumption. Someone opens the 480 V machine disconnect, watches the machine-side I/O go dark, and expects the whole box to be dead — while a receptacle-fed 120 V supply keeps the robot I/O alive at 24 V behind the same door. OSHA's control-of-hazardous-energy standard calls for isolating every energy source, so a live feed with no lock point is the problem to fix.
| Cell state | Machine side | External feed | Robot I/O |
|---|---|---|---|
| Machine running | Live at every level | Live | Energized |
| Main disconnect open | Dead | Still live | Energized |
| Both points locked out | Dead | Dead | Dead |
UL's guidance on machine supply circuits places the disconnect for such a circuit in an enclosure within or adjacent to the control enclosure, which tells you where the second handle belongs. When you have to work inside with both feeds hot, the mixed voltage control panel safety for troubleshooting walkthrough covers the sequence.
What color wire marks circuits that stay live when the main disconnect is off?
Orange, on the ungrounded conductors of the independent feed, from the disconnect all the way to the power supply terminals: NFPA 79 gives orange to ungrounded AC conductors that stay energized when the main supply-circuit disconnect is off, and reserves it for that single use.
Re-identify the entire run, with sleeves or marker bands at each terminal the conductors pass, so nobody ends up tracing a faded tag at one end. Orange carries one meaning — live with the main off — so keep it off the DC control wiring, which keeps its own DC color convention. Grounding conductors keep their color regardless: NEC 250.119 requires insulated equipment grounding conductors of 6 AWG and smaller to be green or green with one or more yellow stripes. For where that ground lands, the enclosure grounding and bonding guide covers the bus, the bonding path and the paint rule.
Where does the second feed need its own disconnect?
In or on the cabinet, ahead of the first terminal of that feed, fitted with a handle that takes a padlock: for a robot I/O supply drawing about 0.3 A, a 5 A disconnect carries the load with margin to spare.
The fuses on that circuit clear faults and nothing else. They offer no handle for a lock and no visible open position, so they serve as neither an isolation point nor a switch, and a lockout placard cannot point at them. Keep them for short-circuit protection downstream of the new disconnect.
Work the sizing with real numbers before you order anything. Sixteen inputs at 15 mA each draw 0.24 A; eight outputs driving solenoid valves at 100 mA each draw 0.80 A; the module itself takes about 0.10 A. Total: 1.14 A on the DC side, which is 1.14 × 24 ≈ 27 W at the load. At 85% supply efficiency the line side draws 27 ÷ 0.85 ≈ 32 W, and 32 ÷ 120 ≈ 0.27 A. That disconnect on 14 AWG conductors runs near 5% of its rating.
Placement follows two NEC rules you can measure. NEC 240.24(A) keeps the center of the handle grip, at its highest position, no more than 2.0 m [6 ft 7 in] above the floor. NEC 110.26(A) keeps clear working space in front of equipment serviced energized: at least 3 ft [900 mm] deep for 0–150 V to ground, at least 30 in wide or the equipment width, and room for the door to swing 90 degrees; the machine side of your cabinet needs more depth than that. The enclosure mounting guide works through the clearances, and the supply-side disconnect rules for this voltage class cover exactly that.
How do you install the second disconnect and barrier?
In eight steps, with the cabinet dead at both feeds before you start and about half a day for one person with a drill, a multimeter and a label printer.
- Lock out the machine disconnect and the external feed, then test for absence of voltage at the supply input terminals before your hands go in.
- Route the external feed through its own gland, low on the enclosure wall, and dress it away from the machine-side conductors.
- Mount the new lockable disconnect with its handle grip center at or below 2.0 m [6 ft 7 in] and the handle visible with the door open.
- Land the equipment grounding conductor on the ground bus and strip paint from the contact surface, as NEC 250.12 directs for grounding connections.
- Re-identify both ungrounded conductors orange along the full run, with fresh markers at every terminal they cross.
- Fit the finger-safe cover over the supply, its fuses and its terminals, fastened with the screws the cover was drilled for.
- Apply the four labels at the disconnect area and the live-when-off label on the cover itself.
- Update the lockout placard with both isolation points, naming the location of each.
How do you label a cabinet with two power sources?
With four marks at the disconnect area: a warning that the enclosure is fed from more than one source, a voltage marker naming the machine feed, a voltage marker naming the external feed, and a line directly under the multi-source warning naming where the second feed comes from.
The placard on the machine then lists both lock points — the machine disconnect and the new feed disconnect — with a line for each location. OSHA's lockout rule puts a personal lock on every energy-isolating device before work starts, so the placard has to make every device findable. If you are weighing whether the machine's E-stop counts as one of them, the write-up on lockable E-stop vs LOTO disconnect for CNC machines separates the two jobs cleanly. For the rest of the panel's markings, the control enclosure label checklist gives you the full set.
Does a plexiglass cover make the live section safe?
Partly: a screwed-down clear cover keeps hands and dropped tools out, but the protection that counts is finger-safe terminal hardware plus a cover labeled "live when the main disconnect is off", not the plastic sheet on its own.
Build the barrier from 3 mm [1/8 in] clear polycarbonate over the supply, the fuses and the input terminals, fastened at all four corners with screws you actually have to remove. Everything inside that boundary stays live when the main disconnect is open, which is exactly what its label says. At the boundary itself, touch-safe terminal covers stop a finger or a stray strand from finding a live point, and what makes an electrical enclosure finger safe explains what that hardware is tested to.
If the cabinet is being rebuilt around this scheme, a custom enclosure can come with the second gland plate, the barrier bosses and the label positions already in place — delivered in 10 business days, counted from your approval of the drawing. Choose 304 stainless for an indoor cell, or 316L for a washdown cell, rated NEMA 4X / IP66 by the manufacturer; not UL listed.
What checks prove the two-source isolation works?
Two measurements close it out: full line voltage still present at the robot supply terminals with the main disconnect locked open proves the marking matches reality, and 0 V at those same terminals with both points locked open proves the lockout boundary holds.
Then run the availability check you kept the second feed for. With the cell idle at the machine, open the main disconnect and confirm the robot I/O status LEDs stay lit and the inputs still read back from the pendant. That single test shows the independent feed, the new disconnect and the barrier working together. Close the door, photograph the finished disconnect area and placard, and file the photo with the updated one-line.
When is rewiring to the single machine feed the better choice?
When nobody needs the robot while the machine is down: move the supply input to the machine side of the main disconnect, and the second live section, the orange run and half the labels disappear in one afternoon.
Decide it by what the cell has to do during a machine fault, not by what the cabinet looks like today:
| Your case | Keep the second feed? | What you order and build |
|---|---|---|
| Robot I/O must stay available through machine trips | Yes | Lockable disconnect, orange run, four labels, finger-safe barrier, placard listing both points |
| Nobody needs the robot during machine downtime | No — rewire to the machine side | Single-source labels only, second gland sealed |
| A cell controller with its own supply exists | Feed the robot from the cell side | One isolation point per source, with the boundary at the cell instead of inside the machine cabinet |
If you rewire, distribute the machine-side power from one input so the panel keeps a single boundary: one power input for multiple devices inside an enclosure shows the layout. And if the retrofit has outgrown the box, run your inside dimensions and DIN count through the enclosure selector — it sizes the body, and you still owe that body a separate heat check.
Enclosures for this job
Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 60 × 36 × 16 in | 304 stainless | 60 x 36 x 16 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 48 × 36 × 16 in | 304 stainless | 48 x 36 x 16 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 60 × 36 × 12 in | 304 stainless | 60 x 36 x 12 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Is a fuse holder good enough as the isolation point for the second feed?
No. A fuse clears overcurrent and nothing else: it has no handle for a padlock and no visible open position, so it counts as neither a lockout point nor a switch. Keep the fuses for short-circuit protection, and size a lockable disconnect from the load — the 0.3 A supply in this scheme runs even a small disconnect near 5% of its rating.
Does the orange conductor rule apply to the DC side of the robot I/O?
No. NFPA 79's orange rule covers ungrounded AC conductors that stay energized when the main supply-circuit disconnect is off, which here is the feed to the power supply. NFPA 79 assigns blue to DC control conductors, so the PNP wiring to the robot I/O keeps blue, and the PNP-versus-NPN distinction belongs on the drawings rather than in the wire color.
Can the second disconnect sit outside the cabinet, near the receptacle that feeds it?
It can — UL's guidance on machine supply circuits allows the enclosure adjacent to or within the control enclosure — provided you can see both handles from the open door and the lockout placard names its exact location. One lock per person per point still applies, so a second handle out of sight doubles the lockout steps and the chance one gets missed.
How much extra space does the second feed add inside the enclosure?
Plan roughly 6 in [150 mm] of clear DIN rail for the lockable disconnect, two fuse terminals and the feed terminals, and about 2 in [50 mm] of depth for the finger-safe cover over them. If the cabinet is already tight, a wider body built to your layout is delivered in 10 business days, counted from your approval of the drawing.
