Yes to both cuts, with one change: the fuse moves instead of disappearing. Put it where the UPS hot leaves the UPS, because a 250 VA inverter's output figure is a continuous rating, not a fault-current limit, and the code wants each conductor protected where it receives its supply. The neutral can stay out of the 480 V cabinet: the SSR switches only the hot, so the return runs from the lamp straight back to the UPS. Run the supply hot and the switched hot through the panel as a yellow-marked pair. Label the door for the external source and hand the shop three redlines. Nothing in that revision fights UL 508A.
Your three redlines at a glance
| Redline | The call | The number that decides it |
|---|---|---|
| Delete the series fuse in the panel | Move it to the UPS end, same rating | 20 AWG ampacity, about 3 A, below the relay's 10 A |
| Drop the neutral terminal | Approved; draw the whole loop | 2 of the circuit's 3 conductors pass through the cabinet |
| Add signage | Yes, plus wire color | The external leg stays live when the main switch is open |
NEC 409.2 defines an industrial control panel, and UL 508A is the standard a shop lists one to. The listing lives as much in the drawings as in the metal, so a documented external circuit of this kind is ordinary work for the fabricator. Each row above turns on its own number, so take them one at a time.
Move the fuse to the UPS end
The 277 V sensing leg and the 120 V switched leg are two separate circuits. The fused 277 V input already protects its own side and it protects nothing on the output. Protection for the UPS leg belongs where that conductor is fed: at the UPS output terminals.
A fuse inside the panel also leaves the cable between the UPS and the panel exposed. A fault in that run sits upstream of the panel fuse, so the shop's device never covered the whole conductor. One fuse at the source covers every inch of the loop with a single device.
The 250 VA nameplate tells you what the inverter delivers continuously. Into a short circuit that inverter may current-limit, fold back, shut down or ride through on a bypass path. The answer sits in the model's documentation, not in its VA rating, and none of those behaviors is overcurrent protection you can size a conductor around. The NEC puts the device at the supply end for exactly that reason: the fuse guards the wire whatever the source does.
Fuse size: the smaller of the wire and the relay
Size the fuse to the lower of two numbers: the ampacity of the loop conductor and the rating of the device the loop feeds. A 20 AWG copper run is good for about 3 A - the NEC's fixture-wire values put 18 AWG copper at 6 A, and 20 AWG works at about half that. The SSR output is rated 10 A, so the fuse is 3 A and it lives at the UPS.
Two things move that number. Lighter wire drops it a step per gauge, and heavier wire stops at the relay's rating unless the SSR datasheet names a maximum protective device for its semiconductor. Many datasheets do, and that figure becomes your ceiling even on thick wire. The same smaller-of-wire-and-device logic runs through fusing both legs of a floating 24 VDC supply when that question comes up on the control side.
The load rides easily. The lamp draws 0.08 A, a trivial load for the fuse, and past about 360 VA of load both the wire and the fuse move up together.
The neutral stays out of the cabinet
Draw the loop: UPS hot, fuse, cable, SSR output terminals, cable, lamp, and the neutral returning from the lamp to the UPS. The cabinet sees two conductors of this circuit - supply hot in, switched hot out - so a neutral terminal in a 480 V assembly serves no connection. Delete it, and say on the drawing where the neutral lands: at the lamp.
Keep the pair in one cable or one raceway and bring them through the same opening. Conductors of one circuit enter a metal enclosure together so their magnetic fields cancel. Send one leg through its own hole and the steel around that hole sits in the field of a live conductor with nothing to cancel it.
Read the relay's wiring diagram before you delete the terminal. A monitoring feature or a different output scheme can want a connection the simplified schematic omits, and the drawing revision is where you catch it.
Equipment grounding travels with the power conductors whatever you decide about the neutral; they are separate decisions. The enclosure grounding and bonding guide covers where grounds land and why the earth is never the fault path.
Yellow wire, labels and segregation
Color the external leg yellow or run it with a yellow stripe. That is the accepted machine-wiring marking, NFPA 79 practice, for a conductor fed from outside the panel: whoever opens the door sees a color the main switch does not control.
The main switch on a 480 V assembly opens its own three-phase supply, as isolating all three phases at the main switch covers, and this external leg stays live right through it. So label as well as color: UL 508A and NFPA 79 both call for marking when a panel is fed from more than one source. A line like 'contains circuits energized from an external 120 V source - open that feed before servicing' belongs at the door, repeated near the SSR terminals. NEC 409.110 already requires an enclosure type number on the panel, so door markings are ordinary paperwork for the shop.
Segregate the run. Give the external pair its own raceway or its own side of the backplate, away from the 480 V wiring; wiring a receptacle inside a control enclosure walks the same layout problem for a receptacle.
Safety, once: 120 V is above the 50 V threshold where NFPA 70E applies, and this leg has its own source. Lock out the UPS feed and the 480 V supply, then test for absence of voltage at the SSR terminals before touching them. The main disconnect alone leaves this leg live, the same trap as the line side of a separate disconnect enclosure.
Sizing the source fuse for a 250 VA UPS on a 20 AWG loop
Inputs, with where each comes from:
- Lamp: 10 W, from the fixture nameplate
- UPS inverter: 250 VA, from the UPS nameplate
- SSR: single-pole, normally open output rated 10 A, from the relay datasheet
- Loop conductor: 20 AWG copper, about 3 A
The rule in words: protect the conductor at its source with a fuse no larger than the smaller of the wire ampacity and the protected device's rating, then confirm the fuse carries the load with margin.
The arithmetic:
- Lamp current: 10 W ÷ 120 V = 0.08 A
- UPS continuous output: 250 VA ÷ 120 V = 2.08 A
- Fuse ceiling: the smaller of the wire ampacity and the relay rating = 3 A
- Margin: the fuse is about 37 times the lamp current, and the full continuous output of the UPS rides under it
Result: a 3 A time-delay fuse in a small holder at the UPS output, feeding the 20 AWG loop.
Thresholds: swap the SSR for a 2 A part and the fuse follows it down to 2 A. Rewire the loop heavier and the ceiling becomes the relay's rating, or its datasheet maximum fuse if that is lower. Push the load past about 360 VA and both wire and fuse move up.
Mount the holder in its own small box: a Type 12 enclosure indoors, Type 4X where it gets washed down. Ours carry manufacturer ratings and are not UL listed. Gland and cutout sizes follow the cable you run, and the cutouts and glands guide has the standard hole dimensions.
What the revised drawing must show
Send the redline as a loop drawing, not a fragment. Five items cover it:
- The fuse at the UPS end, marked time-delay, with the holder's location drawn.
- The full loop end to end: UPS terminals, fuse, cable route, SSR terminals, lamp terminals, and the neutral return at the lamp.
- A note that the neutral lands at the lamp and passes through no terminal in the 480 V cabinet.
- The yellow callout on both external conductors, plus the second-source label text.
- Both conductors through one gland or knockout, sized to the cable.
A terminal drawing for a junction loop shows the same discipline where a loop leaves a smaller box. If the revision grows into new boxes, the enclosure builder takes your sizes, material and gland holes, custom enclosures are delivered in 10 business days from your approval of the drawing, and a custom enclosure drawing package lists everything to send with it.
Fuse at the source, neutral at the lamp, yellow through the panel, label on the door: that is the whole revision. Presented like this, the shop keeps the listing and sends you a drawing to approve.
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 |
|---|---|---|---|
| 16 × 12 × 8 in | 304 stainless | 16 x 12 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 30 × 24 × 10 in | 304 stainless | 30 x 24 x 10 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 60 × 36 × 16 in | 304 stainless | 60 x 36 x 16 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Will a 3 A fuse nuisance-blow when the LED lamp starts?
Use a time-delay fuse and it holds. The lamp runs at 0.08 A, and its driver draws a startup surge lasting well under one cycle at 60 Hz. A time-delay fuse of that rating rides those milliseconds of inrush and still clears a real fault. Specify time-delay on the drawing; a fast-acting fuse of the same rating can open on every start.
Why does the lamp glow faintly when the SSR is off?
A solid-state relay leaks a few milliamps through its internal snubber even when off, typically under 10 mA on a 10 A part. A 10 W LED driver can produce visible light on that. Fit a suitably rated bleeder across the lamp terminals to swamp the leakage, or specify a relay with lower off-state leakage.
What should the external-source label say?
Name the source and the action: what the circuits are energized from and which feed to open before servicing. Mount it at the door beside the disconnect marking and repeat it near the SSR terminals. NEC 409.110 already puts an enclosure type number on the panel, so the shop adds this label in the same pass.
Can the SSR switch the neutral instead of the hot?
It can be wired that way, and it is the wrong way. With the relay in the neutral, the hot runs unswitched all the way to the lamp, so the fixture and its cable sit at line voltage whenever the UPS is on. Switch the hot; the neutral then returns from the lamp directly to the UPS.
