Fused IEC power entry module in a small PLC enclosure

Short answer: yes. A fused IEC power entry module is the tidy way to feed a small PLC box — one 4-20 mA input, a supply, about 1 A — and it clears 3 to 5 DIN modules of rail compared with the terminal-block, breaker and filter train. Two conditions make it right. The protector inside the module is a supplementary device, recognized under UL 1077 in the US and certified to CSA C22.2 No. 235 in Canada, so the branch fuse or breaker upstream keeps doing the branch job. And once you cut the wall, the inlet rather than the enclosure decides the water rating. Here is the sizing, the wiring order, the checks and the sealing rules.

The rule that makes the module legal

A combined inlet does three jobs in one marked part: it connects the cord, it protects the load and it filters the line. The protection job is where the codes draw the line. A protector recognized under UL 1077 in the US, or certified to CSA C22.2 No. 235 in Canada, is a supplementary protector. The Canadian standard covers manual-reset devices from 0.5 A to 63 A at 240 V AC, and 0.5 A to 40 A at 480Y/277 V AC. A true branch-circuit breaker is a different device, listed to UL 489. The NEC treats the supplementary type as a backup: the branch fuse or breaker at the source stays in the circuit, and the module rides behind it.

The upstream device is easy to satisfy. On a cord-and-plug feed from a receptacle, the breaker protecting that receptacle circuit is the branch protection. On a hard-wired feed, the circuit arrives at your box already fused or broken at its source panel. On a control-transformer feed, the branch device protects the secondary. Read the module's marking before you buy: it states the voltage, the cartridge size and the maximum upstream branch rating the device may sit behind.

Sizing is short arithmetic. A 1 A continuous load takes a 2 A cartridge: twice the running current, with headroom for the switching supply's cold-start inrush of tens of amps lasting a millisecond or two. Where the module takes 5 x 20 mm cartridges, fit the time-delay type; a fast cartridge can open on cold start. The fuse-versus-breaker choice inside the module repeats the branch-level trade at small scale; the same comparison at drive power levels is laid out in fuses vs supplementary protectors for branch circuits.

One more marking. A box holding a PLC, its supply and its terminals is an industrial control panel under NEC 409.2's definition — an assembly of two or more power or control circuit components. NEC 409.110 requires the enclosure type number on the panel marking, and how panels get listed and marked walks the rest of that paper.

The space the module hands back

Count the rail the old train eats. The DIN dimensions are fixed at 17.5 mm per breaker pole and 6 mm per terminal block, and a two-stage single-phase filter sized to 6 A runs about 45 mm wide.

Swap in a combined DIN-rail module of about 45 mm and keep one 6 mm PE terminal: 51 mm of rail, 53 mm freed. That is three 18 mm modules back. Flange-mount the combined unit through the rear or bottom wall instead, and the rail keeps only the PE terminal: 104 - 6 = 98 mm freed, five 18 mm modules with 8 mm to spare. A fused terminal block beats both on size, but it gives up the inlet, the switch and the filter — the three things you were combining.

When the backplate is crowded beyond one swap, control panel out of space: add or redesign takes the wider view, and the backplate and DIN rail layout guide sorts the rest of the rail.

What you need

• A combined C14 inlet module: switch, fuseholder or breaker cartridge and filter in one housing, marked UL 1077 recognized (US) and CSA C22.2 No. 235 certified (Canada).
• A 2 A time-delay 5 x 20 mm fuse or a 2 A breaker cartridge, sized to the 1 A load.
• A cord with a C13 connector. IEC 60320 rates the C13/C14 pair at 10 A and 250 V, with a 70 °C (158 °F) temperature rating; add the retention clip the datasheet calls for.
• One 6 mm PE terminal and a lug for the wall bond.
• 18 AWG machine-tool wire for the load side; the machine-tool wiring tables in UL 508A rate 18 AWG copper at 7 A, comfortably above 2 A.
• At a washdown wall: the inlet maker's gasketed flange and a sealed entry cap.

Fitting the module, step by step

Lock the feed circuit out and prove it dead before you land a wire.

1. Add the load. List what hangs on the protector: the PLC supply at about 1 A continuous. The cartridge is 2 A.
2. Read the marking. Match supply voltage (120 or 240 V), cartridge amps and the maximum upstream branch rating printed on the housing.
3. Order the cutout with the box. A flange module sits in the rear or bottom wall, hole per the datasheet. On a custom stainless box you send the cutout layout with the order and it arrives cut; the cutouts, glands and windows guide covers hole sizing and edge finish.
4. Mount the module. Gasket on the inside face, flange screws to the torque printed on the part.
5. Land line and PE. L and N onto the module terminals; PE onto its terminal and onward to a lug on the bare wall. NEC 250.96(A) requires metal enclosures serving as grounding paths to be bonded so they can carry fault current, and bare stainless needs no paint stripped first. The grounding and bonding guide carries the full path.
6. Wire the load side. 18 AWG machine-tool wire from the 2 A protector to the supply input, routed low on the backplate and clear of the supply heatsink. The small-box layout in putting a 120 VAC power supply and relay inside a small enclosure shows the spacing.
7. Label. Cartridge rating at the module, enclosure type number on the panel marking (NEC 409.110).
8. Energize behind the upstream branch device and clamp the cord: about 1 A at 120 V, about 120 VA.

Checks when done

• The protector toggles off and resets cleanly with the load running.
• A clamp meter at the cord reads about 1 A, steady, matching your load list.
• An IR gun at the inlet housing reads within a few degrees of the 40 °C room; the C13/C14 pair is rated 70 °C (158 °F) under IEC 60320, and 1 A through 10 A contacts barely warms it.
• A low-ohm meter from the PE terminal to bare wall metal reads under 0.1 Ω.
• A tug on the cord leaves the C13 seated and the retention clip holding.
• The door closes with even gasket compression and nothing fouling the flange.

What the inlet does to the enclosure rating

Where the box lives decides how much this matters. In a dry control room a plain flanged inlet is an ordinary detail; water never enters the argument.

At a washdown or outdoor stainless wall, the inlet is a hole in a sealed box, and the connector sets the water rating: the weakest part of the assembly governs it. A gasketed flange on the datasheet hole restores the wall itself, but the mating side must seal too. A sealing cap keeps the rating when it is the inlet maker's documented part, fitted at the specified hole size and torque; any other combination, and you rate the finished box at the connector's own rating. Our stainless wall-mount boxes are manufacturer-rated NEMA 4X-type / IP66 and not UL listed, and that rating survives an inlet only under those conditions.

Mind the 70 °C (158 °F) connector limit on hot walls. Put the module on the bottom face, the coolest one, and keep the supply heatsink across the backplate from it. The sealed-entry trade-offs are the same ones worked through in DC power entry into a sealed enclosure for a single board computer.

Mistakes that cost the build

• Using the module as the only protection on a hard-wired feed. Keep the branch fuse or breaker at the source; the module backs it up.
• A fast cartridge on a switching supply. Cold-start inrush opens it in service; the time-delay type rides it.
• An oversize flange hole or a missing gasket. The datasheet hole, cut clean, and the inside-face gasket are the whole seal.
• The inlet crowded against the supply heatsink. The 70 °C connector limit leaves less margin than you think in a 40 °C room.
• PE landed at the module with the wall left unbonded. Run the lug to bare stainless and read under 0.1 Ω.
• A cord hanging with no drip loop outdoors. Bend the cord below the inlet so water sheds before it reaches the cap.

Net: a 2 A supplementary cartridge behind an upstream branch device, a gasketed flange on the datasheet hole, PE bonded to bare metal. That is the module earning its keep — five modules of rail back and an inspector-ready panel.

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