Short answer: land the inlet's ground pin on a single stud bolted to the aluminum wall and leave that conductor unbroken — no switch, no fuse, no plug-in joint anywhere in it. Switch and fuse only the hot. A 520 W device at 120 V draws 4.33 A, so a 6 A time-delay fuse at the inlet protects the circuit with 39% margin and holds through switch-on inrush. Aluminum needs one extra move at the stud: the oxide film on its surface is an insulator, so an external-tooth lock washer has to bite through it, and the nut has to stay locked. Build in that order — ground first and continuous, hot fused and switched at the inlet, neutral straight through — and the wall becomes the fault return a pinched conductor needs. Pull the plug and verify dead with a meter before you crimp or land anything.
Why the ground path carries no switch or fuse
The ground conductor has one job: carry fault current to protection that clears it. NEC 250.4(A)(5) calls that path the effective ground-fault current path, and nothing in it may open. A switch, a fuse or even a plug-in connector in the ground can leave the enclosure live during exactly the fault the ground exists to clear. So run the inlet's ground pin straight to the stud, and feed every load's ground from there.
Switch the hot, and only the hot. On a receptacle of this voltage class the narrow blade is hot and the wide blade is neutral. Opening the narrow one leaves the inside dead to ground with the switch off. Open the neutral instead and the circuit downstream still sits at line potential through the loads, ready to bite you with the switch off. The same habit holds in bigger panels: see switching an external 120 V circuit inside a UL 508A panel.
NEC 250.96(A) covers the wall itself. Metal parts intended to serve as equipment grounding conductors must be bonded so they safely carry any fault current likely to be imposed on them, and your aluminum wall is one such part. That means metal-to-metal contact at the stud, the splitter ground and the inlet pin, not a path through paint. The fundamentals, including why the earth outside is not part of this path, sit in the grounding and bonding guide.
What you need
Six parts cover it:
- IEC C14 power inlet with a built-in switch and fuse holder: one part fuses and switches the hot at the wall, with spade terminals inside; these modules ship with 18 AWG leads.
- A 6 A time-delay fuse in the size the holder takes.
- Ring-tongue lugs crimped for 14 AWG copper; the worked example below uses a #10 stud screw, so #10 lugs.
- Two external-tooth lock washers per stud, one flat washer, and a spare nut if you lock with two jam nuts.
- 14 AWG stranded copper for the ground runs, a crimp tool, heat-shrink tubing and a cable clamp.
- A multimeter with a continuity range.
The stud: teeth through the oxide
Aluminum conducts well and surfaces badly. Within minutes of being cut or scuffed it regrows a hard oxide film that insulates like paint. A lug that reads fine on day one can drift toward open over months. You cut through it once and keep the joint gas-tight so it stays cut:
- Scuff a coin-sized area of wall bright with fine abrasive, right before assembly.
- Set one external-tooth lock washer directly against the bare metal; its teeth emboss both the wall and the first lug, and that bite is what stops the oxide regrowing underneath.
- Land the inlet's ground lug first; it is the primary point. Land the splitter's ground lug on the same stud.
- Add a flat washer and the nut, locked with a second tooth washer or two nuts jammed together.
- Tighten until the teeth bite and no lug turns by hand.
On a painted or anodized box the rule is NEC 250.12's: nonconductive coatings such as paint, lacquer and enamel come off the contact surfaces at grounding connections unless the fitting is designed to cut through them. Bonding a powder-coated aluminum enclosure covers that case in depth, and grounding studs on aluminum walls covers how the studs themselves behave over years. Stacking order is not cosmetic either: why washer and hex nut orientation affects joints walks through how the washer and nut arrangement carries the clamp.
Fusing a 520 W, four-load circuit at 120 V
The inputs: four certified loads of 240 W, 240 W, 25 W and 10 W — a load list that sums to 515 W — and a stated device total of 520 W, which is the number to size to. The supply is a 120 V AC branch circuit through a cord set. The candidate fuse is 6 A time-delay, a standard rating one step above the running current.
The rule is current equals power divided by voltage, per load and for the total:
- 520 W ÷ 120 V = 4.33 A total running current
- 240 W ÷ 120 V = 2.00 A per large branch
- 25 W ÷ 120 V = 0.21 A and 10 W ÷ 120 V = 0.08 A for the small branches
- 6 A ÷ 4.33 A = 1.39, so the fuse sits 39% above full load
| Point in the circuit | Power | Current at 120 V |
|---|---|---|
| Each 240 W branch | 240 W | 2.00 A |
| The two small branches together | 35 W | 0.29 A |
| Splitter input, load list | 515 W | 4.29 A |
| Design total, stated | 520 W | 4.33 A |
| Fuse on the hot at the inlet | — | 6 A time-delay |
The decision: fit the 6 A time-delay fuse in the hot at the inlet. The 1.39 multiple lands inside the 1.25 to 1.5 band a time-delay fuse is usually sized to. It holds the inrush while the loads' switch-mode supplies charge their input capacitors, and it still clears a dead short. Hot to the bonded wall draws multiples of 6 A; in a series chain the smaller device clears first, so the fuse opens before the 15-20 A branch breaker has to. The module's 18 AWG leads carry the 4.33 A running current. Run the ground in 14 AWG: NEC Table 250.122 sizes premises equipment grounding conductors from the overcurrent device, and a 15 A device calls for 14 AWG copper. Treat 10 AWG as bulk that fights you in a small box.
Wiring it, in order
- Mount the IEC C14 fused-switch inlet in its wall cutout and secure it with its retaining nut; the module maker's drawing gives the cutout size.
- Crimp a ring lug on a 14 AWG lead from the inlet's ground pin and land it on the stud first, tooth washer against bare metal.
- Land the splitter's ground lug on the same stud, then run ground from the splitter's ground terminals to each load.
- Wire the hot in one run: line pin, fuse, switch, splitter hot input. The fused-switch module does the fuse and switch on the line pin in one part.
- Run the neutral straight from the inlet's neutral pin to the splitter's neutral terminal with nothing in between.
- Heat-shrink every crimp and dress the leads clear of sharp edges and the stud stack.
- Clamp the bundle just inside the inlet so a pull on the cord opens the ground last.
Checks when you are done
Meter checks prove the bond before you close the box:
- Inlet ground pin to stud: a fraction of an ohm on the continuity range, steady when you wiggle the lead.
- Stud to each splitter ground terminal: the same reading.
- Switch off: open between the line pin and the splitter hot terminal. Switch on: continuous. Neutral: continuous in both switch positions.
- Tug every lug and lead: nothing turns, slides or backs out.
- First power-up on a GFCI-protected outlet: it catches a wiring slip before it bites.
Mistakes that cost the bond
- A fuse or switch in the ground run, including a ground that passes through a plug-in switch module.
- Switching the neutral and leaving the hot unbroken.
- A lug set on powder coat, anodizing or paint without cutting through to metal — NEC 250.12's case.
- A plain flat washer on bare aluminum, where oxide creeps under it and the joint drifts.
- A stud screw with no lock washer or jam nut; vibration works it loose.
- Counting on a door hinge or hinged panel as the ground path.
- Soldered joints on stranded power leads; crimp them instead, because solder wicks the strands and the joint cracks under vibration.
What to put on the order
Order the box with two wall features: the inlet cutout to the module maker's drawing, and a clear area for the ground stud. The custom enclosure builder takes material, size and cutouts and returns a drawing to approve, delivered in 10 business days counted from that approval. Aluminum suits the build when weight or corrosion picks the metal; custom enclosures cut the same layout in 304 or 316L when your site is washdown or coastal. If the box grows into a fixed control panel, the ground logic stays the same. The panel side adds working-space and branch detail: see wiring a 120 V receptacle inside a control enclosure to code and ground bar placement: wall or mounting plate.
The decision, once: one stud, one unbroken ground from inlet pin to splitter to every load, hot fused and switched at the inlet, neutral straight, oxide cut and the nut locked. Everything else is hardware.
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 |
Questions
What fuse size protects a 520 W load at 120 V?
520 W at 120 V draws 4.33 A, so a 6 A time-delay fuse on the hot carries the load with 39% margin and still clears a dead short. The time-delay element holds through the inrush spike when switch-mode supplies charge their input capacitors at switch-on. Fit it at the inlet, before the switch and the splitter.
What wire size suits this 4.33 A circuit?
The 18 AWG leads a fused-switch inlet ships with carry 4.33 A with margin. For the stud bond, 14 AWG copper is the size NEC Table 250.122 gives an equipment grounding conductor on a 15 A circuit. 10 AWG is stiffer than the bond needs in a small box; the contact area at the lug, not the copper bulk, does the work.
How do you get a lasting ground bond to bare aluminum?
Crimp a ring lug on the lead, then stack an external-tooth lock washer against the bare wall so its teeth cut through the oxide into both metal faces, and lock the nut — a second tooth washer under the nut or two nuts jammed together both hold. Aluminum re-oxidizes in minutes, so the teeth must stay embedded in a gas-tight joint. Re-check the bond after any knock or rework; a loosened nut shows up as resistance drift, not as an open circuit.
Can the aluminum wall carry the ground between inlet and splitter?
It can, but run copper anyway. Bond the inlet ground pin to the stud and the splitter's ground terminal to the same stud; the wall then parallels the copper and a fault finds two paths. A copper conductor between inlet and splitter remains the path you trust, because a wall joint depends on its tooth washers staying embedded. On a painted or anodized wall the coating alone can read open until a fault burns through it.
