Short answer: ground the enclosure, not necessarily the board. The stainless box must be bonded to the equipment grounding conductor so a fault, surge or lightning-induced current has a safe path. The board itself can either share that ground through metal standoffs or sit isolated on plastic ones. What you must not do is leave the box floating, as an ungrounded metal case next to electronics is a shock and ESD hazard. Here is how to decide and how to build it.
Bond the box first
The enclosure is a metal part that can become energized, so it belongs in the grounding system. NEC 250.96(A) requires metal parts intended to serve as equipment grounding conductors, including enclosures, to be bonded together so they can safely carry any fault current likely to be imposed on them, and NEC 250.4(A)(5) calls that the effective ground-fault current path; the earth alone is not part of it. A ground rod at the pole is an auxiliary electrode under NEC 250.54 and never a substitute for the conductor.
Our stainless wall-mount enclosures, from the 8 x 6 x 4 in (203 x 152 x 102 mm) 304 box to the 10 x 8 x 6 in (254 x 203 x 152 mm) 316L box, are designed to be bonded through an internal ground stud or boss. Run a bonding conductor from that point to the equipment grounding conductor in the supply cable. Size the conductor from the rating of the overcurrent device ahead of the circuit, and strip any paint or coating from contact surfaces at the connection. The grounding and bonding guide covers conductor sizing and the bonding rules in detail, so we will not repeat them here.
If the box is stainless and the fitting is stainless, you rarely have a coating problem, but check for passivation residue and corrosion film at the stud all the same. Tighten, then tug-test the conductor.
Decide: bond the board or isolate it
With the case grounded, the board has two valid arrangements.
- Bonded. Metal standoffs screw into tapped bosses or through-nuts in the box wall, so the board's mounting holes connect to the case. Use this when the board has designated chassis ground holes, when the vendor's instructions call for a chassis connection, or when you want the mechanical stiffness of a fully bolted stack. This also gives ESD events hitting the case a nearby return path instead of a path through the electronics.
- Isolated. Plastic or nylon standoffs keep the board's ground off the case. Use this when the board mixes grounded and floating circuits, when the design assumes a single-point ground at the power supply, or when you have seen ground loops in the wiring. Isolation does not remove the need to ground the case; it only separates the board's logic ground from it.
Do not mix the two on the same board: three metal standoffs and one plastic one is the worst of both, because the board can be at a different potential from the case at one corner. If you need isolation but also shielding, bond the board at one designated point and isolate the rest.
What you need
- Metal or plastic standoffs and screws sized to the board's mounting holes, with the right thread for the bosses
- A bonding conductor and a ring lug for the enclosure ground stud
- A star washer or contact washer if the stud has any coating on it
- A drill and the correct bit, if the box has no tapped standoffs; see the guide to cutouts, glands and windows for cutting and mounting hardware in the wall
- Sealing washers or grommets for any new penetration
- A multimeter for continuity checks
For a small board in a box like our 8 x 6 x 4 in 304 stainless enclosure, you can usually stand the board off the back panel rather than the wall, which keeps the weather skin untouched.
Steps
- Confirm the enclosure is bonded. Measure resistance from the ground stud to the equipment grounding conductor at the supply side; it should read near zero ohms with the conductor landed.
- Choose the mounting scheme. Mark the standoff positions on the back panel, keeping the board clear of the door swing, the gasket and any gland you plan behind it.
- Drill or tap only where marked, deburr, and blow out the swarf. Every chip left inside a gasketed box is a future leak path or short.
- Fit the standoffs. Metal ones go in with a thread-forming or machine screw plus a sealing washer on the outside if the hole passes through the wall; the sealing detail is the same one that keeps an outdoor box dry, as covered in the outdoor mounting guide.
- Seat the board. Under each plastic standoff, check that no screw tip, trace or shield braid can reach the case. With metal standoffs, confirm each mounting hole is actually a ground hole on the board; a live trace under a metal standoff is a fault waiting to happen.
- Land any chassis or shield connections. If the board or its power supply has a chassis terminal, run a short conductor from it to the enclosure ground stud rather than relying on standoff contact through paint and threads.
- Bond the cable entries. Metal cable glands threading into the wall help, but do not count on them; the same logic appears in the gland and cutout guide. Shielded cable screens and armor should land on the ground system per the manufacturer's practice, not on a scraper contact.
- Close up and test. Recheck continuity from the case to the equipment grounding conductor with the door shut and the conductors dressed. Then pressure-test the sealing: drip loops and gland torque keep the manufacturer's NEMA 4X / IP66 rating meaningful after the work; the NEMA 4X and IP66 guide explains what each rating covers.
Checks before you close the door
- Continuity from the enclosure ground stud to the supply-side equipment grounding conductor, measured with the door closed
- No rotation or lift on any standoff when the board is pushed at a corner
- Gasket fully seated, no hardware or wire under it
- Drip loops on every cable so water cannot wick into the gland
Mistakes that cost the rating or the ground
- Skipping the case bond because the board is isolated. The isolation protects the board, not the person touching the box.
- Counting on a paint-coated thread for the ground path. NEC 250.12 requires nonconductive coatings to be removed from grounding contact surfaces unless the fitting is designed for it.
- Using the external mounting feet or wall screws as the ground path. The mounting guide covers the mechanical side; electrical bonding is a separate, deliberate connection.
- Drilling a pass-through hole and pushing a bare screw through it with no sealing washer, then wondering why the box was rated IP66 at the factory and leaks a year later.
- Mixing metal and plastic standoffs so the board is half-bonded and half-floating.
A grounded case, a deliberate standoff choice and sealed entries give you a box that is safe to touch, kind to the electronics and still weather-tight after the work. If you need a size or a cutout pattern the standard boxes do not offer, our custom enclosure builder starts from your drawing.
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 |
|---|---|---|---|
| 8 × 6 × 4 in | 304 stainless | 8 x 6 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 10 × 8 × 6 in | 316L stainless | 10 x 8 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 10 × 8 × 6 in | 304 stainless | 10 x 8 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 16 × 12 × 8 in | 304 stainless | 16 x 12 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Should a PCB be grounded to a metal enclosure?
The board's logic ground rarely needs to touch the case, but the case must be grounded. Bond the enclosure to the equipment grounding conductor so a fault or surge finds a safe path. Connect the board's chassis or shield points to the case where the vendor calls for it; otherwise mount the board on metal standoffs for mechanical strength or plastic ones for isolation, and keep the decision deliberate rather than accidental.
Are plastic standoffs a problem in a metal enclosure?
They are fine and often the right choice. Plastic standoffs isolate the board from the case, which stops ground loops through mounting points and keeps a live trace from energizing the box. If you use them, the enclosure still needs its own bonding connection, and you should check that no board trace can contact bare metal through an unused mounting hole.
Do cable glands bond the enclosure?
Not reliably. A metal gland threads into the wall and can make incidental contact, but that contact depends on thread condition, paint and corrosion, so treat it as a bonus and not as the grounding connection. Run a dedicated bonding conductor from the ground bar or terminal inside the box to a dedicated ground boss or stud on the wall, and verify it with a meter.
Does grounding the enclosure affect its weather rating?
Not if you do it cleanly. Our wall-mount enclosures are rated NEMA 4X / IP66 by the manufacturer; not UL listed. That rating holds when every penetration is sealed. Use a grommeted stud or a gland with a sealing ring for the bonding conductor, keep the gasket surface clear of hardware, and torque the door screws evenly after the work.
