Bonding stainless hardware on fiberglass enclosures

Bond the stainless parts that are metal-to-metal with the raceway: hubs, locknuts on steel conduit, support steel bolted to metal. Leave the hinges and latches on a fiberglass box unbonded. NEC 314.3 lets a nonmetallic box take metal raceway only when the metal parts are bonded together inside the box. A stainless locknut on a steel hub therefore gets a copper jumper to the equipment grounding conductor (EGC). The same locknut on a fiberglass hub touches nothing conductive. Keep bare stainless off bare copper with plated, bimetallic or insulating washers, because in wet, salty air that couple corrodes, and above about 60 C (140 F) chlorides threaten the stainless itself. Done right, a fiberglass pull box needs one jumper per metal hub — and no screws anywhere else.

Which stainless parts sit in the fault path

The grounding rules in NEC Article 250 have one aim here: keep normally dead metal that could go live on a low-impedance path back to the source. NEC 250.4(A)(5) names that effective ground-fault current path and rules the earth out of it. NEC 250.96(A) adds the bonding duty. Raceways, cables, equipment and enclosures that serve as equipment grounding conductors bond together, so they can carry the fault current likely imposed on them. The grounding and bonding guide works through these rules in depth.

A fiberglass wall conducts nothing, and that pulls the box rules in. NEC 314.3 allows metal raceway or metal-armored cable in a nonmetallic box only where the metal parts are bonded together inside the box. The nonmetallic enclosure materials guide covers the same rule for plastic boxes. A stainless locknut on a steel hub is therefore raceway hardware inside the path. The same locknut on a fiberglass hub is an island. On an all-nonmetallic build, fiberglass RTRC raceway into fiberglass pull boxes, every stainless item sits on nonmetal and the bonding list comes up empty.

Type 4X fiberglass boxes and stainless boxes differ exactly here. Our stainless boxes are rated NEMA 4X / IP66 by the manufacturer, not UL listed. On them, every hinge, latch and screw is metal-to-metal with a conductive wall, so door hardware joins the bonding conversation. A metal door may then need the door bonding jumper; a fiberglass door needs none. The stainless enclosure size finder lists wall-mount and HMI boxes in 304 and 316L, and grounding a fiberglass enclosure walks the nonmetallic wall, back panel and EGC terminations.

Support steel bolted to bonded metal bonds itself through that joint. A medium-voltage, impedance-grounded system changes the fault level, not the shape: each raceway still carries its own EGC, sized from NEC Table 250.122 by the rating of the overcurrent device ahead of it. For how hub and locknut fittings behave at a metal wall, locknuts or hubs at conduit entries covers the metal-box version.

What you need

• Bondable locknuts or hub bonding screws sized to the raceway; a bondable locknut carries a factory bonding cup that grips the metal it clamps.
• Bare copper jumper wire, sized from NEC Table 250.122 for the overcurrent device ahead of the box.
• Zinc-plated steel and bimetallic transition washers for the joints that must bond.
• Insulating washers and sleeves in glass-epoxy (G10/G11) or EPDM, for the parts that must stay apart.
• A 4-wire low-ohm meter for bond quality, and a standard continuity meter for presence.
• Hand tools and a torque driver for the fitting hardware.

How to bond an entry, step by step

1. Sort the hardware before the first wrench turn. Mark every stainless part that is metal-to-metal with the raceway: a hub, a locknut on steel conduit, support steel bolted to metal. Treat the rest as islands.
2. Land the EGC on metal at each raceway entry. Use the hub's bonding screw or a bondable locknut whose bonding cup grips the metal it clamps.
3. Keep the joint copper-to-copper where the layout allows. Lug the jumper to the EGC terminal or a bonding bar, so no stainless sits inside the current path.
4. Where the joint must land on stainless hardware, sandwich a zinc-plated steel or bimetallic washer between the stainless and the copper lug. The plating sacrifices itself before either parent metal does.
5. Separate unbonded stainless from nearby copper with an insulating washer plus a sleeve in the bolt bore. A washer under the head alone leaves the bolt as a bridge across the couple.
6. In washdown service, rinse the hardware on the cleaning round and inspect plated joints about once a year. Swap washers as soon as plating turns white or shows rust-red.

Sizing the bond jumper at a 2 in hub on a fiberglass box

This worked example sizes one jumper, from a steel conduit hub on a fiberglass pull box to the EGC inside. It is a low-voltage case with example inputs; replace each one with your own before you cut wire.

• Jumper: 6 AWG bare copper, 11.8 in [300 mm] long. Example values; your size comes from NEC Table 250.122, your length from the box layout.
• 6 AWG cross-section: 13.3 mm², from the standard wire tables.
• Copper resistivity: 1.724 μΩ·cm at 20 C (68 F), the accepted value for annealed copper.
• Fault level: 5,000 A available ground-fault current, an example figure for a solidly grounded 480 V service.

The rule in words: resistance equals resistivity times length, divided by cross-section. Voltage drop equals fault current times resistance.

• R = (1.724 × 10⁻⁸ Ω·m × 0.30 m) ÷ (13.3 × 10⁻⁶ m²) = 0.00039 Ω = 0.39 mΩ
• Warm at 75 C (167 F): 0.39 %/K × 55 K = +21 %, so R = 0.47 mΩ
• V = 5,000 A × 0.00047 Ω = 2.4 V, against 1.9 V cold

Two volts of drop leaves the path healthy, so a short copper jumper is never the weak link. The threshold that matters is the joint. If the fitted assembly adds more than a few milliohms over the bare wire, or the reading wobbles when you tug the hub, the culprit is coating, gasket or a loose cup. Remake the joint rather than upsizing the copper.

Keeping stainless off bare copper

Galvanic corrosion starts when two different metals touch through an electrolyte, and wet, salty air supplies plenty. In the galvanic series, passive 304 and 316L stainless sit more noble than copper, so the copper is the metal that gives up ions. The same logic drives stainless against galvanized steel, where the zinc side sacrifices first. If chlorides kill the passive film in a crevice, the stainless itself can turn active and corrode. That is why the couple earns respect in a washdown plant and not in a dry office wall.

Heat moves the risk sharply. About 60 C (140 F) is the chloride stress-corrosion threshold for 304 and 316L alike, molybdenum or not. 304 also pits at room temperature under chloride deposits or in gasket crevices. A joint beside a steam line or under hot washdown sits in the worst band.

Pick the washer by the job the joint does. A bonded joint takes the plated or bimetallic washer, so the copper and stainless never touch bare-to-bare and the plating sacrifices first. An unbonded part takes full isolation: a glass-epoxy (G10/G11) washer plus a bore sleeve, the materials compared in insulating washer specs for galvanic corrosion. Unplated stainless resting on unplated copper stays a corrosion cell even with no current flowing.

Checks when the wrench is done

• Zero the meter leads, then read the jumper end to end. The example jumper adds 0.39 mΩ, which needs a 4-wire meter; a standard continuity meter shows presence, not quality.
• Tug the hub while watching the meter. A sound bond holds its reading.
• Look for bare copper riding on bare stainless at every lug and clamp.
• Confirm each bonding cup left a bright ring in the metal.
• Confirm entries went through listed hubs or glands, not field-drilled holes, so the box keeps its rating.

Mistakes that cost the bond or the rating

• Counting on a plain stainless locknut through a fiberglass hub to bond anything. With no metal on the far side, there is nothing to bond.
• Bolting bare copper straight onto stainless hardware in a wet box. The joint eats itself, fastest in the crevice under the lug.
• Drilling a rated wall for a bonding screw. Enter through a listed hub or gland instead. On a stainless box, order the entries as factory cutouts on custom stainless enclosures, delivered in 10 business days, anywhere in the world.
• Leaving a washed-out plated washer in service. Treat plating as a wear part and replace it on the yearly round.
• Bonding hinges and latches for extra safety. On fiberglass the wire ends at an island, and the screws buy nothing.

Sort the hardware, bond only the metal in the raceway path, and let washers handle the corrosion. On an all-fiberglass build that means one jumper per metal hub and nothing more.

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