Separately derived transformer bonding inside a UL 508A panel

Short answer: keep the bond the panel shop made, and add the one thing it left for the field. The transformer's secondary neutral bonds to the enclosure through the system bonding jumper that comes installed, and your job is to land the grounding electrode conductor on the marked terminal at the ground bus before first power-up. The UL 508A label covers the panel as built, not the installation, so the NEC still governs the field wiring. This walkthrough sorts out who wires what, sizes every conductor for a 10 kVA, 460 V to 240/120 V transformer, and checks what the coil adds to the heat budget of a sealed Type 4 panel.

Does the NEC still apply to a UL 508A panel?

Yes, the NEC applies: a UL 508A listing evaluates the panel as the shop built it, and the field installation still follows the NEC, which treats the transformer's secondary as a separately derived system with bonding of its own.

NEC 409.2 defines an industrial control panel as an assembly of two or more power or control circuit components, and UL 508A is the standard a shop lists it to. NEC 409.110 adds the enclosure type number to the panel nameplate. None of that moves the code obligation: the label says the panel passed the listing evaluation, and the installer still grounds the separately derived secondary per the NEC. The UL 508A listing guide shows how listing, OSHA approval and the NEC fit together, and the industrial control panel standards breakdown sets the boundaries between the three documents.

Is the transformer secondary separately derived?

Yes: a 10 kVA transformer stepping 460 V down to a 240/120 V split secondary is a separately derived system, because nothing metallic ties the secondary winding to the primary supply.

A secondary floats until you ground it once, and the arithmetic shows the size of what you are grounding: 10,000 VA ÷ 460 V = 21.7 A into the primary, and 10,000 VA ÷ 240 V = 41.7 A out of each secondary leg. The center tap is the neutral shared by the two halves. A secondary that feeds load circuits is a grounded system in the NEC's eyes, so the neutral gets exactly one connection to ground, at the source. The grounding and bonding guide carries the fundamentals that rule builds on, and a leg left floating hides ground faults instead of tripping them.

What does the panel shop wire, and what waits for the field?

The shop wires two bonds and leaves one terminal for you: the primary EGC lands on the chassis, the system bonding jumper ties the secondary neutral to the ground bus, and an identified grounding electrode conductor terminal sits on that bus waiting for your field run.

UL 508A has the shop provide that terminal; it is evaluated to UL 467 for grounding and bonding equipment and sized for the conductor the panel documentation names. It can be part of the ground bus, and the NEC lets the grounding electrode conductor land on the enclosure at the same point where a wire or busbar jumper bonds the neutral. The shop's neutral bond is correct by design, so leave it in. The transformer inside the enclosure also changes the panel's fault-current math; panel SCCR with an internal transformer covers that calculation.

How do you land the bonds and the GEC, step by step?

Seven steps close the job: confirm the single neutral bond, land the primary EGC, clean the contact surfaces, terminate the 8 AWG copper grounding electrode conductor on the marked terminal, isolate downstream neutrals, bond the door, and record what you measured.

1. Lock out the 460 V feed and confirm absence of voltage at the line terminals before the cover comes off.
2. Measure neutral bus to enclosure with an ohmmeter: about 0 Ω proves the shipped system bonding jumper is in place and single.
3. Land the primary EGC on the ground bus with the rest of the panel grounds.
4. Clean every bond point: NEC 250.12 calls for paint and other nonconductive coatings removed from contact surfaces, so file the landing spot or fit star washers under each lug.
5. Terminate the grounding electrode conductor on the marked terminal and run it to the building's grounding electrode system; 8 AWG copper covers this example.
6. Walk every downstream subpanel on the skid and confirm its neutral stays isolated from ground; keeping the subpanel neutral and ground apart shows what a second bond costs.
7. Refit the door bonding jumper and log the readings; the enclosure door bonding jumper article tells you when the hinge alone is not enough.

What size conductors does a 10 kVA transformer need?

In the example, 10,000 VA at 460 V in and 240/120 V out, the secondary legs carry 41.7 A, 8 AWG copper covers them at 50 A, and the tables ask 10 AWG for the primary EGC and 8 AWG copper for the bonding jumper and the electrode conductor.

The arithmetic assumes a single-phase dry-type unit, copper conductors and 75 °C terminal ratings.

A nameplate that calls out terminal sizes or a different primary protection rule wins over these figures. For the jumper-sizing logic in detail, the transformer neutral bonding jumper sizing guide walks the NEC method step by step.

How much heat does the transformer add to the panel?

Budget about 200 W: dry-type transformer losses run 1–3% of rating, and a 10 kVA coil at 2% dumps 200 W into a sealed panel that has no open vents because it carries a Type 4 hose-down rating.

Stainless steel passes heat through the wall at k = 3.7 W/m²K. A wall-mount box with 2.0 m² of exposed surface sheds that load at ΔT = 200 ÷ (3.7 × 2.0) = 27 K, so a 30 °C boiler room puts the inside at 57 °C — 17 K past the 40 °C ambient that drives, PLCs and most electronics accept. Holding the rise to 10 K takes 200 ÷ (3.7 × 10) = 5.4 m² of stainless wall, which is a bank of cabinets, not one box.

Decide one of three ways: a closed-loop air-to-air core or a washdown-rated air conditioner keeps the seal and moves the heat out; a thermal plate bonds the transformer to the wall so the loss conducts out; or the coil moves to a ventilated housing beside the sealed panel. A custom stainless enclosure sized around the coil, the bus and the factory cutouts for the cooler and the electrode run comes back in 10 business days from drawing approval, and the enclosure sizing guide carries the full heat method.

Type ratings are manufacturer ratings: the stainless enclosures on this site are rated NEMA 4X / IP66 by the manufacturer and are not UL listed.

What checks prove the bond when the job is done?

Three readings close it out: about 0 Ω from the neutral bus to the enclosure with the feed off, 0 V neutral-to-ground at the transformer with no load running, and 120 V from each half to neutral once the panel is up.

Then re-walk the one-point rule: with the electrode conductor landed, neutral-to-enclosure still reads about 0 Ω, and no downstream neutral touches ground anywhere on the skid. A few volts of neutral-to-ground under load point to a loose neutral lug, not a second bond. The reading most often missed lives at the terminal itself: an electrode conductor that never got landed looks identical from outside a closed panel, so photograph the lug under its washer before the cover goes back on and log the torque.

Which mistakes trip this up most often?

The most common failure is also the quietest: the grounding electrode conductor never lands on the field terminal, so the secondary leans on incidental metal and the earth, which the NEC rejects as a fault-current path.

• Electrode conductor omitted. NEC 250.4(A)(5) is blunt: the earth is not an effective ground-fault current path. Terminate the 8 AWG copper run on the marked terminal before first power-up.
• A second neutral bond downstream. Neutral current then splits over two paths and a real fault may not trip; remove the downstream bond and size the conductors as the EGC and bonding jumper after a disconnect shows.
• Bonding through paint or a floating back panel. File the contact spot, use star washers, and bond the bus to the enclosure the way bonding an enclosure to a ground bus lays out.
• A rod mistaken for the electrode conductor. A ground rod driven at the skid is an auxiliary electrode under NEC 250.54 and never substitutes for the conductor run to the grounding electrode system.
• Isolating the shop's neutral bond. Pull the system bonding jumper and the secondary floats ungrounded; the one-bond logic is the same as why the neutral bonds only at the service disconnect.

Decision: in a skid panel with an internal split-secondary transformer, keep the shipped bond, land the grounding electrode conductor on the bus terminal before energizing, and size the cooling around the 200 W the coil adds.

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.

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