4.4 Separately Derived Systems Grounding and Bonding

Redraw the Source

A separately derived system is easiest to understand if you stop looking at the utility service and redraw the source at the transformer, generator, or converter output. For a transformer, the secondary winding is the source for the secondary system. A line-to-case fault on the secondary side must return to that secondary winding. The utility service main bonding jumper on the primary side is not automatically the clearing path for the secondary side.

This is why derived systems have their own grounding and bonding rules. The system may need a grounded conductor, a system bonding jumper, a grounding electrode conductor, and a connection to the equipment grounding conductor system. The exact point of the system bonding jumper is controlled by the NEC arrangement, not by installer preference. The design must avoid both an open fault path and duplicate neutral bonding points.

Identify the Derived System Type

Not every transformer or generator creates the same grounding question. A transformer with a secondary winding electrically isolated from the primary circuit conductors is a classic separately derived system. An autotransformer is different because the windings have an electrical connection. A generator may or may not be separately derived depending on transfer equipment and whether the grounded conductor is switched. The exam may describe a standby generator with a solidly connected neutral through transfer equipment; that detail changes whether a system bonding jumper belongs at the generator.

For a wye secondary, the grounded conductor is often the neutral point. For a delta secondary, the system may be ungrounded, corner-grounded, or grounded through a midpoint depending on the design. Each choice affects overcurrent behavior, ground-fault detection, voltage to ground, and marking. Master-level work includes verifying that equipment ratings match the system grounding method. A slash-rated breaker that is acceptable on one grounded wye system may not be acceptable on a corner-grounded delta or ungrounded system.

Bonding Point Coordination

The system bonding jumper connects the grounded conductor of the derived system to the equipment grounding conductors and metal enclosures. The grounding electrode conductor connects the derived system grounding point to the grounding electrode system. A supply-side bonding jumper may connect equipment enclosures and raceways between the transformer and first disconnecting means when secondary conductors are installed in metal raceways or equipment.

A common field mistake is bonding XO in the transformer because the factory provided a strap, then also installing the bonding screw in the secondary panel. Another mistake is landing the grounding electrode conductor in a panel while the actual system bonding jumper is at the transformer, without checking whether the arrangement is permitted and whether the conductor is routed to the correct point. The right question is not Where is there a green wire?

The right question is Where is the derived system grounded conductor bonded to the equipment grounding system, and does every secondary fault return to the derived winding through that point?

Fault Path Example

Consider a 480 volt primary to 208Y/120 volt transformer. A secondary phase conductor faults to the transformer case. The fault current path runs from the secondary winding through the secondary phase conductor to the fault, into the transformer case, through the bonding connection to the grounded conductor or equipment grounding system, and back to the secondary winding. If the system bonding jumper is missing, the case may be connected to building steel and electrodes but still lack a low-impedance return path.

If the neutral is bonded in two places, normal neutral current can use metal raceways between those points.

For a secondary panel fault, the path may include the panel enclosure, equipment grounding conductor or raceway, supply-side bonding jumper, transformer enclosure, system bonding jumper, grounded conductor point, and secondary winding. Every fitting and conductor in that path matters. Secondary conductors ahead of the first overcurrent device can produce high fault current, so bonding and enclosure ratings are not casual details.

Code Navigation Steps

Use this sequence: identify whether the source is separately derived; determine whether the derived system is grounded or ungrounded; locate the first disconnect; locate the intended system bonding jumper; locate the grounding electrode conductor connection; identify supply-side bonding needs between source and disconnect; then check conductor and equipment ratings. If the question is about a generator, ask whether the transfer switch switches the neutral. If it does not, the generator may not be separately derived, and bonding at the generator can be wrong.

Supervisory Review

For transformer installations, review submittals and field conditions together. Confirm transformer secondary voltage, winding configuration, available fault current, overcurrent location, conductor length limits where applicable, grounding electrode conductor routing, bonding jumper size, enclosure bonding, and panel neutral isolation. Inspect nameplates and wiring diagrams before energizing. A misplaced bonding screw is a small part with systemwide consequences.

For generators, coordinate electrical design with transfer equipment. A separately derived generator with switched neutral has different bonding needs than a nonseparately derived generator with a solid neutral. Portable generator rules, optional standby systems, emergency systems, and legally required standby systems can layer additional requirements, but the grounding logic remains source-based. Draw the normal source, draw the alternate source, draw neutral switching, then decide where the fault returns.

Structured Decision Aid

• Decide first whether the transformer or generator connection creates a separately derived system.
• Place the system bonding jumper at an allowed point and avoid parallel neutral paths.
• Size grounding electrode and bonding conductors from the derived-system conductors, not from the primary alone.
• Coordinate SDS grounding with transfer switches, grounded conductors, and equipment grounding conductors.