Separately derived system: what it means for the Texas exam

Use this diagram

Copy this HTML to show the diagram with a credit link<img src="https://bondingjumper.com/charts/bondingjumper/separately-derived-system.svg" alt="A service panel feeding a transformer primary with phase conductors and an equipment grounding conductor only, the transformer secondary feeding a new panel, a system bonding jumper from the new neutral to the enclosure, its own grounding electrode conductor to an electrode, and a crossed-out dashed line showing no neutral back to the service" width="100%"><p><a href="https://bondingjumper.com/guides/separately-derived-system?utm_source=embed&amp;utm_medium=referral&amp;utm_campaign=figure_embed&amp;utm_content=separately-derived-system.svg">A new neutral starts at the transformer, so the bond and the GEC start there too. by Bonding Jumper</a></p>

How to spot one

Ask one question: is the neutral on the load side the same conductor that runs back to the service? Take a common dry-type transformer with a 480 V delta primary and a 208Y/120 V wye secondary. The primary is fed by phase conductors and an equipment grounding conductor, with no neutral at all. The secondary neutral is created at the wye point inside the transformer. Nothing but the bonding path connects the two sides, so the secondary is a separately derived system. The transformer calculator works out the current on each side of a unit like this.

A standby generator can go either way. When its transfer switch also switches the neutral, the generator's neutral is its own and the generator is a separately derived system. When the transfer switch leaves the neutral solidly connected, the generator shares the service neutral and is not one.

What it needs

• A system bonding jumper. It joins the new neutral to the equipment grounding conductors and the enclosure at one point only: at the source or at the first disconnect, never both. It does the job the main bonding jumper does at the service.
• A grounding electrode conductor. It runs from the new system to a grounding electrode, so the derived system is tied to earth the way a service is. See EGC vs GEC for how that conductor differs from the equipment ground.
• Its own protection. Overcurrent protection for the transformer itself has its own rules in Article 450, and the conductors from the secondary to the panel are feeder conductors. See feeder vs branch circuit.

Why the bond matters here

When a hot conductor on the secondary side touches a metal box, the fault current has to get back to the transformer's secondary winding, not to the utility. The system bonding jumper is the only low-impedance way across from the equipment grounding path to that winding. Leave it out and the fault current has no good path home, the breaker may not trip and the metal stays energized. The equipment grounding conductor on the primary side bonds the transformer case, but it cannot do this job for the secondary.

Common mix-ups

• Skipping the bond because the primary has an EGC. The primary EGC bonds the case. Only the system bonding jumper gives secondary faults a way back to the winding.
• Bonding at the transformer and again in the panel. Two bonds put normal neutral current on the grounding path, the same problem as bonding the neutral in a subpanel.
• Assuming every generator is one. It depends on whether the transfer switch switches the neutral.
• Running the service neutral to the transformer. A delta primary has nowhere to land it, and the secondary needs its own neutral anyway.

On the Texas master exam

The PSI bulletin names separately derived systems in a subject area title: services, service equipment and separately derived systems. It carries 19 scored items on the master exam, 11 on NEC Knowledge and 8 on Calculations, and lists transformers and grounding and bonding inside it. The same area carries 10 scored items on the journeyman exam. Study to recognize a derived system, name its bond and its GEC, and turn to Article 250 for the grounding rules and Article 450 for the transformer.

Sitting a master exam on the 2023 NEC outside Texas? Start with the master electrician exam outline or take the free NEC master check.

Practice it

Every term in this set is in the electrical terms glossary.

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Sources

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• Candidate Information Bulletin, Texas electricians (updated 9/15/2026), PSI for the Texas Department of Licensing and Regulation. Checked 14 Nov 2026.
• NFPA 70, National Electrical Code (2026 edition), NFPA. Free read-only access with an account. Checked 14 Nov 2026.