5.1 Separately Derived Systems
Key Takeaways
- A separately derived system (SDS) has no direct electrical connection to another source other than the grounding and bonding connection — defined in NEC Article 100.
- Common SDS examples include isolation transformers, generators with a transfer switch that breaks the neutral, and PV inverter outputs; a generator whose neutral is not switched in a 3-pole transfer scheme is NOT separately derived.
- NEC 250.30 governs grounding and bonding of SDS: a grounding electrode conductor sized from Table 250.66 based on the largest ungrounded derived conductor (not Table 250.122), a bonding jumper to the source enclosure, and a system bonding jumper connecting the derived neutral to the equipment grounding conductor.
- SDS grounding under 250.30(A) differs from service grounding under 250.24 — a service has only one neutral-to-ground bond at the service disconnect, while an SDS forms its own neutral-to-ground bond at its source or first disconnect.
- The system bonding jumper may be installed at the source or at the first disconnect, but never in both places — installing it in both locations creates a parallel neutral current path.
Separately Derived Systems
Quick Answer: A separately derived system (SDS) is a premises wiring system with no direct electrical connection to another premises wiring source other than the connection to the grounding and bonding means. The definition lives in NEC Article 100, and the grounding and bonding rules are in NEC 250.30. On the Texas Journeyman exam this falls under Area 2 (Services, Service Equipment, and Separately Derived Systems) on the NEC Knowledge part, with related neutral and conductor sizing on the Calculations part.
The Article 100 Definition
NEC Article 100 defines a separately derived system as:
A premises wiring system whose power is derived from a battery, a solar photovoltaic system, or from a generator, transformer, or converter windings, and that has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.
The phrase to underline is "no direct electrical connection". The only permitted connection between an SDS and another source is the grounding and bonding connection — the equipment grounding conductor and the grounding electrode system. The neutral (grounded circuit conductor) of an SDS must NOT be solidly tied to the supply neutral; if it is, the system is not separately derived and 250.30 does not apply.
Examples and Non-Examples
| Source | Separately Derived? | Why |
|---|---|---|
| Isolation transformer (separate primary and secondary windings, no common neutral) | Yes | No metallic connection between primary and secondary neutral |
| Generator with 4-pole transfer switch (switches neutral) | Yes | Transfer switch breaks the neutral, so the generator neutral is independent of the utility neutral |
| Generator with 3-pole transfer switch (does not switch neutral) | No | Generator neutral is solidly connected to the utility neutral — not separately derived; grounding is at the service, not at the generator |
| PV inverter output (utility-interactive inverter isolates from DC) | Yes | Inverter output has no direct connection to the DC source or to another AC source |
| 2-wire transformer secondary feeding a 3-wire load | Yes | Secondary is a new derived voltage with no neutral connection to the primary |
| Auto-transformer (single winding, common neutral) | No | Primary and secondary share a winding — there IS a direct electrical connection |
The exam will test the generator + transfer switch case repeatedly. Memorize the rule: 4-pole (switched neutral) = SDS = 250.30 applies; 3-pole (unswitched neutral) = not SDS = the generator relies on the service grounding.
NEC 250.30 Grounding and Bonding of an SDS
NEC 250.30(A) requires three components to ground and bond a separately derived system:
- Grounding electrode conductor (GEC) — connects the grounded conductor (neutral) of the SDS to a grounding electrode. Sized per Table 250.66 based on the largest ungrounded derived conductor (NOT Table 250.122, which is for equipment grounding conductors).
- Bonding jumper — bonds the SDS enclosure to the grounded conductor and to the equipment grounding conductor. Sized per Table 250.102(C)(1) based on the largest ungrounded conductor of the SDS.
- System bonding jumper — the connection between the grounded conductor (neutral) and the equipment grounding conductor at the SDS source. This is the SDS equivalent of the main bonding jumper at a service.
The system bonding jumper may be located at the source (e.g., at the transformer secondary terminals) or at the first disconnect supplying the SDS — but never in both locations. Installing it in both places creates a parallel path for neutral current on the equipment grounding conductor, which is a Code violation and a shock hazard.
250.30(A) vs Service Grounding 250.24 — What's Different
A common exam trap is confusing SDS grounding with service grounding. Both create a single neutral-to-ground bond, but the location and the rules differ:
| Aspect | Service Grounding (250.24) | SDS Grounding (250.30(A)) |
|---|---|---|
| Where the neutral-to-ground bond is formed | At the service disconnect — main bonding jumper (250.24(B)) | At the SDS source OR the first disconnect overcurrent device — system bonding jumper (250.30(A)(1)) |
| Grounding electrode conductor connection | At the service neutral — sized from Table 250.66 (250.24(A)) | At the SDS neutral — sized from Table 250.66 (250.30(A)(4)) |
| Number of neutral-to-ground bonds downstream of the bond | One only — the service; no downstream neutral-to-ground bonds permitted | One only — the SDS bond; no neutral-to-ground bonds on the load side of the SDS disconnect |
| Equipment grounding conductor source | Derived from the service equipment enclosure and bond | Derived from the SDS enclosure and the system bonding jumper |
| Solidly connected grounded conductor to another system? | N/A — the service is the origin | Prohibited by definition; if present, the system is NOT an SDS |
Single-Point vs Multi-Point Bonding
For an SDS serving a separate building or structure, NEC 250.30(A)(1) Exception No. 2 permits the system bonding jumper to be installed at a single point — typically the SDS source — when the SDS supplies a single disconnecting means. If the SDS feeds multiple disconnecting means, the bond must be at the source and a grounding electrode is required at the separate structure per 250.30(A)(4).
When the SDS is outdoors and serves a separate structure, 250.30(A)(4) requires a grounding electrode at the SDS location and at the structure if the structure has its own SDS disconnect. This is a frequent exam scenario.
Quick Sizing Reference — Table 250.66 (GEC for SDS)
| Largest Ungrounded Derived Conductor (AWG/kcmil) | GEC Size (Copper, AWG/kcmil) |
|---|---|
| 2 or smaller | 8 |
| 1 or 1/0 | 6 |
| 2/0 or 3/0 | 4 |
| 4/0 or 250 kcmil | 2 |
| 251–350 kcmil | 1/0 |
| 351–600 kcmil | 2/0 |
| 601–1100 kcmil | 4/0 |
Note that Table 250.66 values are not the same as Table 250.122 (equipment grounding conductor) values — the GEC is for the electrode connection, the EGC is for fault clearing. The exam will offer both tables as distractors.
Exam Scenarios
- "A 480V to 208Y/120V transformer feeds a 225A panelboard. Where is the system bonding jumper installed?" — Either at the transformer secondary or at the 225A panelboard disconnect, but not both. If the transformer is the source and the panelboard is the first disconnect, either location is permitted; choose based on whether the neutral is landed at the transformer or at the panel.
- "A 75 kVA transformer with 2/0 AWG secondary conductors — what size GEC?" — Largest ungrounded derived conductor is 2/0 AWG → Table 250.66 says 4 AWG copper.
- "A standby generator with a 3-pole ATS — is the generator an SDS?" — No. The neutral is not switched, so the generator neutral is solidly connected to the service neutral; the generator is grounded through the service, not per 250.30.
Takeaways for the Exam
- Know the Article 100 definition cold: no direct electrical connection except grounding/bonding.
- The 4-pole vs 3-pole ATS distinction is the single highest-yield SDS question pattern.
- 250.30(A) requires a GEC (Table 250.66), a bonding jumper (Table 250.102(C)(1)), and a system bonding jumper (one location only).
- A system bonding jumper installed at both the source AND the first disconnect is a violation — pick only one.
- Do not confuse Table 250.66 (GEC) with Table 250.122 (EGC) when sizing the SDS grounding electrode conductor.
Which transfer-switch configuration makes a standby generator a separately derived system under NEC Article 100?
A 75 kVA, 480V-to-208Y/120V isolation transformer has 2/0 AWG copper secondary conductors. What size copper grounding electrode conductor is required for the SDS by NEC 250.30(A)(4)?
Where may the system bonding jumper for a separately derived system be installed?