6.4 Separately Derived Systems & Special Grounding

Why This Matters

A separately derived system (SDS) — most commonly a step-down transformer or a generator with no solidly connected neutral — has its own grounding electrode system and its own system bonding jumper, independent of the service. The exam tests where to place the bond, how to size the GEC, and how second-building feeders differ from SDS grounding. Misplacing the SDS bond creates parallel neutral paths and objectionable current, the same hazard as bonding at a subpanel.

250.30 — Separately Derived AC Systems

An SDS (Article 100) is a premises wiring source with no direct connection to another source except through grounding and bonding. Common examples: a 480-to-208 V transformer, a 4-wire generator with no solidly connected neutral to the service, a PV/battery inverter output.

For a grounded SDS, 250.30(A) requires five elements:

1. System bonding jumper (250.30(A)(1)) — unspliced, installed at exactly one point: either at the SDS source or at the first system disconnecting means — never both. Sized per Table 250.102(C)(1) based on the secondary phase conductor area.
2. Supply-side bonding jumper (250.30(A)(2)) — from the SDS EGC terminal to the secondary disconnect enclosure; sized per Table 250.102(C)(1).
3. System neutral conductor (250.30(A)(3)) — if the bond is at the disconnect (not the source), the neutral is run to the disconnect and sized per Table 250.102(C)(1); not required to be larger than the phase conductors.
4. Grounding electrode connection (250.30(A)(4)) — a GEC from the same point as the system bonding jumper to one or more electrodes per 250.50. Acceptable electrodes include water pipe within 5 ft of entry, building steel, concrete-encased, ground ring, or rods.
5. GEC sizing (250.30(A)(5)) — per 250.66 based on the largest secondary phase conductor. Rod-only installations get the 6 AWG cap (250.66(A)); concrete-encased gets the 4 AWG cap (250.66(B)).

250.30(C) — location: if the SDS is outdoors, the grounding electrode connection must be made at the SDS location. If indoors, it can be at the source or at the first disconnect.

The 2020 NEC added that multiple power sources of the same type in parallel form a single SDS — important for parallel generators and transformer banks.

Diagram: SDS Bonding — One Point Only

graph TD
  T[Transformer Secondary 480/208V 3ph 4w] -->|Secondary phase conductors| D[First SDS Disconnect]
  T -->|System bonding jumper at source OR at disconnect| N[Neutral terminal]
  N -->|GEC| E[Grounding electrode per 250.50]
  D -->|EGC + neutral isolated from case| L[Load panel]
  style T fill:#cfc
  style D fill:#fcc

Worked Scenario — 75 kVA Transformer

A 75 kVA, 480 V primary to 208/120 V 3-phase 4-wire secondary transformer feeds a 200 A panelboard. The secondary phase conductors are 3/0 AWG copper. The system bonding jumper is installed at the transformer.

• System bonding jumper size: Table 250.102(C)(1), row "2/0 or 3/0" → 4 AWG copper.
• GEC to a building steel electrode: Table 250.66, row "2/0 or 3/0" → 4 AWG copper.
• GEC to a driven rod (supplement): 250.66(A) cap → 6 AWG copper.
• The neutral is bonded to the enclosure and EGC at the transformer only; the load-side panel has an isolated neutral bar and a bonded EGC bar.

If the system bonding jumper were placed at the first disconnect instead of the transformer, the neutral from transformer to disconnect would be sized per Table 250.102(C)(1) → 4 AWG, and the supply-side bonding jumper between transformer and disconnect enclosure would also be 4 AWG copper.

250.32 — Building or Structure Supplied by a Feeder

A second building fed by a feeder from the first requires:

• 250.32(A): A grounding electrode system at the second building (Exception: not required for a single branch circuit or multiwire branch circuit).
• 250.32(B)(1): An EGC must accompany the feeder and connect to the second-building disconnect enclosure and to the grounding electrode(s) there. The EGC is sized per 250.122 based on the feeder OCPD.
• Neutral isolation: The grounded (neutral) conductor must NOT be bonded to the disconnect enclosure or EGC at the second building (250.142(B)). This prevents objectionable neutral current on the metallic path between buildings.
• Exception 1 (existing only): A neutral may serve as the ground at the second building only if (a) no EGC is in the feeder, (b) no continuous metallic path exists between buildings, and (c) no ground-fault protection on the supply side. This exception is for existing installations — new work must run an EGC.

250.34 — Portable and Vehicle-Mounted Generators

• 250.34(A) Portable: No grounding electrode required if (1) the generator supplies only equipment mounted on or cord-connected to the generator and (2) the non-current-carrying metal parts and EGC terminals of receptacles are bonded to the generator frame.
• 250.34(B) Vehicle-mounted: Same conditions, with the generator frame bonded to the vehicle/trailer frame.
• A separately derived generator (4-wire, no solidly connected neutral) follows 250.30 — it needs a system bonding jumper and a grounding electrode.

250.36 — Impedance-Grounded Systems

High-impedance grounded neutral systems (typically 5 A ground-fault limiters in industrial 3-phase 480 V systems) require:

• A grounding impedance in the neutral-to-ground connection.
• The neutral not run with the phase conductors except per 250.36(C).
• Ground detectors per 250.36(A).
• GEC sized per 250.66 but not less than the neutral conductor to the impedance.

These are uncommon on the journeyman exam but can appear as a definition/recall question.

Clearing Ground Faults

The fault path is metallic and conductive, not through earth. 250.4(A)(5) requires the fault-current path to be permanent, electrically continuous, and capable of safely carrying the maximum fault current. The impedance must be low enough to trip the OCPD in the time allowed. Earth is NOT an acceptable fault path — a 25-ohm ground rod allows only about 5 A at 120 V to earth, nowhere near the 15+ A needed to trip a 15 A breaker.

Traps

• "Bond the SDS neutral at both the transformer and the disconnect for redundancy" — false; one point only (250.30(A)(1)).
• "The second building can use the feeder neutral as its EGC in new construction" — false; Exception 1 is existing only.
• "A portable generator always needs a ground rod" — false if the 250.34(A) conditions are met.
• "Earth clears the fault if the rod is 25 ohms or less" — false; the EGC and bonding path clear the fault, not the soil.