7.3 Separately Derived Systems

7.3 Separately Derived Systems

Exam Fast Fact: When taking the Colorado Journeyman exam, memorizing the difference between a separately derived system and a non-separately-derived system is essential. A dry-type step-down transformer is always a separately derived system because there is no direct metallic connection between primary and secondary windings. However, an on-site generator is classified as an SDS only if the transfer switch switches the neutral conductor (4-pole switch). If the transfer switch carries a solid, unswitched neutral (3-pole switch), the generator is not an SDS, and bonding neutral to ground at the generator frame violates NEC 250.30 and 250.6!

Separately Derived Systems (SDS) are the workhorses of commercial, institutional, and industrial power distribution. In modern facilities, step-down transformers convert 480Y/277V primary power into 208Y/120V secondary power for lighting, computers, and office appliances. Grounding and bonding these systems correctly under NEC 250.30 requires precision: an installer must correctly locate and size the System Bonding Jumper (SBJ), connect the Grounding Electrode Conductor (GEC) to qualified building electrodes, and strictly prevent circulating neutral currents.

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Definition and Recognition of Separately Derived Systems

Under NEC Article 100, a Separately Derived System is defined as:

"An electrical premises wiring system whose power is derived 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."

                    TRANSFORMER (ALWAYS AN SDS)
 480V Primary                                            208Y/120V Secondary
 ┌──────────────────┐                            ┌─────────────────────────┐
 │ Phase A ─────────┼──────┐              ┌──────┼─────────> Phase A       │
 │ Phase B ─────────┼──────┤  MAGNETIC    ├──────┼─────────> Phase B       │
 │ Phase C ─────────┼──────┤  INDUCTION   ├──────┼─────────> Phase C       │
 │ Primary Ground ──┼──┐   │  (No Direct  │   ┌──┼─────────> Neutral (X0)  │
 │ (EGC / Chassis)  │  │   │   Metallic   │   │  └─────────────────────────┘
 └──────────────────┘  │   │  Connection) │   │
                       │   └──────────────┘   └──[ SYSTEM BONDING JUMPER ]
                       ▼                                    │
              Transformer Enclosure                         ▼
              (Equipment Ground Bus) <──────────────────────┘

1. Step-Down Transformers

In a standard delta-wye dry-type transformer (e.g., 480V Delta Primary to 208Y/120V Wye Secondary), electrical energy is transferred across an air gap purely through electromagnetic induction in the laminated steel core. There is no physical copper connection between the 480V primary conductors and the 208Y/120V secondary windings. The secondary neutral terminal (X0) is born inside the transformer. Therefore, premises step-down transformers are always separately derived systems.

2. Standby and Emergency Generators: 3-Pole vs. 4-Pole Transfer Switches

Whether a standby generator is a separately derived system depends entirely on how the neutral conductor is handled inside the Automatic Transfer Switch (ATS):

• 3-Pole Transfer Switch (Solid Neutral — NOT an SDS): In a 3-phase, 4-wire system, a 3-pole switch switches only the three ungrounded phase conductors (Phases A, B, and C). The neutral conductor connects to a solid, unswitched neutral busbar inside the ATS and runs continuously from the main utility service panel directly to the generator. Because the generator neutral has a direct metallic connection to the utility neutral conductor, the generator is NOT a separately derived system. The neutral must remain isolated from the generator frame.
• 4-Pole Transfer Switch (Switched Neutral — IS an SDS): A 4-pole switch switches the three phase conductors AND the neutral conductor. At any given moment, the load neutral is connected to either the utility neutral or the generator neutral, but never both simultaneously. Because the generator neutral is completely isolated from the utility system supply conductors, the generator IS a separately derived system. It requires an on-site System Bonding Jumper and Grounding Electrode Conductor connection.

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The System Bonding Jumper (SBJ) (NEC 250.30(A)(1))

The System Bonding Jumper (SBJ) is the heart of SDS safety. Under NEC 250.30(A)(1), the SBJ is the conductor, screw, or busbar that connects the derived system grounded conductor (the secondary neutral, X0) to the equipment grounding conductors and the metal enclosure.

Why the SBJ is Essential

Without a system bonding jumper, an SDS floats without a fault return path. If an energized 120V secondary conductor shorts to a metal panel or steel conduit:

• Current cannot return to the X0 terminal of the secondary winding.
• No short-circuit current flows; the secondary breaker never trips.
• The entire conduit system, panel enclosure, and connected appliances remain continuously energized at 120 volts, awaiting human contact.

Sizing the System Bonding Jumper

Under NEC 250.30(A)(1), the system bonding jumper is sized using NEC Table 250.102(C)(1) based on the circular mil area of the largest derived ungrounded conductor (or equivalent area of parallel conductors).

Worked Sizing Example: A 45 kVA 480V-to-208Y/120V dry-type transformer has secondary phase conductors consisting of 3/0 AWG copper THHN. What is the minimum size copper SBJ?

• Look up 3/0 AWG Copper in Table 250.102(C)(1).
• Row '2/0 AWG or 3/0 AWG Copper' dictates a 4 AWG copper system bonding jumper.

Location of the SBJ: The Single-Point Mandate

Under NEC 250.30(A)(1), the system bonding jumper must be installed at a single point on the separately derived system. The installer is granted a choice of two locations:

1. At the Source: Inside the transformer enclosure, connecting the X0 neutral terminal directly to the transformer chassis / ground bar; OR
2. At the First System Disconnecting Means: Inside the secondary panelboard enclosure, connecting the neutral busbar to the panel ground bus via a bonding strap, conductor, or green screw.

The Deadly Double-Bonding Trap: An installer must NEVER install an SBJ at both the transformer and the secondary panelboard! Installing bonding jumpers at both locations creates a parallel circuit between the neutral conductor and the metallic conduit/EGC connecting the two enclosures. Normal 120V load return current divides between the neutral wire and the metal conduit, causing continuous arcing, electrical noise, and hazardous touch potentials that violate NEC 250.6 (Objectionable Current).

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Grounding Electrode Conductor (GEC) for SDS (NEC 250.30(A)(4) & (A)(5))

Every separately derived system must be connected to a grounding electrode to dissipate external surges and stabilize secondary voltage to earth.

            TRANSFORMER (SOURCE)                  SECONDARY PANELBOARD (FIRST DISCONNECT)
 ┌────────────────────────────────────────┐       ┌───────────────────────────────────────┐
 │ [X0 Neutral]                           │       │ [Neutral Bar]                         │
 │      │                                 │       │      │                                │
 │      ├──[ SYSTEM BONDING JUMPER (SBJ) ]│       │      │ (FLOATING - NO BOND SCREW!)    │
 │      │   (Table 250.102(C)(1))         │       │      │                                │
 │      ▼                                 │       │      ▼                                │
 │ [Enclosure Ground Bus]                 │  EMT  │ [Panel Enclosure Ground Bus]          │
 │      │                                 │──────>│      │                                │
 │      ├──[ GEC TO BUILDING STEEL ]      │       │      └── Branch Circuit EGCs          │
 │          (Table 250.66)                │       │                                       │
 └────────────────────────────────────────┘       └───────────────────────────────────────┘
    *SBJ & GEC AT SAME ENCLOSURE!*                   *NO SECOND BOND AT PANELBOARD!* 

Connection Point Rules

Under NEC 250.30(A)(5), the grounding electrode conductor must connect to the derived grounded conductor at the exact same enclosure where the system bonding jumper is installed:

• If the SBJ is installed inside the transformer, the GEC must terminate inside the transformer.
• If the SBJ is installed inside the secondary panelboard, the GEC must terminate inside the secondary panelboard.

Grounding Electrode Selection (NEC 250.30(A)(4))

The GEC must connect to the nearest available grounding electrode from the following hierarchy:

1. Structural Building Steel: The metal building frame, effectively grounded per 250.52(A)(2).
2. Metal Underground Water Pipe: Qualified water piping per 250.52(A)(1), within 5 feet of entrance.
3. Other Existing Electrodes: Concrete-encased Ufer electrode or grounding electrode system.

Sizing the SDS Grounding Electrode Conductor (NEC Table 250.66)

The GEC for an SDS is sized strictly from NEC Table 250.66 based on the size of the largest derived ungrounded secondary conductor.

Common Grounding Electrode Conductor (NEC 250.30(A)(6))

In large multi-story commercial buildings containing dozens of step-down transformers, running individual copper GEC home runs back to the service grounding electrode is cost-prohibitive. NEC 250.30(A)(6) permits installation of a Common Grounding Electrode Conductor:

• The common GEC must be sized at a minimum of 3/0 AWG copper or 250 kcmil aluminum (or structural steel / 2" water pipe).
• Individual grounding electrode conductor taps run from each transformer to the common GEC, sized from Table 250.66 based on that specific transformer's secondary conductors.

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Non-Separately-Derived Generators: The Fatal Bonding Mistake

When a standby generator is installed with a standard 3-pole transfer switch (solid neutral), the generator is a non-separately derived system. Electricians must avoid a widespread jobsite error: leaving the manufacturer's factory-installed bonding jumper intact inside the generator alternator junction box.

                 3-POLE TRANSFER SWITCH (SOLID NEUTRAL - NON-SDS)
                                                                 
  SERVICE DISCONNECT                  TRANSFER SWITCH                   GENERATOR
┌────────────────────┐             ┌────────────────────┐         ┌────────────────────┐
│ Neutral Bus (MBJ)  │             │ Solid Neutral Bar  │         │ Generator Neutral  │
│    │               │             │    │               │         │    │ (FLOATING!)   │
│    └───────────────┼─────────────┴────┴───────────────┼─────────┴────┘    NO BOND!   │
│                    │   Feeder Neutral                 │                              │
│                    │                                  │                              │
│ Enclosure Chassis  │                                  │         Generator Frame      │
│    │               │                                  │              │               │
│    └───────────────┼──────────────────────────────────┼──────────────┘               │
│                    │   Equipment Grounding Conductor  │                              │
└────────────────────┘   (EMT / Copper Wire)            └──────────────────────────────┘

What Happens if the Generator Frame Bond is Not Removed?

If the factory jumper is left connecting the generator neutral to the generator frame:

1. A continuous metallic loop is established between the utility service panel and the generator.
2. Normal 120V unbalanced neutral current from building branch circuits travels to the ATS neutral bar, where it splits: half returns along the insulated neutral wire, while the other half travels over the equipment grounding conductor, conduit fittings, and generator steel frame.
3. Continuous Current on Enclosures: Touching the generator frame, ATS enclosure, or connecting conduit while loads are operating exposes workers to shock.
4. Nuisance Ground-Fault Tripping: If the main service switchboard is equipped with Ground-Fault Protection of Equipment (GFPE per 230.95), the sensor will detect current returning on ground and trip the main service breaker during normal operation.
5. Rule: For non-separately-derived generators, remove the neutral-to-ground jumper at the generator. The generator frame must be grounded solely via an equipment grounding conductor run with the supply conductors from the service.

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Jobsite Scenarios & Common Exam Traps