8.2 Separately Derived Systems (SDS) Transformers & Generator Grounding
Key Takeaways
- A Separately Derived System (SDS) is an electrical premises wiring system whose power is derived from generator, transformer, or converter windings and has no direct electrical connection (including a solid neutral) to supply conductors originating in another system (NEC Article 100).
- Standby generators utilizing a 3-pole transfer switch with a solid (unswitched) neutral are NOT separately derived systems; their neutral is grounded only at the service equipment. Generators with a 4-pole switched neutral transfer switch ARE separately derived systems and require dedicated local bonding and grounding.
- The System Bonding Jumper (SBJ) connects the secondary grounded circuit conductor (neutral X0) to the equipment grounding conductor and metal transformer enclosure under NEC 250.30(A)(1), sized from Table 250.102(C)(1).
- The Single-Point Grounding Rule mandates that the SBJ be installed at EITHER the source (transformer enclosure) OR the first system disconnecting means enclosure, but NEVER at both, to prevent circulating neutral ground loops.
- The Grounding Electrode Conductor (GEC) for an SDS is sized under NEC 250.30(A)(5) from Table 250.66 based on the secondary derived phase conductors and must connect to building structural steel or a metal water pipe within 5 feet of entrance.
Last updated: August 2026
8.2 Separately Derived Systems (SDS) Transformers & Generator Grounding
A Separately Derived System (SDS) is one of the most critical topics in commercial and industrial electrical exam preparation. Governed by NEC 250.30, proper grounding and bonding of separately derived systems ensures that secondary fault currents return reliably to their magnetic source (the transformer or generator windings) without creating hazardous stray currents on building metal or equipment enclosures.
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| SEPARATELY DERIVED SYSTEM (SDS) DEFINITION |
| |
| NEC ARTICLE 100 DEFINITION: |
| "An electrical premises wiring system whose power is derived from a |
| source of electric energy or equipment other than a service. Such systems |
| have no direct electrical connection, including a solidly connected |
| grounded circuit conductor (neutral), to supply conductors originating |
| in another system." |
| |
| [COMMON SDS SOURCES] |
| * Dry-type distribution transformers (e.g., 480V delta to 208Y/120V wye) |
| * Standby generators with 4-pole transfer switches (switched neutral) |
| * Solar photovoltaic inverters / battery energy storage systems (BESS) |
| * Uninterruptible power supplies (UPS) with isolation transformers |
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1. Generator Grounding: Switched Neutral vs. Solid Neutral
Whether a standby generator is classified as a separately derived system depends entirely on how the grounded neutral conductor is handled inside the transfer switch.
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| GENERATOR TRANSFER SWITCH CLASSIFICATION MATRIX |
| |
| [3-POLE TRANSFER SWITCH: SOLID NEUTRAL (NON-SEPARATELY DERIVED)] |
| * Utility neutral and generator neutral are SOLIDLY CONNECTED together. |
| * Neutral is bonded to ground at the MAIN SERVICE EQUIPMENT ONLY. |
| * System Bonding Jumper (SBJ) inside generator MUST BE REMOVED. |
| * Generator frame is grounded via an Equipment Grounding Conductor (EGC) |
| routed with the feeder conductors from the service panel. |
| |
| [4-POLE TRANSFER SWITCH: SWITCHED NEUTRAL (SEPARATELY DERIVED SYSTEM)] |
| * Neutral conductor is SWITCHED simultaneously with phase conductors. |
| * No direct electrical connection exists between utility and generator. |
| * Generator IS a Separately Derived System (SDS). |
| * System Bonding Jumper (SBJ) MUST BE INSTALLED at generator (or switch). |
| * Grounding Electrode Conductor (GEC) connects generator to ground rod. |
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Diagram: Switched Neutral vs. Solid Neutral Wiring
NON-SEPARATELY DERIVED (3-Pole ATS) SEPARATELY DERIVED (4-Pole ATS)
=================================== ================================
Utility Service Generator Utility Service Generator
[Main Bonded] [NO Bond] [Main Bonded] [LOCAL BOND]
| | | |
Phase| Neut Phase| Neut Phase| Neut Phase| Neut
| | | | | | | |
v v v v v v v v
+-------------------------+ +--------------------------+
| 3-Pole Transfer Switch | | 4-Pole Transfer Switch |
| [Switches Phases Only] | | [Switches Phase & Neut] |
| Solid Neutral Bar ---- | | Neut Switched Separately|
+-------------------------+ +--------------------------+
[!CRITICAL] The Objectionable Current Violation (NEC 250.6): If an electrician fails to remove the factory bonding jumper in a portable or standby generator connected through a 3-pole solid-neutral transfer switch, neutral current will split between the neutral wire and the equipment grounding conductor. This violates NEC 250.6, energizes the generator frame during normal operation, and trips upstream ground-fault protection (GFPE/GFCI) devices.
2. The Four (4) Core Components of SDS Grounding (NEC 250.30(A))
Every grounded AC separately derived system requires four foundational grounding and bonding components:
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| THE 4 FOUNDATIONAL SDS GROUNDING COMPONENTS |
| |
| 1. SYSTEM BONDING JUMPER (SBJ - 250.30(A)(1)) |
| * Connects secondary neutral (X0) to transformer enclosure & EGC. |
| * Sized from Table 250.102(C)(1) based on secondary phase conductors. |
| |
| 2. SUPPLY-SIDE BONDING JUMPER (SSBJ - 250.30(A)(2)) |
| * Connects transformer enclosure to secondary disconnect enclosure. |
| * Sized from Table 250.102(C)(1). Required when SBJ is at disconnect. |
| |
| 3. GROUNDING ELECTRODE CONDUCTOR (GEC - 250.30(A)(5)) |
| * Connects derived neutral (X0) to physical grounding electrode. |
| * Sized from Table 250.66 based on secondary phase conductors. |
| |
| 4. GROUNDING ELECTRODE (250.30(A)(4)) |
| * Building steel or metal water pipe within 5 ft of entry point. |
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3. The Single-Point Grounding Mandate (NEC 250.30(A)(1))
The System Bonding Jumper (SBJ) connects the grounded circuit conductor (the X0 neutral terminal on a wye secondary) to the equipment grounding conductor and metal enclosure.
Under NEC 250.30(A)(1), the SBJ is permitted to be installed at only one location:
- Option A (At the Source): Inside the transformer enclosure or generator enclosure, OR
- Option B (At the First Disconnect): Inside the first system disconnecting means / panelboard enclosure.
- PROHIBITION: Installing the SBJ at both the transformer and the secondary panelboard creates parallel neutral paths and is a major code violation.
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| SINGLE-POINT BONDING LOCATION COMPARISON |
| |
| FEATURE | OPTION A: AT TRANSFORMER | OPTION B: AT PANEL |
| ----------------------+----------------------------+--------------------- |
| SBJ Location | Inside Transformer (X0-Gnd)| Inside Secondary Panel
| GEC Connection Point | Inside Transformer (X0) | Inside Secondary Panel
| Conductor to Panel | 4-Wire (3 Ph + N) + EGC | 4-Wire (3 Ph + N) + SSBJ
| Ground-Neutral Bond | Transformer ONLY | Panelboard ONLY |
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4. Master Sizing Tables: Table 250.102(C)(1) and Table 250.66
Table 250.102(C)(1): Sizing SBJ, SSBJ, and Main Bonding Jumpers
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| NEC TABLE 250.102(C)(1) GROUNDED CONDUCTOR & BONDING JUMPER |
| |
| SIZE OF LARGEST UNGROUNDED CONDUCTOR | MINIMUM SIZE BONDING JUMPER |
| OR EQUIVALENT AREA FOR PARALLEL SETS | COPPER WIRE | ALUMINUM WIRE |
| ======================================+=================+==================|
| 2 AWG or smaller Cu (1/0 Al) | 8 AWG | 6 AWG |
| 1 AWG or 1/0 AWG Cu (2/0 or 3/0 Al) | 6 AWG | 4 AWG |
| 2/0 AWG or 3/0 AWG Cu (4/0 or 250 Al) | 4 AWG | 2 AWG |
| Over 3/0 through 350 kcmil Cu | 2 AWG | 1/0 AWG |
| Over 350 through 600 kcmil Cu | 1/0 AWG | 3/0 AWG |
| Over 600 through 1100 kcmil Cu | 2/0 AWG | 4/0 AWG |
| Over 1100 kcmil Cu (Over 1750 kcmil Al)| 12.5% of largest phase area |
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Table 250.66: Sizing Grounding Electrode Conductors (GEC)
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| NEC TABLE 250.66 GROUNDING ELECTRODE CONDUCTOR (GEC) |
| |
| SIZE OF LARGEST UNGROUNDED SERVICE | MINIMUM SIZE GEC |
| CONDUCTOR OR SECONDARY PHASE WIRE | COPPER WIRE | ALUMINUM WIRE |
| ======================================+=================+==================|
| 2 AWG or smaller Cu (1/0 Al) | 8 AWG | 6 AWG |
| 1 AWG or 1/0 AWG Cu (2/0 or 3/0 Al) | 6 AWG | 4 AWG |
| 2/0 AWG or 3/0 AWG Cu (4/0 or 250 Al) | 4 AWG | 2 AWG |
| Over 3/0 through 350 kcmil Cu | 2 AWG | 1/0 AWG |
| Over 350 through 600 kcmil Cu | 1/0 AWG | 3/0 AWG |
| Over 600 through 1100 kcmil Cu | 2/0 AWG | 4/0 AWG |
| Over 1100 kcmil Cu (Over 1750 kcmil Al)| 3/0 AWG | 250 kcmil |
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5. Complete Step-by-Step Worked Transformer Example: 75 kVA 480V to 208Y/120V
Installation Specifications:
- Transformer Rating: 75 kVA, 3-Phase, 60 Hz dry-type transformer.
- Primary Voltage: 480 Volts, 3-Phase, 3-Wire Delta.
- Secondary Voltage: 208Y/120 Volts, 3-Phase, 4-Wire Wye.
- Conductor Insulation: Copper conductors with 75°C terminal ratings.
- The System Bonding Jumper (SBJ) is to be installed inside the transformer enclosure.
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75 kVA STEP-DOWN TRANSFORMER CALCULATION SHEET
===============================================================================
STEP 1: PRIMARY CURRENT & PRIMARY OCPD / CONDUCTOR SIZING
- Primary Full-Load Current (FLC):
I_primary = 75,000 VA / (480V x 1.73205) = 75,000 / 831.38 = 90.21 Amperes
- Primary Overcurrent Protection (NEC Table 450.3(B)):
Maximum primary breaker = 90.21A x 1.25 = 112.76 Amperes
Under NEC 450.3(B) Note 1 & 240.6(A), next standard size up = 125-Ampere OCPD.
- Primary Conductor Sizing:
Minimum 75°C copper ampacity = 125A -> #1 AWG THHN/THWN-2 Copper (Rated 130A).
- Primary Equipment Grounding Conductor (Table 250.122 for 125A OCPD):
Table 250.122 for 125A breaker -> #6 AWG Copper EGC.
STEP 2: SECONDARY FULL-LOAD CURRENT & CONDUCTOR SIZING
- Secondary Full-Load Current (FLC):
I_secondary = 75,000 VA / (208V x 1.73205) = 75,000 / 360.27 = 208.18 Amperes
- Secondary Conductor Sizing (125% continuous duty):
Minimum Ampacity = 208.18A x 1.25 = 260.23 Amperes
Conductor selection from Table 310.16 (75°C Copper):
* 300 kcmil Copper = 285 Amperes (Supplies a standard 225A or 250A panelboard).
STEP 3: SYSTEM BONDING JUMPER (SBJ) SIZING (NEC 250.30(A)(1) & Table 250.102(C)(1))
- Derived Secondary Phase Conductors = 300 kcmil Copper.
- Refer to Table 250.102(C)(1) under "Over 3/0 through 350 kcmil Cu":
>>> Required SBJ = #2 AWG Copper.
STEP 4: GROUNDING ELECTRODE CONDUCTOR (GEC) SIZING (NEC 250.30(A)(5) & Table 250.66)
- Derived Secondary Phase Conductors = 300 kcmil Copper.
- Refer to Table 250.66 under "Over 3/0 through 350 kcmil Cu":
>>> Required GEC to Building Steel / Water Pipe = #2 AWG Copper.
STEP 5: SUPPLY-SIDE BONDING JUMPER (SSBJ) SIZING (NEC 250.30(A)(2))
- If SBJ is installed at secondary panel, raceway bonding jumper = #2 AWG Copper.
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6. Grounding Electrode Selection for Separately Derived Systems (NEC 250.30(A)(4))
Under NEC 250.30(A)(4), the grounding electrode for an SDS must be as near as practicable to, and preferably in the same area as, the grounding electrode conductor connection to the system. The electrode must be one of the following:
- First Choice — Structural Metal: Metal building frame / structural metal that is effectively grounded (NEC 250.52(A)(2)).
- Second Choice — Water Pipe: Metal water pipe electrode complying with 250.52(A)(1) located within 1.5 m (5 ft) of the point of entrance to the building.
- Other Electrodes: Concrete-encased electrode (Ufer), ground ring, or rod/pipe electrodes complying with 250.52(A).
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| COMMON SDS GROUNDING AND BONDING EXAM TRAPS |
| |
| TRAP 1: Sizing SBJ from Table 250.122 instead of Table 250.102(C)(1). |
| TRAP 2: Installing the SBJ at both the transformer AND the secondary panel.|
| TRAP 3: Grounding a portable generator frame when feeding a building with |
| a 3-pole solid neutral transfer switch. |
| TRAP 4: Running a GEC from a transformer all the way outside to a ground |
| rod when structural building steel is available 10 feet away. |
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Test Your Knowledge
A standby generator is installed to provide backup power through an automatic transfer switch. The transfer switch features a solid, unswitched neutral conductor connecting the utility and generator neutrals. Under the NEC, how must this system be grounded?
A
B
C
D
Test Your Knowledge
What is the minimum size copper System Bonding Jumper (SBJ) required under NEC Table 250.102(C)(1) for a 480V to 208Y/120V 3-phase dry-type transformer whose secondary phase conductors are 350 kcmil copper?
A
B
C
D
Test Your Knowledge
Why does NEC 250.30(A)(1) strictly prohibit installing a System Bonding Jumper at both the transformer enclosure and the first secondary disconnect enclosure?
A
B
C
D