3.4 Separately Derived Systems & Transformers

3.4 Separately Derived Systems & Transformers

A Separately Derived System (SDS) is defined in NEC Article 100 as:

"An electrical source, other than a service, having no direct electrical connection(s) to circuit conductors of another system other than those established by grounding and bonding connections."

The most prevalent separately derived systems encountered on commercial and industrial electrical plan sets are isolation and step-down transformers (e.g., 480V Delta primary to 208Y/120V Wye secondary) and standby/emergency generators. Mastering the precise rules of NEC 250.30 (transformers) and NEC 250.35 (generators) is essential for any qualified electrical plans examiner.

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1. Transformer Grounding & Bonding Architecture: NEC 250.30(A)

In a standard dry-type 480V to 208Y/120V step-down transformer, power transfers magnetically across the iron core; there is zero direct physical conductor connection between the 480V primary conductors and the 208Y/120V secondary conductors. Therefore, the secondary constitutes a brand new, separately derived system that requires its own grounding and bonding network.

+-----------------------------------------------------------------------------------------+
|                        TRANSFORMER SDS FOUR-PART BONDING SYSTEM                         |
+----------------------------+------------------------------------------------------------+
| Component & Code Ref       | Location, Function & Sizing Rule                           |
+----------------------------+------------------------------------------------------------+
| 1. System Bonding Jumper   | • Connects X0 neutral terminal to transformer enclosure /  |
| (SBJ) (NEC 250.30(A)(1))   |   equipment grounding terminal.                            |
|                            | • Sized per Table 250.102(C)(1) based on secondary conds.  |
|                            | • LOCATION: At transformer OR first disconnect, NOT BOTH!  |
+----------------------------+------------------------------------------------------------+
| 2. Supply-Side Bonding     | • Required when SBJ is located in the first disconnect     |
| Jumper (SSBJ) (250.30(A)(2)|   enclosure; runs between transformer and disconnect.      |
|                            | • Sized per Table 250.102(C)(1).                           |
+----------------------------+------------------------------------------------------------+
| 3. Grounding Electrode     | • Connects grounded X0 terminal/bus to Grounding Electrode.|
| Conductor (GEC) (250.30(A)4| • Sized per Table 250.66 based on secondary phase conds.   |
|                            | • Landed at same enclosure/point where SBJ is installed.   |
+----------------------------+------------------------------------------------------------+
| 4. Grounding Electrode     | • Must connect to nearest structural metal or metal water  |
| (NEC 250.30(A)(4))         |   pipe (250.52(A)), or complete GES.                       |
+----------------------------+------------------------------------------------------------+

The Single-Location Rule for the SBJ (NEC 250.30(A)(1))

The System Bonding Jumper (SBJ) creates the essential fault-return path for the secondary. However, the NEC mandates that the SBJ shall be installed at a single point on the system:

1. Option A (At the Source): Inside the transformer enclosure, connecting X0 to the transformer metal case. (Most common in commercial design).
2. Option B (At the First Disconnect): Inside the secondary panelboard or disconnect switch enclosure, connecting the neutral bus to the enclosure ground bus.

[!CAUTION] Plan Review Rejection — Dual SBJ Installation: If a plan shows an SBJ installed inside the transformer AND a bonding screw installed in the first secondary panelboard, reject the plan! Dual bonding creates a parallel neutral return path through the metallic secondary conduit and EGC, causing continuous objectionable circulating currents, electronic interference, and raceway overheating under NEC 250.6.

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

In large multi-story commercial buildings containing dozens of dry-type transformers distributed across electrical rooms, running individual GECs from every transformer all the way down to the main service Grounding Electrode System is physically impractical. NEC 250.30(A)(6) provides the Common Grounding Electrode Conductor alternative:

+-----------------------------------------------------------------------------------------+
|                 COMMON GEC BUSBAR / RISER TOPOLOGY (NEC 250.30(A)(6))                   |
|                                                                                         |
|   [GROUNDING ELECTRODE SYSTEM (GES) AT SERVICE]                                         |
|                         |                                                               |
|                         v                                                               |
|   [COMMON GEC RISER CONDUCTOR / COPPER BUSBAR]                                          |
|   • Minimum Size: 3/0 AWG Copper or 250 kcmil Aluminum                                  |
|   • Extends vertically through electrical closets                                       |
|                         |                                                               |
|         +---------------+---------------+                                               |
|         |                               |                                               |
|         v (GEC Tap 1)                   v (GEC Tap 2)                                   |
|   [Transformer 1 (75 kVA)]        [Transformer 2 (150 kVA)]                             |
|   • Sec: 3/0 AWG Cu               • Sec: 500 kcmil Cu                                   |
|   • Tap GEC: #4 AWG Cu            • Tap GEC: 1/0 AWG Cu                                 |
|     (Table 250.66)                  (Table 250.66)                                      |
+-----------------------------------------------------------------------------------------+

Code Sizing Mandates for Common GEC:

1. Common GEC Conductor: Sized per Table 250.66 based on the sum of the secondary conductors, but not smaller than 3/0 AWG copper (or $250\text{ kcmil}$ aluminum).
2. Individual GEC Taps: Sized per Table 250.66 based on the largest ungrounded secondary conductor of that specific transformer.
3. Tap Connections: Must be made by listed irreversible compression connectors, exothermic welding, or copper busbar connections (minimum $1/4\text{ in.} \times 2\text{ in.}$). The common GEC must remain continuous without splice.

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3. Worked Transformer Sizing Calculations

Step-by-Step Engineering Sizing: 150 kVA Commercial Transformer

• Transformer Rating: 150 kVA, 480V 3-Phase Delta Primary to 208Y/120V 3-Phase Wye Secondary.
• Secondary Full Load Amperes (FLA): $I_{\text{sec}} = \frac{150,000\text{ VA}}{\sqrt{3} \times 208\text{ V}} = \frac{150,000}{360.26} = 416.35\text{ Amperes}$
• Secondary Conductors: Two parallel sets of $250\text{ kcmil}$ THHN Copper per phase ($2 \times 250 = 500\text{ kcmil Cu}$ per phase).

+-----------------------------------------------------------------------------------------+
|                  150 kVA TRANSFORMER SIZING SPECIFICATION SHEET                         |
+----------------------------+-----------------+------------------------------------------+
| Component                  | Required Size   | NEC Code Reference                       |
+----------------------------+-----------------+------------------------------------------+
| Secondary Phase Conductors | 2 sets 250 kcmil| 500 kcmil Cu total area per phase        |
| System Bonding Jumper (SBJ)| 1/0 AWG Copper  | Table 250.102(C)(1) (for 500 kcmil Cu)   |
| Grounding Electrode (GEC)  | 1/0 AWG Copper  | Table 250.66 (for 500 kcmil Cu)          |
| Supply-Side Jumper (SSBJ)  | 1/0 AWG Copper  | Table 250.102(C)(1) (if SBJ in panel)    |
+----------------------------+-----------------+------------------------------------------+

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4. Generator Grounding: 3-Pole vs. 4-Pole Automatic Transfer Switches

When reviewing standby and emergency generator single-line diagrams, the plans examiner must determine whether the generator is configured as a Separately Derived System or a Non-Separately Derived System. This is dictated entirely by whether the neutral conductor is switched in the Automatic Transfer Switch (ATS):

+-----------------------------------------------------------------------------------------+
|                    GENERATOR ATS GROUNDING CONFIGURATION COMPARISON                     |
+----------------------------+----------------------------+-------------------------------+
| Feature / Code Element     | 3-Pole ATS (Solid Neutral) | 4-Pole ATS (Switched Neutral) |
+----------------------------+----------------------------+-------------------------------+
| Neutral Conductor in ATS   | Solid (Unswitched)         | Switched (Switches with phase)|
+----------------------------+----------------------------+-------------------------------+
| System Classification      | NON-Separately Derived     | SEPARATELY DERIVED SYSTEM     |
+----------------------------+----------------------------+-------------------------------+
| Neutral-to-Ground Bond     | PROHIBITED at Generator!   | MANDATORY at Generator!       |
| at Generator Enclosure     | (Causes parallel neutral)  | (Sized per Table 250.102(C)(1)|
+----------------------------+----------------------------+-------------------------------+
| Grounding Electrode        | Not required as SDS;       | MANDATORY local GEC & GES     |
| Conductor (GEC)            | frame grounded via EGC     | per NEC 250.30 & 250.35.      |
+----------------------------+----------------------------+-------------------------------+
| Ground-Fault Protection    | Fully compatible with main | Eliminates false tripping;    |
| (GFPE / GFP) Impact        | service GFP sensor         | required for dual-GFP systems |
+----------------------------+----------------------------+-------------------------------+

  3-POLE ATS (Solid Neutral)                   4-POLE ATS (Switched Neutral)
  ==========================                   =============================
  Utility Service                              Utility Service
   [N] ----+-----------------+                  [N] -----+--------\ ---------+
           |                 |                           |        Switched   |
          === (MBJ)          |                          === (MBJ) Neutral    |
          GND                |                          GND                  |
                             v                                               v
                        [3-Pole ATS]                                    [4-Pole ATS]
                             ^                                               ^
                             |                                               |
   Generator                 |                  Generator                    |
   [N] ----------------------+                  [N] -----+--------\ ---------+
    | (NO BOND!)                                         |        Neutral
   GND via EGC only                                     === (SBJ)
                                                        GND to GES (SDS)

[!WARNING] Plan Review Trap — 3-Pole ATS with Generator Bond: When an engineer specifies a 3-pole ATS (solid neutral) but leaves the factory-installed neutral-to-ground bond inside the generator, normal neutral current returning from the load splits across the neutral conductor AND the equipment grounding conductor. This uncontrolled stray current will cause the main service Ground-Fault Protection of Equipment (GFPE) sensor to misread normal neutral return current as a ground fault, tripping the entire main service during emergency transfer tests!