5.3 Separately Derived Systems Grounding

5.3 Separately Derived Systems Grounding

Quick Answer: A separately derived system (SDS) is a premises wiring system with no direct electrical connection to conductors of another supply system except through grounding and bonding connections. For a typical dry-type transformer SDS, you bond XO (neutral) to the metal enclosure / EGC with a system bonding jumper, run a grounding electrode conductor sized from 250.66, and keep only one neutral-to-ground bond at the derived system—do not re-bond neutrals downstream.

Separately derived systems are a high-yield slice of Prov’s Grounding and Bonding six-pack because they combine electrode sizing (250.66), bonding jumper logic (250.30 / 250.102), and equipment grounding continuity. If Chapter 4 taught you service electrode rules, this section is the transformer/generator cousin.

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What counts as separately derived?

Article 100 / 250.30 treat a separately derived system as one 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, other than through grounding, bonding, or grounding electrode connections.

Usually SDS:

• Delta–wye or wye–wye transformers creating a new derived voltage system with a local grounded conductor
• Generators that are not solidly tied to the utility neutral in a non-separately-derived arrangement
• Certain UPS / converter outputs configured as SDS

Often NOT SDS (read the stem):

• Autotransformers that maintain a direct conductive connection to the supply
• Generators that remain solidly interconnected to the service neutral in a non-SDS configuration

On the exam, if the stem says “separately derived 480–208Y/120 V transformer,” apply 250.30.

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Transformer SDS — the required pieces (250.30(A))

For grounded systems, 250.30(A) is the checklist. Mentally install these parts on every transformer SDS question:

1. System bonding jumper (SBJ)

The system bonding jumper connects the grounded conductor of the derived system (transformer XO) to the metal enclosure and to the equipment grounding conductor(s) of the derived system. It establishes the bond that makes a phase-to-case fault return to XO and clear the secondary overcurrent device.

• Location: at the source (transformer) or at the first disconnecting means / first system overcurrent device, as permitted—not both in a way that creates parallel neutral-ground paths.
• Sizing: generally per 250.28 / 250.102(C) supply-side bonding jumper concepts for the derived ungrounded conductors (open the exact references in your NEC for nonflexible vs flexible SBJ).

Critical rule: After the SBJ location, treat the grounded conductor as a neutral only. Do not bond neutral bars to the can again in downstream panelboards on that derived system.

2. Grounding electrode conductor (GEC)

The SDS must connect to a grounding electrode (or electrodes) as required by 250.30(A). Size the GEC from Table 250.66 based on the derived ungrounded phase conductors—just as you size a service GEC from service ungrounded conductors.

Example: Derived secondary conductors are 3/0 copper. Table 250.66 copper GEC for that size is commonly 4 AWG (verify in your table). Do not size this GEC from Table 250.122.

Where a common electrode with the primary system is available, 250.30 prefers using the same grounding electrode (building steel, metal water pipe, concrete-encased electrode, etc.) rather than inventing an unrelated remote rod-only scheme when a better common electrode exists.

3. Supply-side bonding jumper (where used)

If metal raceways or cable assemblies enclose the derived supply conductors between the transformer and the first disconnect, a supply-side bonding jumper may be required to bond those metal parts into the fault path. Size it under 250.102(C) from the derived ungrounded conductors—not from a random EGC guess.

4. Equipment grounding conductors on the load side

Secondary feeders and branch circuits on the derived system still need EGCs (wire-type or qualifying raceway) sized from Table 250.122 based on the secondary overcurrent devices. The SBJ and GEC do not replace load-side EGCs.

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Worked transformer example (exam pattern)

Given: 75 kVA dry-type transformer, 480 V delta primary, 208Y/120 V secondary, separately derived. Secondary conductors to first disconnect are 250 kcmil copper per phase with a 4/0 neutral. First secondary breaker is 225 A. Metal nipple between transformer and secondary disconnect.

Apply 250.30 thinking:

1. SBJ: Bond XO to the transformer enclosure / EGC terminal at the permitted SBJ location. Size from the derived ungrounded conductors under the bonding-jumper rules referenced by 250.30 (250.102(C) family)—not from the 225 A breaker’s Table 250.122 row alone if the Code points you to supply-side sizing.
2. GEC: From Table 250.66 based on 250 kcmil copper derived phases (look up; often 2 AWG copper for that band—confirm in table). Connect to the building grounding electrode system, preferably common with primary.
3. Supply-side bonding jumper: Bond the metal nipple/enclosure path between transformer and first disconnect if required for continuity; size under 250.102(C).
4. EGC beyond the secondary OCPD: Table 250.122 from the 225 A device (copper commonly 4 AWG—confirm).

If a distractor option bonds the neutral again in a downstream 208Y/120 V panel, reject it—that creates objectionable current on grounding conductors.

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Outdoor SDS and electrode notes

250.30 includes outdoor SDS provisions (for example, when electrodes must be installed near outdoor sources). The Journeyman takeaway: outdoor transformers/generators still need the SDS bonding and electrode connections the section specifies; do not assume a building GEC “somewhere inside” automatically satisfies an outdoor SDS without reading the outdoor paragraphs.

Also watch for ungrounded separately derived systems—different bonding of metal parts still applies even when there is no system grounded conductor. Most Prov stems for JW focus on grounded wye secondaries, but the definition still matters.

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Generators as SDS (quick contrast)

A generator is separately derived when its neutral is not solidly interconnected to the service neutral (transfer equipment switches the grounded conductor). Then it needs SDS-style system bonding and electrode connections. If the transfer switch is a three-pole switch on a 120/240 V system that leaves the neutral solidly common, the generator may be non-separately derived, and the bonding pattern changes (often no second neutral-ground bond at the generator). Always let the transfer-switch pole count / neutral switching tell you which ruleset applies.

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Failure modes Prov can test

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Timed open-book tips

Tab 250.30, Table 250.66, 250.102(C), and Table 250.122. On any transformer stem, write four labels on scratch paper—SBJ, GEC, SSBJ, EGC—and fill each from the correct table/section. That four-label habit is the fastest way to finish SDS items inside Prov’s roughly three-minute average pace.