Separately Derived Systems and Transformer Secondaries
Worked Examples (250.30, Table 250.102(C)(1), Table 250.66, 250.104(D), 240.21(C), Table 450.5(B), 450.6)
NFPA 70 National Electrical Code, 2023 edition · Reviewed 2026-09-21
The Separately Derived Systems half of this blueprint subject is where the Unlimited exam spends several calculation items: size the system bonding jumper, the grounding electrode conductor, the transformer secondary conductors and the primary/secondary overcurrent devices for a dry-type transformer, and bond the water pipe and steel to the new system. Lesson 02-03 covered the service electrode system and 07-02 covered transformer installation; this lesson runs the SDS chain end to end. Written from the 2023 text.
1. What is (and is not) an SDS — 250.30
- A transformer, a generator with neutral switching or a UPS output creates a separately derived system — its grounded conductor has no direct connection to the service grounded conductor. Paralleled sources of the same type are one SDS.
- An on-site generator is NOT an SDS where its grounded conductor is solidly interconnected to the service-supplied grounded conductor (transfer equipment that does not switch the neutral) — then no system bonding jumper is installed at the generator.
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