Separately Derived Systems

Fitzemeyer & Tocci
Jun 9
3 min read

When designing an electrical system, one of the most important — and often misunderstood — concepts is the separately derived system (SDS). Whether you're designing a new facility, upgrading existing infrastructure, or troubleshooting grounding issues, understanding what makes a system "separately derived" is essential to code compliance and electrical safety.

What Is a Separately Derived System?

The National Electrical Code (NEC) defines a separately derived system in Article 100 as a premises wiring system whose power is derived from a source other than a utility service, and which has no direct electrical connection — including a solidly connected grounded circuit conductor — to supply conductors originating in another system. In plain terms: a separately derived system is electrically isolated from the utility service. It has its own source and its own ground reference.

Common examples of separately derived systems include:

Transformers (excludes autotransformers)
Generator sets (when the neutral is switched through the transfer switch)
- Common reasons to switch the neutral through the ATS:
  - The circuit breaker at either the service or generator requires ground fault protection. Not disconnecting the neutral leaves multiple paths to ground which can create problems for ground fault protection.
  - Generator has a bonded neutral. Some generators have a bonded neutral and so the transfer switch serving those generators should include neutral switching.

NEC Code Requirements:

NEC Article 250 governs grounding and bonding, and separately derived systems are addressed specifically in NEC 250.30. This section requires that an SDS be grounded at the source with a system bonding jumper, a grounding electrode conductor (GEC), and a grounding electrode. Key requirements include:

System bonding jumper [NEC 250.30(A)(1)]: Connects the grounded conductor (neutral) to the equipment grounding conductor and grounding electrode at the source.
Grounding electrode conductor [NEC 250.30(A)(5)&(6)]: Sized per NEC Table 250.66 based on the derived phase conductors.
Grounding electrode [NEC 250.30(A)(4)]: The building or structure grounding electrode system shall be used as the grounding electrode for the separately derived system. If located outdoors, the grounding electrode shall be in accordance with 250.30(C).

How Separately Derived Systems Affect Electrical Design

Identifying whether a system qualifies as separately derived has significant downstream design implications. Because an SDS must establish its own ground reference, engineers must account for the following during the design phase:

Grounding electrode system design: Each SDS requires its own electrode or connection to the nearest available electrode system, which can affect conduit routing and structural coordination.
Fault current path: The equipment grounding conductor (EGC) must carry maximum fault current back to the SDS source, not the utility transformer. Engineers must recalculate available fault current independently for each SDS.
Neutral-to-ground bonding: The neutral-to-ground bond must only occur at the SDS source. A second bond downstream creates a parallel neutral path, introducing objectionable current on grounding conductors — a direct violation of NEC 250.6.
Generator design: When a generator feeds loads through a transfer switch that switches all current-carrying conductors including the neutral, the generator output qualifies as an SDS and must be grounded accordingly. If only the ungrounded conductors are switched, the generator shares the service neutral — and is not separately derived.

Conclusion

Separately derived systems are a foundational concept in electrical engineering and NEC compliance. Correctly identifying and designing an SDS — with proper bonding, grounding electrodes, and fault current paths — is critical to personnel safety, system reliability, and passing inspection. When in doubt, early coordination between the electrical engineer of record, the authority having jurisdiction (AHJ), and the equipment manufacturer can prevent costly redesigns. If your project involves transformers or generators confirm early whether an SDS condition exists and design accordingly.