9.6 Separately Derived Systems and Separate Buildings

What Makes a System Separately Derived

Article 100 defines a separately derived system as an electrical source, other than a service, having no direct connection to circuit conductors of any other source other than through grounding and bonding connections.

Apply that test carefully:

The transfer-switch question is the one that trips people. If the transfer switch does not switch the neutral, the generator neutral remains bonded through to the service and the generator is not separately derived — the generator neutral must not be bonded at the generator.

Grounding a Separately Derived System — 250.30(A)

250.30(A)(1) System Bonding Jumper. An unspliced system bonding jumper shall comply with 250.28(A) through (D). This connection shall be made at any single point on the separately derived system from the source to the first system disconnecting means or overcurrent device, or it shall be made at the source of a separately derived system that has no disconnecting means or overcurrent devices.

• Exception No. 1: For systems installed in accordance with 450.6, a single system bonding jumper connection to the tie point of the grounded circuit conductors from each power source shall be permitted.
• Exception No. 2: A system bonding jumper at both the source and the first disconnecting means shall be permitted where doing so does not establish a parallel path for the grounded conductor. Where a grounded conductor is used in this manner, it shall not be smaller than the size specified for the system bonding jumper but shall not be required to be larger than the ungrounded conductor(s). For the purposes of this exception, connection through the earth shall not be considered as providing a parallel path.
• Exception No. 3: The size of the system bonding jumper for a system that supplies a Class 1, Class 2, or Class 3 circuit, and is derived from a transformer rated not more than 1000 volt-amperes, shall not be smaller than the derived ungrounded conductors and shall not be smaller than 14 AWG copper.

The practical rule: bond at the transformer, or bond at the first disconnect — pick one. Bonding at both creates a parallel path for neutral current through the raceway and equipment grounding conductors, exactly the objectionable-current condition 250.6 prohibits.

250.30(A)(2) Supply-Side Bonding Jumper. If the source of a separately derived system and the first disconnecting means are located in separate enclosures, a supply-side bonding jumper shall be installed with the circuit conductors from the source enclosure to the first disconnecting means. A supply-side bonding jumper shall not be required to be larger than the derived ungrounded conductors. Where of the wire type, it shall be sized in accordance with 250.102(C).

250.30(A)(3) Grounded Conductor. Where a grounded conductor is installed and the system bonding jumper connection is not located at the source, the grounded conductor shall be routed with the derived ungrounded conductors and shall not be smaller than Table 250.102(C)(1) based on the largest ungrounded conductor of the derived system. Additional requirements apply for parallel conductors and delta-connected systems.

250.30(A)(4) Grounding Electrode. The building or structure grounding electrode system shall be used as the grounding electrode for the separately derived system. Where located in an area without a grounding electrode from 250.52(A)(1) through (A)(4), an electrode from 250.52(A)(5) through (A)(8) shall be used. The grounding electrode shall be as near as practicable to, and preferably in the same area as, the grounding electrode conductor connection to the system.

250.30(A)(5) Grounding Electrode Conductor, Single Separately Derived System. A grounding electrode conductor for a single separately derived system shall be sized in accordance with 250.66 for the derived ungrounded conductors. It shall be used to connect the grounded conductor of the derived system to the grounding electrode as specified in 250.30(A)(4). This connection shall be made at the same point on the separately derived system where the system bonding jumper is connected.

250.30(A)(6) Grounding Electrode Conductor, Multiple Separately Derived Systems. Where more than one separately derived system exists, a common grounding electrode conductor shall be permitted, sized not smaller than 3/0 AWG copper or 250 kcmil aluminum, with taps to each system sized per 250.66 for the individual derived conductors.

250.30(A)(8) Bonding. Structural steel and metal piping shall be connected to the grounded conductor of a separately derived system in accordance with 250.104(D).

Worked Example — Transformer Grounding

A 75 kVA, 480 V delta to 208Y/120 V transformer supplies a panelboard 8 feet away through 3/0 AWG copper conductors. The system bonding jumper is installed at the transformer.

Secondary current = 75,000 / (1.732 x 208) = 208.2 A
Derived ungrounded conductors: 3/0 AWG copper

System bonding jumper, Table 250.102(C)(1), based on 3/0 Cu  = 4 AWG Cu
Grounding electrode conductor, Table 250.66, based on 3/0 Cu = 4 AWG Cu
Grounded (neutral) conductor, Table 250.102(C)(1)            = 4 AWG Cu minimum

The grounding electrode conductor runs from the transformer — the same point as the system bonding jumper — to the building grounding electrode system.

Separate Buildings and Structures — 250.32

250.32(A) Grounding Electrode. Building or structure(s) supplied by feeder(s) or branch circuit(s) shall have a grounding electrode or grounding electrode system installed in accordance with Part III of Article 250. The grounding electrode conductor(s) shall be connected in accordance with 250.32(B) or (C). Where there is no existing grounding electrode, the grounding electrode(s) required in 250.50 shall be installed.

Exception: A grounding electrode shall not be required where only a single branch circuit, including a multiwire branch circuit, supplies the building or structure and the branch circuit includes an equipment grounding conductor for grounding the normally non-current-carrying metal parts of equipment.

So a single 20-ampere circuit to a garden shed needs no ground rod. A feeder to a shed with a subpanel does.

250.32(B)(1) Supplied by a Feeder or Branch Circuit. An equipment grounding conductor, as described in 250.118, shall be run with the supply conductors and be connected to the building or structure disconnecting means and to the grounding electrode(s). The equipment grounding conductor shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded. The equipment grounding conductor shall be sized in accordance with 250.122. Any installed grounded conductor shall not be connected to the equipment grounding conductor or to the grounding electrode(s).

Exception: For existing premises wiring systems only, the grounded conductor run with the supply to the building or structure shall be permitted to serve as the ground-fault return path if all of the requirements of the following are met: (1) an equipment grounding conductor is not run with the supply to the building or structure; (2) there are no continuous metallic paths bonded to the grounding system in each building or structure involved; and (3) ground-fault protection of equipment has not been installed on the supply side of the feeder.

This is a critical point of Code history. Through the 2005 NEC, a three-wire feeder to a detached structure with the neutral bonded at that structure was permitted for new work. The 2008 NEC removed that allowance. Today the exception is available only for existing installations. Every new feeder to a separate structure must be four-wire with an isolated neutral and a separate equipment grounding conductor.

250.32(B)(2) Supplied by Separately Derived System. Where a separately derived system supplies a building or structure, the requirements of 250.30(A) apply, with (a) covering the case where overcurrent protection is provided where the conductors originate and (b) covering the case where it is not.

250.32(C) Ungrounded Systems. The equipment grounding conductor run with the supply conductors shall be connected to the grounding electrode(s), with specific rules for feeders and branch circuits.

250.32(D) Disconnecting Means Located in Separate Building or Structure on the Same Premises. Where one or more disconnecting means supply one or more additional buildings or structures under single management, and where these disconnecting means are located remote from those buildings or structures, all of the conditions in (D)(1) through (D)(3) shall be met.

250.32(E) Grounding Electrode Conductor. The size of the grounding electrode conductor to the grounding electrode(s) shall not be smaller than given in 250.66, based on the largest ungrounded supply conductor. The installation shall comply with Part III of Article 250.

Worked Example — Detached Alaska Shop

A 100-ampere feeder runs 120 feet underground from a dwelling's main panel to a detached heated shop with its own subpanel. What is required at the shop?

Feeder conductors  ...... 3 AWG copper (100 A at 75 C), sized for the load
Grounded conductor ...... run with the feeder, ISOLATED at the shop panel
Equipment grounding
  conductor ............. Table 250.122 at 100 A = 8 AWG copper
Disconnecting means ..... 225.31, at a readily accessible location nearest
                          the point of entrance; 225.36 suitable for use
                          as service equipment
Grounding electrode ..... 250.32(A) - required; typically two driven rods
                          6 ft apart, or a concrete-encased electrode
Grounding electrode
  conductor ............. Table 250.66 based on 3 AWG Cu = 8 AWG Cu,
                          but 250.66(A) caps it at 6 AWG if it terminates
                          solely on rod electrodes; 8 AWG is smaller than
                          the cap, so 8 AWG copper governs
Burial .................. Table 300.5(A) cover; 300.5(J) arrangement to
                          prevent damage from frost heave and settlement

The single most important item is the isolated neutral. Bonding the neutral to the enclosure at the shop panel puts return current on the equipment grounding conductor, on any metal water or fuel line between the buildings, and on the earth path between the two electrode systems. In a frost-heaved Alaska yard with a buried water line, that is a genuine shock hazard on exposed metal.

Ungrounded and Impedance-Grounded Systems

250.21 Alternating-Current Systems of 50 Volts to 1000 Volts Not Required to Be Grounded lists systems that may be operated ungrounded, including certain industrial electric furnaces, separately derived systems supplying rectifiers, and systems supplied by transformers with a primary voltage of less than 1000 volts under specified conditions.

250.21(B) Ground Detectors. Ground detectors shall be installed for ungrounded AC systems operating at 120 volts or more and less than 1000 volts, in accordance with the section.

250.21(C) Marking. Ungrounded systems shall be legibly marked "Ungrounded System" at the source or first disconnecting means of the system. The marking shall be of sufficient durability to withstand the environment involved and shall not be handwritten.

250.36 High-Impedance Grounded Neutral Systems are permitted for three-phase AC systems of 480 to 1000 volts where the conditions of maintenance and supervision ensure that only qualified persons service the installation, continuity of power is required, ground detectors are installed, and line-to-neutral loads are not served.