Separately Derived Systems Practice Questions

B. An electrical power supply output, other than a service, having no direct connection to the circuit conductors of any other source, apart from connections established by grounding and bonding

One test carries the whole topic: does a circuit conductor run through uninterrupted from another source. A service is excluded by name, so a service is never a separately derived system however it is wired. The carve-out at the end is what stops every grounding and bonding path in the building from making two systems into one.

NEC 2026 Article 100, 250.30

D. Yes, since none of those is a connection to a circuit conductor

This is the objection everybody raises and the definition answers it in the same sentence that raises it. Connections established by grounding and bonding are excluded, and every path named in the stem is one of those. An older book words the same idea as a list of four connections that do not count, naming the earth, metal enclosures, metallic raceways and equipment grounding conductors.

NEC 2026 Article 100

A. No, a service is excluded by name in the definition

The definition opens by describing an electrical power supply output other than a service, so the exclusion is not inferred, it is stated. That is why the service grounding rules and the separately derived system rules sit in different sections and take different bonding jumpers. An item that describes a service transformer on the premises is checking whether you know where the service point falls.

NEC 2026 Article 100, 250.24

C. The secondary of an ordinary two-winding transformer

Two separate windings mean power crosses by magnetic field and nothing on the secondary is electrically continuous with the primary, so the secondary is a new system. An autotransformer has one winding with a tap, so a circuit conductor is common to both sides and the test fails outright. A buck-boost arrangement wired as an autotransformer is the thing that looks like a separately derived system and is not.

NEC 2026 Article 100, 250.30

B. No, since the grounded conductor stays solidly interconnected with the service-supplied grounded conductor

The informational note that travels with the section says an on-site generator is not a separately derived system while its grounded conductor stays solidly interconnected with the service-supplied one, and it gives transfer equipment with no switching action in the grounded conductor as the example. So the bond stays back at the service and the generator gets none of its own. The neutral is a circuit conductor and it is running through.

NEC 2026 250.30

D. Yes, since on that system three poles opens the two ungrounded conductors and the neutral as well

Count poles against the number of wires the system runs on rather than against a remembered rule. A 120/240-volt system has two ungrounded conductors and a neutral, so the switch that leaves the neutral solid has two poles and the switch that opens it has three. Carry the three-pole shortcut from a three-phase job into this one and you get it backwards on the paper and backwards on the truck.

NEC 2026 250.30, 250.35

A. Yes, since three poles opens every circuit conductor the system has

What the code keys on is the switching action rather than the pole count, and here nothing is left connected through. This is the second place the pole-count shortcut fails, and it fails in the opposite direction from the single-phase case. Ask the same question every time: is any circuit conductor of the new source still joined to the other source while the source is carrying load.

NEC 2026 250.30

C. The grounded conductor, the supply-side bonding jumper, the disconnecting means enclosure and the equipment grounding conductors

Where the jumper lands decides what it connects. At the source it ties the grounded conductor to the supply-side bonding jumper and the metal enclosure, and nothing else exists yet to tie in. At the first disconnecting means the equipment grounding conductors are there, so they join the connection. The jumper is unspliced either way and it stays inside the enclosure where it originates.

NEC 2026 250.30(A)(1), 250.28

B. A system bonding jumper at both the source and the first disconnecting means, provided doing so establishes no parallel path for the grounded conductor

This is a narrow allowance for one arrangement rather than a general license to bond twice, and its condition does the work: no parallel path for the grounded conductor. Where a grounded conductor is used that way it carries a minimum size of its own. A related exception then releases the supply-side bonding jumper between enclosures for installations made this way.

NEC 2026 250.30(A)(1), 250.30(A)(2)

D. Not smaller than the derived ungrounded conductors, and not smaller than 14 AWG copper or 12 AWG aluminum

The exception is written for a system supplying a Class 1, Class 2 or Class 3 circuit derived from a transformer rated not more than 1000 volt-amperes, and 900 clears that. It sets two floors at once and the larger of them governs. A companion exception releases the grounding electrode conductor for the same kind of system where the grounded conductor is bonded to the frame or enclosure with a jumper sized this way.

NEC 2026 250.30(A)(1), 250.30(A)(5)

A. A supply-side bonding jumper installed with the circuit conductors

On the supply side of the first overcurrent device there is no equipment grounding conductor yet, so the supply-side bonding jumper does that job. It may be a nonflexible metal raceway or the wire or bus type, and a wire type is sized from the bonding jumper table based on the derived ungrounded conductors. It is never required to be larger than the derived ungrounded conductors.

NEC 2026 250.30(A)(2), 250.102(C)

C. The building or structure grounding electrode system

The section sends you to the electrode system the building already has rather than asking for a new one. Where the source sits outdoors, the electrode connection is made at the source location instead under a subsection of its own. An exception lets a system that originates in equipment listed and identified for use as service equipment use that equipment's electrode.

NEC 2026 250.30(A)(4), 250.30(C)

B. At the same point where the system bonding jumper is connected

Tying the two connections to one point is what keeps the system from having two references and a circulating path between them. So once you have chosen where the bonding jumper goes, the electrode conductor connection is decided for you. Installation of that conductor follows the general grounding electrode conductor rules for material, securing, continuity and enclosures.

NEC 2026 250.30(A)(5), 250.30(A)(7)

D. A common grounding electrode conductor, not smaller than 3/0 AWG copper or 250 kcmil aluminum where it is the wire type, with a tap from each system sized from Table 250.66 for that system's derived ungrounded conductors

The common conductor may also be a metal water pipe or the metal structural frame of the building where those qualify under the electrode rules. Each tap connects the grounded conductor of its system to the common conductor at the same point where that system's bonding jumper is connected. The connections are made so the common conductor itself stays without a splice or joint.

NEC 2026 250.30(A)(6), 250.66

A. Three of them are provisions of the separately derived systems section

Counting the carve-outs tells you how differently the code treats these systems, since the continuous-length rule at a service is relaxed mainly for this one situation. The reason is physical: a building can hold several separately derived systems and an unspliced run from each of them to the same electrode is not a practical installation. The fourth carve-out points at the connections to grounding electrodes section.

NEC 2026 250.64(C), 250.30(A)(5)

C. The neutral conductor

The section lists five system shapes and names the conductor to be grounded for each one. Where a single wire is common to all phases it is the neutral, and where one phase is grounded instead it is that phase conductor, which is the corner-grounded delta case. The separately derived section sends you here by name, alongside the sections that decide which systems have to be grounded at all.

NEC 2026 250.26, 250.30

B. 4 AWG copper

System bonding jumpers are sized from the same table that sizes main bonding jumpers and grounded conductors, and the row covering 2/0 or 3/0 copper gives 4 AWG copper. The 6 AWG answer comes from the row above, for 1 or 1/0, and 2 AWG from the row below, for over 3/0 through 350 kcmil. Read the row by the size of the derived ungrounded conductors rather than by the transformer rating.

NEC 2026 250.28(D), Table 250.102(C)(1)

D. Systems used exclusively for rectifiers supplying only adjustable-speed industrial drives, and systems from a transformer with a primary rated 1000 volts or less used exclusively for control circuits where only qualified persons service the installation and continuity of control power is required

The sentence carries two statements at once, a permission and a release from an obligation, and an answer choice will hand you only one of them. Both entries are industrial and both exist for the same reason, since a first ground fault on an ungrounded system does not shut the process down. The subsection also reaches industrial electric furnaces and any other system not required to be grounded under the general rule.

NEC 2026 250.21(A), 250.20

A. Ground detectors, connected as close as practicable to where the system receives its supply

The requirement reaches ungrounded ac systems operating at not less than 120 volts and at 1000 volts or less, which is a bracket worth reading rather than assuming. The location instruction is part of the rule: the sensing equipment goes near the supply rather than out at a load. The system is also legibly marked at the source or first disconnecting means to say it is ungrounded and at what voltage.

NEC 2026 250.21(B), 250.21(C)

C. A legible marking at the source or the first disconnecting means saying the system is ungrounded and naming the voltage between conductors

The marking goes where somebody working on the system will meet it first, which is the source or the first disconnecting means. It names the voltage between conductors, not the voltage to ground, since there is no defined voltage to ground on an ungrounded system. Ground detectors and the marking are two separate obligations in consecutive subsections.

NEC 2026 250.21(C)

B. Sized from Table 250.66 for the largest derived ungrounded conductor, connecting the metal enclosures of the derived system to the electrode, at any point from the source to the first system disconnecting means

An ungrounded system still gets an electrode connection, which is what the last wrong answer misses, and what changes is the thing being connected. Here the conductor ties the metal enclosures to the electrode rather than tying a grounded conductor to it. A supply-side bonding jumper still runs from the source to the first disconnecting means under the same rule that governs grounded systems.

NEC 2026 250.30(B)

D. A grounding electrode connection made at the source location to one or more grounding electrodes

An outdoor source gets its own electrode connection where it stands, and the rest of the section still applies on top of that. This is also why the system bonding jumper rule sends you here where the source is outside the building supplied. Impedance grounded systems take their electrode conductor connection from a different section instead.

NEC 2026 250.30(C), 250.50

A. It runs with the supply conductors from the generator to the first disconnecting means, and where the generator is installed as a separately derived system the separately derived rules apply

The section splits by whether the generator is separately derived and points each branch at the rules that govern it. That means the identification question decides the whole installation, from where the bond lives to what the electrode connection looks like. A separate informational note in the separately derived section points at the minimum size of conductors that carry fault current for a generator.

NEC 2026 250.35, 445.13

C. Not smaller than Table 250.102(C)(1), and never required to be larger than the derived ungrounded conductors, with a 1/0 AWG floor where conductors are connected in parallel in two or more raceways

The grounded conductor carries the fault current back to the jumper when the jumper is not at the source, which is why it gets sized from the bonding jumper table rather than the equipment grounding one. A 3-phase, 3-wire delta system is the exception to the general sizing, since there the grounded conductor takes an ampacity not less than the ungrounded conductors. In parallel installations the grounded conductor goes in each raceway or cable and each one is at least 1/0.

NEC 2026 250.30(A)(3), Table 250.102(C)(1)

A. As a single separately derived system installed under one set of rules, with an exception permitting the system bonding jumper at the paralleling equipment

Treating them as one system is stated in the opening paragraph of the section, and the matching exception lets the jumper sit at the paralleling switchgear, switchboard or other paralleling connection point rather than at each source disconnect. Putting a jumper at each source would give the combined system more than one bond. Structural steel and metal piping are separately connected to the grounded conductor of the system under the bonding subsection at the end of the list.

NEC 2026 250.30, 250.30(A)(8), 250.104(D)