Texas Electrician ExamTexas journeyman & master

Checked against primary sources 2026-08-24

A transformer secondary usually is one. A generator behind a transfer switch depends on what the switch opens.

This is the most confused topic in the services area, and the confusion is upstream of the requirements. Candidates who can recite what a separately derived system needs still cannot tell you whether they are looking at one.

On this page
  1. What makes a system separately derived?
  2. The transformer answer
  3. The transfer switch answer
  4. Why the answer changes everything downstream
  5. The code writes the exceptions into the electrode conductor rule
  6. Not every separately derived system is grounded
  7. Where this sits on the paper, and how to prepare it
  8. What this page cites

What makes a system separately derived?

No direct connection to the circuit conductors of any other electrical source. That is the whole test, and Article 100 puts it in one sentence about an electrical source other than a service.

The definition then adds the carve-out that settles most items. Connections established by grounding and bonding are excluded, so none of them makes one system continuous with another.

Read that again, because it answers the objection everyone raises. The transformer enclosure is bonded to the same metal as everything else, an equipment grounding conductor runs back to the panel, and the earth sits under both. Every one of those is a grounding or bonding connection, and not one of them is a connection to a circuit conductor.

Two things in that sentence get skipped. A service is excluded by name, so a service is never a separately derived system however it is wired. And a book older than the 2014 edition words the carve-out as a list of four connections that do not count, naming the earth, metal enclosures, metallic raceways and equipment grounding conductors. If that is how you learned it, the current sentence is the same idea in fewer words, and it is the sentence your 2026 copy prints.

So the question to ask is always the same. Does a circuit conductor run through from the other source, uninterrupted? If yes, it is not separately derived. If no, it is.

The transformer answer

An ordinary two-winding transformer has no metallic path between primary and secondary. Power crosses by magnetic field. Nothing on the secondary is electrically continuous with anything on the primary, so the secondary is a new system and it is separately derived.

An autotransformer is the opposite case and it is the one items are built on. It has one winding with a tap, so the input and the output share conductors. A circuit conductor is common to both sides, which fails the test outright, so an autotransformer does not produce a separately derived system.

That single contrast answers most transformer items in this subject area. A dry-type transformer in an electrical room feeding a panel is the commonest separately derived system on the paper, and a buck-boost arrangement wired as an autotransformer is the commonest thing that looks like one and is not.

The tell in the question is usually a description of separate windings, or the pointed absence of one.

The transfer switch answer

A standby generator behind a transfer switch is sometimes separately derived and sometimes not, and the fact that decides it is what the switch does to the grounded conductor. Not how many poles the switch has.

Take the ordinary three-phase, four-wire case first. A three-pole switch moves the three ungrounded conductors and leaves the neutral connected straight through, from the generator, past the switch, to the service. That neutral is a circuit conductor, it is continuous, and it joins the two sources. So the generator is not separately derived, and the bond stays back at the service.

Now switch the neutral as well. A four-pole switch on that same system opens the grounded conductor along with the three ungrounded ones, so while the generator carries the load there is no circuit conductor in common with the utility system at all. The generator has become the source of a separately derived system, and it gets its own bond and its own connection to an electrode.

Where counting poles gives you the wrong answer

Count poles on a single-phase service and the shortcut inverts. 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. A three-pole transfer switch there does exactly what a four-pole switch does on a three-phase system, and the generator behind it is separately derived.

That is not an edge case. It is the ordinary residential and light commercial standby generator, which is most of the generators anyone in Texas actually wires. Carry three poles equals not separately derived into that job and you get it backwards on the paper and backwards on the truck, and getting it backwards on the truck means a bond in the wrong place or no bond at all.

The shortcut fails a second time on a three-wire delta system that has no neutral. Three poles there open every circuit conductor the system has, nothing is left connected through, and the answer is separately derived again.

What the code keys on is the switching action. The informational note that travels with 250.30 says an on-site generator is not a separately derived system if its grounded conductor stays solidly interconnected with the service-supplied grounded conductor, and the example it gives is transfer equipment with no switching action in the grounded conductor.

An item here rarely says the words separately derived. It describes what happens to the neutral, or it hands you a pole count sitting next to a system voltage and waits. A pole count on its own is not enough to answer from.

Why the answer changes everything downstream

The neutral and the ground are joined at exactly one point and separated everywhere after it At the service, the grounded neutral conductor and the equipment grounding conductor are bonded together by the main bonding jumper. Downstream of that point, in every subpanel, the two are kept apart: the neutral floats on an insulated bar and the grounding conductors land on a bar bonded to the enclosure. If they are joined again in a subpanel, normal load current returns on the equipment grounding conductor and on any metal in its path, which is the fault this rule exists to prevent. SERVICE A B from the utility neutral bar grounding bar main bonding jumper the one legal connection feeder: ungrounded and grounded plus the grounding conductor SUBPANEL neutral bar, ISOLATED grounding bar, bonded to the can no jumper here joining them twice puts load current on metal that people touch to the electrode system one bond, one place. everything downstream stays apart.
Bond once, at the service. After that, separate. Joining them twice puts load current on metal people touch. Drawn against NEC 250.24(B), 250.28 and 250.142(B). In editions before 2026 the load-side rule was 250.24(A)(5).

The figure above is the service rule, and it is the rule most candidates carry into every question. Bond once, at the service. Separate everywhere after it.

A separately derived system creates a second, entirely legitimate once. The system bonding jumper ties the grounded conductor to the equipment grounding conductors at any single point on the system from the source to the first system disconnecting means or overcurrent protective device (NEC 250.30(A)(1)). At the transformer, at the first disconnect, or anywhere between the two. Pick one place.

Past that point the grounded conductor goes back to being an ordinary circuit conductor. It is not reconnected to ground, not connected to equipment grounding conductors, and not connected to metal parts of equipment (NEC 250.30(A)), which is the same separation that applies downstream of a service.

One exception is worth knowing rather than memorizing. Where a building is supplied by a feeder from an outdoor separately derived system, a jumper at both the source and the first disconnecting means is permitted, so long as doing it creates no parallel path for the grounded conductor (NEC 250.30(A)(1), Exception No. 2). That is a narrow allowance for one arrangement, not a general license to bond twice.

The system gets its own connection to earth as well. The grounding electrode conductor runs from that same point, the one where the system bonding jumper is connected, to the grounding electrode (NEC 250.30(A)(5)).

And where the source and the first disconnecting means sit in separate enclosures, a supply-side bonding jumper runs with the circuit conductors between them (NEC 250.30(A)(2)). That is the bonding path on the supply side of the first overcurrent device, doing the job an equipment grounding conductor does on the load side of it.

The consequence to carry into the room: if a question describes a panel and asks whether the neutral bonds to the can, the answer depends on what is upstream. Another panel, and the neutral floats. A service, and it bonds. A transformer, or transfer equipment that opens the neutral, and you are at a new source, so you start again from there.

The code writes the exceptions into the electrode conductor rule

A small piece of evidence in Article 250 shows how differently the code treats these systems, and it sits in a sentence about splices.

Except as provided in 250.30(A)(5), 250.30(A)(6), 250.30(B)(1), and 250.68(C), grounding electrode conductors shall be installed in one continuous length without splices or joints.

NEC 250.64(C)

Count the carve-outs. Four are named, and three of them are provisions of the separately derived systems section. The continuous-length rule that governs the grounding electrode conductor at a service is relaxed mainly for one situation, and that situation is this one.

The reason is physical. A building can hold several separately derived systems on several floors, and an unspliced conductor from each of them to the same electrode is not a practical installation, so the code permits a tapped arrangement instead.

Not every separately derived system is grounded

This catches people who know the bonding rules well, because those rules assume a grounded system.

The following ac systems of 50 volts to 1000 volts shall be permitted to be grounded but shall not be required to be grounded.

NEC 250.21(A)

Two of the systems on the list that follows are named as separately derived systems. One is a system used exclusively for rectifiers supplying adjustable-speed industrial drives. The other is a system supplied by a transformer with a primary rated 1000 volts or less, used exclusively for control circuits, where only qualified persons service it and continuity of control power matters.

Both are industrial, and both exist for the same reason: a first ground fault on an ungrounded system does not shut the process down. The code balances that in the next subsection by calling for ground detectors, connected as close as practicable to where the system receives its supply (NEC 250.21(B)).

For an exam item, the useful shape is that 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.

Where this sits on the paper, and how to prepare it

Electrical Services, Service Equipment, and Separately Derived Systems carries six of the fifty-six scored items on the journeyman knowledge portion and four of the twenty-four scored items on the calculations portion, where it ties with branch circuit calculations for the largest block on the outline. It is not a corner of the syllabus.

The identification question sits in front of everything else. Get it wrong and you apply a set of rules correctly to a system they do not govern, which produces a confident wrong answer rather than a blank.

  1. Read the definition of a separately derived system in Article 100, including the words that exclude connections established by grounding and bonding. Two minutes, and it is the whole topic.
  2. Read 250.30 in your own copy once, in place, and mark three things: where the system bonding jumper goes, where the grounding electrode conductor connects, and when a supply-side bonding jumper is called for.
  3. Draw the same generator twice, once with the grounded conductor switched and once with it solid, and mark on each where the bond lives. Then write the system voltage beside each drawing and work out how many poles that switch actually needs. You will not confuse pole count with switching action again.
  4. Ask one question on every grounding item you meet: where is the source for this panel. Service, another panel, or a new source. Everything follows from the answer.

Texas adopts the 2026 National Electrical Code effective 1 September 2026 under 16 TAC 73.100, and the examinations reference that edition from the same day. Article 250 kept its number, and several sections inside the book moved this cycle, so read 250.30 in the 2026 copy rather than trusting a tab from an older one.

What this page cites

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