Short answer: A separately derived system is an electrical source, other than a service, whose circuit conductors have no direct connection to the circuit conductors of another source, except through grounding and bonding connections. The usual exam example is a transformer. Because it starts a new system, 250.30 asks you to ground it again, in this order: make the system bonding jumper at one point only, either at the source or at the first disconnect (NEC 250.30(A)(1)); use the building grounding electrode system as its electrode (NEC 250.30(A)(4)); and size the grounding electrode conductor from Table 250.66 using the derived ungrounded conductors, connected at the same point as the system bonding jumper (NEC 250.30(A)(5)).
PSI lists separately derived systems in the Services, Service Equipment and Separately Derived Systems subject area of the Texas Journeyman exam: 6 scored NEC Knowledge questions and 4 scored Calculations questions. PSI does not say how many cover 250.30 alone. The Texas Journeyman Practice Pack has 1,000 original questions and 10 timed mock exams in the real two-part format, with step-by-step solutions for every calculation. Try free practice questions without signing up. Get the full pack for US$39 with one payment.
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Is it separately derived? Ask about the neutral
The test is whether any circuit conductor of the new source connects directly to the circuit conductors of another source, other than through grounding and bonding. For a generator with a transfer switch, the neutral is the clue to check.
| Source | Usually separately derived? | Why |
|---|---|---|
| Dry-type transformer, for example 480 V to 208Y/120 V | Yes | The windings are separate, so the 208Y/120 V neutral is a new conductor |
| Generator through a transfer switch that also switches the neutral | Often yes | The neutral is not solidly connected to the utility neutral |
| Generator through a 3-pole transfer switch with the neutral solidly connected | No | The neutral is shared with the service, so it is not separately derived (NEC 250.30 Informational Note No. 1) |
The last row is the classic trap. When the generator neutral is solidly connected to the service neutral through a 3-pole transfer switch, you do not add a new neutral-to-ground bond at the generator. Supply-side bonding jumpers run to the first disconnect, and the grounded conductor is not grounded again on the load side (NEC 250.35(B)).
The three steps for a transformer
- One system bonding jumper, one point. Bond the derived neutral to the equipment grounding conductors either at the source or at the first disconnect, not at both. A limited exception allows bonding at both the source and the first disconnect only for an outdoor source, and only where no parallel path for grounded conductor current is created; it does not apply to the indoor transformer examples on this page (NEC 250.30(A)(1) Exception No. 2).
- Use the building electrode system. The grounding electrode for the new system is the building or structure grounding electrode system (NEC 250.30(A)(4)).
- Size and land the grounding electrode conductor. Size it from Table 250.66 using the derived ungrounded conductors, not the primary conductors, and connect it at the same point where the system bonding jumper is connected (NEC 250.30(A)(5)).
When several transformers share one common grounding electrode conductor, each tap from a transformer to that common conductor is sized from Table 250.66 for the derived conductors of the system it serves (NEC 250.30(A)(6)(b)).
Worked example 1: GEC for a 250 kcmil secondary
A transformer’s derived ungrounded conductors are 250 kcmil copper, one set. The electrode is building structural steel.
- Find the row. 250 kcmil falls in the “over 3/0 through 350 kcmil” copper row of Table 250.66.
- Read the GEC. That row gives 2 AWG copper (NEC Table 250.66).
- Check for a limit. The smaller limits in 250.66(A) to (C) apply to rods, pipes, plates, concrete-encased electrodes and ground rings. Structural steel is not one of them, so 2 AWG stands.
Worked example 2: parallel secondary conductors
The derived conductors are 2 sets of 600 kcmil copper per phase.
- Add the areas. 2 × 600 = 1,200 kcmil equivalent area, which is how Table 250.66 treats parallel sets.
- Find the row. 1,200 kcmil is over 1,100 kcmil copper.
- Read the GEC. That row gives 3/0 AWG copper (NEC Table 250.66).
As a separate Table 250.66 point: where the GEC connects only to a rod, pipe or plate electrode, 250.66(A) says that conductor need not be larger than 6 AWG copper. That is a sizing rule, not permission to use a rod alone in place of the building grounding electrode system (NEC 250.66(A)).
Mistakes that cost points
- Sizing the GEC from the primary. Table 250.66 is entered with the secondary, derived conductors.
- Bonding at both ends. Two bonding points on an indoor transformer create a parallel path, so neutral current flows on the equipment grounding conductors and raceways.
- Calling every generator separately derived. Check how the transfer switch handles the neutral first.
For the difference between the GEC and the equipment grounding conductor, and the subpanel neutral rule, see the five common grounding and bonding mistakes.
Check yourself
- A 480 V to 208Y/120 V transformer has 3/0 copper secondary conductors. Which GEC row do you use? The 2/0 or 3/0 copper row, which gives 4 AWG copper.
- Where may the system bonding jumper be made for an indoor transformer? At the source or at the first disconnect, at one point only.
- A generator neutral is solidly connected through a 3-pole transfer switch. Is it separately derived? No.
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Find 250.30 faster on exam day
NFPA 70, National Electrical Code, 2026 Edition (softcover). Highlight 250.30(A) and Table 250.66 so you can move quickly between the bonding rules and the GEC sizing table. The Texas exam is open book. PSI allows a soft-bound NEC with highlighting, underlining and notes made in advance. Loose-leaf, spiral-bound and ring-bound copies are not allowed, and only permanent index tabs made or supplied by the NEC publisher are permitted; homemade and third-party tabs are not.
FAQ
What is a separately derived system?
A source, such as a transformer with separate primary and secondary windings, whose circuit conductors have no direct connection to another source’s circuit conductors except through grounding and bonding connections. For a grounded system, 250.30 sets where the system bonding jumper and grounding electrode conductor connect.
Where does the system bonding jumper go on a transformer?
At one point: either at the transformer or at the first disconnect it supplies, not both. A limited exception allows both only for an outdoor source where no parallel path for grounded conductor current is created.
How do you size the GEC for a transformer?
From Table 250.66, using the derived ungrounded conductors on the secondary side. Parallel sets are added together to find the row.
Is a generator always a separately derived system?
No. If its neutral is solidly connected to the service neutral through a 3-pole transfer switch, it is not separately derived.
Are separately derived systems on the Texas Journeyman exam?
PSI lists them in the Services, Service Equipment and Separately Derived Systems subject area: 6 scored NEC Knowledge questions and 4 scored Calculations questions.
Sources checked October 2026
- PSI Candidate Information Bulletin, Texas electrician examinations (updated September 15, 2026): https://test-takers.psiexams.com/api/content/bulletin/1682
- NFPA 70, National Electrical Code, 2026 Edition, 250.30, 250.35, 250.66 and Table 250.66
