Separately derived system grounding under 2017 NEC 250.30(A) comes down to three sized conductors: a system bonding jumper and a supply-side bonding jumper from Table 250.102(C)(1), and a grounding electrode conductor from Table 250.66, all sized by the derived (secondary) ungrounded conductors. Make the neutral-to-enclosure bond at one point between the transformer and the first disconnect, land the GEC at that same point, and connect it to the building grounding electrode system. With 3/0 copper secondaries, all three conductors are 4 AWG copper.
If the words still blur together, read grounding vs bonding in plain English first. A transformer secondary is bonded much like a service, which JCR covers in main bonding jumper size and where the neutral bond goes. To see how you do on a timed set, take the free 25-question 2017 NEC mock.
Tennessee, Nevada, Arizona and Kansas still grade on the 2017 NEC, and Table 250.102(C)(1) is a 2017 table that older study guides don't have. JCR's 2017 course teaches grounding and bonding from that edition.
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Separately derived system grounding: the sizing table
Size everything from the largest derived ungrounded conductor (or the equivalent area of parallel sets). Copper values from the 2017 NEC:
| Largest secondary ungrounded conductor (Cu) | System or supply-side bonding jumper, Cu (Table 250.102(C)(1)) | Grounding electrode conductor, Cu (Table 250.66) |
|---|---|---|
| 2 AWG or smaller | 8 AWG | 8 AWG |
| 1 or 1/0 | 6 AWG | 6 AWG |
| 2/0 or 3/0 | 4 AWG | 4 AWG |
| Over 3/0 through 350 kcmil | 2 AWG | 2 AWG |
| Over 350 through 600 kcmil | 1/0 | 1/0 |
| Over 600 through 1100 kcmil | 2/0 | 2/0 |
| Over 1100 kcmil | At least 12½% of the conductor area (Note 1) | 3/0 |
The GEC also gets the 250.66 limits: it never has to be larger than 6 AWG copper when it is the sole connection to a rod, pipe or plate electrode, or larger than 4 AWG copper to a concrete-encased electrode. JCR's Table 250.66 guide walks through the aluminum column too. These transformer lookups show up in every one of the 2017 NEC exam states.
Transformer grounding and bonding: what each conductor does
A typical 480 V to 208Y/120 V dry-type transformer has no direct connection between primary and secondary circuit conductors, so its secondary is a separately derived system with a neutral that must be bonded and grounded on its own.
| Conductor | Job | 2017 rule |
|---|---|---|
| System bonding jumper (250.30(A)(1)) | Joins the neutral to the enclosure and equipment grounding path, at one point | Table 250.102(C)(1) through 250.28(D)(1) |
| Supply-side bonding jumper (250.30(A)(2)) | Bonds the transformer enclosure to the first disconnect when they are in separate enclosures | Wire type from Table 250.102(C)(1); never required larger than the derived ungrounded conductors. A nonflexible metal raceway may serve instead. |
| Grounded conductor (250.30(A)(3)) | Carries fault current back to the source when the bond is at the disconnect, not the transformer | Not smaller than Table 250.102(C)(1), and large enough for the neutral load |
| Grounding electrode conductor (250.30(A)(5)) | Connects the neutral, at the bonding point, to the electrode | Table 250.66 |
Where the system bonding jumper goes
- One point. Anywhere from the source to the first system disconnecting means or overcurrent device: in the transformer (250.30(A)(1)(a)) or in the first disconnect (250.30(A)(1)(b)). A second bond puts neutral current on raceways and equipment grounding conductors. A narrow exception covers some outdoor sources where no parallel path results.
- Stay in the enclosure. The jumper remains within the enclosure where it originates.
- Same point for the GEC. The grounding electrode conductor connects to the neutral where the system bonding jumper does (250.30(A)(5)).
- Separate downstream. Past the bond, neutrals and equipment grounding conductors stay apart, the same as after a service.
Which grounding electrode? 250.30(A)(4) and (A)(6)
For an indoor separately derived system, the 2017 NEC uses the building or structure grounding electrode system. An outdoor source follows 250.30(C). Some older texts describe hunting for the nearest water pipe or structural steel; that is not the 2017 rule.
Several transformers may share one common grounding electrode conductor if it is at least 3/0 copper or 250 kcmil aluminum. Each tap to it is sized from Table 250.66 for that transformer's own secondary conductors (250.30(A)(6)).
Worked example: 3/0 copper secondary conductors
A 208Y/120 V secondary runs in 3/0 THWN copper to a separate panelboard with a main breaker. The bond is made in the transformer.
- System bonding jumper: 2/0 or 3/0 copper row of Table 250.102(C)(1), so 4 AWG copper.
- Supply-side bonding jumper (wire type) from transformer to panel: same row, 4 AWG copper, or a nonflexible metal raceway such as RMC.
- GEC: 2/0 or 3/0 copper row of Table 250.66, so 4 AWG copper, landed at the same terminal as the bonding jumper and run to the building grounding electrode system.
- Panelboard: no neutral-to-case bond, because the bond already exists at the transformer.
Move the bond to the panel instead and the 3/0 neutral from transformer to panel must be at least 4 AWG copper under 250.30(A)(3), which it easily is. The supply-side bonding jumper is still needed to bond the transformer enclosure.
Exam traps on 250.30
- Bonding at both ends. One system bonding jumper. Two create a parallel neutral path.
- Mixing up the tables. Bonding jumpers: Table 250.102(C)(1). GEC: Table 250.66. They agree up to 1100 kcmil copper, so the trap appears on big secondaries.
- Capping a bonding jumper at 3/0. Table 250.66 stops at 3/0 copper. Table 250.102(C)(1) keeps going: above 1100 kcmil copper, use 12½% of the largest ungrounded conductor's area.
- Sizing from the primary. Every size here comes from the secondary ungrounded conductors.
- Using an older book. Table 250.102(C)(1) first appeared in the 2017 NEC. Before that, these jumpers were sized from Table 250.66 plus a 12½% rule.
Testing on the 2020 NEC? JCR's 2020 NEC changes vs. 2017 shows what moved between editions.
Separately derived system quiz: 5 questions
Use your 2017 NEC, then reveal each answer.
Question 1
Secondary conductors are 500 kcmil copper. What is the minimum copper system bonding jumper?
- 2 AWG
- 1/0 AWG
- 2/0 AWG
- 3/0 AWG
Question 2
Secondary conductors are 4/0 copper, and the GEC is the sole connection to a concrete-encased electrode. What is the largest size the NEC requires?
- 6 AWG copper
- 4 AWG copper
- 2 AWG copper
- 1/0 AWG copper
Question 3
A transformer is installed indoors. Under 250.30(A)(4), what is its grounding electrode?
- A new ground rod at the transformer
- The nearest water pipe within 5 ft of the transformer
- The building or structure grounding electrode system
- The primary equipment grounding conductor
Question 4
Where may the system bonding jumper be installed?
- At both the transformer and the first disconnect
- At any single point from the source to the first disconnecting means or overcurrent device
- Only at the building's service equipment
- In every panelboard the transformer supplies
Question 5
Each phase of a secondary is one 1,500 kcmil copper conductor. What is the minimum copper system bonding jumper?
- 2/0 AWG
- 3/0 AWG
- 4/0 AWG
- 250 kcmil
Answer key
Show the answer to question 1Hide the answer to question 1
1. B, 1/0 AWG copper (Table 250.102(C)(1))
500 kcmil sits in the over 350 through 600 kcmil row, which calls for 1/0. Choice A is the row above.
Show the answer to question 2Hide the answer to question 2
2. B, 4 AWG copper (250.66(B))
Table 250.66 alone gives 2 AWG for 4/0 copper, but a sole connection to a concrete-encased electrode never needs more than 4 AWG copper. Choice A is the rod limit in 250.66(A).
Show the answer to question 3Hide the answer to question 3
3. C, the building or structure grounding electrode system (250.30(A)(4))
Choice B resembles the nearest-electrode approach of older editions, not the 2017 text.
Show the answer to question 4Hide the answer to question 4
4. B, any single point from the source to the first disconnect (250.30(A)(1))
Bonding at both ends puts neutral current on the grounding path.
Show the answer to question 5Hide the answer to question 5
5. C, 4/0 AWG copper (Table 250.102(C)(1), Note 1)
1,500 × 0.125 = 187.5 kcmil. 3/0 is 167.8 kcmil, too small; 4/0 is 211.6 kcmil (Chapter 9, Table 8). Choice B is the Table 250.66 ceiling for a GEC.
Transformer grounding FAQ
What size system bonding jumper do I need for 3/0 copper secondary conductors?
4 AWG copper, from the 2/0 or 3/0 copper row of 2017 NEC Table 250.102(C)(1). The Table 250.66 grounding electrode conductor is also 4 AWG copper.
Can I bond the neutral at both the transformer and the panel?
No. 250.30(A)(1) allows the system bonding jumper at a single point, at the source or at the first disconnecting means. A second bond creates a parallel path for neutral current.
Where does the grounding electrode conductor connect on a separately derived system?
To the grounded conductor at the same point as the system bonding jumper (250.30(A)(5)), and from there to the building or structure grounding electrode system (250.30(A)(4)).
How is a supply-side bonding jumper sized?
When the transformer and first disconnect are in separate enclosures, a wire-type supply-side bonding jumper is sized from Table 250.102(C)(1) by the derived ungrounded conductors, and it never has to be larger than those conductors (250.30(A)(2)). A nonflexible metal raceway may serve instead.
Lock in the grounding tables
Transformer questions pull from two tables and four subsections. A few dozen reps make them routine.
For the complete course, see the 2017 NEC self-study bundle for Tennessee, Nevada, Arizona and Kansas, or the Tennessee-specific package.
Sources
- NFPA 70, 2017 edition: 250.28(D)(1), 250.30(A)(1) through (A)(6), 250.30(C), 250.66, Table 250.66, 250.102(C), Table 250.102(C)(1), Chapter 9 Table 8
Reviewed November 6, 2026. NEC references are to the 2017 edition.
