How to size the grounding electrode conductor and system bonding jumper for a transformer (NEC 250.30, CEC 10-616)

Every dry-type transformer that feeds a 208Y/120 V or 480Y/277 V panel creates a new neutral, and the NEC treats that secondary as a separately derived system. NEC 250.30(A) then asks for three conductors that are routinely undersized: a system bonding jumper tying the neutral to the enclosure at one point, a grounding electrode conductor (GEC) to the building electrode, and a supply-side bonding jumper where the transformer and the first disconnect sit in separate enclosures. All three are sized from the area of the derived phase conductors, not from the breaker. On a 150 kVA, 480 V to 208Y/120 V transformer with two sets of 350 kcmil copper, the answer is a 2/0 copper GEC, a 2/0 copper system bonding jumper and a 2 AWG jumper in each raceway. Above 1100 kcmil the GEC stops at 3/0 copper but the jumper keeps growing at 12.5%. Canada sizes the same jumper from the overcurrent device instead. The secondary currents and device ratings these steps start from are the ones the transformer sizing calculator produces.

Which conductors does NEC 250.30(A) require at a transformer?

A two-winding dry-type transformer has no direct electrical connection between the primary and secondary circuit conductors, so its secondary neutral is not the service neutral. The NEC therefore makes the secondary bond and ground itself, under the subsections of NEC 250.30(A) (numbering per the 2023 NEC; verify against the edition your AHJ enforces):

• 250.30(A)(1) system bonding jumper, installed at any single point from the source to the first system disconnecting means or overcurrent device, sized under 250.28(D) from Table 250.102(C)(1).
• 250.30(A)(2) supply-side bonding jumper, run with the derived conductors from the transformer enclosure to the first disconnect when they are separate enclosures; a wire-type jumper is sized from Table 250.102(C)(1) on the derived phase conductors in that raceway.
• 250.30(A)(4) grounding electrode, which for an indoor transformer is the building or structure grounding electrode system.
• 250.30(A)(5) grounding electrode conductor, landed at the same point as the system bonding jumper and sized from Table 250.66.
• 250.30(A)(6) common GEC for several transformers, an unspliced conductor of at least 3/0 AWG copper or 250 kcmil aluminum, with a tap from each transformer sized from Table 250.66.

The equipment grounding conductor (EGC) that leaves the secondary main is a different conductor under a different rule: NEC 250.122 sizes it from the overcurrent device rating. Confusing the two tables is a common error in this part of a design.

What do Table 250.66 and Table 250.102(C)(1) require?

Both tables key on the largest ungrounded conductor, or the equivalent area for parallel conductors — the circular-mil areas of one phase set added together. Up to 1100 kcmil copper the two tables carry identical rows. Above it they part ways.

Two limits sit outside the table. NEC 250.66(A) says the portion of a GEC that is the sole connection to a rod, pipe or plate electrode need not exceed 6 AWG copper, and 250.66(B) caps the sole connection to a concrete-encased electrode at 4 AWG copper. Neither cap applies to the run to building steel or a metal water pipe.

Equivalent area, parallel sets = number of sets × area of one conductor

Bonding jumper above 1100 kcmil Cu = 0.125 × equivalent phase area

The physics behind the split is simple. The GEC carries little fault current — the ground fault returns to the transformer neutral through the bonding path, not through earth — so its size is capped. The system bonding jumper is that return path: it carries the full secondary ground fault, so it scales with the phase conductors with no upper cap in the table. For the voltage-class and grounding-scheme decision that precedes this, see choosing a power system grounding scheme.

Worked example: 150 kVA, 480 V to 208Y/120 V dry-type transformer

Take a three-phase dry-type distribution transformer indoors, 480 V delta primary, 208Y/120 V secondary, feeding a panelboard 20 ft away with a 600 A main. The system bonding jumper is installed at the transformer, so a supply-side bonding jumper runs to the panel. Secondary conductors are copper, 75 °C terminations, in two PVC conduits. The conductor selection is carried over from the secondary conductor tap-rule article.

1. Secondary full-load current. 150 000 ÷ (√3 × 208) = 150 000 ÷ 360.3 = 416.4 A.
2. Secondary device and conductors. 125% × 416.4 = 520.5 A, rounded up under NEC Table 450.3(B) Note 1 to the NEC 240.6(A) standard rating of 600 A. From NEC Table 310.16, 75 °C copper, 350 kcmil = 310 A, so two sets give 2 × 310 = 620 A ≥ 600 A. The device derivation is in sizing transformer overcurrent protection.
3. Equivalent area. 2 × 350 kcmil = 700 kcmil copper per phase.
4. Grounding electrode conductor. 700 kcmil falls in the over 600 through 1100 kcmil row of Table 250.66: 2/0 AWG copper (or 4/0 aluminum), landed at the system bonding jumper point under 250.30(A)(5).
5. System bonding jumper. Same row of Table 250.102(C)(1): 2/0 AWG copper, under 250.30(A)(1) and 250.28(D).
6. Supply-side bonding jumper, per raceway. Each PVC conduit carries one 350 kcmil set; 350 kcmil falls in the over 3/0 through 350 row, so each raceway gets a 2 AWG copper jumper under 250.30(A)(2).
7. EGC downstream of the 600 A main. Table 250.122, 600 A row: 1 AWG copper. It is smaller than the 2/0 jumper because it is sized from the device, not the conductor.

I = kVA × 1000 ÷ (√3 × V)

The conductors decide; the transformer kVA does not appear in either table. Two sets of 3/0 for a 400 A main, for instance, give 2 × 167.8 = 335.6 kcmil, which drops the GEC and the jumper to 2 AWG copper.

Second example: 500 kVA and the 12.5% rule

A 500 kVA, 480 V to 208Y/120 V transformer — dry-type indoors, or a 500 kVA pad-mounted transformer outdoors — puts the derived conductors above the 1100 kcmil line:

1. Current. 500 000 ÷ (√3 × 208) = 1387.9 A; 125% × 1387.9 = 1734.8 A, next standard rating 2000 A.
2. Conductors. Six sets of 400 kcmil copper at 335 A: 6 × 335 = 2010 A ≥ 2000 A; equivalent area 6 × 400 = 2400 kcmil.
3. GEC. Over 1100 kcmil, so Table 250.66 caps it at 3/0 AWG copper.
4. System bonding jumper. Note 1 to Table 250.102(C)(1): 0.125 × 2400 = 300 kcmil copper.
5. Supply-side jumper per raceway. 400 kcmil falls in the over 350 through 600 row: 1/0 AWG copper in each of six raceways.

The jumper is now larger than the GEC (300 kcmil against 3/0) and larger than the 250 kcmil EGC that Table 250.122 gives for the 2000 A main.

Results across common transformer ratings

All rows assume 480 V to 208Y/120 V, copper, 75 °C terminations, one set per raceway, and a secondary device at the next 240.6(A) standard rating at or above 125% of full-load current.

The 300 kVA row shows the rounding step: 0.125 × 1400 = 175 kcmil, and because 3/0 is only 167.8 kcmil, the jumper goes to 4/0 (211.6 kcmil). The 75 kVA row uses 350 kcmil because 300 kcmil at 285 A fails a 300 A device. Run your own rating in the transformer sizing calculator, then read the two tables against the conductors it implies.

Where Canada differs: CEC Rules 10-212 and 10-616

The Canadian Electrical Code (CSA C22.1, quoted here from the 2021 text of Section 10) reaches the same bond by a different basis.

The conductors between the transformer and the secondary main are protected only by the primary device, so the Appendix B method multiplies the primary device rating by the voltage ratio and reads that current in Table 16. On the 150 kVA case, CEC Rule 26-254(1) gives 1.25 × 180.4 = 225.5 A, raised under 26-254(3) to a 250 A primary device; 250 × (480 ÷ 208) = 250 × 2.31 = 577 A is the Table 16 entry point. The NEC jumper, by contrast, is read from 700 kcmil of copper. The device choice matters: in a published 50 kVA, 600 V to 120/240 V example (ratio 2.5), primary devices of 100 A, 125 A and 250 A give 250 A, 312.5 A and 625 A and jumpers of #4, #3 and 1/0 copper. Rule 10-212(2) is the larger practical difference: since the 2018 edition, a 600 V-class transformer in a Canadian building may be grounded through its system bonding jumper to the primary supply’s bonding conductor, with no new electrode run. Where a grounding conductor is installed, Rule 10-114 (2018 and later numbering) governs its size. Confirm the current subrule text against the edition your AHJ (ESA in Ontario) enforces, and see how transformers get approved for use in Canada for the product side.

Common mistakes

• Sizing the system bonding jumper from Table 250.122. On the 150 kVA example the 600 A row gives 1 AWG copper; the correct jumper from Table 250.102(C)(1) is 2/0 copper. An inspector checking 250.28(D) against the derived conductors catches it.
• Capping the jumper at 3/0 because the GEC is capped. Six sets of 400 kcmil need a 300 kcmil jumper under Note 1, not 3/0; the 3/0 cap belongs only to Table 250.66.
• Sizing from the largest single conductor instead of the parallel set. Reading 350 kcmil instead of 700 kcmil gives a 2 AWG GEC; the correct answer is 2/0. The table heading’s “equivalent area for parallel conductors” is the test.
• Bonding the neutral at both the transformer and the panel. 250.30(A)(1) permits a single point outside its narrow exceptions; a second bond puts normal neutral current on the supply-side jumper and metal raceways in parallel with the neutral.
• Applying NEC area sizing on a Canadian project. CEC 10-616(1) reads Table 16 by the device rating — 577 A for a 250 A primary at 480 to 208 V — so a design that cites Table 250.102(C)(1) is likely to be questioned in plan review.

What to specify

• Derived phase conductor size and number of parallel sets, from the conductor schedule and NEC Table 310.16 — the input to both grounding tables.
• System bonding jumper location (transformer or first disconnect) and size, citing 250.30(A)(1) and Table 250.102(C)(1), or CEC 10-616(1) and the primary device rating times the voltage ratio.
• GEC size and electrode, citing Table 250.66 and 250.30(A)(4); name building steel or the water pipe, and whether a common GEC under 250.30(A)(6) is used.
• Supply-side bonding jumper per raceway, sized on the conductors in that raceway, where PVC or flexible raceway is used.
• EGC on the secondary feeder, from the device rating in Table 250.122 — a separate line item, not the jumper.
• Secondary device rating and available fault current, from the calculator and available fault current and short-circuit ratings.

Where Entogo fits

The grounding conductors above have to terminate somewhere: neutral and ground bus provisions, the system bonding jumper terminal and lug ranges are set on Entogo transformers and distribution switchboards and panelboards to the conductor schedule at order, so a 2/0 or 300 kcmil jumper lands on hardware sized for it. Entogo is a Toronto-headquartered supplier backed by its own source factory; dry-type units are designed and built to ANSI/IEEE C57 and UL 1561 / UL 1562, pad-mounted units to IEEE C57.12.34, and panelboards and switchboards to UL 67 and UL 891; UL (cULus) / CSA certifiable on request.

Start with the transformer sizing calculator, then carry the result into the transformer configurator or the low-voltage switchboard range. For packaged lineups, see substations and power distribution; for plant work, industrial EPC. For the kVA decision itself, see how to size a transformer.

The grounding conductors at a transformer follow the phase conductors, not the transformer and not the breaker. Add up the parallel set, read Table 250.66 for the GEC and Table 250.102(C)(1) for both jumpers, apply 12.5% above 1100 kcmil, and keep Table 250.122 for the EGC downstream. In Canada, start from the primary overcurrent device, scaled by the voltage ratio, and Table 16 instead.