Transformer Bonding, Grounding Electrodes, and a Detached-Building Feeder
A machine shop installs a 75 kVA dry-type transformer (480 V delta primary, 208Y/120 V secondary) to supply a 225 A panelboard 25 ft away. Secondary conductors are 4/0 copper in EMT. The crew installs the system bonding jumper in the transformer, brings a 4/0 neutral to the panelboard, and installs a second bonding jumper from the panelboard neutral bar to the enclosure. They drive a single 5/8 in. by 8 ft ground rod at the panelboard and connect it to the neutral bar with 6 AWG copper. The building's metal water service pipe is 20 ft away, but they see no need to use it because 'the rod is dedicated.' The EMT is made up tight and serves as the equipment grounding conductor. From the 225 A panel, a 60 A feeder runs 90 ft to a detached storage building with a 60 A main-lug panel; there the crew plans a 3-wire feeder with the neutral bonded to the panel enclosure and a ground rod at the building.
1. Evaluate the system bonding jumpers as installed. What is wrong and how is it corrected?
Read the worked answer
There is one bonding point too many. Under 250.30(A), a separately derived system gets a single system bonding jumper, installed at the source or at the first disconnecting means, but not at both. As built, the neutral is bonded at the transformer and again at the 225 A panel, so neutral current splits between the 4/0 grounded conductor and the EMT, enclosures, and panel cans. That is a parallel path and objectionable current under 250.6, and it is the classic source of shock complaints and erratic VFD behavior in a shop like this.
Correct it by removing the bonding jumper at the panelboard and keeping the neutral bar isolated from the enclosure there. The EMT remains the equipment grounding conductor, which is permitted by 250.118 for a continuous, tight metallic raceway, so the fault path is intact. Size the system bonding jumper and any supply-side bonding jumper from 250.102 and 250.66 based on the 4/0 secondary conductors, which is 2 AWG copper; verify the factory-installed jumper in the transformer meets that minimum.
2. Is the 6 AWG grounding electrode conductor to the rod a violation? Evaluate the electrode selection as a whole.
Read the worked answer
The 6 AWG itself is legal. Under 250.66(A), the grounding electrode conductor connection to a rod, pipe, or plate electrode is not required to be larger than 6 AWG copper, so the size is not the defect. The defects are about the electrode system, not the wire.
First, 250.53(A)(2) requires a supplemental electrode for a single rod unless the rod is documented at 25 ohms or less. Nobody measured it, so add a supplemental electrode. Second, 250.50 requires all present grounding electrodes to be bonded into one grounding electrode system. If the metal water service pipe qualifies under 250.52(A)(1) with at least 10 ft in contact with earth, it must be part of the system with a full-size conductor per 250.66, and any qualifying concrete-encased electrode must be used as well. The 'dedicated rod' reasoning does not exist in the Code.
Also confirm the grounding electrode conductor runs to the grounding electrode system rather than being landed only on the neutral bar, and separate the neutral and equipment grounding terminations at the panelboard as described above.
3. Is the 3-wire feeder to the detached storage building acceptable? What must change?
Read the worked answer
No. For new work, 250.32(B)(1) requires an equipment grounding conductor run with the feeder to a separate building or structure, with the grounded conductor isolated at the separate disconnecting means. Bonding the neutral to the storage panel enclosure creates a parallel return path through the earth, the rod, and any metallic path between the buildings, which is exactly the condition the rule exists to prevent. The exception permitting a 3-wire feeder with a bonded neutral is limited to existing installations where no EGC exists; it does not license this new feeder.
The fixed design: pull a fourth conductor sized per 250.122 for a 60 A overcurrent device (10 AWG copper), or use the continuous metallic raceway as the EGC if that is how it is being installed, and keep the neutral bar isolated in the storage panel. The building still needs its own grounding electrode system connected to the EGC under 250.32(A), which the rod and the water bond do provide, so that part of the plan can stay. Update the drawing before the trench is backfilled.