Separately Derived System Bonded at Both Ends in a Detached Shop
A 45 kVA dry-type transformer in a detached shop steps 480 V delta down to 208Y/120 V. Primary: 54 A calculated, protected by a 70 A breaker, fed with #6 Cu THHN. Secondary: 3/0 Cu THHN in 2 in EMT, 40 ft to a 200 A panelboard; a #6 Cu green conductor is run with the phase conductors. X0 is bonded to the transformer enclosure with a #6 Cu jumper, and at the 200 A panel the neutral and the green conductor land on the same terminal bar. The shop has a metal underground water pipe and one 8 ft ground rod, neither of which is connected to anything. Reported symptoms: 6 A measured on the green conductor, warm conduit, a GFCI receptacle that trips at random, and a tingle when touching the panel cover while standing on a wet floor.
1. Identify the defect and the code rule that is violated. Where must the system bonding jumper be, and what size does it need to be for 3/0 Cu secondary conductors?
Read the worked answer
The defect. The system is bonded at two points - at X0 in the transformer and again at the 200 A panel. That makes the green conductor and the EMT a parallel return path for neutral current. The 6 A on the green wire is neutral load current that has found a second route back to X0, which is exactly what 250.6(A) prohibits (objectionable current over grounding conductors) and what 250.30(A)(1) prevents by requiring a single system bonding jumper for a separately derived system.
Where it goes. One connection, either at the source (the transformer) or at the first disconnecting means supplied by the secondary - not both, and not elsewhere. Because the neutral is switched or opened at the panel in many layouts, most designers keep the bond at the transformer and use a solid, unspliced neutral. Whichever is chosen, install it once and label it.
Size. 250.30(A)(1) requires the system bonding jumper to be sized per 250.102(C)(1) from the derived phase conductors. For 3/0 Cu the table gives #4 Cu. The installed #6 jumper is undersized even before the double-bond problem is considered.
Also verify the separate-structure rules: with an equipment grounding conductor run to the shop, 250.32(B) requires the grounded conductor to be isolated from the enclosure and the EGC at that building.
2. Size the grounding electrode conductor and the supply-side bonding jumper, and state which electrodes must be used at the shop.
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Grounding electrode conductor. 250.30(A)(5) sizes it from Table 250.66 using the largest derived ungrounded conductor: 3/0 Cu gives #4 Cu.
Apply the 250.66(A) and (C) limits where they apply:
- Connection to a rod or pipe electrode need not be larger than #6 Cu
- Connection to a concrete-encased electrode need not be larger than #4 Cu
Practical result: #4 Cu to the metal water pipe, #6 Cu to the ground rod.
Supply-side bonding jumper. 250.102(C)(1), sized on the derived conductors: #4 Cu for 3/0 Cu.
Electrodes at the separate structure. 250.32(A) requires a grounding electrode at the separate building, and 250.50 requires all available electrodes to be bonded into one electrode system:
- Metal underground water pipe with at least 10 ft in contact with earth (250.52(A)(1)) - present here, must be used
- The 8 ft rod (250.52(A)(5)) - present, must be used
- Metal building frame and any concrete-encased electrode, if present - check the drawings
A single rod also needs a supplemental electrode unless it is proven at 25 ohms or less (250.53(A)(2)); the water pipe normally satisfies that. Right now neither electrode is connected, so a fault on the secondary has no intentional path to earth at the shop - that is a second, independent defect from the double bond.
3. Explain the hazard the 6 A represents, list the repair sequence, and evaluate the #6 Cu primary feeder.
Read the worked answer
The hazard. The green conductor and EMT are carrying neutral current, so every enclosure they touch sits above earth potential by the voltage drop in that path. That produces the tingle at the panel cover and the touch potential between the cover and a wet floor. It also heats terminations, can arc at a loose lug, and can ignite anything the conduit touches. The GFCI trips because the neutral potential difference between the two bond points drives current on paths the GFCI did not expect. This is a shock and fire condition, not a nuisance.
Repair sequence
- De-energize the 70 A primary and lock it out. Do not open the neutral under load.
- Remove the neutral-to-ground bond at the 200 A panel and install an isolated neutral bar; land the green EGC on the enclosure with a listed lug.
- Keep exactly one system bonding jumper - here, at the transformer - and size it #4 Cu.
- Run and terminate the GEC: #4 Cu to the water pipe, #6 Cu to the rod, both to the same electrode system.
- Re-energize and measure. Green conductor current should be essentially zero; any residual means a second neutral-to-ground connection downstream.
Primary feeder check. 45,000 / (1.732 x 480) = 54.1 A. At 125% that is 67.6 A. #6 Cu THHN at 75 C is 65 A, which does not meet the 125% continuous-load requirement, so the correct answer is #4 Cu (85 A at 75 C). Flag it: the #6 is a real deficiency, and the transformer primary/secondary protection scheme should also be checked against 450.3(B) before the inspector gets there.