Sheet E-306: The 6000 x 6000 Earth Grid Is a Candidate, and the Conductors Stay Separate

What this sheet shows

E-306 is the local plan of the earth electrodes at one OEM transformer. The coordination line on the sheet is 26 SEP 2026. The view is titled a proposed 6000 x 6000 electrode envelope, and note 4 names the candidate for this grid as 6 x 6 m with four rods. ER-1, ER-2, ER-3 and ER-4 sit at the four corners. The OEM transformer is drawn inside the envelope at 1900 x 2220 overall, with the radiator on one face of the tank and the HV and LV compartments on the other. One side is labeled HV / MV-P1; E310 SEQ-1, and the other side is the LV tray exit. On the door and operating side the plan notes transverse access through the HV / LV direction, and it refers the piers to E312, MV-P1 to E310 SEQ-1..5 / E313-03, and the OEM opening to T1. EB-TX, the accessible earth bar, is on that side; the tank bond is marked TX ground-pad bond / G4, and the line continues to the site grounding system. The open area of the envelope states that no driven rods or stakes go in the reserved cable route, and it refers the ground pad, the rods and the earth bar to E307. The reference line cites NEC 2017 Article 250, OEM GA 08 Sep 2026, and the transformer installation specification.

Rods, ring, and the accessible bar

Note 2 is headed configuration and installation. ER-1..4 are four listed copper-bonded steel rods, and the size line calls the rod a candidate: 3/4 in diameter by 10 ft long. Each rod uses a listed direct-burial bolted clamp, and the clamp is selected for the actual rod, conductor and strand class. The ring is 2/0 AWG bare copper, with a candidate depth of 800 mm. The plan perimeter is a nominal 24 m, and that line says to add both risers and leads. One continuous conductor is required, and both ends terminate at EB-TX, which keeps the ring landed on the accessible bar. That bar is an accessible copper bar of at least 1/4 x 2 in, with separate terminal positions and no operating disconnect. The grid is connected to the site system and is not isolated earth; the plan shows the same continuation to the site grounding system. No welding is required by this connection arrangement. The note says to protect an existing ring and not to cut-and-rejoin it with ordinary bolts, to use listed buried fittings for the actual use, and to make GEC splices under 250.64(C).

Separate conductor functions

Note 3 keeps the conductor functions from sharing one size. The GEC entry is a general-table screen, 3/0 Cu for 8 x 1000 Al per phase, and electrode-specific sizing exceptions remain applicable. SBJ / SSBJ are to be calculated separately under 250.102(C). The note says not to use 3/0 Cu as a universal bonding-jumper limit, so the screen is not a number that can be copied onto the jumpers. The MV EGC is sized from 250.190(C) and 250.122: at least 6 AWG Cu, then sized for the protection and the fault duty. That sentence says the ring is not the fault return. Neutral bonding stays at one coordinated point per TX, and a second N-PE bond is not to be added in the indoor cabinet. The heading keeps the 2/0 AWG bare copper ring, the 3/0 Cu GEC screen, the SBJ / SSBJ and the MV EGC as separate conductor functions.

Status

Published at Design Development — Public Edition P1 (title block: 2026-09; design coordination: 26 SEP 2026). This is a reference design for study and evaluation, and note 4 prints the limits on the grid in the same column as the candidate. The 6 x 6 m envelope and the four rods are a design candidate, not a code prescription and not a verified touch/step-voltage result. The note says to confirm soil resistivity, fault split and clearing time. Rebar and the four structural piles are supplementary, and they are not to be credited to make the base earth grid pass. The note says to check all electrode ties, continuity and the test method. Grounding acceptance is stated as a verification of touch/step and fault-clearing performance, and 25 ohms alone does not establish MV-yard acceptance. The final site position avoids poles, crane supports and ducts. G-X1, on the plan, is a continuous crossing after MV duct and backfill acceptance, at E310 SEQ-2; its location and levels follow the actual MV route / C1 and are to be recorded before covering.

Sheet guide — the devices on this drawing

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