400 A Service: Three Red Tags on One Grounding System
A 400 A, 3-phase, 208Y/120 V service is fed by two parallel sets of 250 kcmil copper THWN per phase. The electrician runs a 1/0 Cu grounding electrode conductor to the metal underground water main, bonds around the water meter with a 6 AWG jumper, and drives a single 8 ft ground rod with a 6 AWG conductor, measuring 38 ohms to ground. A 200 A subpanel is fed 180 ft with 3/0 Cu phase conductors and a 6 AWG Cu equipment grounding conductor. For voltage drop, the crew decides at the last minute to pull 4/0 Cu phase conductors instead, keeping the same 6 AWG equipment grounding conductor and the same 200 A breaker. The inspector red-tags the job on three points.
1. Evaluate the grounding electrode conductor to the water main and the 6 AWG jumper around the water meter.
Read the worked answer
Service conductor equivalent size: two parallel 250 kcmil Cu sets = 250 + 250 = 500 kcmil equivalent. Where service conductors are in parallel, the grounding electrode conductor is sized on the equivalent area, not on one set.
Table 250.66, copper: over 350 kcmil through 600 kcmil → 1/0 Cu. The 1/0 run to the water main is therefore correct. Had the electrician sized from a single 250 kcmil set, the table would have called for 2 AWG - a common and dangerous shortcut.
The jumper around the water meter is a violation. The water meter is part of the grounding electrode path; the bonding jumper that carries the electrode conductor around it must be at least as large as the grounding electrode conductor it replaces. A 6 AWG jumper in a 1/0 Cu path is a choke point on the fault-clearing and lightning path. It must be 1/0 Cu and made with a listed, irreversible or properly tightened connection - not a split-bolt wrapped in tape.
Also confirm: the connection to the water pipe is within 5 ft of the point of entry, is on the street side of any meter or insulating fitting where practicable, and that any insulating section or union is bonded around with the same full-size jumper.
2. The subpanel feeder phase conductors were upsized from 3/0 to 4/0 Cu for voltage drop. What must happen to the 6 AWG equipment grounding conductor, and why?
Read the worked answer
The equipment grounding conductor must be upsized proportionally.
Step 1 - What 200 A requires. For a 200 A overcurrent device, the minimum Cu equipment grounding conductor is 6 AWG (26,240 circular mils). That was correct with 3/0 phase conductors.
Step 2 - Why upsizing matters. The equipment grounding conductor's job is to hold the voltage drop along the return path low enough during a ground fault that the overcurrent device trips promptly. If the phase conductors are made larger, the circuit impedance drops and fault current rises; the grounding conductor must be increased in the same proportion to keep the same ratio.
Step 3 - The ratio.
- 3/0 Cu = 167,800 cmil; 4/0 Cu = 211,600 cmil
- Ratio = 211,600 ÷ 167,800 = 1.261
- Required EGC = 26,240 × 1.261 = 33,090 cmil minimum
- 8 AWG (16,510) - no. 6 AWG (26,240) - no. 4 AWG (41,740) - yes.
Verdict: the 6 AWG must be replaced with at least 4 AWG Cu. Leaving the 6 AWG in place while enlarging the phase conductors is a code violation and defeats the purpose of the upsizing: the conductor would be larger, but the ground-fault return path would not be.
3. The ground rod reads 38 ohms. Is the installation acceptable, and what is the correct fix?
Read the worked answer
Not acceptable, but note what the 25-ohm threshold actually means. A single rod reading 38 ohms fails the 25-ohm criterion. The remedy is not to keep driving deeper or to add chemical treatment as a first step: the code requires a supplemental electrode - a second made electrode, bonded to the first with a full-size conductor - when a single rod exceeds 25 ohms.
Important distinction the inspector is testing: once the second rod is installed, the resistance is not re-measured against 25 ohms. The two-rod requirement is satisfied by installing the second electrode; the 25-ohm value is not a pass/fail number after that.
Conductor size to the rods: a grounding electrode conductor to a rod, pipe, or plate need not be larger than 6 AWG Cu, regardless of service size. So the existing 6 AWG to the rod is fine - the error is the single rod, not the conductor.
Overall picture of this service:
- Water main: the primary electrode, and the 1/0 Cu electrode conductor is the one that must be intact and full-size.
- Ground rods: supplemental electrodes only. Do not let the crew believe the rods are carrying the service; the water pipe is.
- All electrodes are bonded together into one grounding electrode system.
- The 6 AWG jumper around the water meter must be replaced with 1/0 Cu before the system is complete.