320-Amp Service, Two 200-Amp Panels: GEC Sized on 4/0 or 320-Amp Rating?

Electrician Questions — technical NEC analysis for electricians, apprentices, and inspectors.

The question: 320-amp service, 2× 200-amp breakers in pedestal, 2× separate 4/0 SER cables to 2× 200-amp panels in basement, 2× ground rods tied with #4 bare (tied back to both panels). Table 250.66 sizes GEC on largest ungrounded conductor — parallel conductors use equivalent area. These are individual feeders, not parallel. I sized #4 bare copper. Inspector says GEC must reflect the 320-amp service rating. Who is right?

Short Answer

• You are correct that separate 4/0 SER runs to individual panels are not parallel conductors under the Table 250.66 note — parallel applies to multiple conductors per same phase of the same circuit.
• However, #4 copper is undersized for 4/0 phase conductors per Table 250.66 — the table requires #2 AWG copper minimum for 4/0 or larger.
• Inspector’s “320-amp” aggregate is debatable as a single Table 250.66 lookup, but the install still fails on per-feeder 4/0 sizing alone.
• 250.66(A) exception: conductor to ground rod alone need not exceed #6 copper — but bonding electrode system and main bonding jumper rules still apply at each panel.

NEC Table 250.66 — Grounding Electrode Conductor Size

Parallel vs Separate Feeders — NEC Note

Your installation:

• Pedestal: 320A meter/main with 2× 200A overcurrent devices
• Feeder A: 4/0 SER → Panel A (complete set of ungrounded conductors)
• Feeder B: 4/0 SER → Panel B (separate set)

These are two distinct feeder circuits, not parallel sets of the same phase. Do not sum cross-sectional areas. Size GEC at each service disconnect / bonding point based on the largest ungrounded conductor for that path — here, 4/0 copper → 2 AWG copper GEC per table.

System Diagram

[320A PEDESTAL — 2× 200A breakers]
        |                           |
   4/0 SER (Feeder 1)          4/0 SER (Feeder 2)
        |                           |
  [200A Panel A]              [200A Panel B]
        |                           |
        +-------- #4 bare to rods ---+  ← common electrode (250.58)
        
Table 250.66 per 4/0 feeder → 2 AWG Cu GEC (not #4)
250.66(A) rod-only tap → max #6 Cu required to rods

250.66(A) — Rod Connection Exception

This is why #6 to rods is common — but the main bonding jumper / GEC to water pipe / Ufer / metal frame may still require full Table 250.66 sizing based on 4/0. #4 is neither the rod exception maximum nor the full table size.

Inspector “320-Amp Service” Argument

Worked Sizing — Your Install

Comparison Chart — GEC Size vs Conductor

What to Install

1. Run #2 AWG copper (or larger per table) as common bonding conductor between electrodes and panels where Table 250.66 applies.
2. Bond both panels to common electrode system per 250.58.
3. Size equipment grounding conductors in each 4/0 SER per 250.122 — typically #2 Cu EGC for 200A/4/0 feeder (verify cable assembly listing).
4. Do not treat dual feeders as parallel for 250.66 note — document separately for inspector.

Bottom Line

General consensus: Separate 4/0 feeders are not parallel — size from 4/0 per set. That yields #2 AWG copper GEC, not #4. The inspector is wrong to force an abstract “320A” lookup if NEC table uses conductor size — but the inspector is right that #4 is undersized for 4/0 service conductors.

References: NEC 250.24, 250.58, 250.64, 250.66, 250.122, Table 250.66. Verify SER assembly EGC sizing with cable UL listing.

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