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    Calculates locally in your browser · NEC 250 grounding conductors
    Electrical / grounding

    Grounding electrode and equipment conductors

    Two tables that look alike and are not. Table 250.66 sizes the grounding electrode conductor from the largest ungrounded service conductor — it is keyed to the service capacity. Table 250.122 sizes the equipment grounding conductor from the overcurrent device — it is keyed to what will clear a fault. Confusing the two is the most common grounding error in a panel schedule.

    Service4/0OCPD200 ACap appliedYes
    Grounding electrode conductor
    6 AWG
    capped by §250.66 for a rod electrode
    Table 250.66 result
    2 AWG
    for a 4/0 copper service
    Equipment ground
    6 AWG
    for a 200 A device, Table 250.122
    Aluminium equivalent
    4 AWG
    if the EGC is aluminium
    Two tables, two different questions
    250.66 sizes the electrode conductor from the service; 250.122 sizes the equipment ground from the breaker.
    6 AWG
    Two tables, two different questions
    ItemSizeSource
    Largest service conductor4/0 copperas entered
    Table 250.662 AWG copperkeyed to the service capacity
    Electrode typerodrod, pipe or plate
    Electrode cap6 AWG§250.66(A) or (B)
    GEC required6 AWG copperthe larger of the table and the cap
    Largest feeder OCPD200 Aas entered
    Equipment ground6 AWG copperTable 250.122
    Aluminium EGC4 AWGTable 250.122

    Table 250.66 calls for 2, but §250.66(A) caps the requirement at 6 for that electrode, so 6 is the size to run.

    Inspect the model
    Choose a quick result view, a scaled comparison, or the complete diagnostic set.
    GEC against service conductor
    The steps are coarse on purpose — the table has only six rows.
    GEC against service conductor0.0002.605.197.7910.3812.984.00225.001/01/06.003/03/08.0035035010.0060060011.0010001000GEC size
    Equipment ground against breaker rating
    Linear in rating up to 60 A, then in steps.
    Equipment ground against breaker rating0.0002.244.486.728.9611.2015 A15 A20 A20 A30 A30 A40 A40 A60 A60 A100 A100 A200 A200 A300 A300 A400 A400 A500 A500 A600 A600 A800 A800 ASize index
    Canonical equations
    The exact relationships implemented by this engine.

    Table 250.66(largest service conductor) · Service capacity.

    min(table, 6 AWG copper) · §250.66(A).

    min(table, 4 AWG copper) · §250.66(B).

    Table 250.122(OCPD) · Fault-clearing capacity.

    engineering methodology

    A clear path from inputs to results.

    Two tables that look alike and are not. Table 250.66 sizes the grounding electrode conductor from the largest ungrounded service conductor — it is keyed to the service capacity. Table 250.122 sizes the equipment grounding conductor from the overcurrent device — it is keyed to what will clear a fault.

    Step-by-step methodWorked exampleAssumptions explained

    How to use

    1. Choose the largest service conductor and its material.
    2. Set the largest feeder breaker rating.
    3. Pick the electrode type.
    4. Read the grounding electrode conductor and the equipment ground.

    The formula

    GEC=min⁡(Table 250.66,  electrode cap)\text{GEC} = \min\big(\text{Table 250.66},\; \text{electrode cap}\big)

    Reference: NFPA 70 Table 250.66, Table 250.122, §250.66(A) to (C).

    Worked example

    A 4/0 copper service with a rod electrode

    Inputs
    • Largest service conductor 4/0 copper.
    • Largest feeder breaker 200 A.
    • Rod, pipe or plate electrode.
    Calculation path
    1. 1

      Table 250.66 gives 2 AWG copper for a service conductor over 3/0 through 350 kcmil.

    2. 2

      A rod electrode caps the requirement at 6 AWG under §250.66(A).

    3. 3

      So 6 AWG is what is actually run, even though the table asks for 2 AWG.

    4. 4

      The equipment ground for a 200 A device is 6 AWG copper from Table 250.122.

    5. 5

      The two numbers happen to agree here, which is exactly why the confusion persists.

    Result

    2 AWG from the table · 6 AWG after the cap · 6 AWG equipment ground.

    Interpreting the result

    The cap exists because a rod cannot use more copper than its surface area can transfer into the earth — a 3/0 conductor to a 5/8 inch rod buys nothing. The equipment ground has no such cap, because its job is different: it has to carry fault current long enough for the breaker to trip.

    Assumptions & limitations

    • Service conductors are sized per Article 310.
    • The electrode is a single rod, pipe, plate, concrete-encased electrode or ground ring.
    • The equipment ground is sized from the largest overcurrent device on the feeder.
    • The §250.122(B) proportional increase for voltage-drop upsizing is not applied.

    Frequently asked questions

    Why is the electrode conductor sometimes smaller than the table?

    Because §250.66(A) to (C) cap the requirement by electrode type: 6 AWG for a rod, 4 AWG for concrete-encased, 2 AWG for a ground ring. Above that the extra copper does nothing.

    Can I use aluminium for the grounding electrode conductor?

    Not where it will be in contact with masonry or earth, and never below the sizes in the table. Copper is the default for a reason.

    What if the service is upsized for voltage drop?

    The equipment ground must be increased proportionally under §250.122(B), which is a separate calculation from the table.

    What is the difference between the two tables?

    250.66 is keyed to the service capacity and protects the electrode connection. 250.122 is keyed to the breaker and ensures a fault has a low-impedance path back to trip it.

    Does a metal water pipe count as an electrode?

    Yes, if it is in contact with the earth for 10 feet or more, and it must be supplemented by another electrode under §250.53(D)(2).

    Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.