A 4/0 AWG copper service takes a #2 AWG copper grounding electrode conductor — but only #6 AWG where that conductor runs to ground rods and no other kind of electrode, or #4 AWG to a concrete-encased electrode.
Calculator
Grounding Electrode Conductor
#2 AWGCU
NEC Table 250.66
Row 4: 4/0 AWG is 211,600 cmil.
Per NEC 2023 Table 250.66
How it’s calculated
A = area of one ungrounded service conductor × number of parallel sets
Size = the table row that area falls in, in the column for the conductor's own metal
Then: the limit for the electrode this conductor alone connects to, where it is smaller
A
circular mils of one phase, all its parallel conductors together
table row
read in the copper or aluminum service-conductor column, whichever the service is
limit
#6 AWG copper or #4 AWG aluminum to rods, pipes or plates; #4 AWG copper to concrete-encased electrodes; the ring's own size to a ground ring
A service larger than the table's last row takes the last row's size.
A limit is lost only where the conductor runs on to another kind of electrode that needs a larger conductor; such a conductor is sized from the table.
Assumptions
The conductor entered is the largest ungrounded conductor supplying the system: for a service, a service-entrance conductor. For a separately derived system, enter its derived conductors instead.
All the parallel conductors of one phase are the same size.
The metal of the electrode conductor is chosen separately from the service conductors': the row comes from the service, the column from the conductor being sized.
The limit for a concrete-encased electrode names a copper conductor. An aluminum conductor to one keeps the table size here.
Where the conductor serves several electrodes, or the electrode is unknown, no limit is applied and the table size stands.
Size only. Where the conductor may run, how it is protected, and how it is spliced are separate rules.
Worked example
A 400 A service in two parallel sets
Two 4/0 AWG copper conductors per phase, and a copper conductor to the electrodes.
The table is entered with one phase's total area, not one conductor's:2 × 211,600= 423,200 cmil
That lands on:row 5 of the table= 1/0 AWG copper
One set of the same conductor would have given:211,600 cmil, row 4= #2 AWG copper
And if this conductor runs only to ground rods, the rod limit applies:= #6 AWG copper, whatever the table says
Answer: 1/0 AWG copper to the electrode system; #6 AWG where it runs only to rods.
Entering a single 4/0 AWG where two run per phase reads row 4 instead of row 5. The table's own column asks for the equivalent area of the whole set.
Check: whether the other tool asked how many conductors per phase.
Missing the electrode limits
A table-only answer sends a large copper conductor to ground rods that are allowed #6 AWG. It is not wrong, but it is expensive.
Check: whether the other tool asked what the conductor connects to.
Applying a limit to a conductor that runs on
The rod and concrete-encased limits hold only where the conductor goes no further than that kind of electrode. One that continues to another kind needing a larger conductor is sized from the table.
Check: where the conductor ends.
Using the service rating instead of the conductors
The table is read from conductor area, not from amperes. Two services of the same rating with different conductors can land on different rows.
Check: what the other tool asked for.
Confusing it with the equipment grounding conductor
The conductor run inside the raceway with a feeder is sized from the overcurrent device, by a different table. The two are rarely the same size.
Check: which of the two the answer was for.
Questions
What size grounding electrode conductor for a 200 amp service?
A 200 A service is usually 2/0 AWG copper or 4/0 AWG aluminum, and both read the same row: #4 AWG copper. Where that conductor runs to ground rods and nothing else, #6 AWG copper is enough.
Why is 6 AWG enough for a ground rod?
The code caps it: a conductor that runs to rod, pipe or plate electrodes — one or several — and on to no other kind of electrode needing a larger conductor need not be larger than #6 AWG copper or #4 AWG aluminum, however big the service is. Run it on to a concrete-encased electrode, say, and that limit no longer holds.
And for a concrete-encased electrode?
#4 AWG copper, where the conductor goes no further than concrete-encased electrodes. The limit names copper only, so an aluminum conductor to one stays at the table size on this page.
What about a ground ring?
The conductor to a ground ring need not be larger than the ring itself. A ground ring is bare copper, #2 AWG or larger, at least 20 ft long and encircling the building in contact with the earth, so with the smallest ring that is #2 AWG.
Can it be aluminum?
Yes, with limits on where it runs: without an extruded polymer covering it may not be in direct contact with concrete or where it is subject to corrosion, and outside a building or enclosure it may not be terminated within 18 in. of the earth. The aluminum column is larger than the copper one at every row.
Is this the same as the ground wire in the feeder?
No. This conductor connects the system to the earth. The one run with a circuit to bond metal parts is the equipment grounding conductor, sized from the overcurrent device by a different table.
Written and maintained by Yusuf Can Samiloglu: MEP/HVAC design background, mechanical-engineering degree, not a licensed electrician.
How it’s checked: the calculation is the method of the Electric Master app, ported to the web and tested case by case against the app’s source code. Every table value was checked against the NEC 2023 values in at least two independent published sources.
The App Store version of Electric Master (2.2.0) predates the NEC corrections this page follows; until its next update, its result can differ from this page for some inputs. Version 2.2.0 has no parallel-set entry, no aluminum column for the electrode conductor and no electrode limits, so it shows the table size for a single conductor.
: A ground ring is offered at 2 AWG and larger only (250.52(A)(4)), and a ring entered below that caps the conductor at 2 AWG with a note.
: First version, NEC 2023. Table 250.66 entered with the service conductor, or with the combined area of one phase where the service is in parallel; a copper or aluminum electrode conductor; and the 250.66(A)–(C) limits for a conductor that runs on to no electrode needing a larger conductor than a rod, a concrete-encased electrode or a ground ring.