Two jobs the word "ground" is doing
Open a service panel and you will find two entirely different wires that both get called "the ground." One leaves the panel and heads for the earth itself — driven rods, a metal water pipe, or the rebar in the footing. The other travels with every branch circuit, from the panel to each box and appliance frame. The National Electrical Code refuses to confuse them: it gives each wire its own name, its own sizing variable, and its own table. The wire to earth is the grounding electrode conductor (GEC), sized from the service conductors per Table 250.66. The wire that rides along with circuits is the equipment grounding conductor (EGC), sized from the breaker or fuse ahead of it per Table 250.122.
The two names track the two jobs hiding inside the word "ground." Grounding, strictly, is the connection to earth — the path that deals with lightning and surge gradients, phenomena that genuinely involve the planet. Bonding is the connecting of metal enclosures together so that a fault inside any of them has a low-impedance metallic path back to the source, which is what actually trips the breaker. The EGC is the workhorse of the bonding side; earth has almost nothing to do with it. This guide sizes both wires for one running example — a 200 A residential service with a single 50 A branch circuit — and shows why the two answers come from different pages of the code.
The grounding electrode conductor: the earth connection
The GEC runs from the service to the grounding electrode system, and Table 250.66 sizes it from the largest ungrounded service-entrance conductor — the fatter the service, the fatter the earth connection, because this wire must be proportional to the system it grounds, not to any particular breaker. The table is keyed to conductor size ranges: 2/0 or 3/0 copper service conductors land in the row that returns a 4 AWG copper GEC, while a service over 3/0 through 350 kcmil copper returns 2 AWG copper. Parallel service conductors are counted by their combined circular-mil area per the table note — two 4/0 copper conductors per phase add to 423,200 cmil, which reads from the "over 350 through 600 kcmil" row and returns a 1/0 copper GEC even though each individual conductor is only 4/0.
Then come the reliefs of 250.66(A) through (C), which surprise almost everyone the first time. Where the GEC is the sole connection to a rod, pipe, or plate electrode, it never needs to be larger than 6 AWG copper (4 AWG aluminum), no matter how big the service. A run to a concrete-encased electrode caps at 4 AWG copper, and a ground ring connection follows the ring conductor itself. That is why the bare copper wire clamped to the ground rods behind a big house is so often 6 AWG: the code recognizes that a larger wire cannot push more current into the earth than the electrode itself can dissipate.
What earth resistance actually buys — and the 25-ohm target
How good is the earth connection, once made? NEC 250.53(A)(2) requires a single made electrode — a rod, pipe, or plate — to achieve 25 ohms or less to earth; if it does not, a second electrode must be added. The physics behind the number is dominated by soil: the Dwight equation of IEEE Std 142 puts a single 3 m (roughly 10 ft) rod, 16 mm in diameter, at about 33.5 ohms in 100 ohm-meter soil — already over the threshold — and soil resistivity swings from under 50 ohm-meters for wet clay to several thousand for dry sand or rock, which is why a measured value beats any assumption.
Adding rods helps, but less than intuition says: closely spaced rods compete for the same soil volume, so four rods at 3 m spacing behave like roughly 2.8 independent ones and bring that 33.5-ohm rod down to about 12 ohms rather than a quarter of it. The important disillusionment is what the 25-ohm figure is for. It is a code minimum for the electrode, not a performance target for equipment grounding — the earth path exists for lightning and surge duty, and it is neither sized nor intended to clear a fault on a branch circuit. That job belongs entirely to the other wire.
The equipment grounding conductor: the fault-clearing path
The EGC runs with the circuit conductors and bonds every metal enclosure along the way — boxes, raceways, appliance frames — into one continuous metallic path back to the source. When a hot conductor touches any of that metal, the EGC carries the fault current home so the overcurrent device trips fast. Because its whole purpose is to survive long enough to operate a specific breaker or fuse, Table 250.122 sizes it from the rating of that device, not from the circuit conductors and not from anything about the earth: a 20 A circuit gets a 12 AWG copper EGC, a 100 A feeder gets 8 AWG copper, a 200 A feeder gets 6 AWG copper.
The table's ampere column is a ceiling — the code reads it as "not exceeding" — so a rating between rows rounds up to the next row. A 50 A breaker has no row of its own and uses the 60 A row: 10 AWG copper or 8 AWG aluminum. A 45 A device lands in the same place, a 90 A device uses the 100 A row, and a 175 A device the 200 A row. Reading the printed row below your rating is the quiet, wrong shortcut this rule exists to prevent.
When the EGC must grow: the 250.122(B) upsize
There is one situation where the circuit conductors do drag the EGC with them. When the ungrounded conductors are made larger than ampacity requires — the classic reason is voltage drop on a long run — NEC 250.122(B) requires the EGC to grow by the same circular-mil ratio, using the conductor areas of Chapter 9 Table 8. The logic is impedance: a long run has a long fault loop, and if the phase conductors grow while the EGC stays at its table minimum, the loop impedance can stay high enough to slow the breaker when it matters most.
Worked with real numbers: a 40 A branch circuit needs 8 AWG copper for the load but gets 6 AWG installed for voltage drop. The area ratio is 26,240 ÷ 16,510 = 1.589. Table 250.122 gives a 10 AWG copper EGC (10,380 cmil) at 40 A, so the upsized EGC must have at least 10,380 × 1.589 = 16,498 cmil — and the next standard size up is 8 AWG copper at 16,510 cmil. The grounding conductor size calculator reads both tables in both metals and runs exactly this 250.122(B) arithmetic, including the parallel-set equivalent-area treatment on the Table 250.66 side.
A 200 A service and one 50 A branch circuit, both wires sized
Now the running example, both wires at once. The service: 200 A, wired with 2/0 or 3/0 copper service-entrance conductors — the usual residential pairing — which lands in Table 250.66's third row and returns a 4 AWG copper (or 2 AWG aluminum) grounding electrode conductor. If that GEC runs only to ground rods, the 250.66(A) relief caps it at 6 AWG copper regardless. Out at the rods, the earth check runs separately: a single 3 m rod in 100 ohm-meter soil estimates near 33.5 ohms, over the 25-ohm line of 250.53(A)(2), so a second rod goes in — and a four-rod array at 3 m spacing lands near 12 ohms with margin to spare.
The branch circuit: a 50 A breaker feeding a subpanel or range circuit. Table 250.122 has no 50 A row, so the 60 A ceiling row applies — a 10 AWG copper or 8 AWG aluminum equipment grounding conductor, run with the circuit conductors and bonded to every enclosure on the way. Notice what never happened: the 50 A circuit's ground wire took no interest in the ground rods, the soil, or the service size, and the earth connection took no interest in any breaker. Two wires, two variables, two tables — and neither answer could have come from the other one's page.
The two tables in one pass
Recap the example build. Earth side: 200 A service, 2/0–3/0 copper service conductors → Table 250.66 → 4 AWG copper GEC, relieved to 6 AWG copper where it serves rods alone; a single rod at about 33.5 ohms fails the 25-ohm test of 250.53(A)(2), and four rods at about 12 ohms pass. Fault-clearing side: 50 A breaker → Table 250.122's 60 A ceiling row → 10 AWG copper EGC, growing by circular-mil ratio under 250.122(B) whenever the circuit conductors are upsized for voltage drop. Grounding connects the system to earth for the phenomena that involve the earth; bonding ties the metal together so the breaker can do its job. Keep the two names straight and the two tables stop looking interchangeable.