The ground wire in a branch circuit or feeder is the equipment grounding conductor, and NEC Table 250.122 sizes it by the rating of the breaker or fuse ahead of the circuit, not by the size of the hot wires. Each row covers every rating up to its own, so you read the first row at or above your breaker. So the sizes come in steps: a 15 A circuit takes a 14 AWG copper ground and a 20 A circuit 12 AWG, but every breaker from 25 to 60 A takes 10 AWG, 70 to 100 A takes 8 AWG and 110 to 200 A takes 6 AWG. The aluminum column, which also covers copper-clad aluminum, is the next size up at each step, and its footnote points to 250.120 for where aluminum grounds may be run.
The table is a minimum. A note to it says the ground has to be larger where that is needed for an effective ground-fault path, and 250.122(A) says it never has to be larger than the circuit conductors supplying the equipment.
Factory cable already follows the table. Southwire's and Cerrowire's NM-B specs give 14/2 a 14 AWG ground, 12/2 a 12 AWG ground and 10/2 a 10 AWG ground. 8/2 and 6/2 carry a 10 AWG ground too, which is enough: 8/2 is rated 40 A and 6/2 is rated 55 A at 60 °C, both inside the 25 to 60 A row. The 4/3 and 2/3 cables carry 8 AWG, the 70 to 100 A size.
Upsizing the hot wires changes the ground. When the ungrounded conductors are made larger than the minimum for any reason other than the ampacity adjustment and correction of 310.15(B) and (C), such as voltage drop on a long run, a wire-type ground has to grow in proportion to their circular-mil area. A 20 A circuit needs 12 AWG copper. Run it in 10 AWG and the hots grow from 6,530 to 10,380 circular mils, so the 12 AWG ground grows by the same ratio to 10,380: a 10 AWG ground, which is what 10/2 NM-B has.
The calculator uses the exact area ratio, with no rounding. On a 60 A circuit that needs 6 AWG copper but is run in 4 AWG, the ratio is 41,740 ÷ 26,240, or 1.5907. Applied to the 10 AWG ground's 10,380 circular mils, it gives about 16,511, just over the 16,510 of 8 AWG, so the result is 6 AWG. Rounding the ratio to 1.59 would give 16,504 and an 8 AWG ground. The calculator shows the exact, conservative size. An exception lets a qualified person size the ground to give an effective ground-fault path instead, and whether it applies is for the electrician or inspector to decide.
The grounding electrode conductor is a different wire. It runs from the service, from a building supplied by a feeder or from a separately derived system to the grounding electrodes, such as ground rods or a concrete-encased electrode. Table 250.66 sizes it by the largest ungrounded service-entrance conductor, with parallel sets added together, not by the main breaker. 250.66 then sets caps, not minimums: the part that runs only to rods, pipes or plates need not be larger than 6 AWG copper or 4 AWG aluminum or copper-clad aluminum, the part that runs only to a concrete-encased electrode need not be larger than 4 AWG copper, and the part to a ground ring need not be larger than the ring conductor.
Some nearby conductors use neither table. Since the 2014 edition, the main bonding jumper, the system bonding jumper, supply-side bonding jumpers and the grounded conductor are sized from Table 250.102(C)(1), not Table 250.66; bonding jumpers between electrodes do use 250.66. The proportional rule is 250.122(B) in the 2023 NEC and 250.122(D) in 2026, where the section was reorganized with no technical change, and both tables have the same values in both editions. The 2026 edition moves systems over 1000 V AC or 1500 V DC to Article 270, so the tables on this page are the ones for systems up to those voltages. Your local code edition and its amendments apply.