Looks up the breaker or fuse in Table 250.122 (round up to the next row). In parallel runs, every raceway or cable gets its own full-size EGC sized off that same breaker — 250.122(F). If the hots were upsized for voltage drop, the EGC goes up by the same proportion of circular-mil area — 250.122(B). The EGC never has to be larger than the circuit’s own hots (250.122(A)).

All three are sized from the largest ungrounded service conductor (for parallel sets, the total circular-mil area). GEC: Table 250.66, but the run to a rod/pipe/plate never needs to be bigger than #6 Cu / #4 Al (250.66(A)), to a concrete-encased electrode never bigger than #4 Cu (250.66(B)), to a ground ring never bigger than the ring conductor (250.66(C)). Main bonding jumper (250.28(D)) and supply-side bonding jumper (250.102(C)) use Table 250.102(C)(1); above 1100 kcmil Cu / 1750 kcmil Al they must be at least 12.5% of the service conductor area. With parallel sets in separate raceways, each raceway gets its own supply-side bonding jumper sized from the conductors in that raceway (250.102(C)(2)).

The main bonding jumper (MBJ) connects the service-neutral to the equipment ground at the service disconnect. Per NEC 250.28(D), MBJ sizing follows the same table as the GEC and is based on the largest ungrounded service conductor. For services over 1100 kcmil copper or 1750 kcmil aluminum, size per 12.5% of the largest phase area.

Supply-side bonding jumpers live between the service neutral and enclosures ahead of the service disconnect (like meter cans and CT cabinets). Per NEC 250.102(C), they're sized from the largest ungrounded supply-side conductor using the GEC table, then upsized proportionally for parallel runs based on circular-mil area.

System bonding jumper (X0 to the enclosure) per 250.30(A)(1) using Table 250.102(C)(1). The SDS grounding electrode conductor per 250.30(A)(5) using Table 250.66. Both are based on the largest derived ungrounded conductor (total area for parallel sets). Electrode-type limits from 250.66(A)–(C) still apply to the GEC.

The 25-ohm rule: if a single rod electrode reads over 25 ohms to ground, NEC 250.53(A)(2) requires a supplemental electrode. Most installations default to two rods regardless of test results, which satisfies the rule without measuring. Rods must be at least 6 feet apart (8 ft is typical). This tool compares your measured resistance and rod count against the rule.