Bonding Jumper & Grounded Conductor Size Chart
One table sizes four conductors: the main bonding jumper, the system bonding jumper, the supply-side bonding jumper, and the grounded (neutral) conductor of the service or separately derived system. All key off the largest ungrounded supply conductor — find it below and read across. Values per NEC Table 250.102(C)(1), including Note 1's 12.5% rule for the largest services.
Minimum size by supply-conductor size
| Supply — copper | Supply — aluminum/CCA | Jumper — copper | Jumper — aluminum/CCA |
|---|---|---|---|
| ≤ 2 | ≤ 1/0 | 8 AWG | 6 AWG |
| 1 to 1/0 | 2/0 to 3/0 | 6 AWG | 4 AWG |
| 2/0 to 3/0 | 4/0 to 250 | 4 AWG | 2 AWG |
| 4/0 to 350 | 300 to 500 | 2 AWG | 1/0 AWG |
| 400 to 600 | 600 to 900 | 1/0 AWG | 3/0 AWG |
| 700 to 1100 | 1000 to 1750 | 2/0 AWG | 4/0 AWG |
| over 1100 | over 1750 | 12.5% of phase area | 12.5% of phase area |
What these four conductors are
The main bonding jumper ties the grounded conductor to the enclosure at the service disconnect; the system bonding jumper does the same job at a separately derived system like a transformer secondary; the supply-side bonding jumper bonds equipment on the line side of the service disconnect, where there is no overcurrent device to size from; and the grounded (neutral) conductor of the service must meet this same minimum even when the load calculation would allow smaller. They share a table because they share a job: every one of them is in the fault-current path back to the utility transformer, so they scale with the supply conductors that determine how much fault current can flow.
Why it splits from Table 250.66 on big services
Through 1100 kcmil copper (1750 kcmil aluminum) this table is row-for-row identical to the grounding electrode conductor chart. Above that they part ways: the GEC tops out at 3/0 copper no matter how large the service grows, because the earth connection is not a fault-clearing path. Bonding jumpers and the grounded conductor are — so Note 1 keeps them growing at 12.5% of the phase-conductor area. On a 4,000 A service with twelve 600 kcmil copper conductors per phase (7,200 kcmil equivalent), that is a 900 kcmil minimum, nowhere near anything on the printed table. Don't borrow between the tables in either direction, and as always, verify against the code edition your jurisdiction enforces.
Common questions
What size main bonding jumper for a 200 amp service?
With the typical 2/0 copper service-entrance conductors, Table 250.102(C)(1) puts the minimum main bonding jumper at 4 AWG copper (or 2 AWG aluminum) — the same row a 4/0 aluminum service lands on. In practice the factory bonding screw or strap supplied with a listed panel satisfies this; the table matters when you build the jumper yourself, on the supply side, or for a separately derived system.
How does the 12.5% rule work?
When the largest ungrounded supply conductor (or the summed area of a parallel set) exceeds 1100 kcmil copper or 1750 kcmil aluminum, the table hands off to Note 1: the jumper or grounded conductor must have at least 12.5% of the phase-conductor area. Example: four parallel 500 kcmil copper per phase is 2,000 kcmil equivalent, and 2,000 × 0.125 = 250 — so the minimum is 250 kcmil copper.
Is this the same as Table 250.66?
Row for row it is identical through 1100 kcmil copper / 1750 kcmil aluminum — then the tables split. The grounding electrode conductor caps at 3/0 copper forever, because a GEC is not expected to clear faults. The conductors in this table ARE the fault-clearing path, so above 1100 kcmil they keep growing at 12.5% of the phase area with no ceiling.
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