Bonding Jumper Size Calculator

NEC Table 250.102(C)(1) bonding jumper

Values from NEC 2020 Table 250.102(C)(1) (identical in the 2017 and 2023 editions). Sizes are AWG unless marked kcmil. The first six rows match Table 250.66; the difference is the top row, where the bonding jumper follows the 12.5% rule instead of a fixed size.

The 12.5% rule above 1100 kcmil

Table 250.102(C)(1) stops listing discrete sizes once the ungrounded supply conductors pass 1100 kcmil copper or 1750 kcmil aluminum. Above that, Note 1 takes over: the grounded conductor or bonding jumper must have an area not less than 12.5 percent of the area of the largest set of ungrounded supply conductors. Take that area, multiply by 0.125, and round up to the next standard conductor size. A 2000 kcmil copper phase gives 0.125 x 2,000,000 = 250,000 cmil, so a 250 kcmil copper jumper. Two 600 kcmil copper conductors per phase equal 1,200,000 cmil, and 12.5 percent of that is 150,000 cmil, which rounds up to a 3/0 copper jumper.

This is exactly where Table 250.102(C)(1) parts ways with Table 250.66. The grounding electrode conductor caps at a fixed 3/0 copper (250 kcmil aluminum) no matter how large the service gets, because it feeds the earth electrode. The bonding jumper keeps scaling with the phase conductors because it has to carry fault current back to the source.

One table, four conductors

Table 250.102(C)(1) sizes four different conductors with the same numbers: the grounded (neutral) service conductor where it doubles as the fault-current path (250.24(C)), the main bonding jumper that ties the grounded conductor to the enclosure at the service (250.28), the system bonding jumper at a separately derived system (250.30(A)(1)), and the supply-side bonding jumper on the line side of the service (250.30(A)(2), 250.102(C)). Pick the one you are sizing from the dropdown; the calculator cites the governing section but the required size is identical for all four.

Parallel supply conductors

When the supply conductors are run in parallel, Table 250.102(C)(1) is read on the combined circular-mil area of the paralleled conductors in a single phase, not on one conductor. Enter the number of conductors per phase and the tool multiplies the area for you: two 250 kcmil copper conductors per phase equal 500,000 cmil, which lands in the over-350-through-600 kcmil row and calls for a 1/0 copper bonding jumper.

Bonding jumper vs. the grounding conductors

Three of Article 250's conductor tables are easy to confuse. The bonding jumper (this tool, Table 250.102(C)(1)) is sized on the supply-conductor area and bonds the fault-current path. The grounding electrode conductor (Table 250.66) is also sized on the supply-conductor area but ties the grounded service to the earth electrode, and it caps out at 3/0 copper. The equipment grounding conductor (Table 250.122) is sized on the overcurrent-device rating instead. On a 200 A service with 3/0 copper conductors, the main bonding jumper and the grounding electrode conductor are both 4 AWG copper, but the equipment grounding conductor is only 6 AWG copper. Size the other two with the Grounding Electrode Conductor Size Calculator and the Equipment Grounding Conductor Size Calculator.

Frequently asked questions

What size main bonding jumper for a 100 amp service?

A 100 A service is commonly 4 AWG copper or 2 AWG aluminum ungrounded conductors, which Table 250.102(C)(1) sizes with an 8 AWG copper (or 6 AWG aluminum) main bonding jumper. Anything 2 AWG copper or smaller lands in the same top table row.

Is the supply-side bonding jumper sized the same as the main bonding jumper?

For a wire-type jumper, yes: 250.102(C) sends both to Table 250.102(C)(1), sized on the largest ungrounded supply conductor. A supply-side bonding jumper of the busbar or raceway type is a separate construction rule, but the wire-type minimum is the same figure this calculator returns.

Can the bonding jumper be aluminum when the phase conductors are copper?

Yes. For the six tabulated rows, read the aluminum jumper column of the same row the copper conductor lands in; the table's paired columns are already ampacity-equivalent, which is what Note 2 requires. Above 1100 kcmil copper the different-material case needs the Note 2 ampacity conversion by hand, so this tool asks you to keep the jumper the same material as the supply there.

Why does the bonding jumper keep growing when the grounding electrode conductor stops at 3/0?

Because they carry current for different reasons. The bonding jumper is part of the effective ground-fault current path and must be able to carry fault current proportional to the phase conductors, so 250.102(C)(1) uses the 12.5% rule above 1100 kcmil. The grounding electrode conductor only has to reach the earth electrode, so 250.66 caps it at 3/0 copper.

Related calculators

• Grounding Electrode Conductor Size Calculator: the earth-electrode conductor from the service conductor (Table 250.66).
• Equipment Grounding Conductor Size Calculator: the equipment ground from the breaker rating (Table 250.122).
• Service Size Calculator: size the service disconnect, conductor, and grounding electrode conductor from a known load.
• Wire Size Calculator: size the ungrounded service or feeder conductors themselves.