Main Bonding Jumper Size and Where the Neutral Bond Goes (NEC 250.28)

This question is from JCR's 500+ question mock exam. 47% of students got this wrong. Try it with your 2020 NEC open, then check your answer below.

The rule

• Where the bond goes: at a service, the grounded conductor (neutral) is bonded to the equipment grounding conductors and the service-disconnect enclosure by an unspliced main bonding jumper inside the enclosure, not on the load side.
• Size: use Table 250.102(C)(1), based on the largest ungrounded service conductor.
• Large services: above 1100 kcmil copper (1750 kcmil aluminum), the jumper must be at least 12½% of the area of that conductor, or of the parallel set.

Where the bond goes

• At the service. The grounding electrode conductor connects to the grounded service conductor at any accessible point from the load end of the overhead service conductors, service drop, underground service conductors, or service lateral, up to and including the terminal or bus where the grounded service conductor lands at the service disconnecting means (250.24(A)(1)).
• Not downstream. On the load side of the service disconnect, the grounded conductor may not be connected to normally non-current-carrying metal parts or to equipment grounding conductors, or be reconnected to ground, unless Article 250 permits it (250.24(A)(5)). The Informational Note points to 250.30 (separately derived systems), 250.32 (separate buildings or structures) and 250.142.
• Main bonding jumper required. An unspliced main bonding jumper connects the equipment grounding conductor(s) and the service-disconnect enclosure to the grounded conductor within the enclosure for each service disconnect (250.24(B)).
• Bring the neutral to the service. Where an ac system of 1000 V or less is grounded at any point, the grounded conductor must be run with the ungrounded conductors to each service disconnecting means and connected there (250.24(C)). Size it from Table 250.102(C)(1). In parallel in two or more raceways or cables, it can't be smaller than 1/0 AWG (250.24(C)(2)).

The jumper can be copper, aluminum, copper-clad aluminum or another corrosion-resistant material, and it can be a wire, bus, screw or similar conductor (250.28(A)). A screw used alone as the main bonding jumper must have a green finish that is visible with the screw installed (250.28(B)).

How to size it

Find the largest ungrounded service conductor, then read Table 250.102(C)(1) (250.28(D)(1)). The two copper rows the examples below use:

For every other size, and for aluminum, read Table 250.102(C)(1) in your code book.

For parallel conductors, use the combined area of the conductors in one phase. The table's first column and Note 1 both work from this equivalent area.

Note 1 says the jumper's area can't be less than 12½%, so if the result lands between sizes, go to the next larger size. It also never has to be larger than the largest ungrounded conductor or set.

Worked examples

• 3/0 copper service conductors: main bonding jumper = 4 AWG copper.
• 4 sets of 500 kcmil copper in parallel: 4 × 500 = 2,000 kcmil. Over 1100, so 2,000 × 12½% = 250 kcmil copper.
• 8 sets of 500 kcmil copper in parallel (the exam question above): 8 × 500 = 4,000 kcmil. 4,000 × 12½% = 500 kcmil copper.

Aluminum service conductors

Table 250.102(C)(1) has a separate column for aluminum or copper-clad aluminum. With 4/0 aluminum service conductors, the minimum main bonding jumper is 2 AWG aluminum or 4 AWG copper. With 500 kcmil aluminum, it's 1/0 aluminum or 2 AWG copper.

Above 1750 kcmil aluminum, Note 1's 12½% rule applies to an aluminum jumper. For a copper jumper on aluminum conductors, use Note 2: take 12½% of the copper conductors that would have the same ampacity.

Check yourself

For a 120/240 V single-phase dwelling service, is either of these allowed?

1. The service conductors don't include a grounded conductor.
2. The grounding connection is made on the load side of the service disconnect.

Want more practice? Try JCR's free 25-question practice exam.

Common questions

Where is the neutral bonded to ground?

At the service. The main bonding jumper inside each service disconnect enclosure bonds the neutral to the enclosure and the equipment grounding conductors (250.24(B)). The grounding electrode conductor can connect to the neutral anywhere from the load end of the service drop or lateral up to the service disconnect (250.24(A)(1)). On the load side of the service disconnect, the neutral stays separate from the equipment grounding conductors, except where Article 250 allows otherwise.

What size main bonding jumper for 2/0 copper service conductors?

4 AWG copper. Table 250.102(C)(1) puts 2/0 and 3/0 copper in the same row.

Can a screw be the main bonding jumper?

Yes. A screw used by itself as the main bonding jumper must have a green finish that you can see with the screw installed (250.28(B)).

Main bonding jumper vs. system bonding jumper: what's the difference?

Both connect the neutral (grounded conductor) to the equipment grounding conductor. The main bonding jumper does it at the service (250.24(B)). The system bonding jumper does it at a separately derived system, a power source other than the service (250.30(A)(1)). Both follow 250.28 for material, the green screw and sizing.

Written to the 2020 NEC. To follow along, pick up the 2020 NEC or open it online through a paid NFPA LiNK subscription. Our state requirements page shows the edition each state tests.

JCR checked this against the 2020 NEC in NFPA LiNK on October 21, 2026.

Related guides: Dwelling service load calculation, step by step · The 5 most-missed questions on JCR's free practice exam

Free JCR grounding videos

• Bonding and grounding explained (Article 250)
• Neutral vs. grounded conductor: what it actually does
• How to size the equipment grounding conductor (250.122)
• See more on JCR's YouTube channel

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