NEC 250.90-250.102 Explained: Bonding on the Line & Load Side

Pull the cover off a 400-amp meter main and look at where the service raceway lands. Nine times out of ten there is a locknut inside, a locknut outside, and a set of concentric rings still sitting in the sheet metal around the hole. The conduit is mechanically solid. Electrically, on the line side of the disconnect, it is not bonded — and that is a violation of NEC 250.92(B).

This catches experienced electricians because the same detail is perfectly legal six inches downstream. Part V of Article 250 splits at the service disconnect: everything on the line side gets one set of rules and one sizing table, and everything on the load side gets a different set and a different table. Use the wrong one and the jumper is either undersized or missing entirely.

This guide walks NEC 250.90 through 250.102 section by section — what each one requires, which of the four bonding jumpers you are sizing, which table to open, and the specific details that fail inspections.

What NEC 250.90 actually requires

Section 250.90 is two lines long and it is the reason every other section in Part V exists: bonding shall be provided where necessary to ensure electrical continuity and the capacity to conduct safely any fault current likely to be imposed on it.

Read the second half of that sentence carefully. It is a performance requirement, not a connection requirement. Two pieces of metal touching are electrically continuous. They are not necessarily capable of carrying 20,000 amps for the few cycles it takes an upstream device to open. Everything from 250.92 to 250.102 is the Code spelling out where ordinary hardware fails that test and what to install instead. If you want the wider context first, our overview of NEC Article 250 grounding and bonding maps all ten parts of the article.

Line side and load side — what the words mean here

Everything in Part V hinges on which side of the service disconnecting means you are standing on.

• Line side. Upstream of the service disconnect — the service raceway, the meter enclosure, the wireway, the disconnect enclosure itself. There is no overcurrent device between this metal and the utility transformer. A ground fault here is cleared by the utility’s primary protection, or by the conductor burning open.
• Load side. Downstream of the service disconnect, and downstream of any separately derived system. A ground fault here is cleared by a breaker or fuse whose rating you know, which is why the load-side jumper can be sized from that rating.

That asymmetry explains the whole part. On the load side the Code knows the maximum current the fault path will see. On the line side it does not, so it sizes the bonding path off the supply conductors instead — the only thing that limits the fault.

The four bonding jumpers and where each one lives

Article 250 defines four separate jumpers. They are not interchangeable names for the same conductor, and three of the four share a sizing table:

The equipment bonding jumper is the odd one out, and it is the one people reach for by habit. If you are bonding a raceway on the line side of the service disconnect and you sized the jumper off the breaker, you sized it from the wrong table. Our dedicated guide to bonding jumper types and sizing works through each one with numbers.

NEC 250.92 — bonding at the service

250.92(A) lists what must be bonded: service raceways, cable trays, cablebus frameworks, auxiliary gutters, service equipment enclosures, metering enclosures, and any enclosure or raceway containing a grounding electrode conductor. If service conductors pass through it and it is metal, it is on the list.

250.92(B) is the part that gets cited on correction notices. Electrical continuity at those enclosures must be ensured by one of four methods:

1. Bonding the equipment to the grounded service conductor in a manner provided in 250.8
2. Connections using threaded couplings or threaded hubs on enclosures, made up wrenchtight
3. Threadless couplings and connectors made up tight, for metal raceways and metal-clad cables
4. Other listed devices — bonding-type locknuts, bonding bushings, or bushings with bonding jumpers

And then the sentence that decides most inspections: bonding jumpers meeting the requirements of Article 250 shall be used around impaired connections, such as reducing washers or oversized, concentric or eccentric knockouts. Standard locknuts or bushings are permitted to make the mechanical connection, but they shall not be the only means of bonding.

The same logic covers reducing washers. A 2-inch knockout filled with a reducing washer to take a 1-inch connector has two extra sheet-metal interfaces in the fault path, and the Code does not accept them on the line side. Bond around it. If you are choosing hardware for this, our comparison of grounding bushings versus bonding bushings covers what the listing on each one actually buys you.

NEC 250.94 through 250.100 — the four sections people skip

Between the service rules and the sizing rules sit four short sections. None of them is long, and each one covers a situation that is easy to miss on a walkthrough.

250.94 — intersystem bonding termination

Every service needs an accessible means for the telecom, CATV and satellite people to bond their systems to yours, so nobody drives their own rod in the back yard and creates a difference in potential. The device must be accessible, must provide terminations for at least three communications systems, and must be mounted at the service or at the meter. Our write-up on the intersystem bonding termination covers the mounting and listing requirements in full.

250.96 — bonding other enclosures

This is the general load-side counterpart to 250.92. Metal raceways, cable armour, boxes and enclosures that serve as an equipment grounding conductor must be bonded together to form an effective ground-fault current path. The clause that gets missed: nonconductive coatings such as paint, lacquer and enamel must be removed at threads, contact points and contact surfaces, or the connection must be made with fittings designed so that removal is unnecessary. Painted strut, painted enclosures and powder-coated gear all fall under this.

250.96(B) is the exception that permits an isolated grounding circuit: a listed nonmetallic raceway fitting may interrupt the metal raceway to reduce electromagnetic interference, provided the equipment grounding conductor inside is continuous. Sensitive electronic equipment is the usual reason.

250.97 — bonding for over 250 volts to ground

This is the section that pulls 277/480 V work into service-grade bonding. For circuits over 250 volts to ground, where ringed knockouts are present, the raceway or cable must be bonded using one of the methods in 250.92(B)(2) through (B)(4) — threaded hubs, threadless couplings, or listed bonding devices. A plain double-locknut connection into a ringed knockout does not qualify.

The exception relieves you where the box or enclosure is listed to provide a reliable bonding connection despite the rings — outlet boxes evaluated for that purpose — or where the knockout is punched to leave a smooth hole with no rings remaining. Cabinets, pull boxes and junction boxes generally are not listed this way, so on 480 V feeders the safe assumption is that you need a bonding device.

250.98 and 250.100 — loose joints and classified locations

250.98 covers loosely jointed metal raceways: expansion fittings and telescoping sections must be made electrically continuous with a bonding jumper or an equivalent means. An expansion coupling on a rooftop run is designed to slide, which means it is designed not to be a reliable electrical connection. Jumper it.

250.100 is the strictest section in Part V. In any hazardous (classified) location, regardless of the voltage, continuity must be ensured by one of the methods in 250.92(B)(2) through (B)(4). Standard locknuts and bushings may hold the raceway on, but the bonding has to come from a threaded hub, a threadless coupling made up tight, or a listed bonding device. There is no 250-volt threshold and no exception for clean knockouts.

NEC 250.102 — sizing the bonding jumper

Section 250.102 handles material, attachment, size and installation. (A) requires copper or another corrosion-resistant material. (B) sends you to 250.8 and 250.70 for how the connection is made — listed pressure connectors, exothermic welding, listed lugs, and so on. Sheet-metal screws are not on that list.

250.102(C) is the line-side size. A supply-side bonding jumper of the wire type is sized from Table 250.102(C)(1), using the largest ungrounded supply conductor in the raceway:

Largest ungrounded conductor — copper	Largest ungrounded conductor — aluminium	Jumper, copper	Jumper, aluminium
2 AWG or smaller	1/0 AWG or smaller	8 AWG	6 AWG
1 or 1/0 AWG	2/0 or 3/0 AWG	6 AWG	4 AWG
2/0 or 3/0 AWG	4/0 AWG or 250 kcmil	4 AWG	2 AWG
Over 3/0 through 350 kcmil	Over 250 through 500 kcmil	2 AWG	1/0 AWG
Over 350 through 600 kcmil	Over 500 through 900 kcmil	1/0 AWG	3/0 AWG
Over 600 through 1100 kcmil	Over 900 through 1750 kcmil	2/0 AWG	4/0 AWG
Over 1100 kcmil	Over 1750 kcmil	12.5% of phase area	12.5% of phase area

250.102(D) is the load-side size, and it points at Table 250.122 instead — the same table you use for an equipment grounding conductor, based on the rating of the overcurrent device ahead of the equipment. Where a single jumper bonds two or more raceways, size it from the largest overcurrent device supplying circuits in them. The jumper is never required to be larger than the circuit conductors it is protecting.

OCPD rating	Copper	Aluminium	OCPD rating	Copper	Aluminium
15 A	14 AWG	12 AWG	600 A	1 AWG	2/0 AWG
20 A	12 AWG	10 AWG	800 A	1/0 AWG	3/0 AWG
60 A	10 AWG	8 AWG	1000 A	2/0 AWG	4/0 AWG
100 A	8 AWG	6 AWG	1200 A	3/0 AWG	250 kcmil
200 A	6 AWG	4 AWG	1600 A	4/0 AWG	350 kcmil
300 A	4 AWG	2 AWG	2000 A	250 kcmil	400 kcmil
400 A	3 AWG	1 AWG	2500 A	350 kcmil	600 kcmil
500 A	2 AWG	1/0 AWG	3000 A	400 kcmil	600 kcmil

The four-step method

Step 1 — find the service disconnect. Stand at the enclosure you are bonding and ask whether there is an overcurrent device between it and the utility. If there is not, you are on the line side and everything below changes.

Step 2 — pick your table. Line side: Table 250.102(C)(1), and the input is the largest ungrounded supply conductor. Load side: Table 250.122, and the input is the overcurrent device rating. Never mix them.

Step 3 — read the row. For a 200-amp residential service run in 4/0 aluminium SER or 2/0 copper, Table 250.102(C)(1) gives a 4 AWG copper jumper. For a 200-amp feeder on the load side, Table 250.122 gives a 6 AWG copper jumper. Same amperage, different answer — that is the whole point of the split.

Step 4 — check the material. Note 2 to Table 250.102(C)(1) handles mixed metals: if the supply conductors are aluminium and the jumper is copper, size the jumper from an aluminium conductor of equivalent ampacity rather than reading the aluminium column straight across. Our guide to bare copper ground wire sizing covers the material and installation rules that follow.

Where a bonding jumper is mandatory — and where it is not

Part V does not require a jumper at every raceway entry. It requires one wherever the ordinary hardware cannot be trusted to carry fault current.

A jumper or a listed bonding device is required

• Concentric or eccentric knockouts at any service enclosure — 250.92(B)
• Oversized knockouts and reducing washers on the line side — 250.92(B)
• Ringed knockouts on circuits over 250 volts to ground — 250.97
• Any raceway or enclosure in a hazardous (classified) location, at any voltage — 250.100
• Expansion fittings and telescoping raceway sections — 250.98
• Enclosures and raceways containing the grounding electrode conductor — 250.92(A)

Standard fittings alone are acceptable

• Load-side circuits at or below 250 volts to ground, into clean punched knockouts
• Threaded hubs and threaded couplings made up wrenchtight
• Threadless couplings and connectors made up tight on metal raceway and metal-clad cable
• Service enclosures where every concentric and eccentric ring has been removed and the locknut bears on solid metal
• Outlet boxes specifically listed to provide reliable bonding at ringed knockouts — the 250.97 exception

Installing the jumper — 250.102(E)

Inside a raceway, the jumper follows 250.120 and 250.148 like any other equipment grounding conductor. Outside a raceway is where the restriction bites: under 250.102(E)(2), a jumper installed outside the raceway or enclosure must not exceed 6 feet (1.8 m) in length and must be routed with the raceway. A jumper looping across the back of a gear lineup to a convenient lug is a violation even if the wire size is right.

The exception covers outdoor pole locations: jumpers installed for bonding isolated sections of metal raceway or elbows in exposed risers are not limited in length and need not be routed with the raceway.

Table 250.102(C)(1) vs Table 250.122 vs Table 250.66

Three tables in Article 250 take a conductor size or a breaker size and hand you back a wire size, and mixing them up is the single most common Article 250 error on exams and on the job:

If you are working the electrode side of the system rather than the bonding side, start with Table 250.66 and grounding electrode conductor sizing and our overview of the eight grounding electrodes in NEC 250.52. For the load-side conductor itself, see equipment grounding conductors, and for transformer secondaries, grounding and bonding separately derived systems.

Bonding bushing or bonding locknut — which to reach for

Both satisfy 250.92(B)(4). The choice is practical, not legal, and it comes down to whether you need a lug.

A bonding-type locknut is faster and cheaper where the knockout is the only problem and you do not need to run a conductor anywhere. Where you need a wire — a supply-side bonding jumper back to the neutral bar, or a jumper around a reducing washer — the bushing is the tool.

Working with Part V in the field

Frequently asked questions

What does NEC 250.90 require?

That bonding be provided wherever it is necessary to ensure electrical continuity and the capacity to safely conduct any fault current likely to be imposed. It is a performance rule — contact alone is not enough if the connection cannot carry the available fault current.

Is a bonding jumper required at every service knockout?

No. It is required around impaired connections — concentric knockouts, eccentric knockouts, oversized knockouts and reducing washers. If the knockout is clean and the locknut bears on solid metal, the double-locknut connection is one of the permitted methods under 250.92(B).

What size supply-side bonding jumper do I need for a 200-amp service?

For 2/0 copper or 4/0 aluminium supply conductors, Table 250.102(C)(1) gives 4 AWG copper or 2 AWG aluminium. Size from the conductors in the raceway, not from the 200-amp rating.

What is the difference between 250.102(C) and 250.102(D)?

(C) covers supply-side bonding jumpers on the line side of the service disconnect and sizes them from Table 250.102(C)(1) using the supply conductors. (D) covers equipment bonding jumpers on the load side and sizes them from Table 250.122 using the overcurrent device rating.

Can a bonding jumper be installed outside the conduit?

Yes, but under 250.102(E)(2) it is limited to 6 feet in length and must be routed with the raceway. The exception for bonding isolated raceway sections and elbows at outdoor pole risers has no length limit and does not have to follow the raceway.

Do 480-volt circuits need bonding bushings?

Where ringed knockouts are present, yes — NEC 250.97 requires one of the methods in 250.92(B)(2) through (B)(4) for circuits over 250 volts to ground. A 277/480 V system is over 250 volts to ground, so a plain double-locknut connection into a ringed knockout does not satisfy it.

How do I size a bonding jumper for parallel service raceways?

Under 250.102(C)(2), a jumper in each individual raceway is sized from Table 250.102(C)(1) based on the ungrounded conductors in that raceway. A single jumper bonding all the raceways together is sized from the sum of the circular mil areas of every ungrounded supply conductor.

Does bonding in a hazardous location depend on voltage?

No. NEC 250.100 applies regardless of the voltage of the system. Every raceway and enclosure in a classified location must be bonded by one of the methods in 250.92(B)(2) through (B)(4), and standard locknuts and bushings cannot be the bonding means.

The bottom line

Part V of Article 250 earns its length because a bonding path that merely exists is not the same as one that works. NEC 250.90 sets that bar, 250.92 through 250.100 mark out the specific hardware the Code refuses to trust, and 250.102 tells you how big the conductor has to be and where you are allowed to run it.

Two restraints do most of the work in the field. First, decide which side of the service disconnect you are on before you open a table, because Table 250.102(C)(1) and Table 250.122 will both give you a plausible answer and only one of them is right. Second, stop treating a tight locknut as a bond at a ringed knockout — on the line side, on 480 volts, or in a classified location, it is not one. For the wider picture of how the electrode system, the grounded conductor and the bonding path fit together, start with our NEC Article 250 overview, and check what changed in the 2026 NEC before you quote a section number on a plan review.