How to Size a Bonding Jumper
Updated July 24, 2026 • Written by the field team at Arizona Electrical Solutions. All field guides →
Bonding jumpers are the backbone of the fault-current path. When a phase conductor faults to a panel can or a raceway, the jumper carries that fault current back to the source so the overcurrent device opens fast. Undersize it and the jumper becomes the fuse — it burns open, the breaker never trips, and everything metal downstream sits there energized.
On commercial work you size these constantly: the main bonding jumper at every service, the system bonding jumper at every transformer secondary, supply-side jumpers at CT cabinets and meter stacks, load-side jumpers at concentric knockouts and flex.
The whole trick is knowing which side of the first overcurrent device you're on. Supply side has no OCPD ahead of it, so the jumper is sized from the ungrounded conductors — Table 250.102(C)(1). Load side has a breaker or fuse ahead of it, so it's sized from that OCPD rating — Table 250.122. Sort that correctly and the rest is a table lookup.
We do this work. Arizona Electrical Solutions self-performs commercial electrical on commercial projects across Arizona. Grounding and bonding on new services is work we self-perform and stand behind at inspection.
Safety first. This work is for qualified, licensed electricians only. Bonding jumpers land in service equipment where incident energy is at its worst — de-energize, apply lockout/tagout, and verify absence of voltage with a tested meter before touching any conductor or bus. Wear appropriate PPE selected per NFPA 70E. Service and transformer work typically requires a permit and inspection, and the locally adopted NEC edition and local amendments govern — confirm with your AHJ.
Simplified concept diagram for training and illustration — not a construction document. Equipment layouts vary; manufacturer instructions and the locally adopted code govern.
Text description of this trainer
One-line diagram of an electrical service split into two zones by a dashed vertical divider at the first overcurrent protective device. Supply side, left: utility service conductors pass through a CT cabinet and meter enclosure; a supply-side bonding jumper bonds that enclosure to the grounded conductor and is sized from the ungrounded service conductors per Table 250.102(C)(1). Center: the service disconnect enclosure contains the neutral bus and ground bus joined by the main bonding jumper per 250.28, highlighted; a grounding electrode conductor per 250.66 runs down to a grounding electrode. The service disconnect breaker is the first OCPD. Load side, right: a feeder leaves the breaker to a downstream panel; the equipment grounding conductor running with the feeder is sized from the OCPD rating per Table 250.122.
What you'll need
- Current NEC codebook or code app (2023 numbering used here)
- Bare or green-insulated copper conductor in the calculated size, or the listed bus/strap/screw
- Listed grounding and bonding lugs rated for the conductor size and material
- Irreversible compression or listed mechanical connectors per the job specs
- Calibrated torque wrench or torque screwdriver — torque values per the manufacturer
- Crimper with correct dies if using compression lugs
- Antioxidant compound for aluminum terminations, per the manufacturer
- Voltage tester rated for the system, plus LOTO kit
Step by Step
Size a Bonding Jumper — step by step
1. Identify which bonding jumper you're sizing
Four animals, two sizing paths. The main bonding jumper (MBJ) connects the grounded (neutral) conductor to the enclosure at the service disconnect — 250.24(B), sized per 250.28(D). The system bonding jumper (SBJ) does the same at a separately derived system like a transformer secondary — 250.30(A)(1). Both use Table 250.102(C)(1).
Supply-side bonding jumpers bond enclosures and raceways ahead of the service disconnect or an SDS's first OCPD — CT cabinets, meter stacks, wireways — per 250.102(C), same table. Load-side equipment bonding jumpers bond raceways and equipment downstream of an overcurrent device — 250.102(D) sends you to Table 250.122.
2. Supply side: size from the ungrounded conductors with Table 250.102(C)(1)
Find the largest ungrounded (phase) conductor, then read the jumper straight out of Table 250.102(C)(1). For copper phases: 2 AWG or smaller takes an 8 AWG copper jumper; 1 or 1/0 takes 6 AWG; 2/0 or 3/0 takes 4 AWG; over 3/0 through 350 kcmil takes 2 AWG; over 350 through 600 kcmil takes 1/0; over 600 through 1100 kcmil takes 2/0.
Example: a 400 A service fed with 500 kcmil copper per phase lands in the over-350-through-600 row, so the MBJ is 1/0 copper. If the phase conductors and jumper are different metals, use the table note: size from an assumed conductor of the jumper's material with equivalent ampacity.
3. Over 1100 kcmil: apply the 12.5% rule
When the ungrounded supply conductors are larger than 1100 kcmil copper (or 1750 kcmil aluminum), the table hands off to a calculation: the jumper must be at least 12.5% of the area of the largest ungrounded conductor — with parallel sets, 12.5% of the total circular-mil area per phase.
Example: a 3000 A service with eight sets of 500 kcmil copper per phase. 8 × 500 = 4000 kcmil; 12.5% of that is 500 kcmil, so the MBJ is 500 kcmil copper. If the math lands between standard sizes, round up, never down.
4. Parallel raceways on the supply side: one big jumper or one per raceway
250.102(C)(2) gives two legal options for paralleled supply conductors. Option one: a single jumper sized from the total circular-mil area of one phase, all sets added, using the table or the 12.5% rule. Option two: an individual jumper on each raceway, each sized from Table 250.102(C)(1) for the conductor in that raceway only.
Example: an 800 A service, two parallel sets of 500 kcmil copper in two PVC conduits. Single-jumper method: 1000 kcmil total lands in the over-600-through-1100 row — one 2/0 copper. Per-raceway method: each conduit has 500 kcmil, so each gets its own 1/0 copper. Both comply; routing and cost decide.
5. Load side: size from the OCPD with Table 250.122
Downstream of an overcurrent device, the bonding jumper sizes exactly like an equipment grounding conductor — enter Table 250.122 with the rating of the breaker or fuse ahead of the circuit. Common copper values: 20 A takes 12 AWG, 60 A takes 10 AWG, 100 A takes 8 AWG, 200 A takes 6 AWG, 400 A takes 3 AWG, 800 A takes 1/0, 1200 A takes 3/0.
Example: bonding around a flex section on a feeder behind a 200 A breaker — 6 AWG copper, no matter how big the feeder conductors are. One wrinkle from 250.122(A): the jumper never has to be larger than the ungrounded circuit conductors.
6. Parallel runs on the load side
For paralleled feeders, 250.122(F) requires that where a wire-type EGC or bonding jumper is run in each raceway, each one is sized full-size from Table 250.122 based on the feeder OCPD — you don't divide by the number of sets. A 1200 A feeder in four conduits needs a 3/0 copper EGC in every one of them.
That's the opposite instinct from paralleling phases, and a common plan-review correction. The reason: a fault in any single raceway must clear through that raceway's ground path alone.
7. Pick the form and land it right
Per 250.28(A), an MBJ or SBJ can be a wire, bus, screw, or similar suitable conductor. The factory bonding screw or strap shipped with service-rated gear is listed for that enclosure — use it, and 250.28(B) requires a bonding screw to be green and visible after installation. At transformers you'll usually run a wire-type SBJ from XO to the enclosure or ground bus.
Terminate on listed lugs and torque to the equipment label value or per the manufacturer — 110.14(D) requires a calibrated tool, not a calibrated elbow. Keep the jumper short and direct.
8. Verify before you close up
There should be exactly one neutral-to-ground bond per system — the MBJ at the service or the SBJ at the separately derived system, never extras downstream. Multiple bonds put neutral current on raceways and will nuisance-trip ground-fault protection on larger 480Y/277 V services.
Photograph the jumper, torque values, and conductor markings before covers go on. Inspectors ask, and six months from now so will you.
FAQ
Frequently asked questions
What's the difference between a main bonding jumper and a system bonding jumper?
Same job, different places. The main bonding jumper connects the neutral to the enclosure at the service disconnect; the system bonding jumper makes that connection at a separately derived system such as a transformer secondary. Both are sized from NEC Table 250.102(C)(1).
How do I know whether to use Table 250.102(C)(1) or Table 250.122?
Ask whether there's an overcurrent device ahead of the jumper. Supply side of the service disconnect means Table 250.102(C)(1), sized from the ungrounded conductors. Load side of a breaker or fuse means Table 250.122, sized from the device rating.
How does the 12.5% rule work?
When the ungrounded supply conductors exceed 1100 kcmil copper or 1750 kcmil aluminum, the jumper must be at least 12.5 percent of the circular-mil area of the largest phase, counting all parallel sets. Eight sets of 500 kcmil copper is 4000 kcmil, so the jumper is 500 kcmil copper.
Do parallel feeders need a full-size bonding jumper in every conduit?
On the load side, yes. NEC 250.122(F) requires each raceway's wire-type equipment grounding conductor or bonding jumper to be sized from Table 250.122 at the full feeder overcurrent device rating, not a divided share.
Can the green bonding screw in a panel serve as the main bonding jumper?
Yes, if it is the screw or strap listed and supplied for that enclosure. NEC 250.28 permits a screw as the main bonding jumper and requires a green finish visible after installation. Be precise about where it belongs: at the service disconnect, and never in an ordinary subpanel fed by a 4-wire feeder. It is not true that a bond never happens downstream of the service - a separately derived system such as a transformer secondary gets its own system bonding jumper, which is a different animal from the subpanel case and is required by 250.30(A)(1).
Does a bonding jumper ever need to be bigger than the phase conductors?
No. NEC 250.122(A) says a load-side bonding jumper is never required to be larger than the ungrounded circuit conductors. On the supply side the jumper is always smaller than the phase conductors too, but be careful how you say why: 12.5 percent is not a ceiling on the table. It is the floor for the calculation branch that takes over above 1100 kcmil copper or 1750 kcmil aluminum. Below that threshold the fixed rows of Table 250.102(C)(1) run well above 12.5 percent - 2 AWG phase conductors call for an 8 AWG jumper, which is about 25 percent of the area. Size from the table row, not from a percentage.
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