Bonding Jumper Sizing Trainer
Watch a fault find its way home through the bonding jumper, size the jumper at every enclosure on a service lineup, and finish with a scored quiz.
This trainer walks a service lineup — CT cabinet, meter, service disconnect, downstream panel — and animates what a bonding jumper is for: nothing on a normal day, everything in the moment a phase conductor lands on a steel can. You watch the fault current find its way back to the source through the jumper, work the sizing at each location, and finish with a scored quiz that sends you back to the diagram, not the answer key.
The code is one sorting question and two tables. Ahead of the first overcurrent device there is nothing to trip, so the main bonding jumper (250.24(B), 250.28), the system bonding jumper at a separately derived system (250.30(A)(1)) and any supply-side bonding jumper are sized from the ungrounded conductors in Table 250.102(C)(1) — paralleled sets summed first. On the load side a breaker or fuse is ahead of the jumper, so it is sized from that device’s rating in Table 250.122 and never has to be larger than the circuit conductors (250.122(A)); a feeder in parallel raceways gets a full-size equipment grounding conductor in each one (250.122(F)). Terminations follow 110.14(D).
The mistakes it drills: sizing a load-side jumper from the feeder conductors instead of the breaker; entering one set of a paralleled service into the table instead of the total; a second neutral-to-ground bond downstream, which puts neutral current on raceways and trips ground-fault protection; and a green bonding screw installed with its green finish hidden, so the inspector cannot see that the bond exists.
Simplified concept tool for training and illustration — not a construction document. Equipment layouts and bender take-ups vary; manufacturer instructions and the locally adopted code govern.
This page frames an interactive trainer that does not print. Open https://www.aztucsonelectricalsolutions.com/trainer-bonding-jumper-sizing in a browser to use it.
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; 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. A quiz follows the diagram.
How to use it in a class
Stop the animation at the fault and ask the room what happens if the jumper is undersized — the trainer’s answer, that the jumper becomes the fuse and the breaker never sees the fault, is the whole reason the sizing is a rule and not a preference. Then put three enclosures on the board (CT cabinet, service disconnect, a 200 A subpanel) and have students say which table applies at each before they open the code. The quiz can be run individually or as a team round.
Section numbers follow the 2023 NEC; the edition adopted by your jurisdiction and its amendments govern, and the AHJ has the final say. The trainer is a teaching simplification of work for qualified, licensed electricians — on a real installation, de-energize, apply lockout/tagout, verify absence of voltage with a tested meter, and wear PPE selected per NFPA 70E before touching anything.
Learn it here, do it with us
Arizona Electrical Solutions trains apprentices on exactly this work — services, grounding and bonding, transformers, conduit — on commercial projects across Arizona. If you are planning the work itself rather than learning it, send us the job.
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The guide this trainer came from, the calculator that pairs with it, and the rest of the set.