How to size supply-side bonding jumpers with NEC Table 250.102(C)(1)

Line-side bonding, explained

Look up Table 250.102(C)(1) in the free NFPA viewer. NFPA offers read-only access to current NEC editions at no cost. This page explains how to use the table; it does not reproduce it.

On the line side of a service or transformer secondary disconnect there is no overcurrent device protecting the conductors, so a bonding jumper there can’t be sized by a breaker rating. Instead, Table 250.102(C)(1) sizes it from the largest ungrounded supply conductor. The same table sets the main bonding jumper, the system bonding jumper at a separately derived system, and the minimum size of the service neutral — because on the line side, the neutral is the fault-return path to the source.

The 12.5% rule. The table covers phase conductors up to a top row (1100 kcmil copper, 1750 kcmil aluminum). Past that, a table note requires the jumper to have at least 12.5% of the area of the largest phase conductor, or the equivalent area of a parallel set. That’s the key difference from the grounding electrode conductor table (250.66), which stops growing at its top row: the GEC only has to carry current to earth, while a bonding jumper must carry the full available fault current back to the source.

Worked example. A 1200 A service runs three parallel sets of 500 kcmil copper — 1500 kcmil per phase, which is past the top row. 12.5% of 1500 is 187.5 kcmil. The next standard size with at least that area is 4/0 AWG copper (211.6 kcmil), so that’s the minimum main bonding jumper, the minimum service neutral, and the size of a single SSBJ serving all three conduits. The GEC for the same service tops out at 3/0 copper. If you bond each conduit with its own jumper instead, 250.102(C)(2) lets you size each one from the 500 kcmil set in that conduit.

Once you are past the disconnect, switch tables: load-side bonding jumpers and EGCs follow the OCPD rating — see the equipment grounding conductor chart (NEC 250.122). Size the phase conductors themselves with the NEC 310.16 ampacity chart. Field PM keeps grounding and bonding inspections, photos, and as-builts with the job file.

Not engineering advice. Edition note: this page follows the 2023 NEC. Table 250.102(C)(1) arrived in the 2017 edition; older editions sized these jumpers by referring to Table 250.66 plus the same percentage rule, so older textbooks may point you to the wrong table. Verify the edition your AHJ enforces.