Grounding & Bonding

The supply-side bonding jumper and the main bonding jumper use Table 250.102(C)(1) — enter the ungrounded conductors in section 1. The result is shown here as well. Load-side equipment bonding jumpers are sized from Table 250.122 (section 2) based on the OCPD ahead of the circuit.

• A rod, pipe, or plate electrode must have a resistance to earth of 25 ohms or less, or be supplemented by an additional electrode (250.53(A)(2)). In the field this is the “25-ohm or two-rod” rule: if you do not prove 25 ohms or less, drive a second rod.
• The supplemental electrode must be at least 6 ft from the first (250.53(A)(3)). More spacing is better when the site allows it.
• Rods are typically 8 ft long and must be driven to full length unless rock is encountered; then 250.53(A)(4) / 250.53(G) permit burial or an angled drive as specified. Keep the connection accessible or use a listed buried clamp suitable for direct soil burial.
• Bond the electrodes of the grounding electrode system together (250.50). Do not use a rod as a substitute for a concrete-encased electrode when one is present (250.50 requires all available electrodes to be bonded together).

• A concrete-encased electrode is at least 20 ft of bare copper conductor not smaller than 4 AWG, or 20 ft of one or more bare or zinc-galvanized or other electrically conductive coated steel reinforcing bars or rods not smaller than ½ in., encased in at least 2 in. of concrete, near the bottom of a concrete foundation or footing in direct contact with the earth.
• Where present in the building (new construction typically has rebar in the footing), it is an electrode that 250.50 requires you to use. The GEC to it need not be larger than 4 AWG copper (250.66(B)).
• Do not paint or insulate the connection in a way that defeats the electrode. If the AHJ requires an inspection window or an accessible stub, provide it before the pour.

• An intersystem bonding termination (IBT) is required at the service (and at a disconnecting means for other buildings) so communications, CATV, and similar systems can bond to the building grounding electrode system.
• The IBT must be listed, accessible, have at least three terminals (more on many listed bars), and be bonded to the service enclosure, GEC, or electrode as 250.94 permits. Do not land those bonds on a random enclosure screw or a water pipe in lieu of the IBT where 250.94 applies.
• Bonding these systems together reduces voltage differences during a surge or fault. It is not a substitute for each system’s own grounding electrode rules in Chapters 7 and 8.