When to use: Size the bonding/grounding electrode conductor based on the largest ungrounded AC service-entrance conductor, per NEC Table 250.66. This same table governs three related conductors: the Grounding Electrode Conductor (GEC, 250.66), the Main Bonding Jumper (250.28(D)) at service equipment, and the Supply-Side Bonding Jumper (250.102(C)(1)) between service equipment and a separate source.
NEC 250.94 requires an intersystem bonding termination for communications, CATV, and similar systems at the service, and TIA-607-style structured cabling standards separately specify a Telecommunications Bonding Backbone (TBB) from the entrance facility to the telecom main grounding busbar (TMGB). Unlike the AC power table above, TBB conductor sizing in TIA-607 is driven primarily by run length — longer backbone runs generally need a larger conductor to keep bonding impedance low. Because exact length breakpoints vary between standard editions and specific site conditions, treat this as a general design consideration and confirm the current TIA-607 table and any AHJ/telecom provider requirements before finalizing a TBB conductor size.
This tool sizes AC power-system bonding conductors from the largest service-entrance conductor per NEC Table 250.66 — a single table that governs the Grounding Electrode Conductor, the Main Bonding Jumper at service equipment, and the Supply-Side Bonding Jumper to a separate power source. It also summarizes how telecom/low-voltage bonding (TIA-607-style) differs from this AC power table.
NEC Table 250.66 was written to size the Grounding Electrode Conductor (GEC) — the conductor connecting the grounded system conductor (or equipment, for ungrounded systems) to the grounding electrode system. NEC 250.28(D) reuses this exact table to size the Main Bonding Jumper, which connects the grounded (neutral) conductor to the equipment grounding system at service equipment. NEC 250.102(C)(1) reuses it again to size the Supply-Side Bonding Jumper, used on the line side of service equipment or between a separately derived system and its source. All three conductors serve the same underlying purpose — providing a low-impedance path for fault current to return to its source — so the NEC sizes them consistently from the same service-conductor-size input.
Two exceptions cap the GEC size below what Table 250.66 alone would otherwise indicate. Per NEC 250.66(A), if the sole grounding electrode is a concrete-encased electrode (rebar or a 20-foot #4 AWG copper conductor encased in the footing), the GEC connecting to it never needs to be larger than #4 AWG copper (or the equivalent). Per NEC 250.66(B), if the sole electrode is a ground ring, the GEC need not be larger than the ground ring conductor itself, and never larger than #2 AWG copper. These caps exist because these electrode types cannot electrically benefit from an oversized GEC.
Structured cabling and telecom bonding (TIA-607-C and similar BICSI/ANSI guidance) uses its own infrastructure — a Telecommunications Main Grounding Busbar (TMGB) at the entrance facility, Telecommunications Grounding Busbars (TGBs) at each telecom room, and a Telecommunications Bonding Backbone (TBB) conductor connecting them. Unlike NEC Table 250.66, TBB conductor sizing is primarily driven by the physical run length of the backbone, since longer conductors have more impedance at the frequencies relevant to noise and surge current. Because published length breakpoints differ between standard editions and site-specific engineering judgment often applies, always confirm the current TIA-607 edition and any AHJ or carrier requirements before finalizing a TBB conductor.
The GEC connects the system to the earth grounding electrode(s); the Main Bonding Jumper connects the grounded (neutral) conductor to the equipment grounding system within the service equipment, establishing the fault-current return path. Both are sized from the same NEC Table 250.66 based on the largest service conductor, but they serve different connections within the same overall grounding and bonding system.
The Supply-Side Bonding Jumper (NEC 250.102(C)) is used on the line side of service equipment, or on a feeder from a separately derived system (e.g., a step-down transformer) to that system's first disconnect, whenever an equipment grounding path must be established outside of a service disconnecting means. It is sized identically to the GEC and Main Bonding Jumper per Table 250.66.
A concrete-encased electrode (Ufer ground) already achieves very low resistance due to concrete's conductivity and moisture retention. NEC 250.66(A) recognizes that oversizing the GEC beyond #4 AWG copper provides no meaningful benefit when this is the only electrode used, so the code caps it there regardless of what Table 250.66 would otherwise call for based on service size.
No. NEC Table 250.66 sizes AC power-system bonding conductors. Telecom and low-voltage bonding (the Telecommunications Bonding Backbone under TIA-607-style standards) uses separate, length-driven sizing criteria and connects into the AC system only at the intersystem bonding termination required by NEC 250.94 — the two systems are bonded together but sized by different rules.
Try our Electrical Studio
More calculators, simulators, and guides for this discipline.