Telecom Grounding & Bonding — TIA-607
The ANSI/TIA-607-E bonding system: the busbars and conductors by both their legacy names (still the field vernacular) and the current ones, the full bonding-backbone sizing table, and the NEC side — protector grounding (800.100) and the intersystem bonding termination (250.94). TIA-607 is a voluntary standard that specs invoke; the NEC rows are law where adopted.
The bonding system, component by component
| Legacy term | Current term (607-C+) | Role |
|---|---|---|
| TMGB | PBB (primary bonding busbar) | The telecom extension of the building grounding electrode system — typically one per building, at the entrance facility |
| BCT | TBC (telecommunications bonding conductor) | Bonds the PBB to the electrical service ground; sized at least as large as the largest TBB |
| TBB | TBB (unchanged) | The bonding backbone from PBB to every SBB; no splices where possible; never a cable shield or water pipe |
| TGB | SBB (secondary bonding busbar) | The busbar in each telecom room; racks and equipment bond here |
| GE | GE (grounding equalizer) | Ties multiple TBBs together — at the top floor and at least every third floor between |
| RGB | RBB (rack bonding busbar) | In-rack busbar on stand-offs for multiple equipment bonds; racks bond individually, never in series |
TBB / GE conductor sizing by length
| TBB length | (feet) | Minimum size |
|---|---|---|
| 0–4 m | 0–13 | 6 AWG |
| 4–6 m | 14–20 | 4 AWG |
| 6–8 m | 21–26 | 3 AWG |
| 8–10 m | 27–33 | 2 AWG |
| 10–13 m | 34–41 | 1 AWG |
| 13–16 m | 42–52 | 1/0 AWG |
| 16–20 m | 53–66 | 2/0 AWG |
| 20–26 m | 67–84 | 3/0 AWG |
| 26–32 m | 85–105 | 4/0 AWG |
| 32–38 m | 106–125 | 250 kcmil |
| 38–46 m | 126–150 | 300 kcmil |
| 46–53 m | 151–175 | 350 kcmil |
| 53–76 m | 176–250 | 500 kcmil |
| 76–91 m | 251–300 | 600 kcmil |
| > 91 m | > 301 | 750 kcmil |
Build rules that pass inspection
Busbars are predrilled copper, listed, insulated from their supports — the main busbar at least 0.25″ × 4″, room busbars 0.25″ × 2″. Conductors are green, or marked with distinctive green, run as short and straight as practicable with sweeping bends (≈8″ radius at busbars, never sharp), and cable shields and water piping never serve as the backbone. On the code side, the intersystem bonding termination (NEC 250.94) gives telecom, CATV, and every other system at least 3 accessible terminals at the service — the point where the TIA system and the electrical service meet. Shielded cabling bonds into this same system at the patch panel — see the shielded cable chart.
Common questions
What is the difference between the TMGB and a TGB?
The TMGB (renamed PBB — primary bonding busbar — in current TIA-607) is the single busbar, typically at the entrance facility, that extends the building's electrical grounding electrode system to telecom. Each telecom room then gets a TGB (now SBB — secondary bonding busbar) where racks and equipment bond. The TBB — the bonding backbone — ties every room busbar back to the main one.
What size is the telecom bonding backbone?
Minimum 6 AWG, sized at 2 kcmil per linear foot of run, up to 750 kcmil. A widely reproduced version of this table stops at 3/0 — that was the pre-2011 standard; TIA-607-B extended it. Under 4 m a 6 AWG suffices; a 25 m riser needs 3/0; past 91 m the answer is 750 kcmil.
Does the NEC require an insulated grounding conductor for the protector?
No — it requires a LISTED conductor (NEC 800.100); insulated, covered, or bare are all permitted. The size floor is 14 AWG, and it is never required to be larger than 6 AWG. In one- and two-family dwellings it must be as short as practicable — 20 ft maximum, with the ground-rod-plus-bonding-jumper exception when that is impracticable.
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