Intersystem Bonding Termination Calculator
The National Electrical Code (NEC) 250.94 requires an intersystem bonding termination (IBT): the common point where telephone, CATV, antenna, and other communications systems bond to the power grounding electrode system, provided external to enclosures at the service and at each structure disconnect. Enter the location and how the termination is built, and this tool checks it against the six 250.94(A) requirements, the 250.94(B) busbar option, and the existing-building exception.
How to use this calculator
- Pick the location: the service equipment or meter enclosure, or the disconnecting means of an additional building or structure.
- Choose how intersystem bonding is provided: a listed IBT device (250.94(A)), a busbar or other means (250.94(B)), the existing-building exception, or none.
- For a device or busbar, enter the number of terminals and set whether it is accessible, whether it blocks the enclosure opening, and whether it is listed as grounding and bonding equipment.
- Pick how it ties to the grounding system: bonded directly to the enclosure or raceway, or connected with a bonding conductor. If a conductor is used, pick its size and material.
- Read the result: each applicable 250.94(A) requirement marked OK or Not OK, plus an overall compliance verdict.
NEC reference
This calculator implements NEC 2020 Section 250.94, Bonding for Communications Systems. An intersystem bonding termination must be provided external to enclosures at the service equipment (or meter enclosure) and at the disconnecting means of each additional building or structure. Under 250.94(A) it must be accessible for connection and inspection, have capacity for not less than three intersystem bonding conductors, not interfere with opening of any enclosure, be mounted and bonded to the enclosure or raceway (or connected with a minimum 6 AWG copper conductor to the enclosure or grounding electrode conductor), and have terminals listed as grounding and bonding equipment. Section 250.94(B) permits a busbar or other approved means. NEC 2020 is the site default; the 2023 edition adds a direct-to-structure mounting option that is out of scope here.
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
The six 250.94(A) requirements
250.94(A) spells out what an intersystem bonding termination device has to be and do. This tool checks each one and tells you which, if any, fails.
| Requirement | Source |
|---|---|
| Accessible for connection and inspection | 250.94(A)(1) |
| Terminals with capacity for not less than three intersystem bonding conductors | 250.94(A)(2) |
| Does not interfere with opening of any enclosure | 250.94(A)(3) |
| Mounted and bonded to the service enclosure, meter enclosure, or exposed metallic service raceway, or connected with a minimum 6 AWG copper conductor | 250.94(A)(4) |
| At a structure disconnect, mounted and bonded to its metallic enclosure, or connected with a minimum 6 AWG copper conductor | 250.94(A)(5) |
| Terminals listed as grounding and bonding equipment | 250.94(A)(6) |
Why one common bonding point
Before 250.94 required a single intersystem bonding termination, communications installers landed their grounding conductors wherever they could find a connection: a ground rod driven separately, a water pipe, a cold-water clamp, a sheet-metal screw into the meter can. Two grounds that are not bonded together can sit at different potentials during a fault or a lightning surge, and that difference drives current through the coax, the phone pair, or the data cable, where it damages equipment and can start fires. Giving every system one listed place to bond, tied to the power grounding electrode system, closes that gap. That is why the IBT must have room for at least three conductors and must be accessible: the cable, phone, and antenna installers who arrive later each need a code-compliant point to land on without opening the service.
Worked examples
A new house has a meter-main at the service. The electrician mounts a listed intersystem bonding termination block with five terminals beside the meter, bonded directly to the meter enclosure, clear of the cover and reachable from the ground. Five terminals is at least three, it is accessible, it does not block the cover, the direct bond needs no separate conductor, and the block is listed as grounding and bonding equipment. It complies with 250.94.
A detached shop is fed by a feeder from the house. At the shop disconnect the installer adds a three-terminal IBT and runs an 8 AWG copper conductor from it to the grounding electrode conductor. The terminal count, accessibility, and clearance are fine, but 250.94(A)(5) requires a minimum 6 AWG copper bonding conductor, so the 8 AWG run fails. Replace it with 6 AWG copper (or bond the block directly to the metallic disconnect enclosure) and it complies.
An existing home already has the cable and telephone bonding conductors landed on an older grounding point as required by 820.100(B) and 800.100(B). Under the 250.94 exception for existing buildings, a new intersystem bonding termination is not required because those conductors already exist. On new construction the same arrangement would not qualify; a listed IBT or a 250.94(B) busbar would be required.
Common mistakes
- A single lug instead of three terminals. 250.94(A)(2) requires capacity for not less than three intersystem bonding conductors, so one cable clamp is not enough.
- Blocking the meter or service cover. 250.94(A)(3) forbids interfering with opening the enclosure; the device has to sit clear of the cover.
- Undersizing the bond. Where a conductor is used it must be a minimum 6 AWG copper (250.94(A)(4)/(5)), not 8 AWG and not aluminum.
- An unlisted terminal strip. 250.94(A)(6) requires terminals listed as grounding and bonding equipment, not an improvised bus.
- Forgetting the second location. An IBT is required at the service and at each additional building or structure disconnect, not just at the main service.
- Confusing the IBT with the communications grounding conductor. The 6 AWG conductors from each communications system are sized by Articles 770/800/810/820/830, not by 250.94.
Frequently asked questions
What is an intersystem bonding termination?
The single listed point required by 250.94 where communications systems bond to the power grounding electrode system, provided external to enclosures at the service and at each structure disconnect.
How many terminals are required?
At least three (250.94(A)(2)): capacity for connection of not less than three intersystem bonding conductors. A one- or two-terminal device does not comply.
What conductor bonds it to ground?
A minimum 6 AWG copper conductor where a conductor is used (250.94(A)(4)/(5)), or a direct bond to the enclosure or raceway with no separate conductor.
Is there an exception for existing homes?
Yes. An IBT is not required in an existing building where the communications bonding and grounding electrode conductors required by 770.100(B), 800.100(B), 810.21(F), 820.100(B), and 830.100(B) already exist.
Related calculators
- Grounding Electrode Conductor Size Calculator: size the grounding electrode conductor the IBT ties into, per NEC Table 250.66.
- Bonding Jumper Size Calculator: main, system, and supply-side bonding jumpers per NEC Table 250.102(C)(1).
- Equipment Grounding Conductor Size Calculator: size the EGC run with a feeder per NEC Table 250.122.
- Separate Structure Grounding & Bonding Calculator: grounding and bonding for a detached building fed by a feeder per NEC 250.32.
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