Metal Water Pipe & Structural Metal Bonding Calculator
Size the bonding jumper for a metal water piping system, other metal piping likely to become energized, or a structural metal building frame under the National Electrical Code (NEC) 2020 section 250.104. The calculator picks the governing subsection, reads the right table, and applies the subsection-specific cap: Table 250.102(C)(1) with the 3/0 copper / 250 kcmil aluminum cap for water piping, Table 250.122 off the overcurrent device for other piping, and the uncapped Table 250.102(C)(1) for structural steel.
How to use this calculator
- Pick what you are bonding: metal water piping (250.104(A)), other metal piping likely to be energized such as gas (250.104(B)), or exposed structural metal (250.104(C)).
- For water piping, say whether the building is fed by a service (250.104(A)(1)) or is a separate structure on a feeder or branch circuit (250.104(A)(3)).
- For the water and structural paths, enter the largest ungrounded supply conductor size and material, and how many are run in parallel per phase.
- For other metal piping, enter the rating of the overcurrent device of the circuit likely to energize the pipe.
- Choose the bonding jumper material and read the minimum size, the table and row it came from, and whether a cap applied.
NEC reference
This calculator implements NEC 2020 section 250.104. Subsection (A) bonds the metal water piping system and sizes the jumper from Table 250.102(C)(1) on the largest ungrounded service conductor, not required to be larger than 3/0 copper or 250 kcmil aluminum; (A)(3) handles a building supplied by a feeder or branch circuit and sizes from the feeder conductors. Subsection (B) bonds other metal piping likely to become energized and sizes the jumper from Table 250.122 on the overcurrent device of the circuit likely to energize it. Subsection (C) bonds exposed structural metal and sizes from Table 250.102(C)(1) with no cap. The sizing tables themselves are unchanged across the 2017, 2020, and 2023 editions; the explicit 3/0 copper / 250 kcmil aluminum water-pipe cap is the 2020 wording this tool uses. NEC 2020 is the site default.
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
Which 250.104 subsection and table applies
The hardest part of 250.104 is not the arithmetic, it is picking the right subsection. Each one sends you to a different table and a different basis.
| What is bonded | Subsection | Table / basis | Cap |
|---|---|---|---|
| Metal water piping (service) | 250.104(A)(1) | Table 250.102(C)(1), largest ungrounded service conductor | 3/0 Cu / 250 kcmil Al |
| Metal water piping (separate building on a feeder/branch) | 250.104(A)(3) | Table 250.102(C)(1), feeder/branch conductor | Not larger than the supply conductor |
| Other metal piping likely to be energized (gas, etc.) | 250.104(B) | Table 250.122, OCPD of the energizing circuit | None |
| Exposed structural metal building frame | 250.104(C) | Table 250.102(C)(1), service/supply conductor | None |
Why the water-pipe cap matters
Table 250.102(C)(1) runs up to the Note 1 12.5% rule for ungrounded conductors above 1100 kcmil copper or 1750 kcmil aluminum. The main, system, and supply-side bonding jumpers take that uncapped value. The water-pipe bonding jumper does not: 250.104(A) caps it at 3/0 copper or 250 kcmil aluminum. So a 1500 kcmil copper service needs a 4/0 main bonding jumper but only a 3/0 copper water-pipe bonding jumper. Below 1100 kcmil copper the cap never binds, because the largest tabulated jumper is 2/0 copper, already under 3/0. That is the single most common place the two calculations diverge, and the reason this tool is separate from the Bonding Jumper Size Calculator.
Worked examples
A 200 A dwelling service runs 250 kcmil aluminum ungrounded conductors. Water piping under 250.104(A)(1) reads the aluminum column of Table 250.102(C)(1) at the 4/0 AWG or 250 kcmil row and takes a 4 AWG copper bonding jumper. The 3/0 cap does not bind.
A commercial service runs 400 kcmil copper. Water piping reads the Over 350 through 600 kcmil copper row and takes a 1/0 copper bonding jumper. Structural steel in the same building under 250.104(C) uses the same row and the same 1/0 copper, because at this size neither path is near a cap.
A large service runs 1500 kcmil copper. The water-pipe jumper is 3/0 copper, held there by the 250.104(A) cap even though the 12.5% rule would otherwise call for 4/0. A structural steel bond on that same service is 4/0 copper, because 250.104(C) has no cap and the 12.5% rule governs: 12.5 percent of 1,500,000 circular mils is 187,500, which rounds up to 4/0.
A gas furnace is fed by a 60 A circuit. The gas piping bond under 250.104(B) reads Table 250.122 at the 60 A row and takes a 10 AWG copper jumper, and the equipment grounding conductor of that furnace circuit is permitted to serve as the bonding means.
Common mistakes
- Sizing the water-pipe jumper off the breaker. 250.104(A) is based on the ungrounded service conductor and Table 250.102(C)(1), not the main breaker rating.
- Sizing gas piping off the service. 250.104(B) uses Table 250.122 on the circuit likely to energize the pipe, which is almost always far smaller than the service.
- Carrying the 3/0 cap onto structural steel. The cap is in 250.104(A) for water piping; 250.104(C) structural metal has none.
- Missing the cap on a large service. Above 1100 kcmil copper the water-pipe jumper stays at 3/0 even though the general bonding jumper grows past it.
- Forgetting the points of attachment must be accessible. 250.104 requires the bonding connections to be accessible; this tool sizes the conductor, it does not place it.
Frequently asked questions
What size wire bonds a water pipe?
Per 250.104(A), from Table 250.102(C)(1) on the largest ungrounded service conductor, capped at 3/0 copper or 250 kcmil aluminum. A typical 200 A dwelling service takes a 4 AWG copper bonding jumper.
Does gas pipe need bonding, and what size?
Metal gas piping likely to become energized is bonded under 250.104(B), sized from Table 250.122 on the overcurrent device of the circuit likely to energize it. The equipment grounding conductor of that circuit may serve as the bonding means.
Why is my water-pipe answer smaller than the main bonding jumper?
Because 250.104(A) caps the water-pipe jumper at 3/0 copper or 250 kcmil aluminum, while the main bonding jumper under Table 250.102(C)(1) is uncapped and follows the 12.5% rule above 1100 kcmil copper.
What about a detached garage water line?
Under 250.104(A)(3) the jumper is sized from Table 250.102(C)(1) on the feeder or branch-circuit conductors and need not be larger than that conductor. Bond at the structure disconnect or to an equipment grounding conductor.
Related calculators
- Bonding Jumper Size Calculator: the main, system, and supply-side bonding jumper and grounded conductor from Table 250.102(C)(1), uncapped, including the 12.5% rule.
- Equipment Grounding Conductor Size Calculator: the Table 250.122 lookup that the 250.104(B) other-piping bond uses.
- Grounding Electrode Conductor Size Calculator: the grounding electrode conductor from Table 250.66, the companion grounding-conductor size.
- Separate Structure Grounding & Bonding Calculator: the 250.32 grounding-electrode and neutral-isolation side of a detached building fed by a feeder.
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