Short definition
A bonding clamp is a code-listed metal clamp that secures a bonding (grounding) wire to a metal pipe. The National Electrical Code requires metallic water piping (and gas piping where applicable) to be bonded to the building’s grounding electrode system. The clamp is what makes the connection.
What it is
A bonding clamp has two parts: a U-bolt or saddle that grips the pipe, and a listed conductor lug that accepts a bare copper bonding wire (typically #6 or #4 AWG, sized per NEC Table 250.66). The clamp must be UL-listed for both the pipe size and the conductor it bonds. A plain pipe strap without a conductor lug is not a bonding clamp and will fail inspection.
The relevant code citations:
- NEC 250.104(A). Metallic water pipe in or attached to a building must be bonded to the service equipment, grounded conductor, grounding electrode conductor, or grounding electrodes.
- NEC 250.104(B). Other metal piping systems (including gas) must be bonded where likely to become energized — this is the rule that drives CSST gas-line bonding.
In Washington, the NEC is adopted via the WA-amended electrical code (RCW 19.28). Plumbing inspectors and electrical inspectors overlap on bonding compliance.
Why it matters to a homeowner
If you have a pre-1970 home in Seattle, Tacoma, Olympia, Everett, or Spokane and you’ve ever had a partial repipe with PEX, your bonding may be broken. Here’s how it happens: the original galvanized cold-water service line was the bonding electrode connection. A repipe replaces the galvanized with PEX (non-conductive). The bonding clamp on the old line gets cut off. Nothing replaces it. The bonding-system continuity is silently lost, and the next remodel inspection turns it up.
This is one of the most common things a pre-purchase home inspection flags in older WA homes. The fix is small — $50–$200 — but it requires the plumber and electrician to coordinate. Cheap contractors don’t always check, and the homeowner ends up paying for the corrective on the next permit.
When you’ll encounter this term
- A pre-purchase home inspection report noting “no main bonding visible.”
- A galvanized-to-PEX repipe quote that itemizes “re-bonding” or “ground bridge.”
- A CSST gas-line install where the inspector specifically checks for direct bonding.
- Replacing a water heater where the bond on the inlet must remain continuous to the new tank.
Common variants and not the same as
- Bonding vs. grounding. Grounding connects a system to earth; bonding connects metal parts together at the same electrical potential. On a residential job, a single clamp on the cold-water line often does both jobs.
- Bonding clamp vs. ground rod clamp. Ground rod clamp is for a bare copper electrode driven into earth; pipe bonding clamp is for piping. Different geometry, different listing.
- Bonding clamp vs. plain pipe strap. Strap holds pipe to structure. Bonding clamp has a listed conductor lug. They are not the same product.
Common failure modes
- Plain pipe strap used in lieu of bonding clamp. Fails inspection.
- Bond broken by a non-conductive section downstream. A PVC repair coupling, a dielectric union, a plastic check valve — any non-conductive component breaks continuity. Bond on both metal sides.
- Bond cut off during a partial repipe. New PEX is non-conductive; the bond evaporates silently.
- CSST without direct bonding. Lightning-strike fires in residential CSST systems drove NEC’s direct-bonding requirement. WA inspectors check for this.
Washington note
CSST (corrugated stainless steel tubing) gas-pipe bonding is a load-bearing WA topic. CSST has been widely used in WA-permitted gas installs since the early 2000s. Direct bonding is required by NEC 250.104(B) and by CSST manufacturers (Gastite, OmegaFlex). WA inspectors check this routinely. A gas-line install that omits CSST bonding is code-noncompliant — confirm written sign-off from both the plumber and the electrician on any new CSST work.