Short definition
NEC bonding of metallic water piping is the National Electrical Code requirement (§250.104) that interior metal water pipes be electrically bonded to the building’s grounding electrode system. This eliminates voltage potential between water pipes and electrical ground, protecting against shock hazard if a fault energizes a water line. Common WA inspection finding because PEX repipes break the bond and CSST gas requires its own bonding rule.
What it is
NEC §250.104 has two parts that show up on WA plumbing projects:
Part A — Metallic water piping. Interior metal water pipes (copper, galvanized, etc.) must be bonded to the electrical service grounding electrode system using a bonding jumper sized per NEC Table 250.66. For typical residential 200-amp service, this is #4 AWG copper. The jumper connects from the water pipe to the building’s grounding electrode (Ufer concrete-encased rebar, driven ground rod, etc.) — making the metallic water piping at the same electrical potential as the electrical ground.
Part B — CSST (corrugated stainless steel tubing) gas piping. CSST requires a separate bonding conductor to the gas piping system, typically #6 AWG copper. This rule was added after lightning-strike fire research showed CSST is more vulnerable than traditional black iron gas pipe to lightning-induced damage. Without proper bonding, lightning can arc through CSST walls and ignite the gas.
The terms get confused in casual usage. “Grounding the water pipe” is what people say; “bonding the water pipe to the grounding system” is what the code actually requires. Grounding means connection to earth; bonding means connection between metal parts to equalize potential. Modern NEC requires the building grounding electrode system to use a Ufer or driven ground rods — metallic water pipe is bonded to that system, not used as the primary electrode.
Why it matters to a homeowner
This rule is one of the most common WA inspection findings, for two reasons.
PEX repipes break the bond. A typical scenario: a 1960s WA home has copper supply lines from the meter to the panel area, with the original water-pipe bond still in place. The homeowner repipes in PEX. The new PEX section breaks electrical continuity to the building’s water-pipe bond — creating a section of metallic pipe (the remaining bits past the PEX) that’s no longer bonded to ground. The fix is a bonding jumper across the PEX section, restoring continuity. Inspectors flag missing or improperly placed jumpers regularly.
CSST gas requires bonding. When a gas line is run in CSST (very common in modern WA installs because CSST is fast to route and good for retrofits), a separate #6 AWG bonding jumper must connect the CSST manifold to the building’s grounding electrode system. This is non-optional — lightning-strike fire risk is the documented basis. A WA gas-line install without proper CSST bonding fails inspection.
When a contractor’s repipe quote doesn’t address bonding (jumper across plastic sections; CSST bonding if gas was touched), ask. The bonding is part of code-compliant work, not an extra. A reputable plumber working with a licensed electrician handles both at install.
When you’ll encounter this term
- A repipe project quote with a “bonding jumper” line item.
- An electrical inspection finding “missing water pipe bond” after a PEX repipe.
- A CSST gas-line install requiring separate bonding.
- A pre-purchase home inspection flagging missing or undersized bonding.
Common variants and disambiguation
- Grounding vs. bonding. Grounding = connection to earth. Bonding = connection between metal parts to equalize potential. NEC §250.104 is bonding, even though casual usage says “grounding the water pipe.”
- Pre-PEX vs. post-PEX house. A 1960s WA home with copper supply may have a robust water-pipe bond. After a PEX repipe, the metal continuity is broken; a bonding jumper across the new plastic section is required.
- Water-pipe bond vs. Ufer / driven rod. Modern NEC requires the building grounding electrode system to use a Ufer (concrete-encased rebar) or driven ground rods. Metallic water pipe is bonded to that system, not used as the primary grounding electrode.
Washington note
WA adopts the 2020 NEC via WAC 296-46B, enforced by the WA L&I Electrical Section on every electrical permit and inspection. NEC §250.104(A) requires bonding interior metallic water piping; §250.104(B) requires CSST bonding. The WA-amended version generally tracks the NEC base text on these provisions.
The CSST bonding rule is enforced rigorously in WA because of the lightning-strike fire research that drove the change in earlier NEC cycles. A CSST gas install without proper bonding will fail any WA electrical inspection. If you’re getting a new gas appliance installed and the gas line includes any CSST, verify bonding is included in the scope.
For PEX repipes, the bonding fix is straightforward: the licensed electrician adds a bonding jumper across the PEX section, sized per Table 250.66 (typically #4 AWG copper for residential). The jumper restores electrical continuity from the cold-water inlet through the building’s metallic plumbing to the grounding electrode system. WA homeowners on a repipe project should expect a small electrical line item alongside the plumbing scope.