In a previous electrical inspection, I was told to add a 4 AWG CU bonding wire from the main panel to the hot and cold pipes on my water heater. I'm in the process of replacing my water heater with a tankless one, and the pipes leading to it with PEX. So, this bonding wire really has nothing on the water heater to connect to. I could add a short piece of copper pipe to the water inlet/outlet (currently planning on braided flex pipe, then ball valve, then PEX, then (inside wall) galvanized), but bonding to these does nothing for most of the galvanized pipes elsewhere in the house (which, over time, I'll replace with PEX).

There is some galvanized pipe in the walls that I'll connect the PEX to the water heater to. I'm assuming buried in the wall is not accessible and hence not compliant.

Can it be anywhere in the water system where there is galvanized pipe or does it need to be within 10' of the water entrance to the building (as I've read elsewhere, but can't find in NEC)? If I do that, I don't know I can do both cold and hot - there may not be hot water within 10' of where cold water enters the crawlspace. I could feed a bonding wire from the panel to within 10' of where cold enters the crawlspace, and then somewhere else bond just the hot and cold water pipes together. Of course, eventually, all hot will be PEX, at which point, I assume bonding is no longer required.

  • Is the water heater itself gas or electric? Nov 24, 2020 at 12:40
  • This seems to be 2 questions in one: 1) What does "accessible" mean in terms of code. 2) Where/how should I bond my panel since bonding to the water supply isn't going to be viable in the long run due to plumbing upgrades. Please split it into 2 more focused questions.
    – FreeMan
    Nov 24, 2020 at 13:22
  • Electric. Will do on the split. Nov 24, 2020 at 16:13
  • @FreeMan -- there's a difference between grounding to the water supply and bonding the water piping system, and part of the OP's question hinges on that Nov 25, 2020 at 1:01
  • 1
    I haven't dug it up to verify, but I'm fairly certain it's galvanized steel. Until I start fixing things, it's pretty much all galvanized except a few places where it was transitioned to copper (like to the old WH). Nov 25, 2020 at 17:52

3 Answers 3


Accessible means "no demolition needed"

The definition of "accessible" as applied to wiring methods (vs. equipment) is fairly simple:

Accessible (as applied to wiring methods). Capable of being removed or exposed without damaging the building structure or finish or not permanently closed in by the structure or finish of the building.

As a result, having an access hatch, cover plate, or such that needs to be opened to access the bonding connection is not an issue.

Bonding and grounding are two different things

Your situation, with a galvanized service line but a mix of metal and plastic interior piping, does put an emphasis on the distinction between grounding to a water service and bonding your water piping to ground. In houses with all-metal plumbing, all-plastic plumbing on a metal service, or all-metal plumbing on a plastic service, this is handled through a single connection to the water system. This leads people to ignore the distinction, but since you have a mix of plastic and metal plumbing on a metal service, you need to pay attention to what you're doing insofar that a bond at one point won't be sufficient.

You'll need something fatter than 4AWG, too

The other problem is since you have a 400A service landing on service equipment on the exterior of the house (vs. a "maypole" service where the service equipment's on a pole with the meter), you need to size the bonding and grounding system accordingly. While a 4AWG copper conductor is adequate for grounding to ground rods or an Ufer electrode due to the NEC 250.66(A) and (B) exceptions to normal grounding electrode conductor sizing, water service line electrodes have no such exception, so its grounding electrode conductor needs to be the full size required by your service, or 1/0AWG for an overhead 400A service (600kcmil total area) as per table 250.66 in the NEC, and also needs to connect to the plumbing within 5' (not 10') of where the service line enters the building as per NEC 250.68(C) point 1:

(C) Grounding Electrode Conductor Connections. Grounding electrode conductors and bonding jumpers shall be permitted to be connected at the following locations and used to extend the connection to an electrode(s):

(1) Interior metal water piping that is electrically continuous with a metal underground water pipe electrode and is located not more than 1.52 m (5 ft) from the point of entrance to the building shall be permitted to extend the connection to an electrode(s). Interior metal water piping located more than 1.52 m (5 ft) from the point of entrance to the building shall not be used as a conductor to interconnect electrodes of the grounding electrode system.

However, since you have metal pipework that's not electrically continuous with the service line, you also need to bond that piping to the grounding electrode system or service equipment as per NEC 250.104(A)(1). Fortunately, the Table 250.104(C) requirements for bonding jumper sizing also require 1/0AWG wire for your application, so you don't need yet another size of wire, at least. Furthermore, unlike grounding electrode conductors, these water system bonding jumpers do not need to be run without splice or joint, so that gives you much more flexibility as to how you arrange them. The caveat here, though, is you can't just bond the cold water pipe and call it done; that "island" of metal hot water piping must be bonded as well, even if you have to make the bonding connection under a sink or such.

  • NEC doesn't define "metal water piping system". As I replace parts of what was one MWPS with PEX, I can see how it becomes multiple and needs bonding jumpers. When can I stop? I assume I don't need to bond every brass fitting connecting 2 pieces of PEX. What about metal shutoff valve, flex pipe to faucet, and metal faucet itself? What if those are in a cabinet that also has an electrical receptacle (e.g., for a garbage disposal)? What if PEX goes to just below the cabinet, then there is galvanized from there up to cabinet (because I don't want to remove the cabinet and demolish walls)? Nov 26, 2020 at 21:42
  • 250.52(A)(1) defines when a metal underground water pipe is permitted as a grounding electrode. I don't intend mine to be that. I have a rod and plate all connected correctly. Given that, and just wanting to ground the MWPS, what size conductors do I need? E.g., to ground any pipe sections not connected to underground water pipe, what is smallest I can get away with? Nov 26, 2020 at 21:51
  • @TomGetzinger -- I'd say it'd also be 1/0 as bonding jumpers of that nature are sized IAW Table 250.102(C)(1) (see the last sentence of NEC 250.104(A)(1) for details) Nov 26, 2020 at 21:55
  • @TomGetzinger -- really good question about the extent of bonding required in a mixed metal/plastic piping system, though -- I'm pretty sure the NEC doesn't intend for bonding wires to be run to metal fixtures or isolated metal fittings in otherwise nonmetallic plumbing systems, but otherwise, I'm not sure...might be something to discuss with the AHJ Nov 26, 2020 at 21:57
  • 1
    I hear you, but my point is that they rejected it because they felt it was already addressed. By my reading of NEC, if it is addressed, that sure isn't clear or we wouldn't still be having this discussion over a decade later :) Nov 27, 2020 at 18:07


Accessible means you don't have to demolish anything to inspect it. You can have it behind a drywall access panel, in an attic, under a crawlspace, in a mechanical closet, etc. as long as it isn't covered by a finished wall.

Plumbing bonding

You really need to check with your building official. This varies. Here's a good article in The Spruce with some explanations.

Effectively, the metal parts of your plumbing system should be bonded to the Grounding Electrode Conductor, and in copper distribution systems, that bonding is achieved by the plumbing parts themselves. If you have a run of PEX between different parts of a copper system, in some jurisdictions, they want you to bond the separate copper parts together. Same for water heaters; that's why some inspectors will require a bond at the water heater, to connect the cold pipe to the hot one.

However, this really does vary, and you need to check with your building official for guidance.


The person that told you #4 was wrong to start off with

A rod,pipe or plate grounding electrode is only required to be 6awg max NEC 250.66.A. The connection to the grounding electrode or pipe in this case is to be within 5’ of entering the building. I do normally bond the incoming cold pipe to the outgoing hot pipe if copper or galvanized, once you go to plastic you are correct bonding is not required. I believe the 10’ you are thinking of is the minimum length of pipe in contact with earth to be a grounding electrode this is still code but it sounds like you are planning on removing some of the pipe.

I would suggest updating your pipe electrode to add at least 1 driven rod 8’ long better would be 2 rods 6’ or more apart further is better but 6’ is the minimum spacing. This electrode can be connected with the #4 copper if long enough if not #6 is all that is required.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.