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.
Best Answer
Accessible means "no demolition needed"
The definition of "accessible" as applied to wiring methods (vs. equipment) is fairly simple:
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:
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.