On 2009-11-27, Existential Angst wrote:

The current in the main neutral *should* be the difference in current
between the main hot legs, right?

If there are no other return paths, that is correct. Although as you
have observed, in communities with interconnected water supply piping,
the water supply piping may be a very good return path (via the
various neighbors' neutral to grounding system bonds).

However, it is usually off by between 1 and 1.5 amps, which seems to
correspond to the current flow through/around the water meter. This
would seem to be a *necessary* relationship, right?

Again, if those are the only two return paths. However, there may be
more or less current on the metallic water supply, contributed by your
neighbors.

My impression is that prior to the utility replacing the old wires
along the poles and to my house, this current flow through/around
the water meter was a lot higher -- 3-5 amps.

That makes sense. If the service drop was higher resistance, less
current goes on the service drop neutral, more current on the water
supply ground to your neighbors' neutrals.

So the Q is: What is "acceptable" ito current flow through the water
meter to the street?

There's not a good answer to this question. Cetainly your service
drop neutral should be taking more of the current than your water
supply pipes; if not, there's a problem with your service drop
neutral. Beyond that, it is what it is.

[Depending on the local practices for your water serive, you could
take steps to eliminate the return current on the water supply. For
example, here the water meters are in shallow vaults in the sidewalk;
if you install a non-conductive coupling on the house side of the
water meter, then you disrupt the return path, while retaining the
buried water lateral as a grounding electrode. But be careful about
disrupting the return path, see below.]

My assumption is that if I took out the water meter, then all of the
neutral current would necessarily flow through the main neutral

Yes, but you would have to be very careful about doing this, since as
soon as you disconnect the water meter, there will be a voltage
difference between the two sides of the water supply, representing the
voltage that was driving the current. You could be injured or killed
if you get across the two sides of the water piping. This can happen
even if your main breaker is off, since a neighbor's neutral could be
faulty and could be using your neutral as the principal return path,
via this water pipe bond.

But again, what are acceptable limits ito of current flow through
grounds? Is there an NEC ratio, a percentage?

Nope, the NEC doesn't directly provide a ratio or say to what extent
this is acceptable. It is, however, a side effect of the NEC
requirements on grounding and bonding.

Cheers, Wayne