ECN Forum Forums General Topics of the Trade General Discussion Area Where is the neutral-to-ground bond?

The more I look at that picture (gec_tap.jpg), the more I'm scratching my head. It appears that 250.24(B) in fact requires a ground-neutral bond in each service disconnect enclosure. However, the installation illustrated gec_tap.jpg will have as much as 50% or more of the neutral current flowing in the steel of the nipples and the cabinets. In fact, if there is poor contact at, say, the connection of a nipple to the gutter, it is possible that that point will get dangerously hot.

In any case, it is clearly wrong to have any of the service current, let alone a substantial part of it, flowing through the metal of the raceway. Yet that appears to be what the Code requires.

These are from NFPA so that is what they are thinking. I agree the whole cludge around the meter to service disconnect(s) is one big conductor but they stopped worrying about that sometime around 1999 or 2002. They are specific that you can't reground the neutral on the load side of the service disconnect but they are silent on the line side. The utility will ground it in the meter base if nothing else.
For that matter, you also carry current on the ground electrode system too. We just don't like to think about it.

I guess that's the only advantage of the European TT system - no ground loops. The PoCo grounds the neutral at the substation and that's it, local grounds are only connected to a ground rod. Of course the higher impedance of such a system requires additional ground fault protection, for example 100mA RCDs.

Wait, WHAT? The neutral bar in the gutter makes sense, but I really disagree with separately bonding the neuetral bars in the panels to the enclosure. Neutral current flowing through the enclosures and conduit is completely unsat.

250.24(C) is pretty clear that this is the way NFPA wants to see it, though. The only exception is if all the service disconnects are in the same enclosure.

FYI, NEC 2008 handbook retains gec_tap2.jpg, but does not include gex_tap.jpg.

Thinking more about this, I think I see why this works OK. (And I'm intentionally saying "OK", rather than "well." It's only just, "OK.")

The key is that the white neutral wires going from the neutral bar in the gutter to the neutral bars in the panels are very low impedance paths. That establishes an "upper bound" on the voltage difference between the three neutral bars show, and that upper bound is well under one volt.

All the additional bonding serves to reduce the impedance, making that voltage differential even smaller.

So even though this whole thing is a mess, the maximum voltage differential that results from the mess is substantially less than one volt.

And "substantially less than one volt" is small enough that it doesn't result in any serious problems in practice.

Still, I would much rather see separate neutral and ground bars in the panels, fed by separate neutral and ground wires going to neutral bar in the gutter.