Residential wiring question

[zahc]

In my house's main panel, there is a big bus where the neutral wires and the ground wires are all bonded together (if memory serves). From what I understand, this is the only place the neutral wires and the ground wires can be connected (in the panel). Because the ground wire is not supposed to carry any current and all.

In the same main panel, there is a 40A 2-pole breaker which is hooked to 8 gauge wire which runs out to the detached garage's sub-panel. In the garage panel, all the neutral wires are connected together, and all the ground wires are bonded to the box, but the grounds/box and the neutrals are NOT bonded together, as far as I can tell. Is this correct/normal? I kind of expected the garage neutrals and grounds to be tied together since it's 'a breaker box'. I'm wiring a 4-wire 240V receptacle out there, and I'd like to correct things if I can.

Also, the 8-ga wire that feeds the garage panel appears to have a substantially smaller ground wire, like 12-guage, even though it looks like it's in a typical romex jacket with the other 8-ga wires. Was this allowed under older codes? I've never seen romex where the ground wire is smaller.

[Hawkmoon]

That is correct for a sub-panel.

Don't ask me why, because no matter how many times the State Electrical Inspector explains it to me, it doesn't make sense 15 seconds after he leaves the room. But that's what the NEC says. When using a main panel as a sub-panel, the little tab thingie that connects the neutral bus to the ground bus is supposed to be removed.

http://ths.gardenweb.com/faq/lists/wiring/2005015726007600.html

Scroll down to the section on "Grounding and bonding of the sub-panel"

This next one says the same thing:

http://www.handymanclub.com/projects/articletype/articleview/articleid/5050/how-to-install-a-subpanel

[CNYCacher]

Think of your sub-panel as any other junction box, but bigger and with breakers in it.  Why would you attach the neutral to ground here?  Just because of the box shape?

[Hawkmoon]

I overlooked the fact that the sub-panel is in an outbuilding. Note that both the articles point out that a sub-panel in an outbuilding must have it's own ground rods (called "grounding electrode system" in the NEC).

[zxcvbob]

Is the cable feeding the outbuilding 3 wires or 4?  Either is legal (at least it was a few years ago) and it determines whether the box is a subpanel or a service entrance.

3 wires = service entrance: grounds and neutrals are connected, must have a grounding electrode. (maybe 2)

4 wires = subpanel: grounds and neutrals are isolated.  Should have a grounding electrode but I don't remember if it's required.

[Triphammer]

The key phase is "First means of disconnect". The first place the power can be shut of, wether that's your main panel in the house or a service entrance outside. It's the last place that grounds & neutrals can be bonded/ same wire. Until 1974, nuetral & grounds could be bonded in sub panels. After that, if the service went to another building, if there were only three wire, you COULD have a seperate grounding electrode & call it a seperate service entrance. Even though it's fed from another metered service. Otherwise, If there is a seperate ground conductor to the main service it's not supposed to have a seperate groung electrode.

[geronimotwo]

general rule is that the neutral is only bonded to ground where there is an approved grounding to earth.  

[never_retreat]

general rule is that the neutral is only bonded to ground where there is an approved grounding to earth.  

Not exactly, the neutrals and ground are only connected together in the main box or the mail disconnect. If you happen to have a disconnect on the outside of the house near the meeter pan than that is the only place. That is also where the ground rods should be attached.
If the sub panel is in a detached structure it needs its own ground rod also. I believe if there is water pipes in that building they also need to be bonded. Also there is some crap in the codes about bonding the outbuilding if it is steel, and bonding the rebar in the slab if it is new construction.