connect al and cu wires together

[Speedy Petey]

Originally Posted by JCA1

Why doesn't the neutral serve as a bonding wire?

Actually in your case it likely can.
The neutral of a sub-feed to a detached structure can also be used as the grounding conductor (unless you are under the 2008 NEC) if certain conditions are met. The main condition is that there are not other metallic paths between structures. Examples would be low-voltage cables or a copper water pipe.
This allowance was removed in the 2008 NEC.

Originally Posted by JCA1

What extra protection does the bare ground bonding wire offer over the ground rod at the building? How?

A ground rod does NOT "ground" anything. It does have a purpose, but that is not it. A ground rod does not "create" a ground, at least not in the sense of the "safety" ground we are all familiar with, the one that causes a breaker to trip if something is shorted to ground. That ground comes from the neutral to ground bond found at the main service.

[JCA1]

OK, thanks. This means then that the bare ground wires for each circuit need to be isolated in this shed panel or not? I'm guessing not? I mean do I need another ground bar in the panel to connect the ground rod wire and all other circuit bare ground wires to or everything goes to the same bar as the neutrals?

[JCA1]

Speedy Petey, do know why the above mentioned allowance was removed in the 08 NEC? Any real problems that came up or just a decision based on educated theory?

[HayZee518]

A ground rod doesn't offer a complete ground per se. this is because of ground resistance. some soils may contribute greatly because of moisture and salts present, others may not like a sandy soil. the isolated neutral has to do with short circuit tripping. the aux ground has to do with fault current. even though the origin of the two cables is the same - your neutral in the main panelbox at its remote end it's isolated. I could never comprehend this myself - just go with the flow of what I'm told by inspectors.

[Speedy Petey]

The reason for disallowing this, same as for the removal of the old "3-wire" dryer and range receptacles, is because if there is a compromise in the neutral any fault current to ground can make metallic parts become live. This will happen even with a ground rod. This is what I meant about a ground rod NOT providing a real safety ground.
With an open neutral, 120v current will try to flow back to it's source. This is the utility transformer. It will do this any way it can. If the neutral is bonded to the grounds they can use this as a path. This will create a situation where current is flowing on exposed parts and also into the earth. The reason for the "no other metallic paths" rule is because these other paths can also become paths for the stray current to flow. Again, NOT a good situation.

Kind of clear?

[JCA1]

OK, yes this is making sense now I think, I'll probably blow it with this:

The grounding circuit is there to carry fault currents back to source. a standard circuit has two ground circuits, the neutral and the ground. Normally the neutral will carry current back to source but if this is compromised the ground will do it. In my case the ground at this shed is a ground rod. So if I do not isolate the neutrals from the grounds then any fault current on the ground will actually go to ground seeking out any path back to source, which could be a phone line, cable wire, water line etc. This then would energize that path possibly putting dangerous voltage on the phones, cable wires, etc. So at this shed I need to isolate the neutrals from the grounds, or isolate the ground rod wire from the other neutrals/grounds? I think I've almost got it, I really appreciate you taking the time to educate me Speedy Petey.

[JCA1]

I guess you have figured by now that I am a little slow, anyway, I'm still trying.

If I bond the neutral and ground at the panel at the shed and the neutral goes bad, then any fault current will go to the ground rod into the earth and look for a way back to the transformer. This however would not trip the breaker because enough current could not flow through the ground rod, because of the resistance between the rod and earth, to trip the breaker. hence I would have a steady current flow on this bare wire and rod into the earth, is this correct?

If I do not bond the ground and neutrals in this shed panel, I would essentially have the same problem right? Because a fault would still make it to the ground through any receps or switches, again energizing the ground rod? having the fourth wire would carry the fault current back to the main panel, but would allow enough current flow through the breaker to trip it? Please don't give up on me yet, you can teach me if you're patient, please keep trying Speedy Petey and HayZee, thanks for everything so far.

[HayZee518]

because you have a solid path back to the main neutral bus [ the bare ground and the white cable] one breaker will trip. [on the affected circuit] the enclosure case will be bonded to the common neutral back at the main panel so any fault to the metal ground will cause it to trip. any short to the white will also trip the affected circuit breaker.

[JCA1]

I went back and read through this some more, it makes more sense each time I read it( I have poor reading comprehension skills it seems) and I think I realy understand now. I swear I'm not as stupid as I make myself out to be, even though I have to be that stupid to sound like I'm that stupid...ha-ha. Or like a good friend of mine told his then girlfriend one time: "If you go around acting stupid people will think you're stupid" As it turns out I was misinformed, we are not under the 08"NEC yet, but will be very soon, so I'm still OK with the 3 wire feed. Anyway, it goes like this, right? The current wants to return to source(transformer), it does this via the neutral. The grounding conductor is there to provide a way for fault currents to get back to source, which still would be the neutral bar in the main panel where the grounding conductors bond with the neutrals.
In my case with the 3 wire feed I have only one path back to source from the sub panel and that is the neutral. Therefore any fault currents in this outbuilding will have to go to the neutral bar so they can follow this path, if not they just energize anything and everything metal that they can. Now if the neutral is compromised I essentially have the same problem, no path back to source, so the current will follow the ground rod and seek a way back to source, but the resistance between the ground rod and earth, as well as the overall resistance of the earth itself, may very well prevent the breaker from tripping. Hopefully this is right.

[HayZee518]

here's a diagram of a typical service from the pole to your house. a single phase pole top transformer has one high side lead connected to the 4160 distribution line at the top of the pole. the secondary of the transformer with terminals known as X1 & X3 are the 240 volt secondary. The X0 is the midpoint of the transformer windings and forms a neutral leg. So from each X1 to neutral you get 120 volts. X3 to neutral you get 120 volts. Across X1 & X3 you get 240 volts.