Wiring subpanel - seperate ground / neutral

I'm wiring a subpanel in free standing garage. 70A circuit, on 4/3 conductor with a ground.
There is something that I don't understand: The main panel has a bus that combinds the neutral and ground. As I read about sub-panel wiring, the neutral and ground should be seperate.
The seperate neutral and ground at the subpanel run to the combind bus at the main panel, so electrically speaking, there is continuity there..
Why wire them seperate at the subpanel?
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DG wrote:

They are not the same. There is current flowing on the neutral wire, and because its resistance is not zero there is a voltage drop from one end to the other. So the neutral bus is not quite at the same potential as the ground when there is a load (unless it is a completely balanced 240V load)
You said it is a free standing garage, so you /can/ combine the ground and neutral if the box is rated as "service equipment" and if you do it right (there's lots of rules.) You're better off in my opinion, since you have the separate ground already, to add a ground bus kit and remove the bonding screw that electrically connects the neutral bus to the box.
Bob
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The neutral and main grounding conductor are only permitted to be bonded in one place; at the main panel or in the meter. If the neutral conductor in the subpanel ever became disconnected, anything that was grounded would become a shock hazard as the grounding conductor would now carry the neutral current. Having them separate reduces that possibility. You will need to purchase a separate grounding bar for your subpanel and have all of your ground wires terminate on it. You also need to drive at least one (Two is most likely) ground rod for the garage and connect them to the ground bar also. If you have any metal piping in the garage it will need to be bonded to the ground bar. Do not install the bonding jumper or bonding screw that is furnished with the panel.
John Grabowski http://www.mrelectrician.tv
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If you combine them at the subpanel, then it is possible that your ground in your feeder (4/3) will become a return path of electricity to the main panels neutral. Equipment Grounding conductors should never carry voltage except as a path of least resistance for carrying ground fault back to the electricy source.
Becare and follow all national and local electrical codes.
later,
tom @ www.NoCostAds.com
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Thanks, guys. I have a ground bar and my neutral bus is not connected. I'll check with my meter to be 100% sure, but I'm planning on wiring them seperately.. I was just curious in regard to the reasoning behind it.
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http://en.wikipedia.org/wiki/Ground_%28power%29
Read the paragraph about combining neutral with earth.
--
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I think the reason for separating the ground and neutral is to insure GFCI breakers work properly. If fault current went to tthe neutral the GFCI may not trip. Remember, for years it was considered good practice to have them tied together.
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" snipped-for-privacy@bellsouth.net" wrote:

The problem with that reasoning is the fact that sub panels are rarely ever fed from GFCI breakers and indeed GFCI breakers are rarely used themselves (in residential) since they cost 5x what a GFCI outlet that provides pretty much the same protection cost.
Pete C.
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