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?

DG wrote:
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?



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

"DG" wrote in message
oups.com...
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?


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

On 1 Apr 2006 07:00:30 -0800, "DG" wrote:

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?


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

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.

"DG" wrote:

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.

http://en.wikipedia.org/wiki/Ground_%28power%29

Read the paragraph about combining neutral with earth.
--
NewsGuy.Com 30Gb $9.95 Carry Forward and On Demand Bandwidth

"DG" wrote in message
oups.com...
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?

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.