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-   -   Why can't you connect Ground/Neutral bus bars on a sub (http://www.diychatroom.com/f18/why-cant-you-connect-ground-neutral-bus-bars-sub-63517/)

secutanudu 02-03-2010 12:52 PM

Why can't you connect Ground/Neutral bus bars on a sub
 
Subject says it all. I understand that there can only be one point of connection between the neutral system and ground system (in the main panel).

My question is...why? I can't figure it out.

What would happen if you were to have a shared neutral/ground bus in a sub panel?

junkcollector 02-03-2010 01:07 PM

Quote:

Originally Posted by secutanudu (Post 394077)
Subject says it all. I understand that there can only be one point of connection between the neutral system and ground system (in the main panel).

My question is...why? I can't figure it out.

What would happen if you were to have a shared neutral/ground bus in a sub panel?


You would have parallel paths back to your main panel; your ground would be carrying some of the current that your neutral should be carrying. The ground is normally a non current carrying conductor.

Bob Mariani 02-03-2010 01:47 PM

The ground must be clean (no current) to be effective to provide a path of least resistance to protect you from errant currents. In a sub panel the neutral carries current from any unbalanced loads. If you connect the neutral and the ground you have current on the ground, thus not a very good grounded system.

DownRiverGuy 02-03-2010 01:59 PM

Ok let's explain this with an example...

Let's assume it's a setup with a properly installed subpanel feeding a load that has some return neutral current (electronic equipment etc.).

3A travels from the outlet neutral to the subpanel neutral buss bar (which is isolated from the ground)
3A travels from the subpanel neutral buss bar to the neutral feeder to the main panel neutral buss bar.
3A travels from the main panel neutral buss bar to the main ground.

So at no point can anyone get shocked from that.

Now what happens if it's a setup with a neutral and ground being shared in a subpanel.

3A travels from the outlet neutral to the subpanel neutral buss bar.
The current then will travel to the ground of the system and will energize anything in the system while it travels back to the ground source.

Thus in the second method you give current a longer path to ground. and something or SOMEONE may interupt that path :-x

secutanudu 02-03-2010 02:18 PM

Got it. Thanks.

Giles 02-03-2010 02:29 PM

Quote:

Originally Posted by DownRiverGuy (Post 394110)
Ok let's explain this with an example...

Let's assume it's a setup with a properly installed subpanel feeding a load that has some return neutral current (electronic equipment etc.).

3A travels from the outlet neutral to the subpanel neutral buss bar (which is isolated from the ground)
3A travels from the subpanel neutral buss bar to the neutral feeder to the main panel neutral buss bar.
3A travels from the main panel neutral buss bar to the main ground.

So at no point can anyone get shocked from that.

Now what happens if it's a setup with a neutral and ground being shared in a subpanel.

3A travels from the outlet neutral to the subpanel neutral buss bar.
The current then will travel to the ground of the system and will energize anything in the system while it travels back to the ground source.

Thus in the second method you give current a longer path to ground. and something or SOMEONE may interupt that path :-x

Does the location if the subpanel matter. If the sub is mounted nex to the main panel as opposed to a sub mounted in a separate building?

DownRiverGuy 02-03-2010 02:40 PM

Per the code no. The moment you have a subpanel of any type (regardless of distance) it must have a seperate ground bus and an isolated neutral buss.

What happens if a person falls on the little piece of conduit connecting the two panels together and there is 30A flowing thru it? (I know not likely but still).

Jim Port 02-03-2010 03:19 PM

The sticky post by Bruto at the top of this forum explains this very well.

Stubbie 02-03-2010 04:06 PM

1 Attachment(s)
Quote:

Originally Posted by Giles (Post 394121)
Does the location if the subpanel matter. If the sub is mounted nex to the main panel as opposed to a sub mounted in a separate building?

It certainly does matter depending on the code cycle your on. In 2008 the NEC withdrew the exception for a 3 wire feeder to panelboards (sub-panels) in detached buildings where no metallic paths other than the feeder existed, no gfpe, and no feeder equipment ground.
Prior to 2008 if you had a three wire feeder consisting of 2 hots and a neutral then you would bond neutral and ground assuming the installation meets the exception requirements. This is very common to see 3 wire feeders to detached structures, neutral and ground bonded.

Only on 4 wire feeders to any panelboard would you not bond neutral and ground.

So if your area is still working with a code cycle other than 2008 you can use 3 wire feeders to panelboards in detached structures not having the service equipment if all 3 requirements listed earlier are met.

Below is an example I drew for the sticky mentioned by Jim in post #8 showing the unwanted result of combining neutral and ground in any panelboard served by a 4 wire feeder....

dougw 10-25-2010 11:01 PM

Stubbie-
If I have a detached subpanel with only a 3 wire(2 hot and 1 ground) hookup with a bonded neutral/ground, do I still need separate ground rods connected to the ground in the subpanel?.....

dougw 10-25-2010 11:33 PM

Stubbie-
Disregard my last question I found your diagrams that explained it perfectly...Thanks-Doug

brric 10-26-2010 06:49 AM

Quote:

Originally Posted by DownRiverGuy (Post 394110)
Ok let's explain this with an example...

Let's assume it's a setup with a properly installed subpanel feeding a load that has some return neutral current (electronic equipment etc.).

3A travels from the outlet neutral to the subpanel neutral buss bar (which is isolated from the ground)
3A travels from the subpanel neutral buss bar to the neutral feeder to the main panel neutral buss bar.
3A travels from the main panel neutral buss bar to the main ground.

So at no point can anyone get shocked from that.

Now what happens if it's a setup with a neutral and ground being shared in a subpanel.

3A travels from the outlet neutral to the subpanel neutral buss bar.
The current then will travel to the ground of the system and will energize anything in the system while it travels back to the ground source.

Thus in the second method you give current a longer path to ground. and something or SOMEONE may interupt that path :-x

Current does NOT normally flow on the grounding system. Current flows back to the source which is usually a transformer.

a7ecorsair 10-26-2010 09:16 AM

Quote:

Originally Posted by DownRiverGuy (Post 394129)
What happens if a person falls on the little piece of conduit connecting the two panels together and there is 30A flowing thru it? (I know not likely but still).

Nothing would happen. Just because there would be 30A of current doesn't mean there is a difference of potential.

sky jumper 03-09-2011 09:00 PM

ok so I understand if neutral and ground are connected at an outlet (or device), ground could become a return path for current. my question is so what? since neutral and ground are tied at the service panel, neutral is always at ground potential. this is AC - so it's the hot that varies +/- 170Vpk above and below ground potential, not the neutral. so what if the chassis of the washing machine is tied to neutral? you and the chassis and neutral are all at ground potential anyway, so nothing will happen. your body's resitance is many magnitudes more than a copper wire, so no current will flow through you even if it does flow through the chassis of the washing machine.

ok now assume there's a fault in the washing machine (or stove top) that shorts "hot" to chassis - it will blow the circuit breaker immediately, even if neutral is tied to hot. no danger there.

ok now assume that nuetral is somehow open circuited somewhere in the house (so the chassis is not grounded or tied back to panel neutral) AND AT THE EXACT SAME TIME there just happens to be a fault inside the device that shorts hot to chassis (you see, if either happened first the device would harmlessly stop working, so you'd know something was wrong and get it fixed) -- ok with simultaneous faults (VERY unlikely) you now you have the potential for a hot chassis. BUT why would you assume that a separate ground wire would neccesarily prevent this? I would argue that whatever disruptive action caused the neutral to open (e.g. a contractor's recip saw) would also cut the ground connection to the device (and probably the hot too) since they are all run together. for the life of me I can't imagine a scenario where only the neutral would open circuit AND at the same time a device suffers an internal short to chassis.

if anyone out there has a real world example of this I'd love to hear it.

a7ecorsair 03-09-2011 09:12 PM

Equipment ground is there as a safety factor. Years ago, nothing was grounded and drills, clocks, toaster, table saws, you name it, all worked just fine on two wires.
Do you have a spare tire in you car just in case you need it? I want my metal framed tools properly grounded just in case I need it.....


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