Installing 240V Service To Your Shop - Price Breakdowns...

[ThomasH]

Sorry for contributing to the off-topic,,,,but this is interesting, educational.

On my "main" panel (200A service house panel) the ground bar and neutral bar are bonded together, bare grounds and white neutrals are mixed on both bars. Inspected and passed. But on the subpanel, grounds and neutrals must remain separate, and the neutral isolated from the subpanel box. ..snip...

I am puzzled and interested in this detail.

Normally the service entrance on the house is a somewhat small distance from the overhead drop or underground feed to the meter. And it is much larger typically than a feed to a sub-panel is. So the thinking was at the time that there wouldn't be much of a voltage difference if you did the bonding in the panel (which is no longer allowed - it is now at the meter along with the ground rod connection).

A barn or shop is often a hundred or more foot run with smaller wire, and has the potential of energizing the ground with enough voltage by the time it gets back to the main panel or meter, to be a hazard. So in order to not use the ground wires as a possible return leg and energizing the grounds that could be a long distance from the earth ground, they don't allow it any more.

And they are reviewing ground rods now and probably are going to require more and longer ground rods in the future. Don't assume that just because something was allowed at some point that it is really as safe as it should be to keep people from being shocked or killed. Grounds have always been an area that have been somewhat unsafe in the past. In our area, they considered water pipes as a valid ground rod in the past, so as long as you had a copper water service line, and your panel bonded to it, it was considered ok. That was just plain dangerous...

 

[Sodo]

ThomasH that helps me understand the need for (more) ground rods at the subpanel. Due to the distance (wire length) there can be potential out at the subpanel that is (hoped to be) reduced, by dumping it into add'l nearby ground rods.

Thanks for the attempts to get thru this thick skull but still can't quite get this part, how either one is safe and the other is not.

If I plug a drill press into a "house" outlet, the eqpt case will be grounded and connected to the neutral bar. And if plug the drill press in at a "garage outlet", then the same grounding condition occurs (at the house panel).

Is there another way to explain it?

 

[JPRambo]

I found this in my copy of the 1990 NEC handbook.
Sorry for the poor scan quality.

 

[newbury]

<snip>how either one is safe and the other is not.
<snip>

I think that no matter how it is done, with electricity and the off chance of a lighting strike or dramatic short nothing is truly "safe". All the precautions attempt to do is minimize the danger.

We are all going to die, the only question is when.

 

[buickanddeere]

ThomasH that helps me understand the need for (more) ground rods at the subpanel. Due to the distance (wire length) there can be potential out at the subpanel that is (hoped to be) reduced, by dumping it into add'l nearby ground rods.

Thanks for the attempts to get thru this thick skull but still can't quite get this part, how either one is safe and the other is not.



Is there another way to explain it?

At the garage there will be a difference in potential between ground and the neutral due to load current causing voltage drop on the neutral.
The isolation of ground and neutral bars in the garage panel prevents neutral current from flowing in the ground circuit and thus raising it's potential above earth.

 

[jlgurr]

We are contemplating running 100 amp service to our workshop/garage from the 200 amp service panel in the house. Feeder length will be about 185 feet unless I cut the driveway. Originally planned on using aluminum conductor for mobile home service. Hadn't considered unbonding the neutral and ground at the sub. Hadn't considered conduit or nicks in the insulation...

..this has been one of thee most helpful and educational threads I have read here at TBN. Thank you all!:thumbsup:

 

[Sodo]

OK I got it. Thanks. Agreed on value of this thread, glad those who know were persistent in passing on this knowledge. It may be hard for others to find this off-topic info by a subject title search but I'm glad to have been privy to the discussion.

Ground is supposed to sit there, un-used, a safety conduit. But if tied together with the neutral, ground circuit will share some of the current carrying capacity. In other words whenever a light is on, or a welder is used, there will be measurable current flowing in the ground wire and that should never be. Because all the metal chassis' of all the other appliances are connected to the ground wire.

You want ALL of the neutral current flow to be in the neutral, not dividing off, energizing the ground circuit.

It's a little easier to understand when you think of these wires as sewer pipes, don't want multiple paths that can back up.

 

[CNC Dan]

ThomasH that helps me understand the need for (more) ground rods at the subpanel. Due to the distance (wire length) there can be potential out at the subpanel that is (hoped to be) reduced, by dumping it into add'l nearby ground rods.

Thanks for the attempts to get thru this thick skull but still can't quite get this part, how either one is safe and the other is not.

Is there another way to explain it?

The neutral wire carries current and therefore will be at a higher voltage at one end and at ground potential (Zero volts) at the other end.

The bare ground wire carries no current (normally) so it should always be at ground potential throughout its entire length.

If you swap or connect the two, they will share current, and will have more than zero volts potential.

You don't want the case of any of your tools/equipment to be charged at ANY voltage. The case of your tools/equipment is connected to the third (ground) prong of the plug.

 

[DarkBlack]

I found this in my copy of the 1990 NEC handbook.
Sorry for the poor scan quality.

You had to show an NEC explanation and ruin all the alternate theories:laughing:

 

[JPRambo]

I had a very good instructor when I went to the Vo-Tech center to learn electrical wiring when I got out of the Army in 1978. I wired houses for 18 years and remembered needing to keep the neutral and ground separated after the first disconnecting means. But I have been working in a hospital for the last 14 years and needed to get the real reason and code reference back into this old head of mine. And I think it helped me and hoped it would help this discussion.