main panel - InspectionNews

Jerome W. Young · #1 (**permalink**) 01-04-2008, 05:45 AM

I know this is probably the most asked question but it always throws me when i see it.

Scenario: Residential home

400 amp SqD handled main breaker.

Less than 4 feet away from it are 2 - 200 amp main/sub-panel cabinets. each panel has connected grounds and neutrals. Should they be floated?

Jerry Peck · #2 (**permalink**) 01-04-2008, 05:58 AM

Jerome,

Once 'not-part-of' the service equipment, yes, the neutrals need to be isolated from ground.

And 4 feet away is 'not-part-of' the service equipment. Even if you had a meter, then a main disconnect (the service equipment), then right next to it a panel, that panel would not be part of the service equipment.

In your case, the 400 amp main is the service equipment. Once outside that enclosure, everything else is 'not-part-of' the service equipment.

Carl Eisen · #3 (**permalink**) 01-04-2008, 06:19 AM

If I properly understand Jim Katen on this subject, the less distance between the panels the less resistance there is in the bond between the two. Since current will look for the easiest path back to ground this would actually increase the chances that current could find its way to the panels' case.

John Arnold · #4 (**permalink**) 01-04-2008, 07:12 AM

Quote:

Originally Posted by Carl Eisen

If I properly understand Jim Katen on this subject, the less distance between the panels the less resistance there is in the bond between the two. Since current will look for the easiest path back to ground this would actually increase the chances that current could find its way to the panels' case.

Carl - I'm confused by your post. Are you saying that if the equipment is wired as Jerry described that there would be greater chance of the enclosures becoming energized and shock hazards?

Carl Eisen · #5 (**permalink**) 01-04-2008, 07:43 AM

Quote:

Originally Posted by John Arnold

Carl - I'm confused by your post. Are you saying that if the equipment is wired as Jerry described that there would be greater chance of the enclosures becoming energized and shock hazards?

No.

I suggest that if the panels are wired improperly, ie. the neutral is bonded in a panel after the service entrance equipment, then there is the potential for current. This potential increases as the resistance decreases between the panels.

Ed Voytovich · #6 (**permalink**) 01-04-2008, 01:23 PM

Quote:

Originally Posted by Jerry Peck

Jerome,

Once 'not-part-of' the service equipment, yes, the neutrals need to be isolated from ground.

And 4 feet away is 'not-part-of' the service equipment. Even if you had a meter, then a main disconnect (the service equipment), then right next to it a panel, that panel would not be part of the service equipment.

In your case, the 400 amp main is the service equipment. Once outside that enclosure, everything else is 'not-part-of' the service equipment.

Jerry: Is this the case even if the panel and subpanels are electrically bonded by rigid metal conduit between them? The question has come up a few times here.

Jerry Peck · #7 (**permalink**) 01-04-2008, 01:29 PM

Quote:

Originally Posted by Carl Eisen

I suggest that if the panels are wired improperly, ie. the neutral is bonded in a panel after the service entrance equipment, then there is the potential for current.

"Potential for current" as in "possible for neutral current on the ground wire", not as in "potential = voltage", right?

Quote:

This potential increases as the resistance decreases between the panels.

This "possibility" (see above) does not increase nor decrease based on distance.

The resistance in the neutral and the ground is reduced, thus, 'along with the "possibility" for current (which is not related to distance) comes the "possibility for greater neutral current on the ground conductor due to the reduced resistance of the conductor".

Along with reducing the resistance of the ground conductor path comes a reduction in the resistance of the neutral conductor, thus the "possibility" for current does not change.

With the neutral bonded to ground at a panel downstream from the service equipment ... there *will be* current on the ground conductor path. Current does not seek the path of least resistance, current seeks *ALL* paths, and, if the resistance of *each* path is known (or can be determined - as 'with an ohm meter'), then the amount of current on *each* path can be calculated.

Distance does, however, affect the relationship between the ground conductor path and the neutral conductor because the the resistance of the ground conductor path (I am using 'ground conductor path' as some of it may be ground "conductor" and some of it may be "metal raceway or enclosure", i.e., "path") will reduce by a greater amount than the resistance of the neutral conductor will (unless both are the same size conductor, of the same material, and of the same length, i.e., a metal raceway/enclosure "path" will have a greater resistance than a "conductor", thus, cutting the distance of each in half will reduce the higher resistance by a greater amount).

Did I make clear that up ... or make or muddier?

Jerry Peck · #8 (**permalink**) 01-04-2008, 01:37 PM

Quote:

Originally Posted by Ed Voytovich

Jerry: Is this the case even if the panel and subpanels are electrically bonded by rigid metal conduit between them?

Yes.

Even if the service equipment enclosure and the panel enclosure are bolted together (but not made as one by the manufacturer) and the wires go through a closed chase nipple (the shortest nipple available) - yes, they are two distinct enclosures and two distinct pieces of electrical equipment, with a separate neutral and ground path running between them.

With combination service equipment / panels in one enclosure, then they are made to be used either as "Suitable For Use As Service Equipment Only" or as "Suitable For Use As Service Equipment" - the difference being that in the "Suitable For Use As Service Equipment Only" enclosure, the neutral *IS* bonded to the enclosure/ground and there is no way to change it.

In "Suitable For Use As Service Equipment" enclosures, the neutral is isolated from the enclosure/ground and must be bonded to the enclosure when used as "service equipment".