Simple question on design of electrical subpanels

Hello,
Is it standard for a residential electrical subpanel to use its chassis as a ground backplane? I.e. the ground from the incoming 4-wire feed terminates at a lug bolted to the back of the panel, and the ground bar is also just bolted to the back of the panel.
Thanks, Wayne
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Yes, that's fine. On a sub panel you don't want the neutral buss to be bonded to the enclosure

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Thanks, I understand the part about the neutral buss being kept separate from the ground buss in subpanels. And obviously the subpanel chassis needs to be tied to the equipment ground. I was just surprised to the subpanel chassis as part of the fault current path for all the circuits served by the subpanel. That's fine?
Thanks, Wayne
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If you have metallic cables or conduits, their connection to the chassis is their path to ground
wrote:

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Hmm, good point--it is permissible to use some conduit systems as the EGC in the branch circuits, so then subpanel chassis is part of the fault current path for those circuits. However, while this may meet the NEC, I would not consider this to be the best practice. So perhaps that is the answer to my original question.
Cheers, Wayne
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Wayne Whitney wrote:

Think about this....if that metal enclosure or any metal conduit is not grounded.....what do you think is going to happen in the event that a hot wire gets a nick in it...or it comes loose from its connector....you dont ever want to have that enclosure or any metal conduit in a situation where it can be hot. Grounding the enclosure and any metal conduit or raceways makes perfect sense.
Ive had the fire knocked out of me before by a 4 by 4 junction box before....hot wire loose touching the box.....box wasnt grounded........thank goodness it wasnt a damp area.
~:>
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I think my original question must not have been clear, of course you have to ground the subpanel chassis. The question is whether it is OK to use the subpanel chassis as part of the fault current path for the circuit served by the subpanel.
Wayne
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Wayne Whitney wrote:

Here 'ya go:
http://ecmweb.com/mag/electric_grounding_vs_bonding/index.html
Just don't blame me 'cause you just lost half a day - I have to re-read the whole thing twice a year to keep it all stright myself <grin>.
Michael Thomas Paragon Home Inspection, LLC Chicago, IL mdtATparagoninspectsDOTcom eight47-475-5668
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On 2006-07-16, MDT at Paragon Home Inspections, LLC

Thanks, I've actually read those articles once upon a time, at least the residential parts. I see that Figure 2 of that article actually shows the chassis of a subpanel as part of an "effective ground-fault path". So this reinforces the idea that this is NEC compliant, although perhaps not best practice.
Cheers, Wayne
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Properly wired "subpanel":
http://www.home-inspect.com/email-images/jan04image.gif
Michael Thomas Paragon Home Inspections, LLC Chicago, IL mdtATparagoninspectsDOT com eight47-475-5668
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cabinet itself. Grounding path for the cabinet should be cabinet-to-bus-to-incoming grounding conductor so the cabinet is not part of other ground fault current paths.
Don Young
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Wayne Whitney wrote:

It would be better practice to have all Equipment Grounding Conductors terminate to the Grounding buss bar so as to avoid using the cabinet as a fault current pathway. If more than one Grounding buss bar is installed they should be bonded to each other by running a conductor that is sized for the largest Over Current Protective Device supplying power to that panel between the buss bars.
--
Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous
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So it sounds like the setup I describe is NEC compliant but not best practice? Another poster pointed out that if any branch circuits are run in metallic conduit and use the conduit as the EGC, then the panel chassis is part of the fault current path anyway. However, I've run a separate EGC for the circuits I've run in EMT.
The subpanel I'm using, the Square D QO132L125G, comes with a ground bar that has no terminals for 1/0 wire and with a single lug bolted to the chassis for the feeder ground wire. So it seems the manufacturer's intention is to the chasis as a ground backplane. Perhaps a better solution would be to replace the single lug with a double lug (terminology?) and to run an appropriately sized jumper wire directly to the ground bar?

Do you mean the largest size OCPD for circuits supplied _from_ that panel, or the size of the feeder circuit OCPD supplying power _to_ that panel? Seems like it should be the former.
Cheers, Wayne
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Wayne Whitney wrote:

There are add on lugs available for Square D ground buss bars for many sizes of wire. Adding one of those to the existing buss bar would be an elegant solution. Your right I should have said "the largest size OCPD for circuits supplied _from_ that panel."
--
Tom Horne

Well we aren\'t no thin blue heroes and yet we aren\'t no blackguards to.
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Ah, thanks for the pointer, that's a good solution. I think I may have one of those lying around, I didn't understand its purpose when I received it.
Cheers, Wayne.
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