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, 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?
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.
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.
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.
Here 'ya go:
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>.
Paragon Home Inspection, LLC
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.
The incoming ground wire should terminate at the ground bus, not at the
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.
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.
"This alternating current stuff is just a fad. It is much too dangerous
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.
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."
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