Are we talking about non metallic raceways here? What grounding
conductor are we speaking of? Is it the Equipment Grounding Conductor
(EGC) of the circuit in question or is it the or the Grounding Electrode
Conductor (GEC) of the entire electric service that is supplying that
Specifically, it's a 240V feed from the service panel to a subpanel, with the
neutral and the two hots in rigid nonmetallic conduit going through the attic
of the garage, and the equipment ground in rigid metal conduit buried in (or
under) the slab.
[Note: I didn't do this. I'm the fourth owner of this house; the original
electrician knew what he was doing, but in between him and me, there's been
work done that was somewhat less than fully Code-compliant.]
If all the current-carrying conductors are in the same raceway, the
magnetic fields will cancel each other out. The equipment ground is not
a current carrying conductor. That's not an excuse for running the EGC
in a seperate raceway, but it does mean the inductive heating problem
that Tom is talking about is not an issue.
RIght -- but could that cause any *other* problems?
Doug Miller (alphageek-at-milmac-dot-com)
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That is a dangerous installation. If a fault were to occur in the sub
panel the impedance of the Equipment Grounding Conductor may be high
enough to delay the operation of the over current protective device.
The longer a fault continues the more damage it is likely to do.
It's not a proper installation, but you're making stuff up about the
danger. (If I can find my code book, I look up where this situation is
specifically allowed for retrofitting old work.) If it was dangerous,
it wouldn't be allowed under any circumstances.
If the EGC is too small, it doesn't matter whether it is run with the
circuit conductors or not, it is too small.
I'm sure Tom P. will be all over this...
Well you locate that code book and then you come back and tell us which
section you think allows you to run the Equipment Grounding Conductor
(EGC) for a feeder in a separate raceway.
Listen, boyo, I have not only been doing electrical work for thirty five
years, I have been fighting fires for thirty. You haven't lived until
you've crawled down a smoke blackened hallway at 0dark30 in the blessed
AM looking for other peoples children because some idiot thought that he
could ignore the basic physics of electron flow. Do you even know why a
breaker has a withstand rating on the label? Do you know what the
instantaneous current flow is likely to be in a sixty ampere feeder
supplied from a panel with an available fault current of say 7500 AIC?
There really is more to electrical work then color to color.
Danger isn't a "yes or no" proposition. Every practise has a distinct
risk factor. There are things that are permitted for rework/renovation
that aren't permitted for new work. There are inspector-permitted
things that aren't officially permitted at all. There are things
that are so dangerous they're not even grandfatherable. Canadian
and US electrical code isn't identical - they just weighed their
risk factors differently.
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.
If the current involved were DC you would be correct but since it is AC
the circuit's reaction to current flow is not just resistive in
character. The inductive reactance caused by the high magnetic fields
that occur during a fault have to be taken into account. The combination
of inductive reactance and resistance is called impedance. Impedance is
a measure of the net opposition to current flow in an AC circuit.
The occurrence of a fault in the sub panel would cause an instantaneous
current flow of several hundred amperes. A current of that magnitude
flowing on the Equipment Grounding Conductor (EGC) will induce a current
into every metallic object that is adjacent to it and that includes the
metal raceway. The induced current causes a counter Electro Magnetic
Field which induces a current that is counter to the original current
flow in the EGC thus slowing the operation of the Over Current
Protective Device (OCPD). The longer it takes the OCPD to open the more
damage occurs, the more heat the arc generates, and the more likely a
fire becomes. If the EGC were in the same raceway as the faulted
conductor the magnetic fields would cancel out and very little such
choking action would occur.
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