Use two 12/2s for 240v?

Page 2 of 2  
Doug Miller wrote:

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 particular circuit? -- Tom Horne
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.]
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Doug Miller wrote:

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.
Bob
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

RIght -- but could that cause any *other* problems?
-- Regards, Doug Miller (alphageek-at-milmac-dot-com)
Get a copy of my NEW AND IMPROVED TrollFilter for NewsProxy/Nfilter by sending email to autoresponder at filterinfo-at-milmac-dot-com You must use your REAL email address to get a response.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Doug Miller wrote:

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. -- Tom H
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
HorneTD wrote:

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...
Bob
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
zxcvbob wrote:

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. -- Tom H
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Define "dangerous".
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.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Can you explain why this is so? It seems to me that if the ECG is properly sized, it would conduct the fault current with no problems.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Doug Miller wrote:

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. -- Tom H
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Got it [I think -- might need to hit the textbooks again :-) ].
Thank you.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Site Timeline

Related Threads

    HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.