Yep, understand that -- but how much of an issue is this, if the conductor that's separated is the equipment grounding conductor (which should never carry any current under normal circumstances)? It's a 240V circuit, with the neutral running in the same raceway as the two hot conductors, so the magnetic fields there should cancel out.
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
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.
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
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
This example is not doing what you are saying. It is not passing separate black conductors to the same circuit using different cables. It is using a two wire cable instead of a three wire one on a three way switch. It's not the worst thing I've seen, but I wouldn't recommend that either.
For those who are interested in the worst thing I've seen, I'll mention it, although it won't help with this problem. There was one house I came across where somebody decided to use two switches to control a light but had only a two wire cable connecting them. He ran a standard two wire cable into the first switch box. It passed the white wire through to the next switch, and the other lead running to the other switch switched between black and white. The second switch passed that switching lead directly to the light, and the second terminal of the light connected to a three way switch that switched between the white coming from the other switchbox and a new black that came in from somewhere else. This had been working fine until the homeowner replaced the switch with a lighted mercury switch, and when he turned the light on or off, the circuit breaker tripped (or it might have been a fuse back then.) It turned out that in a mercury switch, as the mercury flows, it might still be in contact with one pole momentarily when it contacts the other pole before the mercury all runs down. So what seemed like a simple task of changing a switch resulted in a circuit that would not stay on and there was no sane explanation. I suppose who ever did that could have made it even more confusing by using only one of the conductors. Perhaps he did-- it was at least three decades ago.
Any time you do something that may confuse the next homeowner who assumes that it's a straightforward circuit, you are asking for trouble. In the example you showed, a homeowner could easily take a switch out to replace it, not pay much attention to the wires until it's time to put the new switch in, and he'd be left scratching his head trying to figure out what is going on, especially with an unused lead in there.
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.
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
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
According to zxcvbob :
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.
Got it [I think -- might need to hit the textbooks again :-) ].
Thank you.
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