3-wire to 'sub panel'

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electrofelon

electrofelon

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Cherry Valley NY, Seattle, WA
Ok so why do we run 4-wire instead of 3-wire to a sub panel (in same structure)? Well, I think I know the answer, so let me rephrase - Why is it safe to run a 3 wire to feed a service panel, but not to feed a sub panel?
 
One major reason the grounded circuit conductor is NOT permitted to be grounded on the load side of the service is, should the grounded service conductor become disconnected at any point on the line side of the ground, the equipment grounding conductor and all conductive parts connected to it will carry the neutral current, raising the potential to ground of exposed metal parts not normally intended to carry current. This could result in arcing in concealed spaces and could pose a severe shock hazard, particularly if the path is inadvertently opened by a person servicing or repairing piping or ductwork. Even without an open neutral the equipment grounding conductor path will become a parallel path with the grounded conductor, This could involve current flowing through metal building structures, piping, and ducts.
 
Stick,

One major reason the grounded circuit conductor is NOT permitted to be grounded on the load side of the service is, should the grounded service conductor become disconnected at any point on the line side of the ground, the equipment grounding conductor and all conductive parts connected to it will carry the neutral current, raising the potential to ground of exposed metal parts not normally intended to carry current.
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Ok, thats fair BUT, can't the same thing be said of a service panel? Maybe the theory is that service conductors are less lkely to be compromised due to their stricter installation requirements (length, wire methods, etc)?

George,

Actaully this is part of my crusade for 4-wire services. Just kidding, its really just a coincidence that this has come up a few times in my last few posts.
 
The reason we run a 3 wire to a service is because the NEUTRAL is suppose to carry a fault... just not in a sub-panel application... Thats why we BOND the neutral to the ground... make sense?
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Well we run a 3 wire to a service because the power company does not provide a forth wire and the NEC has no authority to make them do so.

I am willing to bet that is at least part of the answer. :)
 
I'd tend to agree with Bob, which probably comes as no surprise. :D

Ethan, my comment was more along the lines of, that is another one of your seemingly innocent questions that could result in a 500 post thread. Keep 'em coming. :D

electrofelon said:
Maybe the theory is that service conductors are less lkely to be compromised due to their stricter installation requirements (length, wire methods, etc)?
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Service conductors have as much or more reason to fail - generally, open neutral conditions on feeders are uncommon, whereas open neutral conditions on services are proportionately common, IMO.

The writers of the NEC want a conductor system that is not used to carry current, dedicated to ground faults. They start this system at the first available opportunity, the service disconnect. If they could start it at the power plant, they probably would. :D
 
Another thought: the neutral is generally larger than a required EGC would be, and would have a lower impedance during a fault.
 
Current seeks a path back to the source. Once it has made it back to the main service panel, it has essentially achieved that goal. What happens after that, whether the current travels along a neutral or a ground or a phase conductor, is an SEP issue.
 
charlie b said:
...What happens after that, whether the current travels along a neutral or a ground or a phase conductor, is an SEP issue.
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What's "an SEP issue". I know, I'm likely the only one here that doesn't know.

carl
 
charlie b said:
Current seeks a path back to the source. Once it has made it back to the main service panel, it has essentially achieved that goal. ...
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Hummm. Let's see, what change happens at the service panel? To paraphrase Don, the electrons now know it is dangerous to have multiple neutral/grounding conductor bonds.

I would have considered the xfm as the source.

carl
 
electrofelon said:
Ok, thats fair BUT, can't the same thing be said of a service panel? Maybe the theory is that service conductors are less lkely to be compromised due to their stricter installation requirements (length, wire methods, etc)?
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We have two POCO's here in Texas that require a #6 bare copper from the meter can to a ground rod. There theory is that if something happens to there neutral and the grounding system on the house there is a back up for a ground. I'm sure in hope to not buy expensive electronics if that should actually happen(loose the house system and there neutral)
Mark
 
Not fair, Jon, beating me to my own punchline. :mad: :wink: :grin:

?Someone Else?s Problem.?

Credit Douglas Adams, author of the five volume trilogy that began with ?The Hitchhiker?s Guide to the Galaxy,? and author of ?Dirk Gently?s Holistic Detective Agency,? for discovering that concept. As an example of an ?SEP Field,? imagine what happens when someone drops a carton of eggs at the grocery store or a glass vase at a department store. All around that area a field is created. The closer you are to the spot, the less comfortable you are, perhaps fearing that someone will think you are at least partially to blame. But the further you get from the spot, the more readily you can relax, knowing that the incident is ?Someone Else?s Problem.?
 
winnie said:
SEP -> Somebody Else's Problem. Often used as a technique for rendering electrical hazards invisible.

http://en.wikipedia.org/wiki/SEP_field

-Jon
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Thanks Jon - i forgot about that. "Hitchhikers Guide to the Galaxy". Spaceship lands in the park and is surrounded with a SEP field. It works about as well as an invisibilty field and takes a lot less energy.

carl
 
coulter said:
I would have considered the xfm as the source.
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True, but irrelevant, as a previous boss of mine was fond of saying. True, current that has made it through a load, and also made it back to the service panel, will not be able to head back out to the load again until after it has passed through the transformer. So it leaves the house, travels up the utility pole, picks up another load of steam from the transformer, heads back into the house, and begins the journey to the load all over again.

But that time spent outside the house is not relevant to the safety of the person standing inside the house. It cannot shock the person, if for example the person touched a section of conduit, because it is heading towards the ?real source,? the transformer, and away from the point at which the person is touching the conduit.

coulter said:
Let's see, what change happens at the service panel?
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What changes is that the direction of current flow is away from the house.

Suppose, by way of counter-example, you bond neutral to ground at a sub-panel. Suppose that there are no EGC wires, but rather the conduit system is used as the equipment ground. Consider a load that is powered from the sub-panel. Current flows from the sub-panel to the load, along an ungrounded conductor, and back to the sub-panel, along the neutral (grounded) conductor. But from that point, it has two paths back to the main service panel. One is along the neutral (grounded) conductor in the feeder to the sub-panel. The other is along the EGC, the conduit surrounding the feeder to the sub-panel. That is the conduit that the person is touching. At that point, current now has three paths back to the source. The original two are still present. The third path includes a human, that human?s shoes, the floor, the house?s structural supports, dirt, the ground rod, the Grounding Electrode Conductor, and the Main Bonding Jumper. OK, so that?s a high resistance path, in relation to the other two. OK, the driving force for current is small at that point (i.e., most of the voltage was dropped within the load). But you don?t need much current for the person to not be happy about the situation.
 
electrofelon said:
Why is it safe to run a 3 wire to feed a service panel, but not to feed a sub panel?
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I don't think it is. The Norwegians have it figured out. The Germans do pretty good. Even the French (bless their souls) don't do to bad.

But, we are Americans - and what we do is RIGHT.

carl
 
Something of an off the cuff babble on this topic:

IMHO there is not a hard dividing line between 'safe' and 'dangerous' when we are talking about grounding the neutral, and especially when we talk about _multiple_ ground-neutral bonds.

When you have multiple ground-neutral bonds, you create a situation where some amount of current will flow on your 'ground' conductors.

However as long as your ground conductors are well bonded and have sufficient ampacity, this does not create much of a hazard.

In the past and in millions of grandfathered installations, the neutral was used to bond the frame of electric ranges and dryers.

Multiple grounding of the neutral appears to have benefits in terms of protection from lightning, at least on the scale of utility distribution of power. Usually this isn't a problem...but not infrequently this causes 'stray voltage' issues.

In urban areas with shared underground metallic pipes and shared utility transformers, each 'ground to neutral' bond in each service is in parallel with other bonds in adjacent houses. Significant current flows on the shared water pipes, and this isn't a problem until the plumber goes to work on the pipes.

Multiple grounding of the neutral appears to cause problems when the bonding of the ground system is not sufficient. 'Sufficient' is a relative term, and really means 'bonding good enough to keep voltage low enough that it doesn't get noticed on the scale of the measurement'. We don't care about a 0.5V difference between two patches of soil 20 feet apart. We care quite a bit about a 10V difference between two metal surfaces that a person could touch simultaneously.

The shell of an individual structure is probably a good dividing line between permitting and prohibiting multiple grounding of the neutral. Outside of the structure the scale of distances is such that problems will be uncommon, and the benefit of multiple grounding more important. Inside of a structure you have utilization equipment in close proximity and greater chance of touch potential, and we prohibit the such multiple bonding.

But at the boundary of the structure (plumbers working on the water main) and in situations sensitive over longer distances (farms where metal structures can 'focus' potential differences), problems with multiple earth bonding can still be noticed.

-Jon
 
charlie b said:
But that time spent outside the house is not relevant to the safety of the person standing inside the house. It cannot shock the person, if for example the person touched a section of conduit, because it is heading towards the ?real source,? the transformer, and away from the point at which the person is touching the conduit.
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However this 'multiple earthing' of the transformer neutral is potentially an issue for someone outside of the house.

For example, someone standing with bare feet on the soil between the house grounding electrodes and the transformer grounding electrodes would be part of a parallel path for the neutral current. As mentioned in my last post, this is usually not a _significant_ issue, because the step potential is low enough.

However I recall a thread started by someone investigating a home where there was sufficient potential between a hose tap on the side of a house, and the soil below, to cause a minor shock. This potential was present even when the main disconnect to the house was _off_.

-Jon
 
Charlie, There are a few places where I'm not understanding the physics you are using.

charlie b said:

... But that time (the current) spent outside the house is not relevant to the safety of the person standing inside the house. It cannot shock the person, if for example the person touched a section of conduit, because it is heading towards the ?real source,? the transformer, and away from the point at which the person is touching the conduit. ...What changes is that the direction of current flow is away from the house.
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This logic completely baffles me. My understanding is if one were to touch a wire with a difference of potential twix the hand and foot, on will get shocked. The direction of current flow or power flow is, ahhh, "not relevent". I recall a couple of guys Thevinen (sp) and Norton that would agree with me.

whoops, hit the submit too soon. will edit and repost.

carl
 
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