Bonding "Again"

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  #1  
Old 01-21-05, 03:43 PM
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Bonding "Again"

For some reason I fail to understand the "bonding" of the neutral and ground in the "main" panel.

If the neutral is for the return of unused electicity to the power company and the neutral and ground bars are bonded, what keeps it from going to ground instead.

I know this is one of those " Dumb a--" questions and will feel stupid when I hear the response, but go ahead.

Another, regardles of the number of sub panels you have the total number of breakers is indifferent as it depends on what you have running all at one time, in other words can I have a a 200 main, 3 100 amp sub panels just as long as the total does not exceed the 200 amps of everything running all at once, which would be impossible in my case??


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John1
 
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  #2  
Old 01-21-05, 03:55 PM
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What keeps the current from going to ground instead is not really the right question. The current needs to get back to the source. The source in this case is the utility. The neutral wire going back to the transformer is the most direct path for this current to follow, and offers the least resistance. Most of the return current will follow this path.
 
  #3  
Old 01-21-05, 04:03 PM
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The fact is that some of the current will go into the dirt. But because the resistance of dirt is about a billion times more than the resistance of copper, 999999999/1000000000 of the electons will flow through the wire back to the power company transformer, and 1/1000000000 of the electrons will flow through your backyard.

Without the bonding, if a fault were to cause the hot wire in your refrigerator to touch the chassis of the refrigerator, the fault current would try to all go into the dirt (since it doesn't have access to the wire to the transformer). Unfortunately, the dirt is not a good enough conductor to allow enough current to flow to trip the breaker. So the next time you wanted a beer, you'd still get killed. We don't really want those 1/1000000000 of the electrons go into the dirt (but it doesn't bother us that much), but the bonding is necessary to make the equipment grounding work to save your life.
 
  #4  
Old 01-21-05, 07:07 PM
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The neutral is a current carrying conductor, the same load (amps) that goes through the hot wire of the breaker also goes throught the neutral, it just completes the path. (Note this does not apply to nominal 240v loads, the neutral in this case carries the larger of the unbalanced load between phases). The ground, on the other case, is a safety protection conductor that will trip the breaker in case the hot wire happens to short against any metal, current carrying components of the appliance. This will keep your butt from getting electrocuted.
As far as the Main load center, you can have a combination of breakers well above the capacity of the panel, as the panel is protected by main breaker/ fuse, and each panel is protected by the breaker in the main panel that feeds the subpanel. If there is an overload in either subpanel, the breaker feeding that panel will trip. If a combination of all the subs exceeds that of the main, the protection of the main will trip, it's perfectly safe and is the norm.
 
  #5  
Old 01-21-05, 09:00 PM
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Then if a 240v cicuit fails (becomes grounded say at the equipment) then why does it not go back the same route as the neutral back from the service panel to the transformer if that is the path of least resistance instead of to the grounding rod? Both the Neutral and Ground at the panel are connected/bonded.

What am I missing here??

Also I guess the answer to the second part of the question is "yes" one
may have as many breakers and pannels, regardless of amp rating, so long as one does not operate the "TOTAL LOAD at ONE TIME" to exceed the maximum service of the main service panel. This includes both the main service panels and any/all subpanel?

Thanks John1
 
  #6  
Old 01-21-05, 10:23 PM
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Except in infintesimal quantities, current does not flow to a grounding rod, whether or not you have a fault (unless you have a downed power line outdoors, or you have an electrical storm in the area). The grounding rod is irrelevant to this discussion of equipment faults. To understand how a fault is cleared, try to forget that the grounding rod even exists.

Faults on 240-volt circuits and 120-volt circuits are cleared in the same manner. A brief but high current flows on the equipment grounding conductor back to the panel, and then on to the power company neutral (because of the bonding) back to the transformer. Without the bonding, the fault would not clear.

I guess I'm pretty much unclear as to what you are unclear about.
 
  #7  
Old 01-22-05, 12:28 AM
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Thanks John

John1
 
  #8  
Old 01-22-05, 11:01 AM
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Bonding and grounding are one of the most complex subjects in the NEC. To put it simply the neutral and the hot wire carry current normally on 120 volt circuits. On 240v circuits the current flows normally between 2 hot wires. Sometimes there may be a neutral with the 2 hot wires but only if some part of the applicance is operating on 120V, such as a timer or oven light on a stove, or a timer on a clothes dryer.
On all circuits the ground wire is there for one function, to carry the current back to the panel in case of a fault (short circuit) so that the breaker will trip. Once the power wires leave the first disconnecting means of a residence, the neutrals and grounds remain separated to keep current from traveling on the grounds, except in case of emergency. So even if several panels are involved downstream of the first panel, neutrals and grounds stay separated and the neutrals are not grounded to the panels (except at the first disconnecting means where power is received from the utlility)
There are exceptions of course, since this is the NEC. Neutrals are grounded to the frame of ovens and dryers for safety reasons. These are large steel appliances that could shock the hounddog out of you if the ground happened to get disconnected somehow and the neutral went to ground.
 
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