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Joined: Oct 2000
Posts: 5,392
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If i may be the thread simpleton here...
all NEC aside, more bonding is better than less?
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Joined: Dec 2000
Posts: 4,294
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I agree with Don here. By the NEC, the conduit could be >200' of EMT and not require bonding on both ends.
But I think more is better.
[This message has been edited by electure (edited 08-20-2001).]
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Joined: Aug 2001
Posts: 5
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Hi, I’m new to the ECN BB. Those who know me from IAEI know I sometimes get wordy, but I feel short posts sometimes raise more questions and confusion than they solve.
Picture the following arrangement of service equipment from top to bottom:
metal weatherhead
2” steel mast
2” bolt-on hub
meter socket
2” nipple
2” bolt-on hub
service disconnect enclosure
There are 7 metallic items that must be bonded so that, if any of them become energized, the bonding system will carry enough current back to the source to clear the OCD ahead of the transformer. Since this is service equipment, the grounded service conductor (neutral) is permitted by 250-94(1) to serve as an equipment bonding conductor.
The service disconnect enclosure is bonded to the grounded service conductor through the main bonding jumper or screw. If the disconnect is “suitable for use ONLY as service equipment” there is no main bonding jumper because the enclosure is already bonded by virtue of its construction. The meter can is also bonded to the grounded service conductor by virtue of its construction.
So far so good. This leaves metal raceways, nipples, hubs and a weatherhead. Since the grounded service conductor is being used to bond the meter can and the disconnect, none of the nipples and raceways are required to be treated as equipment grounding conductors between pieces of equipment. However, 250-92 still requires all metal parts be bonded together and 250-94 says how.
250-92 doesn’t say that the metal parts in the service have to be bonded together at every fitting like a daisy chain, they just have to be effectively bonded together and this can be done star fashion through the neutral conductor as permitted by 250-94(1). They are each required to be bonded only once.
The steel mast is bonded through the threaded hub to the top of the grounded meter can and the metal weatherhead is grounded to the steel mast by its clamp. No problems yet.
This leaves the nipple between the meter and the disconnect. If it goes into the top of an outdoor disconnect, it will enter through a threaded hub and the nipple will be effective grounded (once) as required. There is no requirement to bond it a second time in the meter can, therefore two metal lock rings and a bushing will suffice at the meter can.
If, however, the nipple runs into a disconnect or any other enclosure without a hub, then a bonding bushing or OTHER approved device will be required to bond the nipple to an enclosure or to the grounded service conductor inside it. George C... this applies to your 200 foot long raceway as well.
Once again, since the nipple (or raceway) is not being treated as a grounding conductor between pieces of equipment, it is required to be bonded, but only once. Which end of the nipple or raceway should be bonded? I prefer bonding at the line end because the return path to the source of power will be shorter, but the NEC does not specify and permits bonding at either end. Other factors such as NO BONDING BUSHINGS IN METER CAN may make bonding at the load end necessary.
Fault currents should flow a closed CIRCUIT in the equipment. That is, a fault current will arrive through a legitimate conductor and then, from the fault point, travel back on the metal enclosure, raceway and grounded service conductor to the source of power. As this circuit is self contained it should have a low impedance.
Where the above statement may break down is when a large current from a lightning strike or fault currents imposed from shorts to OTHER SYSTEMS occur. These currents will not be self contained and will travel only in one direction over the metal parts of this service. In this case the same concerns would apply that are dealt with in 250-64(e) and 250-92(a)(3) regarding “transformer” or “skin” effect on a ground electrode conductor. If ALL raceway/enclosure connections are not bonded the impedance presented to this type of current might rise to some appreciable value.
The point becomes, we are required to meet the NEC, are we required to exceed it? The NEC requires the bonding system to be capable of carrying any fault LIKELY to be imposed upon it. Faults in the service conductors would be “likely” but would lightning and currents from other systems be considered likely?
This is my “take” on bonding service equipment from the 1999 NEC. If anyone can provide NEC references to disprove of any of my assertions, I would appreciate hearing from them.
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Joined: Nov 2000
Posts: 2,236 Likes: 1
Member
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Welcome, Max!
Great to have another fellow WV'er here!
No need to apologize for the lengthy posts... It's par for the course here!
Especially when one explains things so well (and in a way I can understand!).
IAEI is sometimes too far over my head for me to keep up!
ECN is a little more laid back than IAEI, but we still get into some heated topics...
We try to keep the flames down (although I'm guilty myself) and the topics interesting.
Bill's got a great thing going here... Enjoy!
-Virgil Kelly - Kelly Electric
Greenbrier County
West "By (the grace of) God" Virginia
-Virgil
Residential/Commercial Inspector
5 Star Inspections
Member IAEI
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Joined: Feb 2001
Posts: 32
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Knock all of them out and use a larger nipple and put just one set screw locknut in meter base side...you can put it on the panel side but alot of inspectors like to see it on the meter side ..
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Posts: 1,158
Joined: May 2003
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