250.24(A)(4), but why?

Christoph

Christoph

Master Electrician, Code Official
Location
Coopersburg, PA
Occupation
Electrical Inspector
I have been wondering about this for a while:

We know that per 250.24(A)(1) the Grounding Electrode Conductor is to be connected to the Grounded (Neutral) Codnductor at the Service. 250.24(A)(4) allows for connection to the Equipment Grounding Bar IF tthe Main Bonding Jumper is a wire or Bus. The screw sinply isn't acceptable. My question is WHY?

While I appreciate any and all input I am mostly interested to hear from someone that knows the substantiation for the rule. I do have several theories myself.

Thank you in advance.
 
Christoph said:
I have been wondering about this for a while:

We know that per 250.24(A)(1) the Grounding Electrode Conductor is to be connected to the Grounded (Neutral) Codnductor at the Service. 250.24(A)(4) allows for connection to the Equipment Grounding Bar IF tthe Main Bonding Jumper is a wire or Bus. The screw sinply isn't acceptable. My question is WHY?

While I appreciate any and all input I am mostly interested to hear from someone that knows the substantiation for the rule. I do have several theories myself.

Thank you in advance.
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Many NEC requirements, particularly around grounding, have no basis in electrical theory or statistics. I assume this is another one of those things that no one really knows, but everyone is too scared to change it.
 
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As far as I got was that this language seems to have been added between 1958 and 1978.

To me the reasoning is obvious, aside from the general overemphasis on the importance of the GEC, (which I think is hard to explain but has at least been historically consistent in the NEC and UL standards). If there is a wire or bus main bonding jumper the the size is clearly proscribed by other rules in the NEC, or perhaps in the case of the bus by UL standards for service equipment. If you are relying on the enclosure then you are potentially relying on field installation of an EGC terminal bar, and the normal rules such connections may leave them undersized especially for high amperage or voltage applications. Probably overkill for the typical single family home.
 
My take:

Bar 1 is isolated from the case and receives the service grounded conductor. Bar 2 is screwed the case. And a bus or jumper runs between the two bars.

Now you can look at that two different ways: bar 1 is the only neutral bar, bar 2 is the ground bar, and the bus or jumper is the MBJ. Or alternatively both bars are neutral bars and the screw to the case is the MBJ.

Clearly in the latter point of view it's fine to land the GEC on bar 2. So even if you take the first point of view, it's also fine to land the GEC on bar 2.

Cheers, Wayne
 
Soares says, "Under certain conditions given in 250.24(A)(4), it is permitted to connect the grounding electrode conductor to the equipment grounding terminal bar rather than the terminal or bus for the grounded service conductor. This scheme is common on larger switchboards and service equipment and is necessary for proper operation of certain types of ground fault protection equipment (see chapter 14 for additional information on this subject)"

From what I can tell in chapter 14 is covering larger equipment. The location of the CTs / sensors on the neutral require the connection of the GEC and MBJ be made upstream of the sensor. "If there is any grounding connection to the neutral downstream of the ground-fault sensor then in a fault situation of the current will be diverted on the the neutral conductor and appear to the ground fault sensing system as "normal" neutral current. ".

Though in most of the photos it is always a wire type MBJ and GEC is always landed on the EGC terminal bar.

I can't find a mention of a screw type MBJ in chapter 14.

I can't think of any reason why it is not allowed except for wanting the GEC to be as close to the neutral as you can get it. So that any weird abnormal phenomenon doesn't travel through the screw, EGC terminal board, enclosure, etc. Like lightning or something. Just from the electrode to the neutral. Or neutral to electrode if it was HV lines or some kind of surge from the utility.
 
LarryFine said:
I believe that the intent was and is that we're bonding it to the neutral, not the EGCs.
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And since we already have allowances to use buses and obviously wires for connecting the GEC, no problem using buses and wires within the service panel for making that GEC to neutral connection. Whereas allowing a screw as part of the conductive path is something usually only done for the EGC.

Cheers, Wayne
 
wwhitney said:
My take:

Bar 1 is isolated from the case and receives the service grounded conductor. Bar 2 is screwed the case. And a bus or jumper runs between the two bars.
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In panels that could be used either as service equipment or subpanels, often there is no Bar 2 from the factory. (Even moreso in the past than now.) So it would be up to the installer to add Bar 2 and a jumper between them. The whole point of the wording under discussion is essentially to prohibit landing a GEC on Bar2 if the installer only bonds it to the case and does not install the jumper.
 
jaggedben said:
The whole point of the wording under discussion is essentially to prohibit landing a GEC on Bar2 if the installer only bonds it to the case and does not install the jumper.
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Sure, that's part of 250.24(A) as a whole. I'm not clear on whether you're disagreeing with anything I said.

Another way to put my take is that the MBJ can be considered the dividing point between the EGC system and the grounded conductor. But when that MBJ is itself a wire or busbar, then at least as far as the "grounding" busbar to which the MBJ is connected, that's a distinction without a difference.

Cheers, Wayne
 
Elect117 said:
t it. So that any weird abnormal phenomenon doesn't travel through the screw, EGC terminal board, enclosure, etc. Like lightning or something. Just from the electrode to the neutral. Or neutral to electrode if it was HV lines or some kind of surge from the utility.
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LarryFine said:
The conductor or bus-bar is of a known gauge or area, and thus a predictable ampacity.
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And its important to have this super good strong infallible electrical path, because once it hits the dirt........ :rolleyes:
 
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