Grounding

[iwire]

However, I'm under the impression that Gmack believes a GEC must be run continuously, which is not entirely correct.

It does have to run continuously from the point of connection to the first grounding electrode or now a buss bar that meets the requirement.

I don't believe that any EGC terminal bar will meet the size regiments for a GEC bus bar.

 

[George Stolz]

It does have to run continuously from the point of connection to the first grounding electrode or now a buss bar that meets the requirement.

Thanks for the correction, I meant to say "continuous from service to electrode" and goofed.

I don't believe that any EGC terminal bar will meet the size regiments for a GEC bus bar.

I'll have to measure one on Monday, I'm pretty sure most are the size required. Square D's weird one (shotgun-hole one) might not be, but the normal-looking one is, I believe.

 

[iwire]

I'm pretty sure most are the size required. Square D's weird one (shotgun-hole one) might not be, but the normal-looking one is, I believe.

I don't have my 2005 inside.

I thought the measurement was 1/4" x 2" x 12" or something like that?

 

[Smart $]

I don't have my 2005 inside.

I thought the measurement was 1/4" x 2" x 12" or something like that?

It's 6 mm x 50 mm (1/4 in. x 2 in.)...

...An implicit reference to cross sectional dimensions only, and assumed to be minimum dimensions. No length specified or implied.

 

[iwire]

It's 6 mm x 50 mm (1/4 in. x 2 in.)...

...An implicit reference to cross sectional dimensions only, and assumed to be minimum dimensions. No length specified or implied.

Thanks, still I was close for going from memory, althouigh I had no idea the metric dimensions.

 

[George Stolz]

Here's the section:

250.64(C)(3) Bonding jumper(s) from grounding electrode(s) and grounding electrode conductor(s) shall be permitted to be connected to an aluminum or copper busbar not less than 6 mm ? 50 mm (1⁄4 in. ? 2 in.). The busbar shall be securely fastened and shall be installed in an accessible location. Connections shall be made by a listed connector or by the exothermic welding process.

Since it does not say which dimensions are depth, width, or length, I think this can be read as 1/4" wide, 2" long, couldn't it?

 

[iwire]

Since it does not say which dimensions are depth, width, or length, I think this can be read as 1/4" wide, 2" long, couldn't it?

No.

Can I 'prove that?

No.

It has to do with the ampacity rating of the bus bar.

Think of the 2" x 1/4" the same as the AWG of a conductor.

 

[George Stolz]

How do I determine the ampacity of a busbar?

 

[iwire]

How do I determine the ampacity of a busbar?

In this case they must have tried to be equivalent of at least a 3/0 CU or 250 AL.

As far as how you determine the ampacity I believe you can find that in the NEC.....I just can't remember where.

But I can tell you that 1/4" x 4" copper is usually good for a 1000 amps.

 

[Smart $]

In this case they must have tried to be equivalent of at least a 3/0 CU or 250 AL.

As far as how you determine the ampacity I believe you can find that in the NEC.....I just can't remember where.

But I can tell you that 1/4" x 4" copper is usually good for a 1000 amps.

I believe one would be hard pressed to find busbar ampacity in the NEC, other than perhaps a couple places, at best, for specific situations and as I recall, an implied ampacity at that... but I can't remember exactly where the text is located.

Regardless, I believe you will find busbar ampacity is usually limited by factors other than the busbar's ability to carry substantial current. In and of itself, it will carry current until it melts to the point of opening the current path... same as wire. Nevertheless, busbar ampacity is given in terms of temperature rise. The mentioned size of 1/4" x 2" as a copper busbar is rated 710A @ 30?C Rise, 60Hz. Compare that to 1100A @ 65?C Rise!!! [Source: copper.org]

Note the cited code spec's either aluminum or copper busbar...

 

[George Stolz]

At this point, I'm skeptical but not disbelieving. Research is in order. I need an intern.

 

[jwelectric]

In a 200 amp main the GEC sized from 250.66 is a #4 copper.
A 200 amp circuit (feeder for subpanel) EGC from 250.122 is a #6 copper.

Would the EGC be large enough for the GEC?

 

[cripple]

I know that the commentary in the NEC Handbook may not be enforced, but I do think it states the intent of the rule
Section 25.64(C)(3) does state ?Bonding jumper(s) from grounding electrode(s) and grounding electrode conductor(s) shall be permitted to be connected to an aluminum or copper busbar not less than 6 mm ? 50 mm ( 1/ 4 in. ? 2 in.)?. Now take look the commentary, second paragraph second sentence and if that is not sufficient look at Mike Holt 2005 Understanding The National Electrical Code Volume 1 page 194 Figure 250.113.

 

[iwire]

At this point, I'm skeptical but not disbelieving.

Skeptical about what?

That a bus bar used to join two conductors together has to have an ampacity at least as high as the conductors it is connecting?

Most of the grounding bars I see in panels are aluminum with a cross section of 1/2" x 3/4" and are perforated with holes.

I highly doubt that a bar like that has the capacity of a 3/0 CU.

 

[George Stolz]

Skeptical about what?

That a bus bar used to join two conductors together has to have an ampacity at least as high as the conductors it is connecting?

I'm just not leaping immediately to the largest GEC size right off the bat. I could understand it if that was the requirement, I just am having trouble accepting it as a fact.

Most of the grounding bars I see in panels are aluminum with a cross section of 1/2" x 3/4" and are perforated with holes.

I highly doubt that a bar like that has the capacity of a 3/0 CU.

So that begs the question: Just what is the ampacity of a regular grounding bar?

 

[Smart $]

...
So that begs the question: Just what is the ampacity of a regular grounding bar?

Well I certainly can't answer that question with one definitive number... and I'm fairly certain no one can. Ampacities of conductors in the typical sense are not of the conductors themselves, but rather that of insulation and connection degradation. With a grounding busbar there is no insulation degradation at issue. The main issue here is heating and the resulting effects: expansion/contraction (connection degradation), resistance, melting, etc. Then you also have to keep in mind that as you land conductors you are increasing the effective cross sectional conductive area, along with adding fairly effective heat sink potential.

 

[George Stolz]

I wasn't asking to be argumentative or anything, if I came across that way, I apologize.

I am genuinely curious about how we find that number.

Bob's right, it's a little bar full of holes. How do we figure it out?

 

[George Stolz]

I just had a thought.

One reason that Bob could be right is that grounding bars generally don't carry incidental neutral current.

A bar for a GEC would, so that would be another argument for a bigger bar.

I'm still mulling it over.

 

[Smart $]

I wasn't asking to be argumentative or anything, if I came across that way, I apologize.

I am genuinely curious about how we find that number.

Bob's right, it's a little bar full of holes. How do we figure it out?

Perhaps ask the manufacturer???

 

[iwire]

I just had a thought.

One reason that Bob could be right is that grounding bars generally don't carry incidental neutral current.

A bar for a GEC would, so that would be another argument for a bigger bar.

I'm still mulling it over.

George lets stay on the topic.

You had suggested that a typical EGC bar could be used as the GEC buss that the 2005 NEC allows.

Now when we look at that section we find the NEC took a 'one size bar fits all' approach.

That said it must have equal ampacity of at least a 3/0 CU or 250 AL as those are the largest GECs that are NEC required.

Don't confuse the issue with 'incidental neutral current' that is a drop in the ocean compared to a service conductor fault to to ground when one of the electrodes is electrical common with other structures and services. (In other words a common underground metal water piping system or a building with multiple services)

As they generally write the code from a worst case point of view we must assume this can happen. The GEC will be useless if the GEC bus melts out.