320.30(B) and 330.30(B)

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bphgravity

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I have noticed a significant change in the securing and supporting sections for type AC and MC in the 2005 NEC.

First of all. With type AC and MC being very similiar in construction and use, why is MC permitted to be secured at intervals as great as 6' where AC is limited to 4?'?

Second, it appears the intention of "securing" is different then "supporting" so they are separated into two sections. For type AC, the section states, "at intervals not exceeding 4? ft. where installed on or across framing members". Type MC doesn't have that specification. Why?

Does that mean type AC does not need to be secured at intervals of 4? ft. if the installation is not on or across framing meners? And if type MC is installed on or across framing members, is that considered to be supported and secured?
 
Re: 320.30(B) and 330.30(B)

Brian
As I understand it Most of chapter 3 has adopted the terms secured and supported.

Supported may be installations where the wiring method is laid on framing members, or ran through framing members (to mention a few).

Secure has become important in such cases to limit the movement of the wiring method for a host of reason one being short circuits that might cause the cable or raceway to whip or jump. "Bad JU JU".

The spacing of supports and points at which supported cable requires securing is essentially the same, with in the wiring methods (320.30 and 330.30 for eg.)

Charlie
 
Re: 320.30(B) and 330.30(B)

Originally posted by bphgravity:
With type AC and MC being very similiar in construction and use, why is MC permitted to be secured at intervals as great as 6' where AC is limited to 4?'?
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I've always wondered myself why the differences between the two just like you mention. Why do we even have the two products? It seems MC makes AC obsolete.
 
Re: 320.30(B) and 330.30(B)

One example may be 517.13(B). The outer armor of MC is for the interlock type is not listed as an equipment ground. Hospital Grade AC provides a redundant ground cable method for such installations. .
 
Re: 320.30(B) and 330.30(B)

Originally posted by sandsnow:
Why do we even have the two products? It seems MC makes AC obsolete.
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Perhaps because it keeps a production line running?

When I worked at a supply store, we sold 2or more pallets of MC for every 250' coil of AC.
 
Re: 320.30(B) and 330.30(B)

Here's another (IMO) peculiarity. Type AC that has a sheath that is recognized in 250.118 as an EGC must use an "anti short bushing or equivalent", yet type MC that doesn't have a recognized sheath doesn't need one.

Wouldn't it make more sense the other way around? ;)

Roger

[ February 02, 2005, 07:13 PM: Message edited by: roger ]
 
Re: 320.30(B) and 330.30(B)

I understand there are fundemental differences between the two, I just wonder what is it about MC that allows 6' support intervals and AC requires 4?' supports?

Why did they have to specify the "where across the framing..." part for AC, yet this is not stated in MC?

The wording just seems to require securing AC only when across frmaing members, while MC is required to be secured no matter where it is. Just doesn;t make much sense to me and I can't find any substantiation for it.
 
Re: 320.30(B) and 330.30(B)

Bryan, the only thing I can think of goes along with my other post. Looking at 320.108 verses 330.108, maybe the concern is keeping the sheath of the AC intact as far as being an EGC and this more rigid securing may help insure this.

Roger
 
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