Nec 348.20 (a)

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erickench

erickench

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I may have discovered another oddity in the NEC. The above section states:

"FMC less than 1/2" shall not be used unless permitted in 348.20 (A) (1) through (A) (5) for trade size 3/8"

Now I have searched the internet and I have'nt been able to find 1/4" flexible metal conduit. So I guess this rule is meant to forbid it's use. But if the rules for
3/8" conduit are layed out in (A) (1) through (A) (5) then should'nt these same rules apply to either 1/2" or 3/4" FMC as well? Again these rules should apply to either the larger or the smaller of the two. Also, the language is'nt appropriate. It should just read:

"FMC less than 1/2" shall not be used".

And then indicate whether rules (A) (1) through (A) (5) apply to FMC larger or smaller than 3/8".
 
The way I read that section is that FMC smaller than 1/2" (ie 3/8 flex) can't be used unless it is permitted in (A)(1) through (A)(5).

So I can't use 3/8" flex in the same manner that 1/2" through 4" FMC is permitted to be used.

Chris
 
raider1 said:
The way I read that section is that FMC smaller than 1/2" (ie 3/8 flex) can't be used unless it is permitted in (A)(1) through (A)(5).

So I can't use 3/8" flex in the same manner that 1/2" through 4" FMC is permitted to be used.

Chris
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I'm using the 2007 NYC Electrical Code which is based on the 2005 NEC.
Did they change it? Because the language in the 2005 NEC appears ambiguos.
I mean they're saying that you can't use smaller than 1/2" unless you use 3/8"!?:-?
 
erickench said:
I'm using the 2007 NYC Electrical Code which is based on the 2005 NEC.
Did they change it? Because the language in the 2005 NEC appears ambiguos.
I mean they're saying that you can't use smaller than 1/2" unless you use 3/8"!?:-?
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They way I read it is that you can't use FMC smaller that 1/2". Unless you use 3/8" flex in one of the 5 ways listed in 348.20(A)(1) through (A)(5).

Chris
 
The general rule is 1/2" FMC is the smallest FMC we can use ... unless ... we decide that we would like to use 3/8" FMC and we can meet one of the five listed methods.

None of that forces us to use 3/8" we can always choose to run larger but they are giving us a break for certain applications.
 
But how is 1/2" FMC to be used? Do these five rules apply? Or do they apply to 3/8" and larger? Also, we have the same problem with NEC table 348.22. The larger side of 3/8" or the smaller?
 
erickench said:
But how is 1/2" FMC to be used? Do these five rules apply? Or do they apply to 3/8" and larger? Also, we have the same problem with NEC table 348.22. The larger side of 3/8" or the smaller?
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The 5 rules of 348.20(A) only apply to the use of 3/8 flex, not to the use of 1/2" thorugh 4" FMC.

In addition Table 348.22 only applys to 3/8" flex, 1/2" through 4" flex would use Table 1, Chapter 9 for number of conductors.

Chris
 
raider1 said:
The 5 rules of 348.20(A) only apply to the use of 3/8 flex, not to the use of 1/2" thorugh 4" FMC.

In addition Table 348.22 only applys to 3/8" flex, 1/2" through 4" flex would use Table 1, Chapter 9 for number of conductors.

Chris
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That's weird. I as an engineer see a problem. The rules should apply to either 3/8" and smaller or 3/8" and larger. As I stated in a previous thread you can't just single out one wire size and not count the others. It either has to be the larger or the smaller.
 
erickench said:
That's weird. I as an engineer see a problem. The rules should apply to either 3/8" and smaller or 3/8" and larger. As I stated in a previous thread you can't just single out one wire size and not count the others. It either has to be the larger or the smaller.
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The rule is really quite simple you can only use 1/2" through 4" FMC, but it lists 5 "exceptions" to that rule when using 3/8" FMC. They're not called exceptions as in the traditional NEC wording but they can be treated that way.
 
erickench said:
That's weird. I as an engineer see a problem. The rules should apply to either 3/8" and smaller or 3/8" and larger. As I stated in a previous thread you can't just single out one wire size and not count the others. It either has to be the larger or the smaller.
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I am confused as to what you are getting at?:confused:

3/8" is the trade size of the flexible metal conduit. To use this size FMC you must do so in accordance with one of the 5 sections listed in 348.20 (A). In addition to 348.20(A) you can only install conductors in 3/8" FMC in accordance with Table 348.22. This Table does not permit you to intermix conductor sizes. This is OK because the limits on the use of 3/8 flex in 348.20(A) makes the need to intermix conductor sizes unnecessary. Also the * in the Table tells us that we are permitted to install 1 insulated, covered or bare equipment grounding conductor of the same size.

I agree that this section could use some clarification.

Chris
 
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erickench said:
That's weird. I as an engineer see a problem. The rules should apply to either 3/8" and smaller or 3/8" and larger. As I stated in a previous thread you can't just single out one wire size and not count the others. It either has to be the larger or the smaller.
Click to expand...


I see That I mis-read your post. Disregard my last answer and go with Raider1's. :)
 
iwire said:
Why do you feel that way?

Would it help you to know that 3/8" is the smallest FMC allowed at all?
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Yes it would help but such is not the case. I've seen 1/2" FMC advertised on the internet. There are reasons why these rules apply to 3/8" FMC. But whatever those reasons are they would have to apply to either the larger sizes or the smaller sizes. This section does need clarification. Unfortunately it's too late to make a proposal.
 
Yes it would help but such is not the case. I've seen 1/2" FMC advertised on the internet.
Click to expand...

3/8" flex is smaller than 1/2".

There are reasons why these rules apply to 3/8" FMC. But whatever those reasons are they would have to apply to either the larger sizes or the smaller sizes.
Click to expand...

Why? :confused:

Chris
 
erickench said:
Yes it would help but such is not the case. I've seen 1/2" FMC advertised on the internet.
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No kidding, I said 3/8" is the smallest.


There are reasons why these rules apply to 3/8" FMC. But whatever those reasons are they would have to apply to either the larger sizes or the smaller sizes.
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The 'special rules' apply only to 3/8" FMC, they can not apply to smller sizes as they do not exsit.

The 'special rules do not apply to larger sizes as we can use the larger sizes in most all instlltions.

The deal is the NEC is giving us a break, allowing the use of 3/8" FMC for specific items.
 
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iwire said:
The deal is the NEC is giving us a break, allowing the use of 3/8" FMC for specific items.
Click to expand...

Agreed, if you don't want to adhere to the requirements for using 3/8" flex, use 1/2" or larger.

Chris
 
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