CMil formula

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KP2

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I have a formula in my code to find the approx inches squared for conductors and I'm not sure it is correct and I can't find were I got this from. Please HELP.
The formula is 3.14 X radius squared / 4 =

I found that if I use the Diamiter instead it works out to the tables but radius does not. If I use the radius but do not divide by 4 it matches.
 
I'm not quite sure what you are trying to calculate, and in particular how it relates to circular mil area. The formula you quoted is simply the formula for the area of a circle.

A = (pi) times (radius squared), which is the same as,

A = (pi) times (diameter squared) divided by 4.

These two are the same because the radius is half the diameter, and when you square the "one half" you get "one quarter." The value 3.14 is an approximation for the value of pi.
 
I formula I have is supposed to be for finding the area of a conductor that is not listed in the tables like 14-3 romex when put in a conduit. I recently took a CEU course and the instructor gave us the same formula that I had and I'm looking at it and realizing that I think its wrong. like you said it is the formula for area of a circle. I cant find my original reference and the CEU instructor had the same one.
 
KP2 said:
I formula I have is supposed to be for finding the area of a conductor that is not listed in the tables like 14-3 romex when put in a conduit. I recently took a CEU course and the instructor gave us the same formula that I had and I'm looking at it and realizing that I think its wrong. like you said it is the formula for area of a circle. I cant find my original reference and the CEU instructor had the same one.
Click to expand...

If you're trying to put NM in a raceway, you need to know the major diameter of the cable. Not all NM is perfectly round.

14-2 NMB .1018
12-2 NMB .1320
10-2 NMB .1917
8-2 NMB .2942
6-2 NMB .3664
14-3 NMB .0740
12-3 NMB .0946
10-3 NMB .1399
8-3 NMB .2507
6-3 NMB .3318
14-2 UF .1405
12-2 UF .1684
10-2 UF .2107
8-2 UF .3610
6-2 UF .4657
14-3 UF .2651
12-3 UF .3078
10-3 UF .4151
8-3 UF .8808
6-3 UF 1.1747
 
The instructor may have been using the explanatory information in the HB the 2008 HB on page 1238 explains the conversion of cir mil to Sq mils

the formular in the book pretty much determines that a 1 cir mil (conductor) has a area of .7845sq mils

this is also applicable to determining the what size rectangled buss is needed to replace a round conductor.
 
I have done half a dozen or so calculations and determined that when using radius I do not divide by 4 otherwise 14-2 romex can go in 3/8 FMC so in conclusion Charlie's formulas are correct and I changed the note in my code book. THANKS AGAIN
 
480sparky said:
If you're trying to put NM in a raceway, you need to know the major diameter of the cable. Not all NM is perfectly round.

14-3 NMB .0740
12-3 NMB .0946
10-3 NMB .1399
Click to expand...

I don't know what the numbers are that you are giving, but there is a problem with at least these 3 ... 12-3 under 1/8 inch? 14-3 under 3/32? What is the source of the data?
 
480sparky said:
If you're trying to put NM in a raceway, you need to know the major diameter of the cable
Click to expand...

I too would like some clarification on those numbers. I dont use romex much so I would have to look it up to be sure, but those numbers given at first glance look like they might be the area in square inches. And I agree something isnt right, the 3 conductor cables are shown as being smaller than the 2 conductor. The numbers cant be the "major dimension" as he says because they look too small.
 
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GeorgeB said:
I don't know what the numbers are that you are giving, but there is a problem with at least these 3 ... 12-3 under 1/8 inch? 14-3 under 3/32? What is the source of the data?
Click to expand...

The numbers are the cross-sectional area required for conduit fill calculations. They are calculated using a circle which circumscribes the widest dimension of the cable.

If you want to verify, go to a manufacturers website and obtain cable dimensions, then for flat cable, square the wide dimension, multiply times Pi and divide by 4. For roundish cable, use the one dimension is given.

If you believe Southwire's data is good enough, click here then click on the SIMpull NM-B tab. Go to page 11 of the catalog (third viewing page) for dimensions.
 
Smart $ said:
The numbers are the cross-sectional area required for conduit fill calculations.
Click to expand...
Thanks. That makes perfect sense with 3 wire often round and the largest dimension used for circumscribed circle area. It was the number being given as diameter that threw me.
 
I measured 14-2 romex to be 3/8 of an inch or .375.
So 3.14 X .375 squared divided by 4 equals = .1074665 which can fit into 1/2 inch EMT. The table 480 Sparky posted is not in inches its in cmil, (cross-sectional area) or inches squared.
 
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