Am I crazy or correct?

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Dsg319

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Location
West Virginia
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Wv Master “lectrician”
Question is asking minimum size RMC nipple less than 24” so obviously 60% fill is allowed.

3-3/0THWN=0.8037
1-#1awg THWN=0.1562
1-#6awg THWN=0.0507
TOTAL= 1.0106sq inches.
1.0106/60%=1.6843 sq in.

so I need to select a RMC trade size of that or larger I selected 2”. Next available size up coming in at 2.045sq in.

Went to check myself in an answer key and it says
1 1/2” should be selected at 60%fill at 1.243sq in.

Either I’ve lost it or this book is wacky.
To me what they have selected brings there fill up to 81%!
 

david luchini

Moderator
Staff member
Location
Connecticut
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Engineer
1.25" RMC has an area of 1.526sqin.
1.5" RMC has an area of 2.071sqin.

Your calculation says you need a raceway with an area of 1.6843sqin.

1.25" RMC is too small. 1.5" RMC would be the next size up.

Or put another way: 1.243sqin (available) is greater than 1.011sqin. (needed.)

Edit: Or put another way, you are applying the 60% twice. Once to the conductor area and once to the raceway area.
 
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infinity

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Location
New Jersey
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Journeyman Electrician
I agree, 1.5" is the minimum size. Chapter 9 Table for RMC says at 60% 1.25" has an area of .916 square inches, 1.5" is 1.243.
According to the numbers in the OP:
"TOTAL= 1.0106sq inches" so 1.25" RMC is too small and 1.5" RMC is large enough.
 

Dsg319

Senior Member
Location
West Virginia
Occupation
Wv Master “lectrician”
So 1.5” RMC could fit those given conductors and be within the 60%fill?

I think I just had my moment, and realized my mess up. I did not need to try to find that 1.0106 is 60% of such number. I just needed to use that number and find equal too or larger (next available size). Guess it wasn’t my day for studying lol. Thanks guys.
 

Dsg319

Senior Member
Location
West Virginia
Occupation
Wv Master “lectrician”
Conclusion—— I’m crazy today, was not my day for book work. Thanks guys for correcting me.
 

don_resqcapt19

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Staff member
Location
Illinois
Occupation
retired electrician
So 1.5” RMC could fit those given conductors and be within the 60%fill?

I think I just had my moment, and realized my mess up. I did not need to try to find that 1.0106 is 60% of such number. I just needed to use that number and find equal too or larger (next available size). Guess it wasn’t my day for studying lol. Thanks guys.
You are just fine doing it that way, but you have to look at the 100% column in Chapter 9, Table 4 for type of conduit you are using.
 

Dsg319

Senior Member
Location
West Virginia
Occupation
Wv Master “lectrician”
You are just fine doing it that way, but you have to look at the 100% column in Chapter 9, Table 4 for type of conduit you are using.
Your exactly right, I was trying to figure out how to explain what I’d done. The way I done it I needed to look at 100% not 60% column.
 

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
You need to remember that RMC is not EMT. It's essentially Schedule 40 pipe, which as we all know isn't its nominal size in any dimension. I learned this lesson as a senior in college, where I was supposed to be building onto a previous senior project. Sadly, there were no plumbers in the family.
 

kwired

Electron manager
Location
NE Nebraska
So 1.5” RMC could fit those given conductors and be within the 60%fill?

I think I just had my moment, and realized my mess up. I did not need to try to find that 1.0106 is 60% of such number. I just needed to use that number and find equal too or larger (next available size). Guess it wasn’t my day for studying lol. Thanks guys.
I think your method would work but you need to calculate the extra free space that needs to be available so you would be calculating the conductor area plus the remaining 40% that needs to be free space in the raceway. Then select any raceway that is larger than your result.
 

kwired

Electron manager
Location
NE Nebraska
I think your method would work but you need to calculate the extra free space that needs to be available so you would be calculating the conductor area plus the remaining 40% that needs to be free space in the raceway. Then select any raceway that is larger than your result.
The fact there is already table column for 60% fill means all you really need to calculate is the conductor area though, but this should work if all you know upfront is 100% of raceway area.
 
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