Breaker size for romex in conduit

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ozark01

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First I'd like to say that I have searched and every post I could find on the site about romex in conduit and could not find a direct answer to my question. I did find some usefull information though.

My question is what size breaker would be allowed if you have nine 12-2 w/g romex cables run in a properly sized conduit longer than 24"? All circuits would be 120 volt. My opinion is that a 20 amp breaker would be allowed. What is the correct answer? :smile:
 

Dennis Alwon

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ozark01 said:
First I'd like to say that I have searched and every post I could find on the site about romex in conduit and could not find a direct answer to my question. I did find some usefull information though.

My question is what size breaker would be allowed if you have nine 12-2 w/g romex cables run in a properly sized conduit longer than 24"? All circuits would be 120 volt. My opinion is that a 20 amp breaker would be allowed. What is the correct answer? :smile:
You have to use T310.15(B)(2)(a). you have 18 current carrying conductors so you derate the nm aat 50% from the 90C col. in Table 310.16
 

infinity

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You would only be permitted to use a 15 amp CB for this setup.

30 amps * 50% = 15 amps
 

ozark01

Senior Member
I was under the impression that the neiutrals did not have to be counted as CCC. For future reference, why do they have to counted in this application? Thanks in advance!
 

Dennis Alwon

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When you have a 2 wire circuit the grounded conductors are always counted because the current travels on the ungrounded and back on the neutral. In a MWBC the grounded conductor only carries the unbalanced portion of the load. Thus if phase A has 12 amps and phase B was drawing 15 amp then the grounded conductor would only draw 3 amps. Although it carries current it cannot carry more than the two phase conductors would at 20 amps.

edited to change grounded to ungrounded.
 
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ozark01

Senior Member
Thanks for the explanation.....

but since when you put an amp meter on the neutral you do not get a reading how can it be considered a CCC? Not trying to disagree...just understand.
 

roger

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ozark01 said:
Thanks for the explanation.....

but since when you put an amp meter on the neutral you do not get a reading how can it be considered a CCC? Not trying to disagree...just understand.

This would only be true in a MWBC.

MWBC

true_neutral.JPG


Two wire circuit

not_a_neutral.JPG


Roger
 

infinity

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Dennis Alwon said:
When you have a 2 wire circuit the grounded conductors are always counted because the current travels on the grounded and back on the neutral. In a MWBC the grounded conductor only carries the unbalanced portion of the load. Thus if phase A has 12 amps and phase B was drawing 15 amp then the grounded conductor would only draw 3 amps. Although it carries current it cannot carry more than the two phase conductors would at 20 amps.


Did you mean ungrounded?
 

Karl H

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Location
San Diego,CA
Dennis Alwon said:
You have to use T310.15(B)(2)(a). you have 18 current carrying conductors so you derate the nm aat 50% from the 90C col. in Table 310.16
So the insulation of the conductors within the outer jacket of NM cable are rated 90c? So why derate to 60c for terminations? Or is the 60c to protect the OUTER insulation of NM?I understand the bundling part but,NM has no more than 3 ccc when run as a single cable. Great post, now when I show up to work tommorrow I'm gonna find all my raceways filled with Romex. thanks guys!:grin:
 

480sparky

Senior Member
Location
Iowegia
Karl H said:
So the insulation of the conductors within the outer jacket of NM cable are rated 90c? So why derate to 60c for terminations? Or is the 60c to protect the OUTER insulation of NM?I understand the bundling part but,NM has no more than 3 ccc when run as a single cable. Great post, now when I show up to work tommorrow I'm gonna find all my raceways filled with Romex. thanks guys!:grin:

334.80.
HoltBlueDot.jpg
 

Karl H

Senior Member
Location
San Diego,CA
480sparky said:
Yep there it is in black and white.Thanks Sparky! Man you are fast dude! It couldn't have been more than 15 seconds after I hit Submit Reply.You were like BAM 334.80. Shows what I know about NM. lol
 

480sparky

Senior Member
Location
Iowegia
Karl H said:
Yep there it is in black and white.Thanks Sparky! Man you are fast dude! It couldn't have been more than 15 seconds after I hit Submit Reply.You were like BAM 334.80. Shows what I know about NM. lol

FWIW, the NEC went to what is called Parallel Numbers a few cycles ago. If you look through various Chapter 3 articles, you will notice that all the sections that deal with a specific wiring method have xxx.80 as ampacity. xxx.60 is grounding, xxx.10 is uses permitted, xxx.30 is support, etc.
 

frizbeedog

Senior Member
Location
Oregon
Does the fact that he has installed cables in a conduit further limit the dissipation of heat from the conductors in the conduit, further reducing the ampacity?

And if so does table 310.15(B)(2)(a) still apply?
 

domnic

Senior Member
Occupation
Electrical Contractor
breaaker size

breaaker size

infinity said:
You would only be permitted to use a 15 amp CB for this setup.

30 amps * 50% = 15 amps
60% fill no de rating for 24" conduit or less.
 
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