Garage Conduit and derating

[smalltown]

I've tried several times to make this short and readable, but can't make it short, maybe I haven't made it readable

I understand the concept of using the EMT conduit as a ground, but I have chosen to run separate grounds with each circuit.

I want to place 4 enclosure boxes around my garage connected with EMT conduit. The "worst" of my conduits will contain 4 separate circuits: 3 for ceiling lighting, and the 4th for my 2 future garage door openers.

Reading other poster's questions, and trying to decipher the NEC code this is what I understand.
Derating with more than 3 current carrying conductors for me will be 70% so 70% of a rated 30 amp wire is 21 amps so I am ok there.

Max physical number of conductors in a 1" EMT conduit is 26.
Max fill however is 40%. So a 1" conduit should have no more then 10.4 wires?

My 4 above circuits have a total of 12 conductors, but only 8 count as current carrying.
Still my fill rate will be 46% so I l'll need to up to 1 1/4" conduit?

Thanks for any help.

 

[pattenp]

The tables already account for the 40% fill when showing the max # of conductors. I assume you are looking at #12 THHN in 1" EMT. #12 THHN area is .0133, so X 26 = 0.3458 sqin. 40% fill on 1" EMT is 0.346 sqin. You can use 3/4" EMT for twelve #12's. But my question is you stated 30A wire which is #10, so is it #10 or #12 you are using.



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[toplessHO]

separate grounds for each circuit is over kill and adds unnecessarily to the fill.
I would however use at least one(not the conduit as ground)
NEC does allow external ground but if you ask me it looks like azz.

 

[smalltown]

The tables already account for the 40% fill when showing the max # of conductors. I assume you are looking at #12 THHN in 1" EMT. #12 THHN area is .0133, so X 26 = 0.3458 sqin. 40% fill on 1" EMT is 0.346 sqin. You can use 3/4" EMT for twelve #12's. But my question is you stated 30A wire which is #10, so is it #10 or #12 you are using.



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pattenp I did not realize that the tables already accounted for a 40% fill.
I was thinking max fill was just how many conductors it was physically possible to fit in a given size conduit.

Here is what I was quoting. It comes from a Southwire web page, which is the wire I was thinking about using.

I don't have the info about the 12 ga THHN wire being rated at 30 amps at my finger tips. I'll have to locate that again and post a copy.

DERATING FOR NUMBER OF CONDUCTORS
Note 8 to the ampacity tables in the NEC contains the requirement columns for derating ampacity because of adjacent current-carrying conductors. This note states that when the number of conductors in a raceway or cable exceeds 3, the ampacities are to be reduced by the appropriate percentage. In the 1999 NEC, a column of factors is given. For example, what is the ampacity of twelve No. 12 copper THHN conductors installed in one conduit? From Table 310-16 the ampacity (in the table) is 30 amperes. From Note 8, the derating factor for 12 conductors is 50 percent. 30 x 0.5 = 15 amperes per conductor

http://www.southwire.com/support/DeratingAmpacities.htm

also found this http://www.inw-training.com/joomla1/images/2008NEC/310.15B16.pdf

Hope I am reading these correctly.

 

[toplessHO]

pattenp I did not realize that the tables already accounted for a 40% fill.
I was thinking max fill was just how many conductors it was physically possible to fit in a given size conduit.

I don't have the info about the 12 ga THHN wire being rated at 30 amps at my finger tips. I'll have to locate that again and post a copy.

save the trouble it wont fly
#12 THHN/THWN 20A
#10 " " 30 A

 

[Slowgsr]

Up here the changed it

10awg - 40a
12awg - 30a
14awg - 20a

These values only rated for deration calculations and no circuit breaker sizes. Fancy.

 

[smalltown]

I hope I did not mislead when I posted. To reiterate I was hoping to use 20 amp breakers with the 12 ga THHN after the wire was derated

 

[rockwithjason]

you can use the 90 deg c column for thhn when derating. the 90 deg c column says 12 thhn s good for 30 a, the exceptions below the table say that 12 ga wire is limited to 20a. you are on the right track

 

[pattenp]

I did not think about the 30 amps was the 90 degree temperature for number 12 but that is the correct temperature to use when derating the amperage.

 

[smalltown]

pattenp thanks for the info. So I am correct that the ground wires do not count toward the 40% fill? Looks like I am good either way, but was curious.

One other question if I could. What colors would normally be run when I have multiple circuits within the same conduit. I saw that Southwire for ex. had the standard Green, White, Red, and Black, but also there are a few others. I was just hoping to be able to identify in the future (or somebody else) already in place circuit wiring, or maybe there is another way to label them?

Thanks to all that answered I have learned quite a bit. Next time I'll ask about lights, and fixtures. That appears to be a whole other beast.

 

[sberry]

The grounds count as fill but not as current carrying conductors. All the math is great but there isn't much to se folks in the sense that these are not heavily loaded conductors such as may be found on machines or in heating circuits etc. You really wont be able to apply enough load to make this a concern.