How many 12 ga circuits can I install in 1/2" EMT?

How many 12 ga circuits can I install in 3/4" EMT?

Many thanks!

Bill

 

Bill,

See this thread:

http://forum.doityourself.com/showthread.php?t=81969

This thread also answers the question about the common neutrals. It's a LONG thread, but well worth it considering the questions you are asking. I know *I* learned a lot from it.

 

Thanks, Chirkware for this tread!
Lota good stuff to learn!
Bill

 

Early in the tread, it says I could use 12 thhn conductors in 3/4"
Is this correct?
How about how many thhn conductors in 1/2"?
Has thhn replaced thw?
Thanks.
Bill

 

You can install 16 THHN #12 awg in 3/4" EMT and 9 THHN #12's in 1/2" EMT as to conduit fill of 40%. You cannot exceed this 40 % in EMT for over 2 wires. If you install 4 to 6 size 12 awg THHN current carrying conductors in 1/2" EMT then you must derate to 80% using the 90 C column of table 310.16. So the 30 amp #12 is now 24 amp rated. 7 to 9 #12 THHN conductors you must derate 70% so you are now rated at 21 amps for that size and insulation rated conductor. You cant install more than 9 THHN #12 awg current carrying conductors in 1/2" EMT so you are finished at this point.
THHN/THWN is the more common insulation used today vs the THW

 

Roger I think you have it only partially correct. The starting point for deration is not 20 amps for this application. You can use up to 9 before you are below the 20 amp threshold.

 

My question had three parts:
1.How many 12 ga conductors can be used in 1/2" EMT with a 20 amp
breaker?
2. " " " 3/4" EMT?
3. Does thhn have a better rating allowing for more conductors?
Thanks
Bill

 

Joed: yes deration for this application would be calculated from the 90 degree column so would start at 30 amps not 20 amps my mistake. I have edited that post. You must also consider ambient temp factors of 310.16 before derating for number of wires in conduit.


1.) 9 THHN #12 awg current carrying conductors before you drop below 20 amp rating.
2.) Still is 9 THHN's before downsizing the breaker to 15 amps you can fill the conduit to 40% of its cross sectional area if over 2 wires so you could put a maximum of 16 THHN #12's in 3/4 EMT. 40% equals .213sq.in. for 3/4 EMT. One #12 THHN is .0133sq.in. so 16 x .0133sq.in. is .2128sq.in. pretty much exactly .213 or 40% of the cross sectional area of 3/4 EMT. If you put over nine wires up to 16 wires then the ampacity derates from a starting point of 30 amps to 15 amps.
3.) THHN is rated at 90 C (194 F) but I am using it here for deration purposes. Many factors enter into the actual temp. column from table 310.16 for which ampacity is calculated. Depends on the temp. rating of the device you are connecting to, whether its a motor, that sort of information.

THW is rated a 75C as opposed to THHN at 90C so the starting point for #12 THW would be 25 AMPS for deration purposes for conduit after considering the ambient temp factors. The cross sectional area is also greater for THW than THHN (.0260sq.in vs .0133sq.in.) so the number of wires in the conduit will be less for THW vs THHN as to the over 2 wires 40% fill restriction.

Example: You have a #8 THW conductor in a conduit along with 5 others for a total of 6 current carrying conductors. The ambient temp for the installation is 50 C. In the 75 C columnn of 310.16. the correction factor for this ambient is .75. The ampacity for 75 C #8 THW is 50 amps. So .75 times 50 is 37.5 amps. Now you must consider the deration due to number of wires in a conduit. In this case the deration percent would be 80% for 6 wires in a conduit. So 37.5 times .80 is 30 amps. Then consider 110.14 if your conductor is less than 1 awg and compare your final ampacity to the 60 degree column of 310.16 and chose the smaller of the two due to termination restrictions. In this case 30 amps is the final ampacity rating for an #8 awg THW conductor in conduit with five other conductors.

Having fun yet?

 

Roger,
Many thanks for the very detailed explanation.
Obviously there's a lot of factors to consider
Short of a chart, it would be great if someone set up a website where entering in the type conductor and the size of the conduit would calculate an answer.
Looks like a max of 16 thhn conductors will work in 3/4" and a max of 9 will work in 1/2". Correct?
Bill

 

Yes......but deration gets pretty serious if more than ten wires in the conduit. I believe there are some calculaters on the web that do this. Probably do a search for conduit fill calculator or the like and see if it will do deration and fill requirements, not sure if one exists though.
Also there would be a difference between "wires in a conduit" and "circuits in a conduit". A circuit could be 2 wires or 3 wires in a residential application. Then you need to know if the white wire is a neutral or grounded leg.

 

Roger,
Again, thanks for your advice
I found a raceway calculator at:
http://www.electrician.com/calculato...lc_rev0326.htm
Looks easy to use.
Are raceway and conduit interchangeable terms?
I'm ok on the difference between conductors and circuits
Bill

 

Yes, raceway and conduit are interchangeble. That calculator is fine and will save you some time when you are intermixing wire sizes. There are many tables on the web similiar to the tables for conduit fill in the NEC code book if using the same size wire. Just need to remember to enter the correct type conduit. Deration for ambient and number of conductors are all part of the equation. Dont confuse the smaller size wires (14 awg thru 10 awg) with the higher ampacities allowed for deration purposes due to insulation types on table 310.16. NEC 240.4.D limits the ampacity of these wires to 15 amps, 20 amps and 30 amps respectively. GL