Sizing EMT For Different Size Wires - Electrical - DIY Chatroom Home Improvement Forum
Advertisement


Go Back   DIY Chatroom Home Improvement Forum > Home Improvement > Electrical

CLICK HERE AND JOIN OUR COMMUNITY TODAY...IT'S FREE!

Like Tree1Likes
  • 1 Post By CodeMatters
Reply
 
Thread Tools
Display Modes
Old 05-09-2019, 03:32 PM   #1
Member
 
Join Date: May 2012
Posts: 31
Rewards Points: 62
Default

Sizing EMT for different size wires


Curious why the percentage of cross section is lower for 2 wires than it is for more than 2 wires?

Am I correct in this calculation?
More than 2 wires @ 40%
1/2" EMT - 0.122 sq. in.

#12 THHN - 0.0133 x 4 = .0532
#10 THHN - 0.0164 x 3 = .0492
Total .1024

.122 - .1024 = .0196

Conclusion: 3 #10 THHN and 4 #12 THHN will fit in 1/2" EMT with .0196 sq.in. free space.
Options is offline   Reply With Quote
Sponsored Links
Advertisement
 
Old 05-09-2019, 04:31 PM   #2
Member
 
Join Date: Jan 2012
Location: IL
Posts: 1,410
Rewards Points: 1,252
Default

Re: Sizing EMT for different size wires


yes but did you consider de-rating of wires due to fill? This may be the more constraining factor.....


Conduit fill is not just a table lookup situation, you need to codnider if derating is a factor to you.
Attached Thumbnails
Sizing EMT for different size wires-derate-table.png  
curiousB is offline   Reply With Quote
Sponsored Links
Advertisement
 
Old 05-09-2019, 05:32 PM   #3
Member
 
CodeMatters's Avatar
 
Join Date: Aug 2017
Location: Canada
Posts: 3,247
Rewards Points: 6,492
Default

Re: Sizing EMT for different size wires


Quote:
Originally Posted by Options View Post
Curious why the percentage of cross section is lower for 2 wires than it is for more than 2 wires?

Because 2 wires take up proportionally more space. Picture 2 pennies
on a table touching each other. Picture the smallest circle that these
two pennies could sit inside. Now picture a third penny placed so that
it touches both of the other pennies. The smallest circle that these 3
pennies could fit in isn't much bigger than the circle for 2 pennies.
Of course, 3 wires won't always be oriented in a pipe that nicely, but
they'll approach that if the pipe is at all tight.
jbfan likes this.
CodeMatters is online now   Reply With Quote
Sponsored Links
Advertisement
 
Old 05-09-2019, 06:42 PM   #4
Member
 
Join Date: May 2012
Posts: 31
Rewards Points: 62
Default

Re: Sizing EMT for different size wires


Please correct me if I am wrong, but I don't believe that derating will be an issue. This is for a woodshop located in a detached concrete block garage. The shop walls and ceiling have been insulated to R20 & R45. I have a surface mounted 200A Square D PON box and all of the conduit (EMT) is exposed within the conditioned air space. The #10 THHN (which I have far too much of) supplies both dedicated 20A/240V circuits (20A breakers) and 30A/240V (30A breakers). #6 THHN will supply two dedicated 50A circuits and be run in separate conduits. The #12 THHN supplies the receptacles for the LED lighting and separately for the general purpose duplex receptacles. Each of the 120V 20A circuits will be protected by 20A PON dual function breakers and when done the panel will be fed by a new 200A underground service from the pole with its own meter.

Last edited by Options; 05-09-2019 at 06:45 PM. Reason: New info
Options is offline   Reply With Quote
Old 05-10-2019, 09:25 AM   #5
Member
 
Join Date: Jan 2012
Location: IL
Posts: 1,410
Rewards Points: 1,252
Default

Re: Sizing EMT for different size wires


Based on fill (7 wires) you have to derate conductors to 70% of their rating. So that means (assuming copper):


12AWG THHN max 21A
10AWG THHN max 28A <<<<<<<<<<<<<<

6AWG THHN max 52A


So you look ok except the 10AWG at 28A
curiousB is offline   Reply With Quote
Old 05-10-2019, 10:50 AM   #6
Member
 
Join Date: May 2012
Posts: 31
Rewards Points: 62
Default

Re: Sizing EMT for different size wires


Does derating apply to the service entrance wire as well? It will be 3 wire underground ] from the pole, but 4 wire from the meter to the inverted panel. Total length of 4 wire will be app 3'.
Options is offline   Reply With Quote
Old 05-10-2019, 11:17 AM   #7
Member
 
Join Date: May 2012
Posts: 31
Rewards Points: 62
Default

Re: Sizing EMT for different size wires


Quote:
Originally Posted by curiousB View Post
Based on fill (7 wires) you have to derate conductors to 70% of their rating. So that means (assuming copper):


12AWG THHN max 21A
10AWG THHN max 28A <<<<<<<<<<<<<<

6AWG THHN max 52A


So you look ok except the 10AWG at 28A
Thank you. I'll either change the 10 to 8 or run separate conduit. Thinking separate though as that will leave me with only the 12AWG having 4 wires. Even then if needed I believe I could eliminate the ground wires as I am running all EMT with weather tite connectors. Took an 11 week class in tech school 20 years ago on the NEC and don't recall any discussion on derating.
Options is offline   Reply With Quote
Old 05-10-2019, 01:37 PM   #8
Member
 
Join Date: Nov 2018
Posts: 403
Rewards Points: 58
Default


The ground wire is not counted in the derating calculations.
Don't get too bogged down on how many wires you can legally put in a conduit, esp. 1/2". You will find the difficulty of pulling that many wires is a nightmare.
Wiredindallas is offline   Reply With Quote
Old 05-13-2019, 04:17 PM   #9
Member
 
Join Date: May 2012
Posts: 31
Rewards Points: 62
Default

Re: Sizing EMT for different size wires


Quote:
Originally Posted by Wiredindallas View Post
The ground wire is not counted in the derating calculations.
Don't get too bogged down on how many wires you can legally put in a conduit, esp. 1/2". You will find the difficulty of pulling that many wires is a nightmare.
Thank you, I have been counting the grounds.

With one exception any pull that consists of more than 3 wires will be in 3/4" EMT and I don't plan to pull more than 6 in any one of these including the insulated grounds. Anything larger than #10 will be limited to 3 conductors including the grounds.

The scenario with 7 wires was due to relocating some equipment and the need for a 220V circuit after I had already installed all of the conduit and wiring. FLA is 10A on the 2hp edge sander that will be located there and I have already relocated the 7th wire which leaves 4 current carrying and 2 grounds in 4' of 1/2" EMT.

Derating shouldn't affect any of the circuits as I am using #12 THHN for the 20A duplexes and #10 THHN for the 20A 220v circuits. Circuits over 20A will be limited to 3 wires in each conduit and that includes the insulated ground.
Options is offline   Reply With Quote
Sponsored Links
Advertisement
 
Reply


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Elimination of telecom wires miamicuse Electrical 7 02-07-2018 08:39 AM
Wires and Size For Pool Sub Panel jduck1 Electrical 15 07-18-2015 03:55 PM
Unfused connectors MELTED House wires WHAT TO DO??? jamiedolan Electrical 63 02-12-2009 06:01 PM
Conduit size for four #8 wires goboomers Electrical 1 11-14-2008 09:00 PM
Number of receptacles and size of wires KUIPORNG Electrical 5 06-28-2006 01:21 PM




Top of Page | View New Posts