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

 DIY Chatroom Home Improvement Forum Sizing EMT for different size wires

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05-09-2019, 03:32 PM   #1
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## 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.

05-09-2019, 04:31 PM   #2
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## 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.
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05-09-2019, 05:32 PM   #3
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## Re: Sizing EMT for different size wires

Quote:
 Originally Posted by Options 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.

05-09-2019, 06:42 PM   #4
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## 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

05-10-2019, 09:25 AM   #5
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## 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

05-10-2019, 10:50 AM   #6
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## 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'.

05-10-2019, 11:17 AM   #7
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## Re: Sizing EMT for different size wires

Quote:
 Originally Posted by curiousB 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.

 05-10-2019, 01:37 PM #8 Member   Join Date: Nov 2018 Posts: 403 Rewards Points: 58 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.
05-13-2019, 04:17 PM   #9
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## Re: Sizing EMT for different size wires

Quote:
 Originally Posted by Wiredindallas 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.

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