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

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

Reply
 
Thread Tools Search this Thread Display Modes
Old 06-16-2011, 09:10 PM   #1
Newbie
 
Join Date: Jun 2011
Posts: 5
Share |
Default

Wire size to 100 AMP subpanel


Looking for the correct 3 conductor w/ ground wire size from 100 AMP breaker in service panel to my 100 AMP subpanel. Romex size vs. THHN size using conduit. What is the benefits of one over the other?

Coqui is offline   Reply With Quote
Old 06-16-2011, 09:18 PM   #2
Electromagician
 
Join Date: May 2011
Posts: 79
Default

Wire size to 100 AMP subpanel


one advantage to conduit and thhn is that I have never seen romex that size before.

powerfactor is offline   Reply With Quote
Old 06-16-2011, 09:51 PM   #3
Electrical Contractor
 
kbsparky's Avatar
 
Join Date: Sep 2008
Location: Delmarva
Posts: 3,368
Default

Wire size to 100 AMP subpanel


I doubt you will be able to find romex large enough to handle 100 amps. #2 is listed in Southwire's catalog, with a 95 Amp rating. I suppose you could special order a reel of the stuf, but it will probably be cost prohibitive.

#3 THHN in conduit, with a #8 ground (if using PVC) is sufficient for 100 Amps.

IF you upsize to #2 THHN, you may be required to upsize your ground to a #6.
__________________
-KB

Life is uncertain -- eat dessert first!!
kbsparky is offline   Reply With Quote
Old 06-17-2011, 01:13 AM   #4
Newbie
 
Join Date: Jun 2011
Posts: 5
Default

Wire size to 100 AMP subpanel


Thanks. You are correct that #2/3 Romex is only rated 95 AMPs, but my town issued a permit to me with this as an option. I was a little uncomfortable, after reviewing the 2011 NEC code, because the Romex did not meet 100% of the ampacity rating. The advice gave me the reassurance to go with THHN #2 w/ #4 ground.

Last edited by Coqui; 06-18-2011 at 10:53 AM. Reason: Typo
Coqui is offline   Reply With Quote
Old 06-17-2011, 03:35 AM   #5
Member
 
Join Date: Mar 2007
Location: Eighty Four, Pa.15330
Posts: 1,247
Default

Wire size to 100 AMP subpanel


Howzabout 3 #2W/g M.C. cable? I like it.
bobelectric is offline   Reply With Quote
Old 06-17-2011, 05:55 AM   #6
Licensed Electrical Cont.
 
Speedy Petey's Avatar
 
Join Date: Feb 2004
Location: NY State
Posts: 6,810
Default

Wire size to 100 AMP subpanel


Quote:
Originally Posted by Coqui View Post
Thanks. You are correct that #3/2 Romex is only rated 95 AMPs,.........
Which is typical and fine for a 100A sub-feeder. There is no such thing as a 95A breaker so we round up to 100A.


Quote:
Originally Posted by Coqui View Post
The advice gave me the reassurance to go with THHN #2 w/ #4 ground.
So you went oversized on both?

What 125% ampacity rating are you referring to?
Speedy Petey is offline   Reply With Quote
The Following User Says Thank You to Speedy Petey For This Useful Post:
electures (06-17-2011)
Old 06-17-2011, 11:50 PM   #7
Electromagician
 
Join Date: May 2011
Posts: 79
Default

Wire size to 100 AMP subpanel


sorry but 3/2 romex is only rated for 85 amps.
powerfactor is offline   Reply With Quote
Old 06-18-2011, 06:35 AM   #8
Lic Electrical Inspector
 
electures's Avatar
 
Join Date: Dec 2009
Location: The Great Police State of New Jersey
Posts: 1,666
Default

Wire size to 100 AMP subpanel


Quote:
Originally Posted by powerfactor View Post
sorry but 3/2 romex is only rated for 85 amps.
It also need to be three conductors plus a ground.
electures is offline   Reply With Quote
Old 06-18-2011, 10:24 AM   #9
Newbie
 
Join Date: Jun 2011
Posts: 5
Thumbs up

Wire size to 100 AMP subpanel


I read #3/2 to be a typo for #2/3 and I inadvertantly followed the typo in my response, for which I apologize and corrected above. Anyone reading this thread should understand that we are not talking about two conductors but three conductor (two hots, one neutral, and the ground). Romex NM-B #2/3 is rated 95 Amps. I went back and reviewed the NEC Code and changed my decision to that recommended by kbsparky (THHN #3, which is rated 115 Amps, with a #6 ground (#8 is the minimum and acceptable). This was extremely helpful, thanks.

Last edited by Coqui; 06-18-2011 at 10:50 AM. Reason: typo
Coqui is offline   Reply With Quote
The Following User Says Thank You to Coqui For This Useful Post:
Speedy Petey (06-18-2011)
Old 06-18-2011, 11:15 AM   #10
Licensed Electrical Cont.
 
Speedy Petey's Avatar
 
Join Date: Feb 2004
Location: NY State
Posts: 6,810
Default

Wire size to 100 AMP subpanel


Quote:
Originally Posted by Coqui View Post
I read #3/2 to be a typo for #2/3 and I inadvertantly followed the typo in my response,
Uhhhh..yup. Me too.
Speedy Petey is offline   Reply With Quote
Old 06-18-2011, 02:09 PM   #11
Electrical Contractor
 
kbsparky's Avatar
 
Join Date: Sep 2008
Location: Delmarva
Posts: 3,368
Default

Wire size to 100 AMP subpanel


Quote:
Originally Posted by Coqui View Post
...I went back and reviewed the NEC Code and changed my decision to that recommended by kbsparky (THHN #3, which is rated 115 Amps....
Ummm ... #3 THHN is rated for 100 Amps not 115, if you use the 75 degree column listed in table 310.16 of the NEC.

#2 is rated for 115 Amps.

Me thinx your typos are still haunting you ...
__________________
-KB

Life is uncertain -- eat dessert first!!
kbsparky is offline   Reply With Quote
Old 06-18-2011, 02:28 PM   #12
Electromagician
 
Join Date: May 2011
Posts: 79
Default

Wire size to 100 AMP subpanel


so is it three twos or two threes and are we using a single two or is it a three? nevermind I will go back and edit an old post to throw the threads whole context off.
powerfactor is offline   Reply With Quote
Old 06-18-2011, 02:47 PM   #13
Electrical Contractor
 
kbsparky's Avatar
 
Join Date: Sep 2008
Location: Delmarva
Posts: 3,368
Default

Wire size to 100 AMP subpanel


Quote:
Originally Posted by powerfactor View Post
sorry but 3/2 romex is only rated for 85 amps.
Thought I'd catch you in a quote before you edit it ...

AFAIK, there ain't no such thing as 3/2 romex. SouthWire makes it in #4, then #2 ... skipping #3 altogether. Click here for the list, and scroll down to page 2. #3 ain't on the chart.
__________________
-KB

Life is uncertain -- eat dessert first!!
kbsparky is offline   Reply With Quote
Old 06-18-2011, 03:35 PM   #14
Electromagician
 
Join Date: May 2011
Posts: 79
Default

Wire size to 100 AMP subpanel


Quote:
Originally Posted by kbsparky View Post
Thought I'd catch you in a quote before you edit it ...

AFAIK, there ain't no such thing as 3/2 romex. SouthWire makes it in #4, then #2 ... skipping #3 altogether. Click here for the list, and scroll down to page 2. #3 ain't on the chart.

check post number two. but don't quote it as I may want to edit it!
powerfactor is offline   Reply With Quote
Old 06-18-2011, 06:54 PM   #15
Member
 
Knucklez's Avatar
 
Join Date: Oct 2007
Location: ontario canada
Posts: 858
Default

Wire size to 100 AMP subpanel


the NEC tables are conservative.. there is a lot of wire in those cables. #3 is fine. there are also derating factors such as duty %, load type etc.. but generally you don't fill your panel more than 80% (80Amps) so a 100A rated cable is just fine.

__________________
"if you can't be handsome, at least you can be handy" -Redgreen
Knucklez is offline   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
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
What size wire to use running underground ms66cadillac Electrical 21 01-13-2011 11:14 AM
What size wire do I need? walnut777 Electrical 1 12-13-2010 06:41 PM
Sub-Panel Wire Size and Conduit Questions Tmb9862 Electrical 3 12-02-2009 11:48 PM
Size of Wire & type of wire agman Electrical 6 11-10-2009 08:13 PM
Aluminum wire size/ guage / amps? walt1122 Electrical 17 07-30-2008 10:44 AM




Top of Page | View New Posts

Copyright © 2003-2014 Escalate Media. All Rights Reserved.