# Wire size to 100 AMP subpanel



## Coqui

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?


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## powerfactor

one advantage to conduit and thhn is that I have never seen romex that size before. :no:


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## kbsparky

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.


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## Coqui

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.


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## bobelectric

Howzabout 3 #2W/g M.C. cable? I like it.


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## Speedy Petey

Coqui said:


> 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.




Coqui said:


> 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?


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## powerfactor

sorry but 3/2 romex is only rated for 85 amps.


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## electures

powerfactor said:


> sorry but 3/*2* romex is only rated for 85 amps.


It also need to be three conductors plus a ground.


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## Coqui

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.


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## Speedy Petey

Coqui said:


> I read #3/2 to be a typo for #2/3 and I inadvertantly followed the typo in my response,


Uhhhh..yup. Me too.


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## kbsparky

Coqui said:


> ...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 ... :laughing:


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## powerfactor

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.


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## kbsparky

powerfactor said:


> sorry but 3/2 romex is only rated for 85 amps.


Thought I'd catch you in a quote before you edit it ... :laughing:

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.


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## powerfactor

kbsparky said:


> Thought I'd catch you in a quote before you edit it ... :laughing:
> 
> 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. :whistling2: but don't quote it as I may want to edit it!


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## Knucklez

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.


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## Speedy Petey

Knucklez said:


> the NEC tables are conservative.. there is a lot of wire in those cables.


Ummmm.......WHAT??




Knucklez said:


> ...there are also derating factors such as duty %, load type etc..


Ummmm.......WHAT??




Knucklez said:


> ...but generally you don't fill your panel more than 80% (80Amps)


Ummmm.......WHAT??


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## Coqui

Me thinx *kbsparky* in wrong column. THHN is rated 90 C not 75 C; and thus, 115 according to 2011 NEC Code Table 310.15(B)(16) formerly 310.16. It can be confusing with all the types of wire.


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## kbsparky

Not quite. You can not use the 90 degree column for final connections unless you have equipment terminations rated at that temperature.

All the breaker box manufacturers that I am aware only have their terminations rated at 75 degrees, thus you are limited to wire with that capacity/rating.

You can start with the 90 degree ratings before any adjustments are made for things like ambient temperature and conduit fill deratings, but your final figure is restricted to the 75 degree values or less.


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## Knucklez

what i mean is.. the 12 gauge wire can take more than 20Amps.. before it has any problems. but your local code does not allow it, so this is what i mean by conservative.

duty %, i am refering to that not all the loads are on at any one time. for instance, a 100A panel with 30 slots, even if they were all just 15 Amp breakers, would be 450Amps. obviously the idea is that not all the breakers will be on giving full power to the load. so this is what i mean by duty.

not filling more than 80%, i am refering to CEC (i don't know if it is in NEC) where it talks about if you don't know what the load is, then don't fill more than 80%. that is just from memory.. section 8, part I. 

didn't mean to lead anyone astray..


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## kbsparky

You posted this in the wrong thread, knucklez


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## Speedy Petey

Coqui said:


> Me thinx *kbsparky* in wrong column. THHN is rated 90 C not 75 C; and thus, 115 according to 2011 NEC Code Table 310.15(B)(16) formerly 310.16. It can be confusing with all the types of wire.


Aside from the fact that we are talking about NM cable, which is limited to the 60 deg C column.


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## kbsparky

No we weren't. As stated previously, you can't get #3 in NM cable. This part of the discussion is definitely about #3 THHN/THWN.


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## Coqui

kbsparky and I are on #3 THHN we dropped the Romex #2/3 line of inquiry.


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