Need help with wire size selection

[Tatternuts]

Wondering if anyone can help me select the correct wire size for my pole barn. I will be running from the house main panel to a sub panel in the barn that is about 130 feet away. The sub panel in the barn is rated for 125 amp, and I will be buring the wire in 2" pvc electrical conduit, 18" below grade. From what I can see its a toss up between #1 and #2 awg copper THWN wire for that distance. I'm not real sure on the nuetral wire size or the ground wire size however.
Also I plan on using two grounding rods about 6ft apart and wire into the sub panel when completed to meet code.

Thanks for any input guys!

 

[pattenp]

It seems a lot of people always think copper for running a feeder to a sub-panel, you should consider aluminum, it's a lot cheaper. For aluminum you are probably looking at less than $2 a foot vs copper at most likely around $5 a foot. That price is for a 4 wire feeder, so on a 130 ft run you are looking at $260 vs $650.

For 125A I believe you will need 1/0 copper or 3/0 aluminum. To be safe 2/0 copper or 4/0 alum would be best.

Check this link for mobile home feeder .. http://www.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet15

 

[MrMark]

No, he's right. Number 2 copper THHN (not nm which must use the 60 degree column and is only rated for 95 amps) is rated for 115 IIRC using the correct 75 degree column. There is the next size up rule which allows you to use the 125 breaker again, as I recall. The issue is also the length of run. Given the marginal 125 amp rating for the #2 and the long run, #1 is a no brainer. Or, drop to a 100 amp breaker with number 2.

Use the same size neutral. I forget on the ground. Look it up in the table in the NEC. I'm going to guess #8 copper.

 

[PRH44]

Please look at the breaker you are installing. You will find a Temp. rating on the name plate. Should you see 75/60 this means the breaker in a stand alone enclosure with no other breakers has a rating of 75 degree C terminations on the lugs. whenusing wire or cable rated for 75 degree and above.
If this breaker is installed in a panel with other breakers where heat can not escape as readily you will default to 60 degree c rating if using romex. Single strand wire such as THHN/THWN will use the 75 degree column.

However 110.14(4)(C) allows equipment rated over 100 amps and marked for conductors larger than #1 AWG to be 75 degree C ampacities.

Thus you may use the 75 degree C if you meet the above
#2 THHN = 115 amps and #1 = 130 amps. Artical 240.4 B allows the next higher rated breaker or fuse so # 2 can be used.



# 2 is fine unless you want you use #1. but please check your lug rating and always torque to proper specification. This will prevent damage to the conductor and or lug.
The neutral conductor will be the same size as the phase conductors #2 if you choose.
The equipment ground will be a # 6 copper

 

[Naq]

Tatternuts you're right in choosing THWN or another wire/cable rated for wet locations since you are running underground.

PRH44 when you say this:
'If this breaker is installed in a panel with other breakers where heat can not escape as readily you will default to 60 degree c rating."

Are you referring to an enclosure containing other devices that are rated for no higher than 60C?

 

[PRH44]

Mr Mark: 310.15(B)(7) is the 2011 code. You are correct he is in fact install a subpanel connect to his main panel and would negate the use of 310.15(B)(7) in the 2011 code. Thank you

Standard size breakers as recognized in the 2008 and 2011 NEC are as follows.

240.6 Standard Ampere Ratings.
(A) Fuses and Fixed-Trip Circuit Breakers. The standard
ampere ratings for fuses and inverse time circuit
breakers shall be considered 15, 20, 25, 30, 35, 40, 45, 50,
60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300,
350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000,
2500, 3000, 4000, 5000, and 6000 amperes. Additional
standard ampere ratings for fuses shall be 1, 3, 6, 10, and
601. The use of fuses and inverse time circuit breakers with
nonstandard ampere ratings shall be permitted.

Yes 240.4 (B) will allow #2 to be used on a 125 amp breaker.

Nag : Any breaker with a 75/60 degree rating will default to a 60 degree rating in an enclosure with any other rated breaker. However this does not apply to breakers rated over 100 amps

This only applies to breakers rated 100 amps or less according to 110-14(c)(1)

 

[MrMark]

PRH, I did not know this on the breakers. So, for #6 cu the max breaker would be 55 amps (bump up to 60 under the rule above), not the 65 amps of the 75 degree column? Thank you.

 

[MrMark]

PRH, now I am confused again. How does #3 cu get breakered at 100 amps as I have commonly seen done? The 100 amps is from the 75 degree column.

 

[Naq]

I used to think that <100A terminations defaulted to 60C but now I'm wondering about NEC 2011-110.14(C)(1)(a)
(3) "Conductors with higher temperature ratings if the
equipment is listed and identified for use with such conductors."

I was browsing the CH load center catalog and all of the panels were rated for 60/75C and the spec section reads:
"Wiring/Termination
...
B. All loadcenters where
marked shall be suitable
for use with 60/75°C
rated wire."

PS: It's NAQ. The sans-serif font knocks the curl off my q.

 

[PRH44]

This is a very confusing part of the code. You can always install higher rated cable but you are always limited to the lowest rated temperature termination in a circuit.

In the right condition #3 can be used as 100 amp. But all Terms have to be rated 75 degree C.

If your 100 amp breaker has a 75 degree c rated termination then it works.

You can fill a panelboard with 75 degree breakers even though the panel is rated 75/60 all that means is the panel is rated for both 75 and 60 rated breakers.

If a breaker of any size has a full 75 degree rating then the 75 degree column will be used. Even if it is installed along side of breakers rated 75/60. it is the duel rated breaker alone that will default back to 60.

If I put and 75 degree breaker in and terminate THHN on it I can use 75 but if Iterminate the other end on a 60 degree rated device I am back to 60 degree in the entire circuit.

I got you guys curious now read up and lets hear your interpretations on this subject. Good code banter is always fun something new always pops up its a good thing.

 

[Charles (in GA)]

This is a very confusing part of the code. You can always install higher rated cable but you are always limited to the lowest rated temperature termination in a circuit.

In the right condition #3 can be used as 100 amp. But all Terms have to be rated 75 degree C.

If your 100 amp breaker has a 75 degree c rated termination then it works.

You can fill a panelboard with 75 degree breakers even though the panel is rated 75/60 all that means is the panel is rated for both 75 and 60 rated breakers.

If a breaker of any size has a full 75 degree rating then the 75 degree column will be used. Even if it is installed along side of breakers rated 75/60. it is the duel rated breaker alone that will default back to 60.

If I put and 75 degree breaker in and terminate THHN on it I can use 75 but if Iterminate the other end on a 60 degree rated device I am back to 60 degree in the entire circuit.

I got you guys curious now read up and lets hear your interpretations on this subject. Good code banter is always fun something new always pops up its a good thing.

This is all well and good as a theoretical discussion, but what we are trying to do here is help someone decide what size wire he needs to use.

Every Square D QO, Siemens, and GE breaker I've ever seen for residential use is dual marked 60°/75°C so the whole thing goes back to 60°C. I'm not a professional, but I've messed with quite a few household systems and components, and the kind of stuff you find in a shop, and I haven't run into anything yet that is simply 75°C rated, so your point, while valid, is useless in this discussion.

Lets help the guy out here, not confuse him.

Charles

 

[Naq]

Lets help the guy out here, not confuse him.

Charles

hahaha Touche. He's over 100A so the terminal discussion is moot in this thread. Sorry for the hijack. He's already on the right track.

Main panel in house:
125A, 2P breaker with 75C terminals
3#2AWG CU THWN, 1#6AWG CU THWN ground
Neutral taped white/gray, Ground green
2" electrical PVC from house panel, outside, 18" underground, into pole barn (2" is double the required size making it easier to pull)
Caution tape 12" above conduit(s)
Pole Barn subpanel:
125A main breaker when using more than 6 circuits.
1#8AWG CU continuous conductor connected from ground bus to both grounding electrode rods
Neutral bus MUST be isolated from ground bus. Check for factory installed wire/strap/or screw bonds

The AL option would be type USE mobile home feeder

 

[MrMark]

This is all well and good as a theoretical discussion, but what we are trying to do here is help someone decide what size wire he needs to use.

Every Square D QO, Siemens, and GE breaker I've ever seen for residential use is dual marked 60°/75°C so the whole thing goes back to 60°C. I'm not a professional, but I've messed with quite a few household systems and components, and the kind of stuff you find in a shop, and I haven't run into anything yet that is simply 75°C rated, so your point, while valid, is useless in this discussion.

Lets help the guy out here, not confuse him.

Charles

I think this guy has been more than taken care of. He has one post. He may never be heard from again. He joined to ask a question and get technical info for free from a very good forum with very good members. He has received more BS free and correct info than anyone I have ever seen on here. Certainly more than I ever have.


I think this discussion is good and better than 98 percent of the typical how can I light my garage threads.

 

[MrMark]

This is a very confusing part of the code. You can always install higher rated cable but you are always limited to the lowest rated temperature termination in a circuit.

In the right condition #3 can be used as 100 amp. But all Terms have to be rated 75 degree C.

If your 100 amp breaker has a 75 degree c rated termination then it works.

You can fill a panelboard with 75 degree breakers even though the panel is rated 75/60 all that means is the panel is rated for both 75 and 60 rated breakers.

If a breaker of any size has a full 75 degree rating then the 75 degree column will be used. Even if it is installed along side of breakers rated 75/60. it is the duel rated breaker alone that will default back to 60.

If I put and 75 degree breaker in and terminate THHN on it I can use 75 but if Iterminate the other end on a 60 degree rated device I am back to 60 degree in the entire circuit.

I got you guys curious now read up and lets hear your interpretations on this subject. Good code banter is always fun something new always pops up its a good thing.

Wow, that explains it. Nice post.

So those #3 run subpanels with 100 amp breakers were not correct because of the dual rating on all residential breakers according to the post Charles made. I'm not surprised. It seems #2 is required for 100 amp breaker and then you can go to 125 (unless nm cable is used which gets stuck on the 60 degree column no matter the terminations) because you are allowed to use the 75 degree column because the breaker is over 100 amps. VERY confusing. No wonder it is always done wrong.

From what I see the only wire breakered at 100 would be nm 2-3, like I used on my subpanel. Is this right?

I have long wondered about this. I learned a lot from PRH and Nags posts on the terminal temps and dual rated breakers and what that means code wise.

 

[Tatternuts]

Great information guys. I figured I was on the right track but wanted to make sure i did everything correctly and by the book. Now the fun part.... going out and buying the supplies, then early this spring I can start trenching the hole to the pole barn and get everything done. Hopefully I can post some pics during the install.

Thanks again!

 

[Naq]

Great information guys. I figured I was on the right track ...
Hopefully I can post some pics during the install.

Thanks again!

Please do! I'm in a similar situation now and have been doing a lot of research on this forum. I can already hear my wallet groaning.

If you're going with #2CU THWN its a very popular size in industry so check prices at the box stores with your local electrical suppliers, the ebay shippers and any contractor friends. You're talking around $1000 worth of wire.

If anyone else wants to continue geeking out on the code conversation I just found

Regardless of what you think of Mike and his forums, he is out there discussing these issues.

 

[Tatternuts]

More then likely I will end up going with 1 awg wire since 2 awg will be slightly undersized for 125 amp service using 75C rated circuit breakers. I looked over NEC table which showed 1 awg wire at 75C was rated to 130amps. I would like to use 2awg wire but there are no circuit breakers available with a 90C rating on them. Oh well, I would much rather be on the safe side and prevent any nuisance tripping of the breaker.

I will definetly start looking into where i can find the best wire prices. Home depot and Lowes dont seem to carry a good selection of large awg wire around here so i may be using a local supply house.

thanks

 

[Norcal]

More then likely I will end up going with 1 awg wire since 2 awg will be slightly undersized for 125 amp service using 75C rated circuit breakers. I looked over NEC table which showed 1 awg wire at 75C was rated to 130amps. I would like to use 2awg wire but there are no circuit breakers available with a 90C rating on them. Oh well, I would much rather be on the safe side and prevent any nuisance tripping of the breaker.

I will definetly start looking into where i can find the best wire prices. Home depot and Lowes dont seem to carry a good selection of large awg wire around here so i may be using a local supply house.

thanks

There is NO equipment that is rated for 90 degree C, BTW be sure that your panel is rated for a 125A circuit breaker, it is common to be limited to 110A per bus stab, plus a 100A breaker is a helluva lot cheaper then a 125A.