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Old 11-12-2008, 08:13 PM   #1
rinker1
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Default What size wire for 100 amp sub panel to detached garage

I have a 200 amp breaker box in the house and running a 1 1/2 inch pvc conduit about 85 ft out to a new detached garage to a 100 amp ckt breaker box.
1: Is #2 al urd ok to run out there?
2: Does it have to be #2 with 4 conducters? (1 for grd)
3: The box in the garage will have a 100 amp main in it, can the breaker in the house feeding this also be 100 amp or does it have to be 90 amp to be the weak link. This is what Ive been told.

Last edited by rinker1; 11-12-2008 at 09:56 PM.
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Old 11-12-2008, 08:18 PM   #2
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Default Re: What size wire for 100 amp sub panel to detached garage

I have ran exactly what you are proposing. It has been in for 2 months no problem, passed final inspection.
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Old 11-12-2008, 08:32 PM   #3
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Default Re: What size wire for 100 amp sub panel to detached garage

Quote:
Originally Posted by SS350Sport View Post
I have ran exactly what you are proposing. It has been in for 2 months no problem, passed final inspection.
Thanks, just trying to be sure what I'm doing I've got to go thru the electrical inspection to.
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Old 11-12-2008, 08:38 PM   #4
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Default Re: What size wire for 100 amp sub panel to detached garage

The ground conductor can be smaller.

Use a 200A box in the garage with a 200A main to serve as the disconnect. Use a 100A breaker in the house panel.

Check this link that some good person posted here before:
http://www.selfhelpandmore.com/home-...arage-2002.php
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Old 11-12-2008, 08:49 PM   #5
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Default Re: What size wire for 100 amp sub panel to detached garage

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Originally Posted by Torque1st View Post
The ground conductor can be smaller.

Use a 200A box in the garage with a 200A main to serve as the disconnect. Use a 100A breaker in the house panel.

Check this link that some good person posted here before:
http://www.selfhelpandmore.com/home-...arage-2002.php
Torque1st- Thanks for that link, that has a lot of great info in it!
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Old 11-12-2008, 08:52 PM   #6
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Default Re: What size wire for 100 amp sub panel to detached garage

I believe 2O is a little overkill. I would have to check my uglies book for the distance, but I think 2GA CU is sufficient maybe even 4GA.
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Old 11-12-2008, 09:01 PM   #7
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Default Re: What size wire for 100 amp sub panel to detached garage

4ga should do 100amps no? as long as its not super far away..
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Old 11-12-2008, 09:04 PM   #8
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Default Re: What size wire for 100 amp sub panel to detached garage

If you insist on using AL make sure your connections are tight and stay tight (tighten them every year) also use no-ox on the connections.
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Old 11-12-2008, 09:26 PM   #9
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Default Re: What size wire for 100 amp sub panel to detached garage

There is some technical debate on 2 for 100A, but it does pass inspections and is fine for services. The box stores sell a "quad" setup just for this application but there is sometimes a slight difference, some is 2-2-4-6 and some 2-2-2-6, I found the latter for less price at Home Depot vs Lowe's. I would rather have 2 inch pipe but it will go 1 1/2 with only a couple 90's. I have done a lot of garages feeding this wire with 60A breaker from the main (its cheap), never had one call back with trip problems.
Yes, I look at this post and its number 2 wire, not 2/0
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Old 11-12-2008, 09:39 PM   #10
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Default Re: What size wire for 100 amp sub panel to detached garage

Quote:
Originally Posted by LoneGunman View Post
If you insist on using AL make sure your connections are tight and stay tight (tighten them every year) also use no-ox on the connections.
Taken from Southwire's website:
http://www.southwire.com/Southwire/S...ide8-15-07.pdf

When terminating aluminum conductors, there are several practices one should be aware of to ensure a
quality connection that will remain trouble free.
1) Always use a connector that is listed for use with aluminum wire.
2) The following is recommended but not required. Always follow the connector manufacturer’s
instructions
a. The surface of the conductor should be wire-brushed to break any aluminum oxide
barrier leaving a clean surface for the connection.
b. The use of anti-oxidant compound, joint compound, is not required unless the
connector’s manufacturer requires it. However, the use of a listed joint compound is
always a good practice.
3) Always tighten set-screw type connectors to the manufacturer’s recommended torque.
4) Most importantly, do not re-torque the terminations as part of routine maintenance. As
with copper conductors, repeated tightening of any set-screw connections can result in the
eventual “biting” through the conductor causing the termination to fail.
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Old 11-12-2008, 09:43 PM   #11
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Default Re: What size wire for 100 amp sub panel to detached garage

This is exactly what I needed to find out myself. I'm also putting in a 100amp feeder breaker. It's time for my electrical rough-in. I'm off to Home Despot tomorrow to get 2-2-4-6. It's only a 20 ft run for me.
Thanks!
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Old 11-12-2008, 10:17 PM   #12
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Default Re: What size wire for 100 amp sub panel to detached garage

Quote:
Originally Posted by thdewey View Post
I'm off to Home Despot tomorrow to get 2-2-4-6. It's only a 20 ft run for me.
Thanks!
just a heads up.. i found Home Despot to be 40% more for wire up here than a local electrical wholesaler..
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Old 11-12-2008, 10:25 PM   #13
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Default Re: What size wire for 100 amp sub panel to detached garage

Quote:
Originally Posted by sberry27 View Post
There is some technical debate on 2 for 100A, but it does pass inspections and is fine for services.
What does a service have to do with a feeder to a garage?

Also, since your clueless inspector is passing code violations, that gives you the right to instruct others to do it in the same wrong way?

In typical forum fashion, sometimes the advice you get on the internet is worth exactly what you paid for it....

Quote:
Originally Posted by thdewey View Post
This is exactly what I needed to find out myself. I'm also putting in a 100amp feeder breaker. It's time for my electrical rough-in. I'm off to Home Despot tomorrow to get 2-2-4-6. It's only a 20 ft run for me.
Thanks!
#1 aluminum is rated for 100 amps, #2 is 90 amp. You can buy the #2, but a 90 amp breaker is what you'll need.
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Old 11-12-2008, 10:42 PM   #14
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Default Re: What size wire for 100 amp sub panel to detached garage

I think there have been two dicussions going on, in my post i referenced 2GA CU (COPPER), so after that not sure what the guys were suggesting. Personally, I dont care for aluminum, it just bothers me with the corrosion problems.

For those that dont know... Copper is a better coductor and generally takes a size or 2 smaller wire...consult the NEC and your local requirements! Voltage drop needs to be considered for longer runs.
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Last edited by PurdueSD; 11-12-2008 at 11:08 PM.
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Old 11-13-2008, 07:33 AM   #15
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Default Re: What size wire for 100 amp sub panel to detached garage

I Ran 3 2 ga. copper strands from my 225 amp house panel to my 100 amp main breaker panel in the shop.
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Old 11-13-2008, 12:34 PM   #16
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Default Re: What size wire for 100 amp sub panel to detached garage

It is a feeder but it is a service to a detached structure too is it not? It is not only my inspector passing them but it seems I cant recall anyone ever being turned down for one? It seems I asked at the forum with Bob Keis and as I remember you may be correct but its been a while. As for it being some kind of risk factor it would certainly be negligible especially in a garage where most loads tend to be intermittent. Are 90A breakers available?
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Old 11-13-2008, 12:42 PM   #17
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Default Re: What size wire for 100 amp sub panel to detached garage

Quote:
You can't use Table 310.15(B)(6) to size the feeder to a detached garage. Table 310.15(B)(6) can only be used for service entrance conductors and main power feeders to dwelling units. So a detahced garage is not a dwelling unit so the Table doesn't apply.
Quote:
Also a change in the 2008 NEC has really clarified that to be able to use this table for a feeder the feeder must carry the entire load of the dwelling unit. Here is the new revisions to 310.15(B)(6)


Quote6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. For individual dwelling units of one-family, two-family, and multifamily dwellings, conductors, as listed in Table 310.15(B)(6), shall be permitted as 120/240-volt, 3-wire, single-phase service-entrance conductors, service-lateral conductors, and feeder conductors that serve as the main power feeder to each dwelling unit and are installed in raceway or cable with or without an equipment grounding conductor. For application of this section, the main power feeder shall be the feeder between the main disconnect and the panelboard that supplies, either by branch circuits or by feeders, or both, all loads that are part or associated with the dwelling unit. The feeder conductors to a dwelling unit shall not be required to have an allowable ampacity rating greater than their service-entrance conductors. The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided the requirements of 215.2, 220.61, and 230.42 are met.



I added the bold to show the new definition of main power feeder. You will also note that the term main power feeder is singular now where in previous editions of the NEC it stated "main power feeder(s)".
So it was allowable under older codes and would be if it was a dwelling unit?
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Old 11-13-2008, 12:46 PM   #18
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Default Re: What size wire for 100 amp sub panel to detached garage

As for alum wire the poco uses it, 90% of the worlds power travels thru it at some point.
I certainly see why they list the wire that way, they dont want the number 2 alum on a circuit where it could be continiously loaded, something like electric heater would be example.

Last edited by sberry; 11-13-2008 at 01:19 PM.
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Old 11-13-2008, 04:14 PM   #19
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Default Re: What size wire for 100 amp sub panel to detached garage

All good info guys. One minor point, when installing a sub panel, use a "Main Lug" box for the sub as opposed to a "Main Breaker" unit. There is usually a signifigant price difference, and in the event of a "Main Feed Breaker Trip", there will be no question where to look.
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Old 11-13-2008, 04:36 PM   #20
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Default Re: What size wire for 100 amp sub panel to detached garage

Quote:
Originally Posted by Aceman View Post
Taken from Southwire's website:
http://www.southwire.com/Southwire/S...ide8-15-07.pdf

When terminating aluminum conductors, there are several practices one should be aware of to ensure a
quality connection that will remain trouble free.
1) Always use a connector that is listed for use with aluminum wire.
2) The following is recommended but not required. Always follow the connector manufacturer’s
instructions
a. The surface of the conductor should be wire-brushed to break any aluminum oxide
barrier leaving a clean surface for the connection.
b. The use of anti-oxidant compound, joint compound, is not required unless the
connector’s manufacturer requires it. However, the use of a listed joint compound is
always a good practice.
3) Always tighten set-screw type connectors to the manufacturer’s recommended torque.
4) Most importantly, do not re-torque the terminations as part of routine maintenance. As
with copper conductors, repeated tightening of any set-screw connections can result in the
eventual “biting” through the conductor causing the termination to fail.

I'll continue to re-torque no matter what Southwire says, if they were tightened PROPERLY the first time odd are no retorque will ever be needed the problem is a good portion of the time they are not tightened properly the first time. The romex monkeys in new construction cookie cutter houses are notorious for not tightening lugs properly. I was taught to tighten the lug, move the wire around a little back and forth and side to side and then tighten a little more, you can usually get at least 3/4 of another turn before it feels right. Some sparkies tighten the lug, back off a little and then tighten it again.

In my opinion Southwire has that on there website because too many people cannot judge what "tight" is (I've never seen a torque wrench in an electricians van) and overtighten until they compress the AL too much or strip the lug. In 15 years I'd take a guess I've tightened 200 lugs that NEEDED to be retightened unless we wanted to change the panel and the wire in the very near future. I'm sure you have seen melted insulation and flickering lights on a service call and found a loose neutral.


*edit, after doing some more reading it seems retightening as scheduled maintenance is frowned upon, learn something new everyday. Still no way around retightening improperly tightened lug set screws.

Last edited by LoneGunman; 11-13-2008 at 04:49 PM.
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