What size wire for 100 amp sub panel to detached garage

[rinker1]

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.

[SS350Sport]

I have ran exactly what you are proposing. It has been in for 2 months no problem, passed final inspection.

[rinker1]

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.

[Torque1st]

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-wiring-usa/accessory-structures-to-dwellings/wiring-a-detached-garage-2002.php

[rinker1]

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-wiring-usa/accessory-structures-to-dwellings/wiring-a-detached-garage-2002.php

Torque1st- Thanks for that link, that has a lot of great info in it!

[PurdueSD]

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.

[brad d]

4ga should do 100amps no? as long as its not super far away..

[LoneGunman]

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.

[sberry]

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

[Aceman]

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/StaticFiles/Text/SIMpullTHHNInstallationandApplicationGuide8-15-07.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.

[thdewey]

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!

[brad d]

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..

[Aceman]

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?:mad:

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

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.

[PurdueSD]

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.

[Minitucking]

I Ran 3 2 ga. copper strands from my 225 amp house panel to my 100 amp main breaker panel in the shop.

[sberry]

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?

[sberry]

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.

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)


Quote:(6) 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?

[sberry]

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.

[2LTim]

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.
Tim

[LoneGunman]

Taken from Southwire's website:
http://www.southwire.com/Southwire/StaticFiles/Text/SIMpullTHHNInstallationandApplicationGuide8-15-07.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.

[sberry]

I usually put a dab of no-lox on the screw. I saw a panel a while back where the lug screws were seized.

[Matt M, PA]

This thread is timely!

I too need to run from the main house box to a panel in our new garage...about 125' away.

So, after reading this thread, it would seem that if we use aluminum, it will have to be #1? Meaning (and forgive my ignorance here...I have never run these feeds before) 1-1-1-6?

[brad d]

My electrician buddy tells me to run 3-3 Tec (3 gauge 3 wire copper) for 100 amps

[sberry]

In some cases it is legal to run 3 wire but its generally not a good idea for feeds to other buildings for a couple of reasons, its legal if there "are no other metal interconnecting pathways", might be ok today but if someone adds a phone wire, cable TV, metal gas or water line then it throws the electric out of compliance. Save yourself some grief and include a separate ground wire at the start and also avoids some confusion about proper ground/bonding.
I dont think the self help forum even shows it as an option anymore, with so many utilities these days its just plain easy to use 4 wire right from the start.

[brad d]

So what is the best bet them? for 100amps about 75' away?

I also have to do this in the next week.. i'm renting a trencher this weekend...

[jmshambarger]

I don't have a code book at home but #2 al sounds about right. Look it up at your supply house. You need to size your breaker no smaller than 125% of your continuous load and for the appropriate wire size you are using. The circuit going to your garage is not a service but a branch or feeder circuit. The breaker at your house panel will be 100 amps and the disconnecting means in your garage can be a switch, a fuse/switch, a panel with a main breaker or a panel with no disconnecting means but not more than 6 breakers in it (a two pole breaker counts as one breaker). 100 amps is fine because you don't care which one trips first, an unlikely event. A complete panel with a 100 amp main is often cheapest since so many are made. Aluminum is the metal equivalent of butter, it flows over time when it is under pressure. It also will react with copper. When you use it be sure that the connectors are marked for Al and you use an anti-oxidant paste. The utility companys and contractors use a compression fitting for aluminum so their fittings stay tight. You will use a bolted connection. It may have some spring action built into it, if not turn off the power and snug-up your connections after a couple of years and then again every few years. A branch circuit unlike a service has the same size neutral and line conductors and you will need four conductors the ground can be smaller. The proper size conduit for the wires you are using is in a NEC code table that your supplier should have available for you to use. Remember that the only point that your ground conductor is tied to the neutral conductor is at your house panel where your meter is. If there is a connection between the neutral and ground in the garage panel from the factory take it off. Put a pull string in your conduit as you put it in the ground.

[dipper]

I would stick with the copper, and if it's going into conduit; you need to run individual THHN wire. I ran four #4 copper for my 100A subpanel because the price was right. No problems with inspection at all. Just call you local inspector and run exactly what they tell you, that's the best way to have it done right the first time.

[sberry]

If you use number 1 alum I believe it requires a 4 ground in alum or 6 in copper.
We might get into a debate here but this is actually a service to a detached structure, it requires disconnecting means, has its own ground electrode. http://www.selfhelpandmore.com/home-wiring-usa/accessory-structures-to-dwellings/wiring-a-detached-garage-2002.php A sub is within the same structure.

[sberry]

For the sparkies. http://www.tchism.proboards49.com/index.cgi?board=general This one is not really a DIY board but code study to some extent. The code book is not really a DIY manual for most people, there are other good ones though.

[Aceman]

If you use number 1 alum I believe it requires a 4 ground in alum or 6 in copper.

#6 aluminum or #8 copper required if using a 100 amp feeder breaker.


We might get into a debate here but this is actually a service to a detached structure.

No debate, it's a feeder. The service is the wiring from the utility connection to the first disconnecting means(i.e. main breaker). It's a feeder for everything after that.

A sub is within the same structure.

NEC doesn't mention subpanel. I call any panel fed from another panel a sub, no matter where it's originates at.

[Charles (in GA)]

Look in the NEC Article 100 - Definitions, for the NEC's def of Service. As Aceman says, it is what the power company or utility gives you, from their transformer to the meter. Once it leaves the meter or a Service Disconnect located right at the meter, it becomes the Main Feeder up to the panelboard. From the Panelboard, branch circuits and feeders travel out.

If the PoCo or utility is not involved in it, its not a service according to the NEC.

From Article 100 - Definitions
2008 NEC

Service - The conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served.

Service Point - The point of connection between the facilities of the serving utilities and the premises wiring.

Service Equipment - The necessary equipment usually consisting of circuit breaker(s) or switche(s) and fuse(s) and their accessories, connected to the load end of service conductors connected to a building or other structure or otherwise designated area, and intended to constitute the main control and cutoff of the supply.

Feeder - All circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch circuit overcurrent device.

Branch Circuit - The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).

Charles

[Todd.Brock]

FWIW,
When I ran a subpanel to my basement, I ran 80 ft. of 2-2-2-4 to a Main lug Only panel, I used a 100a breaker in the main panel. A friend of the family is a construction foreman. His electrician recommended the wire. I was hesitant to run Aluminum. Everyone (electrician, friend, and supply house) asked me why I wanted to run copper? They said any service from the meter is going to be aluminum and any electrician around here running that distance with that load would run it as well. Thats what Noalox is for. Why would I want to spend ~$3 ft for CU when Alum is half the cost, half the weight, but a LOT thicker. That stuff is STIFF...

After I ran my panel and did all my wiring, I had an electrician check all my work, and make sure everything was up to code, etc.

Just my experience.

[rinker1]

Wow, when I started this thread I didn't know there would be so many different variables to do this. Next week the inspector is coming out for a ditch inspection and I think I'll ask him what he wants that will pass. A lot of good info here, thanks for all the replies and keep them coming!!

[sberry]

I am going to debate another point, if you upsize the wire you need to upsize the ground even if the breaker is 100A, just like you would for distasnce and voltave drop.
The 6 that comes in the 2-2-2 quadraplex would need to be number 4 if it was 1 or larger feeders?

[sberry]

Think about this, you can use a 6 space panel in a detatched structure as long at its listed as service equipment.