Wiring a new garage.

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  #1  
Old 06-10-12, 07:52 PM
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Wiring a new garage.

Greetings all.

We are building a new garage and I want to clear up a few things having to do with wiring the building for electricity.

It's a 24x24 2 story building. It will have two garage doors.

Here's what I want to do in the building as far as electricity goes:

2 garage door openers, one for each door.
Lights for up and downstairs.
Exterior light for rear door and top front of building
standard outlet layout for up and downstairs.
I'd also like to have at least 1 240 volt outlet for up and downstairs.
1 external outlet to plug extension cables outside.

I'm planning on installing a 100 AMP panel in the garage.

Here's our current layout for the existing property:

200 AMP service is fed to a 200 AMP Murray load center.

Load center has a 200 amp breaker that serves the residential power and there is also a 20 amp breaker for an outlet on the pole. There are 3 spaces left in this panel.

Power is fed to the house from this panel.

In the house, we have a 200 amp panel installed feeding various circuits.


The building will be roughly 70 feet from the outside load center.

I have read that I need #3 copper to feed the garage because of the distance and current involved.

Am I correct to assume that I will need four conductors to feed the building?

I've tried searching for #3 copper on the local big box home improvement store's website with no luck.

I have a feeling this isn't going to be a cheap endeavor but this is the first wiring job I've ever worked on myself and I want to do it properly so that I don't have to worry about it in the future.


Any advice would be appreciated.

Thanks!

Shawn in Keyser, WV
 
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  #2  
Old 06-10-12, 08:03 PM
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The building will be roughly 70 feet from the outside load center.

I have read that I need #3 copper to feed the garage because of the distance and current involved.
Yes, three 3AWG copper plus one 4AWG copper for the EGC. You will need to install a separate grounding electrode for the garage, and bond both the EGC from the load center and the GEC from the new ground rods to the ground buss inside the subpanel.
 
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Old 06-10-12, 08:14 PM
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While I do know that Big Orange does carry #3 THHN you would likely be better off going to an electrical supplier. Big Orange KILLS you on cut wire.
 
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Old 06-10-12, 10:37 PM
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...plus one 4AWG copper for the EGC.
Why a #4 for the equipment ground? Equipment grounding conductors are based upon the rating of the overcurrent device (fuse or circuit breaker) and for a 100 ampere circuit breaker the EGC would be #8 copper.
 
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Old 06-11-12, 01:07 AM
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Is there a reason you decided to go with copper over aluminum for this? Depending on what code cycle your AHJ is on, you can use either 2-2-2-4 XHHW or USE (NEC2005 or earlier) or 2-2-2-4 XHHW-2 or USE-2 (NEC2008 or later). USE (Underground Service Entrance) is commonly used for mobile home feeds, and can be direct buried without conduit. XHHW must be in conduit to go underground. But any way you slice it even with the larger gauge the aluminum will be half the price of copper.
 
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Old 06-11-12, 07:38 AM
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...plus one 4AWG copper for the EGC.
Why a #4 for the equipment ground? Equipment grounding conductors are based upon the rating of the overcurrent device (fuse or circuit breaker) and for a 100 ampere circuit breaker the EGC would be #8 copper.
As you say. But now, looking back, I don't see that the OP mentioned an OCPD for the feed - just the panel in the new structure.

Maybe I missed this?
 
  #7  
Old 06-11-12, 08:22 AM
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w8sjw, to expand on Nash's post from what you have detailed a 100 amp panel is probably a good idea but from what you have detailed it seems a 60 amp feed would be enough. (Remember whole houses use to run on 60 amps). At 60 amps you could use three #6 copper THWN and a #10 for ground. If you use conduit and size it large enough you could always pull new wires later if you need more power.

Run a load calc for the garage and see what you get. Single Family Dwelling Electrical Load Calculator
 
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Old 06-11-12, 10:00 AM
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@ Jersey, just a helpful note: #2AL URD (Note: URD is not a recognized type, but using it to show an example of an underground rated feed) is only good for 90A’s.
 
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Old 06-11-12, 02:57 PM
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I dont see the significance of that tidbit.. Per 310.16, #2AL USE-2/XHHW-2 is rated [email protected] URD would not be legal to use for this application even if it were recognized because it cannot enter a building.

In the older codes, the wording was ambiguous enough that one could use 310.15(b)(6) - which simply specifies #2AL (including 75c wet location wire like THWN and XHHW) as a 100A feeder - to size subpanel feeds. That was clarified in 2008 to be used for service entrance only, and that 310.16 was to be used for all other.
 

Last edited by JerseyMatt; 06-11-12 at 03:12 PM.
  #10  
Old 06-11-12, 03:21 PM
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I dont see the significance of that tidbit.. Per 310.16, #2AL USE-2/XHHW-2 is rated [email protected]
URD would not be legal to use for this application even if it were recognized because it cannot enter a building.
Lost me?
Maybe someone else will comment. As anytime I say something it gets turned into a debate. Was just trying to help!
 
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Old 06-11-12, 03:57 PM
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Lost me Maybe someone else will comment. As anytime I say something it gets turned into a debate.
Well we need someone to keep us on our toes.
 
  #12  
Old 06-11-12, 04:07 PM
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While I do know that Big Orange does carry #3 THHN you would likely be better off going to an electrical supplier. Big Orange KILLS you on cut wire.
+1.

I just purchased 400' of #10 THHN for the same purpose and would recommend finding a better supplier if you're in the same boat as I was = limited options. I wish I would have been a little more selective and a little less impatient.

I got robbed.
 
  #13  
Old 06-11-12, 04:26 PM
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To touch on fact: When choosing a conductor you should choose it based on knowing the termination rating for all equipment involved.

Note: A 90 degree cel insulation can only be loaded to 60/75 degrees—if the equipment involved is rated at said temperatures. In this case, I believe the OP will be dealing with equipment rated 75 degrees (assuming new electrical equipment). Which means the OP is allowed to use the 75 degree column. Which means the OP is limited to 90 amps- regardless of the USE-2 or other type designations listed under the 90 degree column. Now, selecting a 90 degree cel. insulation does have it’s advantages when derating (adjustment and or correction factors) is needed. Note: This install falls under T.310.16 [Table changed with the 2011. It’s now:Table 310.15(B)(16)]. Note 2: In some cases, the 60 degree column must be used per code. Example: NM cable
In reference to the URD, as I said, I was just using it as an example. I should further say, a simple USE marking alone does not qualify it to enter a structure. In any case, it can terminate directly to the meter or service equipment. Also note that they do make a USE, and I believe a URD that can enter the structure—as it has the required flame retardant designation.
 

Last edited by SeaOn; 06-11-12 at 05:31 PM. Reason: added the 2011 Table number
  #14  
Old 06-12-12, 06:50 AM
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Originally Posted by SeaOn View Post
To touch on fact: When choosing a conductor you should choose it based on knowing the termination rating for all equipment involved.

Note: A 90 degree cel insulation can only be loaded to 60/75 degrees—if the equipment involved is rated at said temperatures. In this case, I believe the OP will be dealing with equipment rated 75 degrees (assuming new electrical equipment). Which means the OP is allowed to use the 75 degree column. Which means the OP is limited to 90 amps- regardless of the USE-2 or other type designations listed under the 90 degree column. Now, selecting a 90 degree cel. insulation does have it’s advantages when derating (adjustment and or correction factors) is needed. Note: This install falls under T.310.16 [Table changed with the 2011. It’s now:Table 310.15(B)(16)]. Note 2: In some cases, the 60 degree column must be used per code. Example: NM cable
The way around that to avoid the expense of having to upsize the entire feed is to terminate it in boxes at both ends, and run the 75c sized cable (#1AL or #3Cu) from the boxes to the panels.

In reference to the URD, as I said, I was just using it as an example. I should further say, a simple USE marking alone does not qualify it to enter a structure. In any case, it can terminate directly to the meter or service equipment.
...As long as it is on the exterior of the structure. It cannot go inside - even if it is through an LB directly into a panel on the inside of the exterior wall. A lot of inspectors would overlook that scenario but by letter of the NEC it would be a red tag.

Also note that they do make a USE, and I believe a URD that can enter the structure—as it has the required flame retardant designation.
RHH.
 
  #15  
Old 06-12-12, 09:30 AM
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The way around that to avoid the expense of having to upsize the entire feed is to terminate it in boxes at both ends, and run the 75c sized cable (#1AL or #3Cu) from the boxes to the panels.
True, but why are you assuming a DIYer knows this. Just recommend the conductor and protection listed, and be done with it. The simply fact: We should be sizing the conductors based on the termination rating on the entire circuit. So 90 amps is almost 99.999% the limit per T.310.16 (2008 NEC).


@w8,
seems we have question regarding the protection for the feeder?
 
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Old 06-12-12, 09:43 AM
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TIME OUT! Until the O/P returns and tells us what amperage feed he is going to use this is all useless speculation. I stand by what I said in post #7. There is no compelling reason to assume w8sjw needs more then a 60 amp feed. When w8sjw has commented on post #7 then we can proceed. W8sjw, I know you were on line last night so you are probably following this thread so please reply to my post #7 so we can proceed.
 
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Old 06-12-12, 10:10 AM
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Originally Posted by SeaOn View Post
True, but why are you assuming a DIYer knows this. Just recommend the conductor and protection listed, and be done with it. The simply fact: We should be sizing the conductors based on the termination rating on the entire circuit. So 90 amps is almost 99.999% the limit per T.310.16 (2008 NEC).


@w8,
seems we have question regarding the protection for the feeder?
I'm not. As Ray said, the OP hasn't justified his need for 100A, but I suggested he use 2-2-2-4 AL because it is commonly available and it is much cheaper than the #3Cu (I believe it was even cheaper than #6Cu at around a buck and change per foot - versus the $2ish per foot I spent on 6/3 Romex to move my stove last year)... From what I am reading, this is a yard-pole installation and the lateral to the garage would be classified as a main feeder - thereby it would fall under 310.15(b)(6) and the #2AL would be suitable for a 100A service.

We also don't know what code cycle the OP is on, so even if I'm wrong and he intends to feed the garage off the house panel we don't know whether he would size per 310.16 or if he could skate with 310.15(b)(6).
 

Last edited by JerseyMatt; 06-12-12 at 11:12 AM.
  #18  
Old 06-12-12, 11:04 AM
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I'm not. As Ray said, the OP hasn't justified his need for 100A, but I suggested he use 2-2-2-4 AL because it is commonly available and it is much cheaper than the #3Cu (I believe it was even cheaper than #6Cu at around a buck and change per foot - versus the $2ish per foot I spent on 6/3 Romex to move my stove last year)... From what I am reading, this is a yard-pole installation and the lateral to the garage would be classified as a main feeder - thereby it would fall under 310.15(b)(6) and the #2AL would be suitable for a 100A service.
Please re-read what you wrote. You said the 90 degree column can be used, then gave us a way of getting around the 75 degree column, so this clearly signifies your intent. If the OP is supplying a non-dwelling, then T310.16 will apply. Changed From 2005 ->>>Table 310.15(B)(6): Revised to specify that the main power feeder is one that supplies the entire load associated with a dwelling unit.
We also don't know what code cycle the OP is on, so even if I'm wrong and he intends to feed the garage off the house panel we don't know whether he would size per 310.16 or if he could skate with 310.15(b)(6).
True!
 
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Old 06-12-12, 11:34 AM
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@ ray, I agree we need more information. But, some topics “may” trigger a deeper conversation (debate) between forum members. This conversation may give everyone a better understanding of any code related issues related to the topic. If we keep closing threads, and stopping general code topics, no one will ever get a deeper—or even general--understanding behind a specific electrical installation. We may seem to be getting off topic, but trust me, it does clarify some issues related to the topic. Also note that the internet is large, and many electrical apprentice and electricians may be reading this thread. Some of them may be learning from these small debates. Heck, I’m learning!!
 
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Old 06-12-12, 11:42 AM
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I started out with one frame of thought and sorta wandered into another. But either way it depends on what code he's on.. And could be moot altogether until we hear back about whether he really needs 100A..

And speaking of learning I didn't even consider the 75c rating on the breakers at first, I only took into account the 90c rating that the panel lugs usually have..
 
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Old 06-12-12, 11:44 AM
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I started out with one frame of thought and sorta wandered into another. But either way it depends on what code he's on.. And could be moot altogether until we hear back about whether he really needs 100A..
Exactly!! I like that you know your way around code!!
 
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Old 06-12-12, 11:56 AM
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I wonder sometimes if the esoteric scares off newbies?
 
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Old 06-12-12, 12:35 PM
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I wonder sometimes if the esoteric scares off newbies?
If a newbie has decided to do it themselves, then they have decided to be subjected to read the small number of pros debate over their question—as there is room for interpretation in “some” installations. And some code needs debate to correct and obtain the correct answer. I do believe long useless debates are non-constructive, and could send a newbie looking elsewhere. But, I’ve learned that you can find posters on more then one site asking the same question<<<<<I’m sure hoping to get a faster answer, as well as a lucid understanding. It could also be to see if answers are similar.
 
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