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Old 03-03-2009, 02:53 PM   #1
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Subpanel - detached garage without a floor


I have a detached garage about 40 feet from the house. It has a dirt floor right now. I'm planning on pouring a slab in it, but before I do I want to get my utilities in place. I have a couple of questions about planning the electrical:
1. I'd like to eventually have a 100 AMP subpanel out there. What size triplex should I pull for this? Or is there a better alternative to triplex?
2. As a followup to question 1: I currently only have a 100 AMP main panel in the house (which I'll upgrade to 200 in a couple of years). For the time being, can I just put a 50 AMP shuttof in the subpanel (garage) powered by a 50 AMP breaker in the main panel (house)? Is this OK even though the wire will be sized for a 100 AMP panel?
3. Since the floor of the garage is currently dirt, can I trench the power lines under the garage foundation and directly up into the subpanel, or do I need to go up the outside wall and then through to the subpanel? What about the ground wire to the grounding rod? Can I trench that under the foundation to the rod outside, or do I need to go through the wall with that? I'd like to put as few holes in the garage wall as possible (it's 18" thick and built with 90-year-old brick).
Thanks for your help.

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Old 03-04-2009, 07:39 AM   #2
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Subpanel - detached garage without a floor


Thanks.

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Old 03-04-2009, 07:52 AM   #3
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Subpanel - detached garage without a floor


If you come up through the floor you will need to change from pvc conduit to rigid conduit and put in a seal off at each end of the conduit as the garage is a hazardous locations. If you penetrate the wall from outside at least 3' up you need not install a seal off. The panel in the garage will nee to be set up as a sub panel with a separate ground bar. Install a ground rod at the garage and also tie to the ground in the main house. Using wire rated for 100 amps and fuseing it at 50 amps is ok and not a code violation. Are you running the wire above the ground or in a trench? If in a trench I would install a 2" PVC conduit and pull THHN or XHHW wire instead of triplex. I would also use copper if you can afford it.
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Old 03-04-2009, 08:20 AM   #4
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Subpanel - detached garage without a floor


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If you come up through the floor you will need to change from pvc conduit to rigid conduit and put in a seal off at each end of the conduit as the garage is a hazardous locations. If you penetrate the wall from outside at least 3' up you need not install a seal off. The panel in the garage will nee to be set up as a sub panel with a separate ground bar. Install a ground rod at the garage and also tie to the ground in the main house. Using wire rated for 100 amps and fuseing it at 50 amps is ok and not a code violation. Are you running the wire above the ground or in a trench? If in a trench I would install a 2" PVC conduit and pull THHN or XHHW wire instead of triplex. I would also use copper if you can afford it.
Thanks. Yes, I'll be running the wire in a trench.
What about the ground wire? Should I run that down out of the panel in a seperate piece of rigid without a seal-off?
Yes, I plan on using copper. What size wire should I use?
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Old 03-04-2009, 10:59 AM   #5
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Subpanel - detached garage without a floor


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If you come up through the floor you will need to change from pvc conduit to rigid conduit and put in a seal off at each end of the conduit as the garage is a hazardous locations. If you penetrate the wall from outside at least 3' up you need not install a seal off. The panel in the garage will nee to be set up as a sub panel with a separate ground bar. Install a ground rod at the garage and also tie to the ground in the main house. Using wire rated for 100 amps and fuseing it at 50 amps is ok and not a code violation. Are you running the wire above the ground or in a trench? If in a trench I would install a 2" PVC conduit and pull THHN or XHHW wire instead of triplex. I would also use copper if you can afford it.
How do you know this is considered a hazardous location. The OP makes no mention of it even possibly being classified. Unless I missed something in the original post.

OP. You have a good opportunity now to save some time and money later. Since the floor is not poured, get as much installed as possible under the new slab NOW.
Run PVC on the ground and stub up for all switches and receptacles. You may have to dig some as the PVC must be covered with 4" of concrete. (I think)?? Secure the PVC where it will stub up. Like inside wall studs ect.... If using rebar, tie it to the rebar with tie wire.
Then after you have decided what size sub panel you will install, you can bury the conduit entering the garage and stub up for the panel. You do not have to run the whole feeder conduit. Just come out under the structure a few feet and mark the spot with a stake ect... Tape up the end real good unless you are going to run it all at one time. In fact, tape up all stub ends to keep concrete from entering. The concrete will then be poured over all the PVC conduit in the garage. Since you do not have any concerns as to how to route the conduit (it will be covered in concrete) you save conduit and wire.

I recommend PVC for the feeder and the garage branch circuits because of your circumstance. Of course you can transition to NM or EMT once inside the structure, but since most all the conduits will be available from the floor, little conduit will be needed. This is actually a good situation, not a problem.

Last edited by J. V.; 03-04-2009 at 11:15 AM.
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Old 03-04-2009, 11:56 AM   #6
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Subpanel - detached garage without a floor


It was my understanding that a garage is always considered a hazardous location due to cars being able to be parked/driven in ??
As long as there is a garage door
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Old 03-04-2009, 12:37 PM   #7
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Subpanel - detached garage without a floor


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How do you know this is considered a hazardous location. The OP makes no mention of it even possibly being classified. Unless I missed something in the original post.

OP. You have a good opportunity now to save some time and money later. Since the floor is not poured, get as much installed as possible under the new slab NOW.
Run PVC on the ground and stub up for all switches and receptacles. You may have to dig some as the PVC must be covered with 4" of concrete. (I think)?? Secure the PVC where it will stub up. Like inside wall studs ect.... If using rebar, tie it to the rebar with tie wire.
Then after you have decided what size sub panel you will install, you can bury the conduit entering the garage and stub up for the panel. You do not have to run the whole feeder conduit. Just come out under the structure a few feet and mark the spot with a stake ect... Tape up the end real good unless you are going to run it all at one time. In fact, tape up all stub ends to keep concrete from entering. The concrete will then be poured over all the PVC conduit in the garage. Since you do not have any concerns as to how to route the conduit (it will be covered in concrete) you save conduit and wire.

I recommend PVC for the feeder and the garage branch circuits because of your circumstance. Of course you can transition to NM or EMT once inside the structure, but since most all the conduits will be available from the floor, little conduit will be needed. This is actually a good situation, not a problem.
Thanks, this is helpful. I had only thought about getting the service in place and then roughing in after the floor was poured, but you're right - there are at least some runs that will be easier to do underground now rather than overhead later.
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Old 03-04-2009, 12:40 PM   #8
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Subpanel - detached garage without a floor


You need a four wire feed.
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Old 03-04-2009, 12:42 PM   #9
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Subpanel - detached garage without a floor


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You need a four wire feed.
What size copper?
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Old 03-04-2009, 12:51 PM   #10
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Subpanel - detached garage without a floor


Use #3 copper for all conductors 2 hot 1 neutral 1 ground. garages are hazardous locations so PVC is not allowed to feed the panel above the slab. Rigid must be used above the slab to the panel and I would recommend EMT for branch circuit if exposed with THHN #12 wire.
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Old 03-04-2009, 02:07 PM   #11
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Subpanel - detached garage without a floor


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Originally Posted by craneservice View Post
Use #3 copper for all conductors 2 hot 1 neutral 1 ground. garages are hazardous locations so PVC is not allowed to feed the panel above the slab. Rigid must be used above the slab to the panel and I would recommend EMT for branch circuit if exposed with THHN #12 wire.
RNC or RMC for above the slab?

Why a 4awg ground?
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Old 03-05-2009, 04:16 AM   #12
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Subpanel - detached garage without a floor


I am in a similar situation.

large shed detached that i use as a shop, but my house panel also could use an upgrade. My house panel is I think a 120 A panel. Id like 200 A if possible plus a 100 for the shed.

Question regarding the pvc conduit. Schedule 80?

I am going to install two 30 A 220 circuits and two 20 A 120's. My saws (table and band,) plasma torch and air compressor demand the juice!
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Old 03-05-2009, 11:42 AM   #13
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Subpanel - detached garage without a floor


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Originally Posted by craneservice View Post
Use #3 copper for all conductors 2 hot 1 neutral 1 ground. garages are hazardous locations so PVC is not allowed to feed the panel above the slab. Rigid must be used above the slab to the panel and I would recommend EMT for branch circuit if exposed with THHN #12 wire.
Code reference please.
Residential garages and some commercial are not considered a hazardous location as far as the NEC goes. You may think it's classified, but it's not.

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