service to new garage

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Old 09-23-07, 11:40 AM
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service to new garage

Hi folks, noobie here with some questions.
I just had a detached 16'x24' garage built in my backyard. While the contractor poured the slab, he installed 2" PVC conduit through the garage floor and wall, closest to the back of the house. He also installed a grounding rod in the slab and attached it to the rebar in the garage foundation. After the work was finished and inspected, I dug a trench between the house and garage and installed additional 2" PVC, 12" underground. I continued the PVC up the back wall of my house to a vent in the attic. I then thoroughly photographed the trench and conduit before covering it with concrete.
Next, I pulled a building permit to run wire from my main panel in the house to a subpanel in the garage. The main I currently have has two breakers. One is marked 100 and it goes to a subpanel in the garage in the front of my home. The other breaker is marked 40 and it goes to my central AC. The meter is marked 240V 3W CL200. I plan to hire a contractor to upgrade my main panel with an additional 50 amp breaker. I then plan to install 6/3 NM cable from the new 50 amp breaker through the garage attic and attic of my house towards the rear of the home where the 2" PVC conduit is. I plan to secure the cable every 4 1/2' and run the cable through 3/4" holes drilled in the rafters. When I reach the conduit, I would like to strip the sheathing from the cable and run the individual wires through the conduit. The wires would then be attched to a new 100amp subpanel in the detached garage. From there I would like to install branch circuits for plugs and lights. No immediate plans for 240V in the detached garage but I would like to have additional wire there in the event the need arises.
My question is..can I run 6/3 non-metallic cable through conduit with the sheathing removed? And if so, what do I use to secure and transition the cable from NM to separate wires at the conduit entrance. I think I will use an LB but what do I use to secure the NM cable as it enters the LB.
If I understand correctly, do I need to separate the the neutral bus bar in the new subpanel from the ground wire?
Finally, how does my wiring plan sound? I'm not skilled by any means but I do have enough handyman knowledge to be dangerous. Please remember this will have to pass muster with a city building inspector. I hope he accepts the photos for the buried conduit..
Thanks in advance for your kind help.
 
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Old 09-23-07, 12:27 PM
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To answer your question, you can't just strip the insulation off the NM cable and run that in the conduit. You should use THWN. I'm sure someone will be along to describe this process in detail. I just didn't want you making a mistake by moving forward with your plan.
 
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Old 09-23-07, 12:49 PM
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My question is..can I run 6/3 non-metallic cable through conduit with the sheathing removed?
absolutely not.

NM cable is listed as a cable and cannot be used in any manner other than what it is. It, as a cable, cannot be placed in the conduit either as NM is not designed for outside use.

As chandltp suggested, you need to use THWN or other wet rated wire in the conduit.

The main I currently have has two breakers. One is marked 100 and it goes to a subpanel in the garage in the front of my home. The other breaker is marked 40 and it goes to my central AC.
So what actually feeds your house? Is this simply a subpanel fed from your house main panel? If not, please describe how your house is fed and how this panel is fed.

If I understand correctly, do I need to separate the the neutral bus bar in the new subpanel from the ground wire?
you understand correctly. Do bond all the grounds together though (the one from the house and the system you have created at the garage.

have you accounted for voltage drop to the new building? You may want to consult a voltage drop calculator to address this.

not a code requirement but a practical point.
 
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Old 09-23-07, 06:40 PM
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Originally Posted by nap View Post
absolutely not.

NM cable is listed as a cable and cannot be used in any manner other than what it is. It, as a cable, cannot be placed in the conduit either as NM is not designed for outside use.

As chandltp suggested, you need to use THWN or other wet rated wire in the conduit.

So what actually feeds your house? Is this simply a subpanel fed from your house main panel? If not, please describe how your house is fed and how this panel is fed.

you understand correctly. Do bond all the grounds together though (the one from the house and the system you have created at the garage.

have you accounted for voltage drop to the new building? You may want to consult a voltage drop calculator to address this.

not a code requirement but a practical point.
The 100 amp breaker in the main panel goes to a subpanel in the front garage that supplies the circuits to the house.

So what is the best way to transition from NM to THWN? Can I start the cable run from the main to the attic with 6/3 NM to a junction box and then connect the NM to 6 gauge THWN inside the conduit for the remainder of the run to the detached garage?

Do you think I should forget the NM idea and just run conduit all the way to the main and use THWN?
 
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Old 09-23-07, 07:05 PM
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Can I start the cable run from the main to the attic with 6/3 NM to a junction box and then connect the NM to 6 gauge THWN inside the conduit for the remainder of the run to the detached garage?
yes.

Do you think I should forget the NM idea and just run conduit all the way to the main and use THWN?
price would dictate if it were me. I prefer conduit but I am a commercial electrician and do not care for "rope" slng term for NM) but there is nothing wrong with it.
 
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Old 09-24-07, 09:43 AM
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[QUOTE=nap;1233695]yes.

rookie mistake, double post
 

Last edited by upflying; 09-24-07 at 09:53 AM. Reason: oops, double post
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Old 09-24-07, 09:48 AM
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Originally Posted by nap View Post
yes.

price would dictate if it were me. I prefer conduit but I am a commercial electrician and do not care for "rope" slng term for NM) but there is nothing wrong with it.
Thanks Nap for some insight. Doing it right is more important than price. I'm inclined to run conduit all the way based on your comments. Another question before I quit bugging you though. As you know the old main panel faces the outside of my stucco house. I removed the wallboard behind it and found two runs of NM extending out the top of it through the "plate" at the top of my stud wall. This NM goes to the central AC and a subpanel in my front garage. 1 1/4" PVC conduit goes into the bottom of the main panel. This conduit is for wire that comes from underground service from the street and is maintained by my utility company.
I assume conduit is intended to be "surface mounted" to the framing inside the house and is not concealed by wallboard except when it enters a sub or main panel.
My concern is drilling a 2" hole for the conduit through the top plate of my garage wall. Making a 2" hole through the middle of a 2"X4" seems like it would weaken it structurally.
Is this correct or should I route the conduit some other way to the main panel? Would it be better to reduce the diameter of the conduit to 1 1/4"?
Thanks again Nap.
 
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Old 09-24-07, 10:31 AM
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Originally Posted by upflying View Post
I assume conduit is intended to be "surface mounted" to the framing inside the house and is not concealed by wallboard except when it enters a sub or main panel.
Conduit may be concealed by any finish material. The only parts of the conduit that must remain accessible are the junction boxes and pull points (condulets, LB fittings, etc).

My concern is drilling a 2" hole for the conduit through the top plate of my garage wall. Making a 2" hole through the middle of a 2"X4" seems like it would weaken it structurally.
I say drill out for the 2". Double up the plate or laminate it with plywood in that section if you want to retain strength (although this type of hole is common with no additional support). A 2" straight conduit is nothing compared to the structural damage from a plumber's assistant, sawzall and 3" toilet flange.

Would it be better to reduce the diameter of the conduit to 1 1/4"?
Don't reduce the conduit size mid-run; it will create a spot where wires can bind and jam making the pull more difficult.
 
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