Yet Another 100 Amp Subpanel Question

[Wiz02]

My new detached garage structure is now built, but I need some electrical advice. The plan is add a 100 amp sub panel fed from the main house 200A panel. I had (2) 2" conduits buried during construction to run the feed into the garage plus a spare for future cable/internet etc.

I have 2 basic questions: First is what wire to run between the house and the sub panel in the detached garage. Of course I've read so many posts that I'm not sure what is the proper wire to run to the garage, given that I'm talking about 130 ft mostly in the basement, but running in conduit underground for about 20 feet. It looks like I can go with #1 THHN/THWN and 100A breaker in my main panel or Mobile Home feeder with a 90 A breaker in the main panel. The cost difference is significant what are the safety and reliability implications between these choice presuming that they are the right choices?

My second question relates to the location of the sweeps coming into the garage. I wasn't there when they put in the underground conduit and the sweeps and of course they used the prime real estate where I want my bench and cabinets to be. How big of a deal is it to continue the panel feed from the sweep sticking up in the first image up and over the door and come down in the stud bay shown in the image? All advice is welcome.

 

[Aceman]

I've never purchased #1 AL. We've always used 1/0 AL when we needed 100 amps.

I can't answer your other questions since the pics didn't show up....

 

[CNGsaves]

Welcome to GJ. Can't see your pics. Can you fix them??

Also, what Country are you in?? Electrical codes vary by Country and State so more information is needed. Now would be good time to Update GJ Profile with location.

 

[theoldwizard1]

Both MHF and THWN require conduit inside. MHF does NOT require conduit underground.

You can continue the cable inside the garage to where ever you want to put the panel, but it must be continuous and it must be in conduit.

Go with whatever is cheaper, which I suspect is MHF. At 90A (or 100A) and 130', you would need 2/0-2/0-2/0-1 MHF. If you drop back to 70A (or 75A) you can use 2-2-2-4 MHF and still be less than 3% voltage drop.

 

[Wiz02]

Thanks for the info, evidently my technique for sharing pics from Microsoft Skydrive didn't work, despite trying it out earlier in the test section. I will definitely price out the 2/0 MHF as the THWN was pretty expensive.

 

[pattenp]

Short of not seeing fully where you are going from and to, I get the feeling your best bet would be to place a junction box at the feed entry point in the garage and change over to SER for the run the the panel. It also seems it may be best to do the same on the house end. Use SER from the main panel to the house exit point and in a junction box switch to MHF or THWN for the 20ft under ground run. That's my SWAG at it.

 

[Wiz02]

Thanks, that's an interesting idea. I guess it is not allowed to transition from SER to MHF in the LB, which is why I would need a junction box. Can the junction box be inside the house or does it have to mount on the outside of the house?

 

[pattenp]

Using MHF the jbox can be inside. The LB should have a listed cubic inch printed on the inside but it usually does not have the required amount of cubic inches to handle the required wire length to do the splicing. The large wire is just a real pain and near impossible to twist and get into an LB even if the LB size just meets the minimum. You are looking at needing a minimum of an 8x8X4 jbox.

Edit: Be prepared to get sticker shock when you price large PVC junction boxes.

 

[Wiz02]

Thanks for the info. Do I need to use a PVC junction box or can I use a metal box?

 

[pattenp]

You can use metal box, but then you get into the hassle of having to ground the box.

Thanks for the info. Do I need to use a PVC junction box or can I use a metal box?

 

[theoldwizard1]

... You are looking at needing a minimum of an 8x8X4 jbox.

Edit: Be prepared to get sticker shock when you price large PVC junction boxes.

A Carlon 8x8x4 is under $25 at HD (online). Of course it has no fittings, so you have to add that on. Is SER that much cheaper than MHF that it would be cost effective over a few more sticks of PVC and some elbows for 100' ? Would 3/4" flexible conduit be adequate for 2-2-2-4 MHF ?

Not that splices in heavy cable are really that hard to do, but for non-professional they can appear "daunting" !

 

[pattenp]

Yeah... but still, it's just a plastic box. As you said you still have to buy the fittings. The SER isn't cheaper than MHF but it doesn't cost much more either. Running the SER is a lot easier than installing conduit. It also depends on the difficulty of the run as to use conduit or not. I haven't done a detailed cost of materials comparison. I'm going from an ease of installation.

A Carlon 8x8x4 is under $25 at HD (online). Of course it has no fittings, so you have to add that on. Is SER that much cheaper than MHF that it would be cost effective over a few more sticks of PVC and some elbows for 100' ? Would 3/4" flexible conduit be adequate for 2-2-2-4 MHF ?

Not that splices in heavy cable are really that hard to do, but for non-professional they can appear "daunting" !

 

[Wiz02]

Thanks for all the input - of course its not a straight shot from where the cable needs to exit the house to the main panel. As a matter of fact, the exit location is almost as far away from the main panel as you can get. I would need at least 3 probably four elbows to get to the main panel.

I'm pretty familiar with residential wiring, and the splices don't particularly scare me but the last time I worked with Service Entrance cable was 20 years ago and then all I did was pull the meter and replace the cable running from the meter to main panel - just a few feet.

I'm going to hit my local electric supply and get some estimates and depending on the $$$, I will talk to my township code enforcement to see if they have any preferences as well.

 

[Magneto]

Wiz,

I just did the same install a few months ago. My local electrical supply house (not the big box morons) gave me all the advice and guidance I needed. Hopefully yours can do the same.

 

[theoldwizard1]

Yeah... but still, it's just a plastic box. As you said you still have to buy the fittings. The SER isn't cheaper than MHF but it doesn't cost much more either. Running the SER is a lot easier than installing conduit. It also depends on the difficulty of the run as to use conduit or not. I haven't done a detailed cost of materials comparison. I'm going from an ease of installation.

Interesting.

I would think that flexible conduit would be very easy to install, although more expensive than PVC conduit and elbows.

 

[pattenp]

The flex would be easy to install but the 2" flex is not cheap. I think it's a few bucks a foot.

Interesting.

I would think that flexible conduit would be very easy to install, although more expensive than PVC conduit and elbows.

 

[theoldwizard1]

The flex would be easy to install but the 2" flex is not cheap. I think it's a few bucks a foot.

Ahh ... I didn't realize you would need 2" flex !

 

[Wiz02]

Thanks everyone for all the advice and input. Barring any changes from the "AHJ" (authority having jurisdiction) gotta love the NEC speak, I think that I will go with the MHF in conduit for the whole run, going with the less connections the better, philosophy.

On another topic, while the general contractor knew that I was planning on running electric to the garage, since he buried the 2" conduit and had the sweeps come up in a stud bay, it would have been nice if someone mentioned ground rods. I was really hoping to avoid having a ground wire run up the side of the foundation, with a hole in the siding. Any suggestions for adding a ground rod after the fact? I read about a "Ufer" (connecting equipment connecting ground to the re-bar in the foundation) but that ship sailed.

It seems that everything I've see about ground rods shows them being installed outside the structure. Are you allowed to drill through the sill plate, block and footing inside the structure or do you have to place the ground round outside?

 

[pattenp]

I think the problem is going to be with rods is that they have to be at least 8' long and all 8' needs to be in contact with the soil. I'm not sure how you would accomplish that going down through the sill and footing without using rods that are way longer than 8' and I don't recall seeing ground rods that are longer than 8'.

 

[soj]

I think the problem is going to be with rods is that they have to be at least 8' long and all 8' needs to be in contact with the soil. I'm not sure how you would accomplish that going down through the sill and footing without using rods that are way longer than 8' and I don't recall seeing ground rods that are longer than 8'.

See here and here.

 

[pattenp]

The first "here" doesn't work. I assume the 10' is long enough to get 8' of it into soil but then the next issue is showing the AHJ that in fact 8' of it is in soil.

See here and here.

 

[theoldwizard1]

The first "here" doesn't work. I assume the 10' is long enough to get 8' of it into soil but then the next issue is showing the AHJ that in fact 8' of it is in soil.

Yeah, I always wonder about that ! Other than have the AHJ stand there while you are driving the rod how does he know if it is 4', 6' or 8' ? One sparky told me it was common for residential garages to have 2 halves pounded in !

 

[Wiz02]

The reason that I wanted to run the ground rod inside the structure is that 6-8 inches below the surface is solid rock. I will have to post pictures, as I needed to have someone come with a pneumatic hammer on an excavator and work for 14 hours to get the footings dug. I live near a quarry and the hillside is rock with a thin soil covering. My thought was that I could get down 4 ft or so through the footing before dealing with the rock. I know that I will need a rotary hammer as my hammer drill didn't make a dent in the rock the last time I tried to drive in an anchor bolt.

I can see the pics in the first post. Can you? If not I will give up trying to display pics via my Microsoft Skydrive account and post the foundation pics via a standard service. I think that seeing that hammer hitting those rock formatios was pretty cool. Even that thing had trouble getting through at times.

 

[Charles (in GA)]

Do you know if there is rebar in the perimeter of the slab, somewhere near the surface? Pic of the forms, etc before the pour? you could have a concrete cutter drill a 2 to 4 inch hole in the slab down to the rebar, and chip around it and weld on to it and connect to that. Just a wild suggestion. Other option is to use 2 ea of... 1ft square copper ground plates, or 8 ft rods placed horizontal in a trench, either the plates or rods being at least 2½ ft deep.

Charles

 

[Wiz02]

Interestingly there was no form per se, the channel cut in the rock was the form. There was no rebar in the footing, just te in the floor. I did hear that in some jurisdictions if there wasn't a ground connected to the rebar they would force the owner to do effectively what you suggested - a ufer installation. I hope that it doesn't come to that, as I would rather pound a ground rod in outside, despite just having sod laid around the perimeter.

 

[Norcal]

Yeah, I always wonder about that ! Other than have the AHJ stand there while you are driving the rod how does he know if it is 4', 6' or 8' ? One sparky told me it was common for residential garages to have 2 halves pounded in !

The inspector is NOT the AHJ.