Questions about underground feed to detached garage

[dtbingle]

This is an older house and currently the garage is fed from a 15A breaker from the main breaker panel with two runs of 14 gauge wire underground. I'm looking to buy a 240v compressor, which requires a wiring upgrade. My thought is to install a 60A subpanel in the garage - 30 amp 240V breaker for compressor and two 15A 120V breakers (lights and then misc. tools). With the mountain of codes and types of materials to use, I have a list of questions.

To start, here's the layout:

1) The run from the garage to the house is ~50 feet if following the concrete patio. Closer to 35-40 feet if ran direct from garage inlet to house entrance. My thought was to use 6/3 cable with a separate 8 gauge bare wire for ground. Any recommendations against or for this?

2) Haven't dug the wire up yet, but expecting a direct bury or 3/4" conduit. This won't be big enough. The plan was to install 1 1/4" pvc conduit buried @ 18 inches. Does this sound correct?

3) What type of wire is allowed for the conduit setup listed above? Seems like I'll be looking for UF cable. Are there other options? The codes/ratings seem to overlap and it's hard to keep track of it all.

4) Per detached building code, 2 grounding rods will be needed next to garage (near inlet) connected to subpanel via 8 gauge bare wire (isolated from neutral bar). Also, these rods need to be buried. Does this sound correct?

 

[teamo]

I would trench around the patio instead of messing with the concrete. 50 feet total will not affect the voltage drop enough to matter. I did a similar project to my garage and ran three 1" pvc conduits. One conduit was for the service, the second was for low voltage (alarm, cable, etc), and the third was for the future in case needed. I ran four #6 THHN/THWN individual conductors although you can down size the ground to #10 if you want to. Two ground rods at the garage and as you already know the neutral bond is lifted from the ground at the sub panel. Use expansion fittings on each end of the conduit where it comes out of the ground and goes into the house and the garage. If I had to do it over I would step up the conduit to larger than the 1" pipe and use 1.5" just to make the pulls easier. Also I would go larger than 60 amps as I have also started welding a little and I may install hvac in the future. If your needs are never going to go beyond the compressor and some general use circuits then 60 amps will be fine.

 

[wyliesdiesels]

FYI 6/3 NM-b includes the bare ground.

But i would use THWN in pipe. U should be able to pull 3 #6 THWNs and a #10 THWN for the EGC (IF the breaker is 60a) in 3/4".

But i would just abandoned the existing and run new conduit. No telling whats in there and whether u can pull the old wire out.

Make sure to put in a few extra conduits for low voltages com wiring...

Yes 2 rods will be needed. U can use bare solid #6 for the GEC to the rods as long as its not subject to damage from such things as a weed wacker. If u use #8 it will need to be in conduit.

And yes the neutral bar needs to be isolated.

 

[matt_i]

I would use the blue pathway, and you might want to go outside of the perimeter of the concrete a couple of feet to avoid undermining its edge.

My fish tape seems to stick in PVC conduits where there is a flared (expanded) pipe that's glued to another, when pushing the tape thru initially. Not sure if my tape-end has gotten smashed flat from use or that's typical experience of others, but paying attention to the way the pull is intended to go and the way you glue up 45 and 90 can make pushing the tape thru easier. Larger conduit will definitely help as well, I think you have a good idea in using a 1-1/4". I didn't consult the tables, but sizing the conduit-only for a 100A service might be a good future planning step.

In your low-voltage conduit(s) I would absolutely put a "string" for easy future pulls (probably around a 1/8" cord with some formulation for wet areas and not an old piece of clothesline)

 

[Fixin'Stuff]

I wouldn't bother with the 15 amp circuits. Go ahead and put in 12 gauge wire and make all of your circuits 20 amps each. The total of all your circuits can be higher than the main breaker. Count up the values of the breakers in your main panel, total the values and compare that against the main breaker. The load calculations (correctly) assume that you'll never be pushing the current limits of all of the circuits at the same time. You don't have to limit the total of the circuits to 60 amps just because you have a 60 amp main breaker. For example, my main panel here is a 200 amp service, but the total value of all of the breakers is just shy of 800 amps. Never have caused the main to trip.

I've added a 60-amp subpanel in the garage for the a/c, compressor, welder, outlets, etc. but I left the feed to the lights powered from the main panel right next to the sub. This way, even if I should ever manage to trip the main breaker in the subpanel, I won't suddenly be left in total darkness with a power tool spinning down.

 

[gtae07]

My fish tape seems to stick in PVC conduits where there is a flared (expanded) pipe that's glued to another, when pushing the tape thru initially. Not sure if my tape-end has gotten smashed flat from use or that's typical experience of others, but paying attention to the way the pull is intended to go and the way you glue up 45 and 90 can make pushing the tape thru easier. Larger conduit will definitely help as well, I think you have a good idea in using a 1-1/4".

I'd use 2" conduit and run MHF for 90A. Might as well.

For pulling the wire, I started by attaching mason's line to a couple of wadded-up shopping bags and sucked that through with the shop vac. I used that to pull the mule tape through, and that pulled the MHF (generously lubed with wire lube).

 

[dtbingle]

Awesome info, thanks everyone. I think for now, I'll run the 6 gauge route to a 60 A subpanel, but use a larger conduit 1-1/2" or 2" in case wire needs to be pulled for a 100A subpanel later. Take the blue path. Few more questions.

1) Do you have an example of this expansion fitting running up from the ground?

2) Currently, the wire enters the garage from the ground through a hole that tunnels through the concrete slab and floor 2x4 sill. This then runs up a stud and to the only outlet in garage + lights. Can I follow same route and come up through 2x4 sill or does the run have to stop outside the garage, come up the side of the garage, and put an entrance box to go through the siding and into the garage?

3) What's more recommended, single 4 wire cable or 4 individual wires? What's cheaper? Finally, where can you get fair priced cable/wire? The online listings at the typical big box stores don't seem to list a lot - maybe they have more in-store which is not listed on the site?

A lot of what I see listed is just THHN wire and not rated for wet location or UF-B 6/3 from online sites for $2-2.50/ft

 

[jreineri]

I got my wire at Cable and Wire To Go: http://www.wireandcabletogo.com/

Prices seem pretty good.

I used individual wires for 100 amp subpanel. 3-3-3-6 THWN copper for a 100 amp circuit.

 

[MushCreek]

I just did this project for my barn. I buried 2" conduit about 30" deep, used a vacuum to pull a pull tape, and used that to pull 2-2-2-4 aluminum. Technically, I could push 100 amps, but due to the distance (120') it is down-rated some. I have a 60A breaker; that should cover my needs. I threw some dirt over the conduit, then ran a separate 1" conduit with a pull tape for future communication/low voltage needs. I have a 40A 240V for the welder, a 30A 240V for machine tools (going to a phase converter) a 20A 240V for the compressor, and three 20A 120V outlet circuits. There is also a 15A 120V for lighting.

I bought my cable on ebay from a big supplier. I used URD, but haven't been able to tell if it's actually code or not. My inspector didn't even look at it. It was WAY cheaper than copper. I use dielectric grease when making up aluminum connections, as I once had a serious corrosion issue with aluminum service conductors in a house.

I put in two ground rods, but my inspector told me I only need one for a subpanel (???)

 

[CNGsaves]

Another option OP . . . . . change your aerial electric service to GARAGE !!

Move meter to mast on side of garage that would be less obtrusive on your property . . . IF . . . trees are not in the way, etc.

Then you'd have 200A panel in garage (main panel) and the house would become subpanel with it's own 100A subpanel. Use MHF Al 2-2-2-4 for the run to house and you'd have 90A breaker in garage to power the house (assuming the house could run on just 90A - - ie furnace/dryer/HWH all on NG or propane).

. . . OR . . .

Convert entire property to BURIED electrical everywhere. Since you're trenching anyway, put in 3" conduit from transformer pole to the house so you'll have buried electrical service entry. Then use blue route to bury 2" conduit over to garage for it's subpanel. Use MHF Al 2-2-2-4 over to garage where it will be powered by 90A breaker in house to a 100A subpanel in garage.

 

[Alchymist]

.........

I've added a 60-amp subpanel in the garage for the a/c, compressor, welder, outlets, etc. but I left the feed to the lights powered from the main panel right next to the sub. This way, even if I should ever manage to trip the main breaker in the subpanel, I won't suddenly be left in total darkness with a power tool spinning down.

You can do this if the garage is attached, but not if it's a detached structure.

I just did this project for my barn. I buried 2" conduit about 30" deep, used a vacuum to pull a pull tape, and used that to pull 2-2-2-4 aluminum. Technically, I could push 100 amps, but due to the distance (120') it is down-rated some. I have a 60A breaker; that should cover my needs. I threw some dirt over the conduit, then ran a separate 1" conduit with a pull tape for future communication/low voltage needs. I have a 40A 240V for the welder, a 30A 240V for machine tools (going to a phase converter) a 20A 240V for the compressor, and three 20A 120V outlet circuits. There is also a 15A 120V for lighting.


I put in two ground rods, but my inspector told me I only need one for a subpanel (???)

"Technically" you could push 90 amps, not 100.

Most jurisdictions require two.

 

[Fixin'Stuff]

You can do this if the garage is attached, but not if it's a detached structure.

Around here they put the main panel in the garage. My sub is 16 inches from the main, simply because there weren't enough spaces left in the main.
That made is super easy. I suppose you could end up with ground loops with feeds from 2 panels where they were in two separate structures.

 

[MushCreek]

The cable I bought is rated at 105A in a conduit, but not at 120'.

 

[Alchymist]

The cable I bought is rated at 105A in a conduit, but not at 120'.

I'm thinking it's rated 100 amps if used as service entrance, otherwise it's 90. Someone with the code book will be along to comment......

 

[md21722]

Another path is to go through the house and put a sub panel on the other side of the garage... Generally speaking wire can't be "too big" so you may wish to consider #4 or #2.

 

[MushCreek]

I'm going by the manufacturer's data- 115A open air; 105A in conduit. When I use an on-line calculator, it goes down to about 80A when the length is factored in. Doesn't matter; it's on a 60A breaker from the house.

 

[Alchymist]

I'm going by the manufacturer's data- 115A open air; 105A in conduit. When I use an on-line calculator, it goes down to about 80A when the length is factored in. Doesn't matter; it's on a 60A breaker from the house.

It's not manufacturer's data - it's NEC allowances you have to meet.

 

[wyliesdiesels]

Awesome info, thanks everyone. I think for now, I'll run the 6 gauge route to a 60 A subpanel, but use a larger conduit 1-1/2" or 2" in case wire needs to be pulled for a 100A subpanel later. Take the blue path. Few more questions.

1) Do you have an example of this expansion fitting running up from the ground?

2) Currently, the wire enters the garage from the ground through a hole that tunnels through the concrete slab and floor 2x4 sill. This then runs up a stud and to the only outlet in garage + lights. Can I follow same route and come up through 2x4 sill or does the run have to stop outside the garage, come up the side of the garage, and put an entrance box to go through the siding and into the garage?

3) What's more recommended, single 4 wire cable or 4 individual wires? What's cheaper? Finally, where can you get fair priced cable/wire? The online listings at the typical big box stores don't seem to list a lot - maybe they have more in-store which is not listed on the site?

A lot of what I see listed is just THHN wire and not rated for wet location or UF-B 6/3 from online sites for $2-2.50/ft

Individual wire is a little easier to pull IMPO but a bundled cable such as MHF will work as well.

THHN is dry only. THWN is rated for wet locations. Most THHN is dual rated THHN/THWN.

UF-B is a PITA in conduit.

I just did this project for my barn. I buried 2" conduit about 30" deep, used a vacuum to pull a pull tape, and used that to pull 2-2-2-4 aluminum. Technically, I could push 100 amps, but due to the distance (120') it is down-rated some. I have a 60A breaker; that should cover my needs. I threw some dirt over the conduit, then ran a separate 1" conduit with a pull tape for future communication/low voltage needs. I have a 40A 240V for the welder, a 30A 240V for machine tools (going to a phase converter) a 20A 240V for the compressor, and three 20A 120V outlet circuits. There is also a 15A 120V for lighting.

I bought my cable on ebay from a big supplier. I used URD, but haven't been able to tell if it's actually code or not. My inspector didn't even look at it. It was WAY cheaper than copper. I use dielectric grease when making up aluminum connections, as I once had a serious corrosion issue with aluminum service conductors in a house.

I put in two ground rods, but my inspector told me I only need one for a subpanel (???)

A few issues here.

#2 AL as a service entrance for a dwelling is good for 100a. For any other application its rated for 90a distance not having anything to do with it.

URD is NOT to be used inside buildings.

The inspector should have looked at it.

And 2 rods are required unless one can prove they have 25ohms resistance ir less to earth. The equipment to prove this is expensive so most people just drive 2 rods.

The cable I bought is rated at 105A in a conduit, but not at 120'.

#2 AL is rated max 100a. Conduit does not change the ampacity of wire. Temperature, termination temp rating and number of conductors does.

I'm going by the manufacturer's data- 115A open air; 105A in conduit. When I use an on-line calculator, it goes down to about 80A when the length is factored in. Doesn't matter; it's on a 60A breaker from the house.

Free air (what youre calling open air) is an entirely different location, often used by utilities for overhead service drops.

URD is for underground distribution NOT free air.

U have wire type locations confused.

Ampacity table for conductors in raceway is 310.15(b)(16).

Free air wire ampacity is table t310.15(b)(17)

And BTW the voltage drop for #2 AL over 120' @ 90a is perfectly acceptable. 6.9v is under 3%...

 

[laser3kw]

Hang a load center with 100 amp main breaker in the garage and feed it with a 60 amp breaker from the house panel. Makes future upgrade much easier and it gives you the ability to shut down all the circuits at once if need be, instead of running back to the main panel.
I like the 2" conduit with the 90amp service idea. Worth a second thought. Won't be any cheaper to do it 5 years from now.

 

[dtbingle]

Putting together the last of the 'shopping list' before starting this job. Stuck on the entrances to the house and garage. The pictures are what's installed now. Can I reuse these entrance points or is the setup in any 1 - 3 not up to NEC code?

1) Entrance into house from outside

2) Wires coming in (inside of house view)

3) Entrance into garage. Wires come up through slab, into garage, and directly to a single outlet and a few lights.