Feeding New Sub Panel Questions

[Poncho Villa]

I've searched the forum and found great information about feeding a 100A sub panel in my detached shop from the 200A main panel in my garage. Here's my basic plan:
* run three #2 CU (2 hots + 1 neutral) and one #6 CU (ground) from a 100A breaker in the main panel;
* it will run up the wall from the main panel into my attic over the garage (~ 5');
* run ~ 38' in the attic to the eave closest to the new shop;
* run ~ 12' down the outside of the garage wall & into the ground ~ 18" - 24";
* run ~ 12' to the new shop;
* run ~ 6' up the outside of the new shop then inside to the sub panel.

I understand that I need to run the wire in conduit at least from the drop from the eave of my house then underground and back up the wall of the shop, but do I need to run the wire in conduit from the main panel, up the wall to the attic, and across the attic?

Also, I believe Al would be less expensive to run. What gauge Al would I run? Lowe's sells feeder wire for mobile homes but I'm confused whether I can run this in conduit (heat issues being the concern). I planned to run 2" conduit by the way.

Thanks for your help.

 

[Norcal]

Individual conductors must be in a conduit, one cannot run them open in a structure. For AL conductors #1 AWG is good for 100A, if using SER cable in a building if on the 2005 NEC #1 AWG is good, but the 2008 NEC sez that 1/0 AL be used due to new requirements that SE/SER be sized w/ the 60 degree column like NM (Romex). #2 AL is only good for 90 amperes, BTW.

 

[hidollartoys]

I do not believe that you can use SER for sud-panel application. I believe you are required to use wire as specified in 310.16.

 

[katit]

Norcal, our county is on NEC 2005 and inspector said to use #2 AL SER for 100A subpanel (which I did). Does it sound wrong?

 

[sberry]

I believe it was allowed, as others have pointed out it was a 2008 code change. Can still use it on service entrance, not a big deal if you are not loading this continuous, electric heaters and pottery kiln on full blast 24/7. For most garages the feed breaker is pretty much a service disconnect.

 

[Poncho Villa]

Thank you all for your prompt replies! It looks like I need to refer to the 2008 NEC 310.16 and go from there.

Also, if I purchase individual wires, I'll need to run it in conduit from the point it leaves the main panel to the point it enters the sub panel. Otherwise, I can look for large gauge cable in 2-2-2-4 CU or 1/0-1/0-1/0-?? in Al. Such cable should be OK to run open in the structure then in conduit from the eave down underground to the new building and up to the new box.

Am I on the right track?

Thanks again!

 

[Norcal]

Norcal, our county is on NEC 2005 and inspector said to use #2 AL SER for 100A subpanel (which I did). Does it sound wrong?

Nope, inspector is flat wrong, table 310.15(B)(6) does not apply to a subpanel,unless it is the main power feeder in a dwelling. Table 310.16 does apply which means 90A max.

 

[W126C]

Poncho Villa,
Get ready for sticker shock on the SER 2-2-2-4 Cu. $6.06 a foot was the quote I got two weeks ago and that was with a minimum order of 150 foot.

 

[Poncho Villa]

Poncho Villa,
Get ready for sticker shock on the SER 2-2-2-4 Cu. $6.06 a foot was the quote I got two weeks ago and that was with a minimum order of 150 foot.

Sticker shock indeed! You can tell I'm a newbie - I have no clue the prices of stuff.

 

[1Garageman]

While $hit that's a lot of money for wires!
My father and I are going to put a 60amp subpanel in my garage. I wonder what kind of wire we'll have to run, and how much that will cost?

 

[elett]

Can you LB out the backside of the 200a panel trench to the new structure back up and LB into the new 100a sub panel? Then you could put everything in PVC, 1 1/4" which is plenty big (3-#4 thhn, 1-#6 thhn green). Don't for get the ground rod !

 

[Norcal]

Can you LB out the backside of the 200a panel trench to the new structure back up and LB into the new 100a sub panel? Then you could put everything in PVC, 1 1/4" which is plenty big (3-#4 thhn, 1-#6 thhn green). Don't for get the ground rod !

#4 is too small for 100A & 2 ground rods are required unless one can prove 25 ohms of resistance or less, per NEC art. 250.56.

THHN is for dry locations only, any conduit outdoors or UG is a wet location.

 

[Poncho Villa]

Can you LB out the backside of the 200a panel trench to the new structure back up and LB into the new 100a sub panel? Then you could put everything in PVC, 1 1/4" which is plenty big (3-#4 thhn, 1-#6 thhn green). Don't for get the ground rod !

Unfortunately, my driveway keeps me from trenching. I figured I could run it across the attic to avoid having to cut the driveway or bore under it. I'm a little on the lazy side...

#4 is too small for 100A & 2 ground rods are required unless one can prove 25 ohms of resistance or less, per NEC art. 250.56.

THHN is for dry locations only, any conduit outdoors or UG is a wet location.

Where would the two ground rods go? There's one directly under the panel outside the house now. I'm guessing one must be installed under the new panel just outside the shop. Is that all?

Thanks again everyone! I appreciate your responses and patience!

 

[Norcal]

The 2nd rod must be at least 6 feet away from the 1st rod.

 

[W126C]

Copied from an other web site discussing sub panels

Grounding and bonding of the sub-panel

At the sub-panel there must be no connection between the neutral bus bar and the equipment ground bus bar or the neutral bus bar and the sheet metal of the breaker box. You may need to purchase an additional bus bar for the equipment ground since many panels, especially those that come with a main breaker, do not come equipped with one. If the panel comes with a bonding screw or bonding strap, do not install it to the neutral. Depending on the brand of panel, you may need it to connect the ground bus to the sheet metal enclosure.


At the remote building, you will need to drive two eight-foot ground rods located a minimum of six feet from each other and connect them to the equipment ground bus using #6 copper wire. The easiest way is to make one continuous run without cutting the wire. The rods must be driven in flush or below grade. The ground rod clamps must be listed for direct burial. It will say so on the clamp or look for the letters "DB". If you have any metal piping (except gas lines) in the remote building, you must bond it to the equipment ground bar, use #6 wire. If the connection is to a metal water line, the connection should be within the first 5 feet of where the water line enters the building. All connections must remain accessible, so if the connection will end up inside a wall, you must provide an access plate.


I realize that some of the things I've included exceed the code minimum, but this is an attempt to make a "one size fits all" response without getting into a really long discussion.


For a sub-panel in the same dwelling there is no need for additional ground rods or connections to metal piping. Also, no need for a main breaker in the sub-panel, regardless of the number of circuits. Unless you are running metal conduit, you will need to run a 4 wire feeder.

 

[Torque1st]

A person can bore under a drive with some 3-5' sections of steel pipe, some couplings, a Tee with a pipe plug and hose fitting. Dig a short trench next to the drive and a hole on the other side of the drive. Assemble the Tee, plug, and hose fitting, with a section of pipe and a coupling on the end. Turn on the water and drive the pipe with a hammer. Add pipe sections as required with couplings. When you have a hole underneath the drive pull the "drill" pipe and insert a conduit. Some people have drilled thru with plastic conduit.

 

[elett]

Norcal, Good catch on the thhn. typicly thhn has a dual listing of thhn/thwn. Also other types you could use would be MTW,RHW RHW-2,TW,THW,THW-2,THHW,THHW-2,THWN,THWN-2,XHHW,XHHW-2,ZW. About the #4 artical 310.15 (B) (6) single phase dwelling services and feeders allows for #4 for 100a feeder.

 

[Norcal]

Norcal, Good catch on the thhn. typicly thhn has a dual listing of thhn/thwn. Also other types you could use would be MTW,RHW RHW-2,TW,THW,THW-2,THHW,THHW-2,THWN,THWN-2,XHHW,XHHW-2,ZW. About the #4 artical 310.15 (B) (6) single phase dwelling services and feeders allows for #4 for 100a feeder.

Most are dual rated THHN/THWN, but have to verify it. On the #4 you need to read the WHOLE article, it only applies to the feeder carrying the entire load in a dwelling, a feeder to a outbuilding or a subpanel do not apply.

 

[Poncho Villa]

Most are dual rated THHN/THWN, but have to verify it. On the #4 you need to read the WHOLE article, it only applies to the feeder carrying the entire load in a dwelling, a feeder to a outbuilding or a subpanel do not apply.

If I understand this correctly (I'm a novice of great degree), this code requires bigger wire to feed the sub-panel than it does to feed the main panel from the electrical source. Is this correct? Out of curiosity, what's the rationale behind this requirement (if my understanding is correct that is)? It would seem to me that the bigger wire would be required from the source to the main panel.

Again, I'm a novice so please bear with me. It's just an interesting topic to me and I'm eager to learn.

Thanks for your input all!

 

[Norcal]

If I understand this correctly (I'm a novice of great degree), this code requires bigger wire to feed the sub-panel than it does to feed the main panel from the electrical source. Is this correct? Out of curiosity, what's the rationale behind this requirement (if my understanding is correct that is)? It would seem to me that the bigger wire would be required from the source to the main panel.

Again, I'm a novice so please bear with me. It's just an interesting topic to me and I'm eager to learn.

Thanks for your input all!

The code allows a SMALLER wire on a dwelling service, that is the way it needs to be looked at.

 

[elett]

Yep. 315(B)(6) only applies to a single feeder that supplies an entire residential panel.
Your use requires the ratings found in Table 316.Unless the feeder carries the entire load for the home you can not use table 310.15(B)(6).
In your case the sub panel is not supplying the entire home so you must use Table 310.16. Under the 2008 NEC SER cable must be sized based on the 60 degree column and therefore the #2 SER AL would have an ampacity of 75 amps and the next standard size breaker would be an 80 amp breaker.The #2 cu you talked about shouldn't be a problem except for the cost.

 

[Poncho Villa]

Thanks! You guys are great. Yes, the Cu will set me back more that I expected but I sure don't want to burn the place to the ground.

I appreciate all of the feedback!

 

[klogan121]

Thanks! You guys are great. Yes, the Cu will set me back more that I expected but I sure don't want to burn the place to the ground.

I appreciate all of the feedback!

Hey Poncho---- Check out the Menards sale ad on their website, they have THHN on sale until March 21st. 2 GA THHN $1.09 linear foot! There are more wiring sizes to choose from also.

AND... If you take the Menards ad to LOWES, they will beat their prices by 10%! Lowes electrical associate was a little angry about that.. said they lose money on price matching wire. Oh well! That's the policy they advertise!

I did that when I bought 4 GA for my pole barn two days ago. 10% markdown price was .58 linear foot... $556.80 for 960 feet of wire. Enough for my 225 foot run to barn.

 

[Poncho Villa]

Hey Poncho---- Check out the Menards sale ad on their website, they have THHN on sale until March 21st. 2 GA THHN $1.09 linear foot! There are more wiring sizes to choose from also.

AND... If you take the Menards ad to LOWES, they will beat their prices by 10%! Lowes electrical associate was a little angry about that.. said they lose money on price matching wire. Oh well! That's the policy they advertise!

I did that when I bought 4 GA for my pole barn two days ago. 10% markdown price was .58 linear foot... $556.80 for 960 feet of wire. Enough for my 225 foot run to barn.

Klogan - THANKS for the heads up!!!