Bottom fed subpanel? and wire type?

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  #1  
Old 08-31-07, 06:44 AM
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Bottom fed subpanel? and wire type?

I've been wiring houses for a few years now under various electricians. This time, my brother asked me to wire his new shop. It's pretty much a six car garage with a second floor. Yea, I know, I wish I had it too. Everything will be done under a homeowners permit. We drew out all the plans and there are two things I have never been presented with before. We're putting a 100 amp panel out there and running the wire underground from his 200 amp main service. My first question is, do I need a bottom fed panel (main breaker). I have never seen these, but I hear the main breaker is literally on the bottom. Sucks, cause I bought him a nice Square D 125amp rated panel for his birthday, but the main is on the top, like most panels. I see running the feeders in the bottom, through the panel to the top breaker being very impractical, and unsafe as well. I'm sure it won't pass inspection.

My other question is the type of feeder. I would love to use SER in conduit, buried underground but I have not found anywhere in the NEC where it says you can or can't do this. Do I have to use USER? And is that with or without conduit? I would usually run it with. Thanks alot everyone, I'm trying hard to get him a car lift so I can use it.
 
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Old 08-31-07, 08:44 AM
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> do I need a bottom fed panel (main breaker).

No, you do not need a bottom feed panel, but you can use one if you wish. Most panels come with instructions how to convert to bottom or top feed anyway -- take the screws out, flip the guts around, re-install.

> through the panel to the top breaker being very impractical, and unsafe
> as well. I'm sure it won't pass inspection.

It's fine to do this, or you can run the conduit up alongside the panel and enter the side or back near the top. If you bring the feeders in the bottom of the panel, form them into a "question mark" shape to maintain proper bend radius. For example, the wire that runs up the left-hand side of the box attaches to the right-hand terminal of the main breaker.

> I would love to use SER in conduit, buried underground

SER cannot be used underground. You can use a burial rated cable like SE type U or USE-2 with conduit for protection only on the vertical portions above grade, or you can pull THWN conductors in continuous conduit. For a 100A subpanel, you need 3 AWG or larger copper, or 1 AWG or larger aluminum. What is the distance from the main panel to the shop panel? You may need to upsize beyond these minimums.
 
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Old 08-31-07, 10:35 AM
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Installing a 4" sq wireway, either 2 or 3 ft in lenth, in a vertical position ,will eliminate routing the Feeder conductors thru the panel, and will provide an excellent "chase" for either pulling the conductors to the panel location, or feeding the conductors into the raceway as they are pulled to the Service location.

THe lenth of the wireway is set parallel to the panel with a 2-3" nipple between the top right / left panel K-O , and the top side of the wireway.
The UG raceway enters the bottom end of the wireway.

If you are pulling an EGC with the Feeder Conductors, then use the EGC to Ground the metal of the wireway as the EGC is routed thru the wireway.
 
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Old 08-31-07, 01:34 PM
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Hmm...well the panel is going to be flush mounted between the studs, so there will be no room behind it. Flipping the guts upside down sounds like a good idea. But I don't remeber the panel mentioning this. I wonder if the numbers and labels would then be upside down from flipping the cover. comming in the side up towards the top is also a good idea. Is it common to come in from the bottom and do the questionmark? I have never seen this but I have only been inside panels or a few years now. Do you think the inspector might have a problem with that or think it looks kind of akward?

Yea, USE was what I was talking about. This is direct burial right? I have never used it before, so I assumed there was USER and USEU available. Does USE contain four conductors? I am trying to stay away from copper. I know its safer and all, but panels will last forever with a good clean aluminum termination and a generous amount of nolox. And I thought you only need number 2 aluminum to run 100 amps? It definitly won't be over 100 feet.
 
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Old 08-31-07, 06:03 PM
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Many commercial (3-phase) panels are made to allow the cover to be reversed along with the internals to allow for bottom feed. This is not as common with single-phase residential panels.

The "question mark" bends are commonly done when a top feed panel has the feeder (or service) conductors entering the bottom.
 
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Old 08-31-07, 06:37 PM
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Do you think the inspector might have a problem with that or think it looks kind of akward?
===========================================

No, they will not have a problem with it.


Curious as to why you believe this may be against NEC.

Just so you know< I do this all the time when I bottom feed a top breaker or the opposite. If I can spin the guts, I do it because it saves wire and is neater but there is no requirement to do so.
 
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Old 08-31-07, 06:56 PM
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You DO NOT have to spin the guts. You do not have to spin anything.
Most all "newer" main breaker panels can be installed either breaker top or bottom. As long as the main breaker throws from side to side you can install the WHOLE panel either way.
I have not seen an "up/down" throw main breaker in a real long time.

Also, the writing on the handle and panel label is typically written sideways so it can be read however the panel is mounted.
 
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Old 08-31-07, 07:09 PM
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I take it that you're not in an apprenticeship. I highly suggest that you get into one if you're doing it for a living. I don't mean any insult, but an apprentice with your amount of time in the field wouldn't be asking these questions. I'm not trying to attack you but, it just sounds like you're not getting proper training and are being taken advantage of. You're just hurting yourself by not getting into an apprenticeship.
To answer your question, you can pick up a 100 amp breaker and mount it on the bottom. You just have to mark it as the main. You can also do the "question mark" as well to feed from the top. I would just suggest that you strip some of the jacketing off your feeders and tape it around the standoffs for the dead front to eliminate the sharp edges that could damage the feeders.
I would reccomend #3 CU THHN in at least 1 1/2" PVC. CU because you can go smaller (also means smaller pipe required), don't need no ox, and it's not as stiff. If you go with AL you'd have to go with #!. Eventhough the wire itself could be rated at 90 deg. C, which would rate #2 AL at 100 amps, you have to go by the rating of your weakest point. Since most lugs are only rated for 75 deg. C, you have to use that rating. That bumps #2 AL down to 90 amps. In the odd chance that you do have 90 deg C rated lugs in both the main and sub panels, then #2 AL would be OK.
As for the PVC, I always use it underground. Neither PVC or RMC are watertight, but RMC will rot out. When this happens it can cause your feeders to get pinched and short out due to expansion and contraction underground. I've seen that happen. I could hear the hot inside the pipe arcing undergound. The scary thing is that the breaker was old and didn't trip. Imagine walking through the backyard after a heavy rain with that going on right below your feet! Because PVC can be a pain to fish, I'd suggest putting in a #12 or #10 pullwire as you're piecing it together. Don't use poly line because it can burn through the inside radius's of your bends and you're SOL. Oversizing you PVC may be worth it. If you have a difficult run it'll make the pull easier.
Hope this helps. Don't forget a ground rod!
 
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Old 08-31-07, 07:23 PM
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Two comments on the last post.
1) If you do install a backfeed main breaker it MUST be secured to the panel somehow. Most mfgs do offer some sort of breaker hold down.
There is NO reason to do this though. ANY main new breaker panel will work fine as-is.

2) In my area, and many others, we can use #2AL or #4CU for a 100A sub-feed, just like a main panel. This is something you should double check with your local AHJ.
 
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Old 08-31-07, 10:08 PM
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> And I thought you only need number 2 aluminum to run 100 amps?

Check with your local inspector. Many areas allow #2 aluminum (or #4 copper) for a 100A subpanel; however the upcoming 2008 code has changed the wording for table 310.15(b)(6) which will narrow the application of that table and require the use of full-sized conductors for many subpanels as per table 310.16.
 
  #11  
Old 08-31-07, 10:14 PM
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Speedy is correct you don't turn the guts. You simply rotate the square d panel and orient the main breaker at the bottom. If you look above the main breaker stamped twice in the metal of the can is the word line. It is stamped such that you can read it either in the breaker top feed position or breaker bottom feed position. You don't remove and rotate the guts. The bolt pattern will not work.

I'm not a fan of bottom feed but what difference does it make other than you might waste the gutter space in the bottom feed orientation.

Is the panel you bought flush mount or surface mount?

If you want to see a square d rotated to the bottom feed orientation go here. Paste it. They don't seem to want this forum to be more than low budget. Auto-Links must be to costly for them.

http://img.photobucket.com/albums/v224/comp1911/House/IMG_0916.jpg

Your going to want a four wire feeder to the sub... neutral and ground not bonded at the shop. Probably 2 ground rods required as I doubt they are going to test for 25 ohms. #6 copper GEC to the rods.

Personally I think you should run down to the big box and get 2-2-2-4 Al mobile home feeder. This gives you an insulated ground (not bare like cables) and it is RHH-RHW-USE-2 rated you can direct bury it or run it in conduit and you can enter a building with it. Worst case it will be rated 90 amps and can be rated 100 amps if your ahj will recognize the dwelling ampacity table as Speedy mentioned.

Keep asking questions....... I am a firm believer that you should be able to work on your homeowners permit.

Stubbie
 
  #12  
Old 09-01-07, 05:33 AM
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I didn't plan on using ground rods as I will be bringing a Grounding Conductor in from the house and I will not be bonding the ground and neutral in the subpanel. Also I will definitly be oversizing the PVC to simplify the pull. I am aware you can use a 100 amp double pole breaker to back feed a main lug panel, but I didn't want to get into having to secure the breaker. The panel I will either flip over or use the questionmark technique. I appreciate all of you explaining these options. As far as the wire, I guess I will just talk to the supply house to see what they have and what they reccomend. Should I just ask for USE? Can you get this with 4 conductors?
 
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Old 09-01-07, 06:02 AM
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I would use what we call quad URD, that or mobile home feeder as Roger suggested.


You MUST have a ground rod(s) at a detached structure supplied by a feeder, regardless if it is 3 or 4-wire. This is mandatory.
 
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Old 09-01-07, 09:38 AM
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Strider

I'm not following what you mean by bringing in a grounding conductor from the house so you don't need ground rods.

Can you explain so we understand what your doing? I would like to make sure your not misunderstanding the GEC vs the EGC of the electrical system.

Typically as Speedy said you run a 3 wire feeder if local codes allow or better a 4 wire feeder.

Roger
 
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Old 09-01-07, 11:31 AM
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When we run a 100 amp subpanel, we normally use number 2 SER and don't bond the ground and the neutral. I must admit however, I have never ran a feeder underground, which is the reason I started this thread...to get some insight on which type of wire to use. I was unaware that you need ground rods on a detacched garage if you bring a grounding conductor from the main service. I'll have to check the nec. I actually remeber, more then once, my old teacher saying you can either bring a 3 wire and make a grounding system (probably rods) or you can save yourself the trouble and just bring a 4th wire as a GEC from the ground bar in the main service. Its a very strange coincidence, but that teacher happens to be the inspector of the jurisdiction this installation will be done in.
 
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Old 09-01-07, 12:27 PM
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You MUST put in grounding electrodes when putting a panel in a different building. Code requires this because there could be a difference in potential between the ground at the service and the ground at the seperate building. The whole point is to eliminate any potential. I would suggest running a ground from the main panel as well. As Roger mentioned before, the ground and neutral are NOT to be bonded at the subpanel. This is to be done at the main panel ONLY.
I believe that there is some confusion on feeder sizing. I am using table 310.16 with a temp rating of 75 deg C. I originally thought that you were using the 90 deg. C rating. I'm wondering now if you and Roger are using table 310.15 (B) (6). According to that it does say that you can use #2 AL or #4 CU for 100 amps. The reason why I did not use this table is that 310.15 (B) (6) applies to "main power feeders". I believe that this is only to feed your main panel (or panels in the case of a multifamily residence). Even if I'm wrong about it including subpanels, this article applies to feeders for panels in dwelling units. I don't belive this applies to feeding subpanels in garages. I am unaware of the possible wording change in this article in the 2008 code, but it doesn't suprise me. That code is greatly outdated. From what I've been told it was originally brought about during WWII when there was a copper shortage. They reduced the wire size needed to save on copper.
If you're doing this for your brother, I'd suggest running THHN wire in 1 1/2" PVC. Direct burial cables are more succeptable to failure. You also have to dig a deeper trench. PVC is easy to install and will last your brother a lifetime. A lot of times cotractor's take short cuts (many of them not even legal) either to make more money or to be able to lower their bids (thus raising their competitive advantage) without lowering their profits. Some samller contractors just don't have the experience or training to know any better. Keep in mind that not only is this family you're doing it for, but your name will be on it.
 
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Old 09-01-07, 12:36 PM
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With regard to the ground rod, here is the code section:

250.32 Buildings or Structures Supplied by Feeder(s) or Branch Circuit(s)
(A) Grounding Electrode
Building(s) or structure(s) supplied by feeder(s) or branch circuit(s) shall have a grounding electrode or grounding electrode system installed in accordance with 250.50. The grounding electrode conductor(s) shall be connected in accordance with 250.32(B) or (C). Where there is no existing grounding electrode, the grounding electrode(s) required in 250.50 shall be installed.


With regard to 310.15(B)(6), many areas DO allow it's use for any residential sub-feed. A garage at a dwelling unit is still a dwelling.
The other ambiguous points in that article have been the topic of MANY internet "discussions". It's not worth going into it again other than to say some areas allow it and some don't.
 
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Old 09-01-07, 12:45 PM
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Strider we are all here to learn. I think possibly you slightly misunderstood what your teacher was saying. The fourth wire is the EGC (equipment ground conductor) that bonds metal parts to an effective ground fault path back to the Service equipment grounded conductor... (service neutral). This allows fault current to get back to the transformer center tap (the lowest impedance path) and facilitates a breaker to trip out on over current....ie....human safety.


The GEC (grounding electrode conductor) is the conductor to electrode(s) that protect equipment from high voltage events like lighting. Property protection.

A 4 wire feeder is required (mandatory) now or in the future if any other metal paths are installed to the garage like water pipes or data cables.

A 3 wire can be run ( neutral and ground are bonded) but no future metal paths can be present and in 2008 they are going to do away with 3 wire feeds period to sub-panels.

Anyway rather than me go through all this... here is an article from Mike Holt that takes you through everything. Copy it and print it out it will be very usefull down the road.


http://ecmweb.com/grounding/electric_grounding_vs_bonding_5/

Roger
 
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Old 09-01-07, 02:56 PM
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First off, I am thankful for all of you and your willingness to help me. That being said, I am well aware of the difference between a EGC and a GEC. I also clearly remeber my teacher stating that you can either use a ground from the main service or set up a system (grid, rod, etc.) and not pull a grounding conductor. I remeber this clearly because he really spent some time driving this into us. I now agree with you, its definitly not the first time a teacher has been wrong. Heck, not the first time an inspectors been wrong either.
 
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Old 09-01-07, 10:06 PM
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Brewcity:

Yes I am refering to table 310.15b6 when I say that 2 al can be 100 amps. This is allowed in some juristictions and used to be in mine but is no longer accepted for conductor ampacity to sub panels. Our codes department investigated and asked for clarification from the NFPA/NEC and the answer they got was that the intent of the cmp for that section of code was the table was to be used for dwelling services and feeders to mains only.... not sub panels. The ROP for that section of code has reported an "agree on principle' to change the the wording of that section of code to state, in short, that it applies to conductors that supply 100% of a dwellings diversified load. This will remove any ambiguity. This is to take effect in the 2008 code cycle.

Strider:

I'm not trying to imply your not aware of any differences between the egc and the gec. Just trying to make sure the correct installation is done for your brother based on the questions and information you are giving. So please don't take anything as a personal attack on what your knowledge is or isn't. I sure don't know everything and never will. Our industry is way to full of information for one person to "know it all". As for what your teacher taught you about running a ground to the detached structure and no ground rods needed I can only speculate but lets don't say he was wrong...only he can explain what he was teaching.

I suggested mobile home feeder because it is an excellent choice due to its qualities for direct burial and also ease to be ran in conduit because of its insulation qualities. It also is aluminum and is a cost savings over copper.
Aluminum is an excellent choice whether or not it is cheaper to buy. And is installed in longer runs to sub panels 10 to 1 over copper. Short runs is a toss up. The big thing is do not run a bare aluminum equipment grounding conductor underground as is with cables. It might be legal but it is asking for trouble due to corrosion of that bare conductor for various reasons.
Thhn/Thwn copper is also an excellent choice as Brewcity suggested but I disagree that it is better than aluminum. I see no advantage other than size of wire.

Roger
 
  #21  
Old 09-02-07, 08:15 AM
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I agree with aluminum vs copper. There are so many panels that have been around for so long with aluminum feeders. With a nice clean connection and some antioxidation formula, aluminum works great. Mobile home feeder sounds like my wire. Thats exactly what I was looking for. So flip the panel, use mobile home feeder, and BE SURE to install a ground rod. Thanks for everyones time and input.
 
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