Wire size for 100 amp sub panel?

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  #1  
Old 05-07-01, 05:31 AM
L West
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What size wire do I need for a 100 amp sub panel? The sub panel will be app. 15 ft. from the main panel in the basement.

 
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  #2  
Old 05-07-01, 09:20 AM
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#4 copper or #2 aluminum per NEC Table 310-15(b)(6). Note that sub-panels must have the neutrals and grounds completely separated, with the neutral bus isolated from (not bonded to) the steel enclosure. If you have any questions about this please let us know.

Juice
 
  #3  
Old 05-07-01, 09:42 AM
L West
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Thanks Juice. Is #4 copper available as cable (3 conductor & ground)?

I have a panel and also purchased a separate ground bar. All the grounds will connect to the ground bar which attaches directly to the cabinet. I WILL NOT install the screw that bonds the neutral bar to to the cabinet.... right?
 
  #4  
Old 05-07-01, 10:08 AM
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Yes, this conductor is available. You want "type SE-R", which contains two insulated hots, one insulated neutral, and a bare ground, all inside an overall sheath, or jacket.

Having bousght a separate ground bus bar, and having described the "bonding screw", you seem to be on top of this requirement already. You are correct, toss that screw! Before installing the ground bus bar, sand the paint completely off the area of the enclosure where you will install it. If it is made by the same mfr. as the panel there may be little studs and pre-drilled and tapped holes to accept the new bar. If not, install in a logical area of the enclosure and use self-tapping screws only, as sheet metal screws are prohibited by the NEC for grounding connections of any kind.

Good luck.

Juice
 
  #5  
Old 05-07-01, 11:05 AM
L West
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One last question... Do you know off hand if #4 copper will fit into a std. Square D "Homeline" 100 amp two pole breaker?
 
  #6  
Old 05-07-01, 12:10 PM
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That's a good question. As luck would have it, I actualy keep a copy of the Square D catalogue at arm's length at my office. (I'm an electrical designer by trade.) Type "HOM" circuit breakers from 80 to 125 amps will accomodate #4 through #2/0 copper conductors. So you're all set!

Your model number is HOM2100. List price is $112, but you'll never pay nearly that much. Plan on $40 to $50 retail.

Juice
 
  #7  
Old 05-07-01, 12:24 PM
L West
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Thanks Juice for all the info :-) By wiring my own basement I will save about $2000 and learn some new skills along the way. I'm getting all the appropriate permits and inspections and when in doubt asking questions. This forum is a great source of information and I appreciate you guys who take time to help us DIY folks out!
 
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Old 05-07-01, 12:47 PM
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You're more than welcome. I did a full service upgrade on my house last year, also including permits/inspections & the lot, so installing this type of stuff and passing inspection are areas where I can certainly both sympathize and help with. (My experience includes years of electrical installation/troubleshooting/repair, though never becoming a licensed electrician, before I became a "drawing board" guy more recently.) So if you have anymore questions, stop by anytime. There are a lot of very smart and very experienced folks who hang out here that will be glad to help.


Juice
 
  #9  
Old 05-08-01, 08:22 AM
L West
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Angry

Do you know how difficult it is to find 4/3 w/ ground CU cable! I live in metro Atlanta and after several calls I found a supplier with a 19' pc (romex). It seems that most folks use #2 AL for this type of application.

 
  #10  
Old 05-08-01, 09:29 AM
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Copper is preferred by many purists, but in the final analysis aluminum is used far, far more. Even tough larger diameter is specified for the same amperage, electricians still save money with aluminum. And in most areas when you hire a pro to wire a panel or a service that's what you'll get automatically unless you specify copper, for which they will charge you extra. For #10, 12, and 14 you will rarely see aluminum, and should not use this in these smaller sizes no matter what, in my personal opinion. In larger gauges I have no problem with aluminum personally, although some folks will strongly disagree with me. But I have gone on numerous service calls where aluminum branch circuit wiring has had serious failures, and saw first hand that some of these deficiencies could have, but miraculously didn't, cause fires.

Since you asked specifically about copper I felt you had a pre-determined preference, so I didn't really discuss it. But due to your persistance you found what you wanted, and should take comfort in knowing you have a superior conductor. Nobody but perhaps an Alcan employee would disagree with me on that!

Good luck.

Juice
 
  #11  
Old 05-08-01, 09:00 PM
RickM
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Cool

While I don't see much of a problem here, you can not use Section 310-15(b)(6) for this type of feeder. Within the body of the paragraph, it states that the feeder must be the "conductors that serve as the main power feeder to a dwelling unit..." which, in this case it is not. You would have to use the table 310-16 to get the wire size, if the main service is anything other than a 100amp service. A 100amp service can use #4 cu or #2al, and Section 310-15(b)(6) says the feeder conductors don't have to be larger than the service entrance conductors.

If the service is larger than 100amp, you will have to go back to Table 310-16.

If you have already ran the wire, just change out the 100amp breaker to a 90amp, and call it good.

Rick Miell
 
  #12  
Old 05-09-01, 06:26 AM
resqcapt19
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Rick,
I have to disagree. Table 310-15(b)(6) does apply to this installation. The conductors in question meet the definition of main power feeders as given in 310-15(b)(6). The conductors in question are between the main service disconnect and a lighting and appliance branch circuit panelboard. This is all that is required to use the smaller conductors permitted in Table 310-15(b)(6). Note that the definition of main power feeder was new in the 99 NEC.
Don(resqcapt19)
 
  #13  
Old 05-09-01, 09:18 AM
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I agree with resqcapt19, which is why I used 310-15(b)(6) in the first place. Actually, I used to quote 310-16 even recently, until Wg explained how it is that 310-15(b)(6) does in fact apply to this particular situation in a dwelling situation. Of course I disagree with your interpretation with all due respect, Rick.

Juice
 
  #14  
Old 05-09-01, 03:23 PM
RickM
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Talking

Hey guys, I agree *** if it is the feeder from the main disconnect to the panel. However, and I quote from LWest's orginal post --- "The sub panel will be app. 15 ft. from the main panel in the basement."---- This leads me believe that he is coming off of a breaker panel, which may or may not have the main disconnect in it, but, if my assumption is correct, also contains other breakers, and therefore the feeder is not the main power feeder.

I guess it's time to ask him just how his setup is configured.

smiling with my teeth closed....

Rick Miell


 
  #15  
Old 05-09-01, 07:30 PM
resqcapt19
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Rick,
It doesn't matter how many other breakers are in the service panel. If the panel where the feeder originates has the service disconnect in it, and the feeder supplies a lighting and appliance branch circuit panelbaord in a dwelling unit, you can use Table 310-15(b)(6). No matter how many other breakers are in the first panel, the feeder is still a feeder between the main disconnect and a lighting and appliance branch circuit panelboard.
Don(resqcapt19)
 
  #16  
Old 05-09-01, 08:19 PM
RickM
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Wink

Ok, Don, I conceed. I really thought it said something about providing the whole power for the dwelling unit. I don't have the 1996 code book in front of me, so I can't say for sure if that is what was changed in the 1999, but something was. There is a verticle line next to the sentence talking about the main power feeder.

When I get back to work tomorrow, I will look it up, both in the 1996 NEC and the ROP & ROC.

Until then....

I'm back at work, so let's look at this.

From the IAEI analysis of 1996 NEC..."the feeder no longer has to "supply the total load to the dwelling unit." It may now be "the main power feeder to a dwelling unit." The term "main power feeder" is not defined in the NEC, but apparently refers to the conductors from the service to a panelboard where the majority of loads are supplied. Loads such as for air conditioning, outbuildings, swimming pools or water pumps can be taken at the service and Note 3 be apllied for sizing the "main power feeder." No guidance is given on how much of the total load the "main power feeder" must supply before the note can be applied."

The 1999 NEC was changed here also, but still no clear direction on how much the total load the "main power feeder" must supply.

I guess that you could have 8 subpanels coming from one service equipment, and use this table.

for now....


Rick Miell

[Edited by RickM on 05-10-01 at 02:48]
 
  #17  
Old 05-10-01, 02:37 PM
Wgoodrich
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Rick, your definition as to "main power feeder" is located in the beginning statement above table 310-15-b-6. Just read that opening statement and you will find what it defines as the main power feeder. Resqcapt19 is right. Just don't tell wirenuts or he will go back to his room again. Just kidding.

310-15-b-6

For application of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and appliance branch-circuit panelboard(s)

Hope this helps

Wg
 
  #18  
Old 05-11-01, 07:59 AM
L West
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After making several calls I finally found a supplier with some #4/3 w/ ground CU cable. After I got home I noticed they made a mistake and gave me #2 CU. So I now have no doubt the cable I have will handle 100 amps!

Talk about work! Getting the # 2 CU cable thru the wall studs and into the main and sub panel box was lots of fun... NOT! I now realize I have many upper body muscles I didn't know about because they are all sore! Hat's off to you guys wo work with this stuff everyday :-)
 
  #19  
Old 05-11-01, 09:13 AM
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Way to go, L West. I just hope they also made the mistake of CHARGING you for #4 Cu!

Juice
 
  #20  
Old 05-11-01, 10:31 AM
L West
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Yep, they did that too. I paid $1.10 / lf :-)
 
  #21  
Old 05-11-01, 10:33 AM
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Cool!

Juice
 
  #22  
Old 05-11-01, 10:45 AM
L West
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Smile

In all fairness I did call the supplier and offered to pay the difference. Since it was only 19' of cable they said not to worry about it. At least I can sleep well knowing that I didn't take advantage of somebodys mistake.
 
  #23  
Old 05-11-01, 11:39 AM
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Truly a scholar and a gentleman!

Juice
 
  #24  
Old 05-11-01, 08:46 PM
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Hey guys !
In all jest you all almost sound like a bunch of lawyers!! Arguing sections and interpretation!!! LOL!!!
 
  #25  
Old 05-11-01, 08:51 PM
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Hey Bob, what do you call a hundred DIY electrical advisors at the bottom of the ocean?

...A GOOD START!

What's the difference between a dead skunk in the middle of the road and a dead DIY electrical advisor?

...THE DEAD SKUNK HAS SKID MARKS LEADING UP TO IT!

LOL!

JuiceHead
 
  #26  
Old 05-12-01, 06:00 AM
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Discussing code with an electrician/diy poster/inspector is like wrestling with a pig in mud, eventually you understand that they like it.

I wonder if within the definition of main power feeder when 'main service disconnect' is used that means the actual wire terminated in said disconnect, or if the intent is the panel in which the main service disconnect is contained. This is the root of the differences of opinion about this code article. If you infer 'panel' or main service disconnect 'enclosure' when the term main service disconnect is used, then any feeder in a residence would fall under 310-15(b)(6) guidelines. This is not the case. If the NEC intended this, they would have used the general term of 'service' or 'service equipment' instead of the specific term of 'main service disconnect'.

In the above example, the 100 amp breaker that will feed the sub panel is a feeder breaker, not a main service disconnect. You can determine this with a simple test. With a main service disconnect off, the only power available will be uninteruptible utility power (no shutoff means available - other than pulling the meter). This 100 amp breaker in question does not pass the test. The power available on the bus can be shut off without pulling the meter.

This means that installing a 100 amp breaker with #4 CU SE-R cable in this instance is a code violation. In fact, due to changes in the 1999 NEC, SE-R cable is now limited to the 60C column in table 310-16, and the cable in question is code limited to only 70 amps (please see 338-4(a), which refers you to 336 part A and B, which contains 336-26 Ampacity). How is that for some serious legal type jargon?

Before those of you that disagree with me start rolling me into my grave, carefully look at the terms used as defined by the NEC. There is no gray are here. All of these items are clearly and specifically defined in the code book, and are not my opinion or interpretation. As such, I am legally bound as an electrician to follow them. If I do not, and something goes wrong, I am liable. It will be my fault that something failed. My personal beliefs that the wire in question will not be harmed by the 100 amp breaker is not important. It is still a code violation, and I will not recommend that you do it.

Look at the 1996 NEC compared to the 1999 NEC. Do they change just for fun? Why did the 1996 NEC allow SE-R cable to use the 75C column (336-26 used to be in part C of article 336, excluding it from applicability to SE-R cable), and in the 1999 code SE-R is limited to 60C column (by the shifting of one article)? Did the construction of SE-R cable change in that time span? I wonder if this type of cable has been attributed to overheating causing fires, and the applicable code committee decided to limit it's use for safety reasons? I wonder if this same thing has been occuring with feeder conductors to sub-panels, which is why 310-16 ampacity must be used for these feeders instead of 310-15(b)(6)? I haven't seen it, but I do not see everything. I also do not sit on the code making panels, and do not have specific insight into why the code is the way it is. Certainly, everyone must make their own choice, but I choose to follow the code.

Enjoy your day!

[Edited by s1nuber on 05-12-01 at 10:07]
 
  #27  
Old 05-12-01, 12:08 PM
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whooooboy!
are you telling me the last 20 yrs i've installed #2 SER @ 100 A is NFG???

I'm gonna stay in my room and hide under the bed!......
 
  #28  
Old 05-12-01, 03:54 PM
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ok gang;

http://www.electrical-contractor.net..._ROP's.htm

look up 280-310

inparticular look at log # 3908 , i beleive the CMP is aware of the confusion here....






[Edited by wirenuts on 05-12-01 at 07:37]
 
  #29  
Old 05-12-01, 11:01 PM
resqcapt19
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s1nuber,
The section, by the use of the word "feeder(s)" clearly allows multiple feeders. Multiple feeders would require the use of feeder breakers. It does not say that the feeders are directly connected to the service disconnect, only that the feeders are between the service disconnect and the lighting and appliance branch circuit panelboard(s).
Don(resqcapt19)
 
  #30  
Old 05-13-01, 02:04 PM
s1nuber
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Wirenuts - what does NFG mean? And how is it under your bed? The world is dying to know.

You are not the only one who has installed #2 al Ser cable under a 100 amp breaker. Just because it has been done in the past, does not mean that it is the best choice. If all old wiring methods were correct, safe, and beyond improvement, why is knob and tube illegal for new installations? I'm not looking for an answer to that question, I'm trying to point out that the code members are trying to limit 'undersized' residential feeders. If all residential feeders were intended to use 310-15(b)(6), why not just use the term feeder? Why bother creating and defining the term main power feeder? Were they bored on a Saturday night? Is this a government conspiracy to make me go mad? Did they see what is under your bed and go insane?

Anyway, I tried to use your link, but it wasn't working. Is it correct? I would like to see what the powers that be have to say on the subject. Thanks.

Enjoy your day!

 
  #31  
Old 05-13-01, 02:45 PM
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Exclamation

s1nuber;
sorry 'bout the link, search up the NFPA, find the ROP's and ROC's.
there are those who seek to clarify the issue in 2002
( as they probably did for the 99')

while we are on the subject, would 310-15(b)have a bearing on this? or is this once again an NEC definitional deficentcy ?

Sorry, I cannot expand on NFG, it is an expletive deletive, the moderator of this forum would lock me in my room for good !
 
  #32  
Old 05-14-01, 09:20 AM
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Took me a second, but NFG did come to me. It means No Flippin' Good.

S1Nuber, FYI knob & Tube wiring is actually still permitted. (See 324-3 - "Uses Permitted")

Wirenuts has obviously demonstrated that he's a REAL MAN! Not afraid that there's monsters under the bed!

Which reminds me of a joke:

Q: How many REAL MEN does it take to screw in a light bulb?

A: NONE! Real men aren't afraid of the dark!

Juice
 
  #33  
Old 05-15-01, 09:25 AM
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Unhappy

I'm with Wirenuts on this one... My supplier only carries #4/0 and #2 SER Al...?!?!? Hmmmmm...

Should we bring this up in the Electrical Contractor's Network Forums?

[Edited by sparky66wv on 05-15-01 at 01:51]
 
  #34  
Old 05-15-01, 02:12 PM
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Virgil;
sure, i can be just as confused anywhere in cyberspace...
 
  #35  
Old 05-15-01, 05:05 PM
s1nuber
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Juicehead - You can only install new knob and tube by special permission or connected to existing knob and tube, not for new construction or new installations. The NEC is 'phasing out' knob and tube as better and safer installation practices are available. I think we agree, but I just wasn't clear enough in my post.

Sparky66 - This same topic has presented itself on Mike Holt's board. On that board I would say it is about 50% for any feeder using 310-15(b)(6), on this board it is about 80% for it. Try it on your site, but I think wirenuts is right, cyberspace is cyberspace, and we are not going to find the end-all answer on the net. If your suppliers are like mine, making a code judgement on what they have in stock would be a grand mistake. I have to special order pvc expansion joints to make transitions from u/g to meter housings for one example.

Wirenuts - I did do some surfin and found what you were linkin' to. When I read the updates, I am even more convinced that the code panel intends to restrict residential feeder 'undersizing' to only main power feeders. The concern appears to be load diversity as expressed in the update about the detached garage (which was rejected) as the feeder would not carry the entire residential load, and not be diverse enough to prevent overheating/overloading. I think WG had posted something along these lines last time we danced on this particular issue, so maybe he'll chime in.

Anyway, I gotta clean the garage, or kill a spider or somethin.

Enjoy your day!

 
  #36  
Old 05-15-01, 06:10 PM
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s1nuber;
I'm glad you could surf that up , sorry i'm not that good at posting links here!...
Whenever i see these types of code "grey areas' or 'interpational debates' i like to visit the ROP's & ROC's, see where it's going...
Even better is having a poster know where it's been.
( Like some major leagers posting here seem to)

For whatever side is advocated, clarity is not 100% to me, my NEC is dogeared from this thread.....

 
  #37  
Old 05-16-01, 05:15 PM
Wgoodrich
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S1NUBER

The sub panel he is talking about is a panelboard. Therefore that panelboard is a general lighting and appliance panelboard if 10% of the breakers protect general lighting circuits. If that sub panel is indeed a general lighting panelboard then 310-15-B-6 applies because in the opening statement of applicability it states to include feeders supplying panelboards that are used as general lighting panelboards. I AGREE WITH resqcapt19 THAT 310-15-B-6 DOES APPLY to the feeder in this post. Please see below for confirmation;

384-14. Classification of Panelboards

(a) Lighting and Appliance Branch-Circuit Panelboard. A lighting and appliance branch-circuit panelboard is one having more than 10 percent of its overcurrent devices protecting lighting and appliance branch circuits. A lighting and appliance branch circuit is a branch circuit that has a connection to the neutral of the panelboard and that has overcurrent protection of 30 amperes or less in one or more conductors.

310-15-b-6

For application of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and appliance branch-circuit panelboard(s)



Please note that the sentence above pertaining to 310-15-B-6 does indeed include feeders to general lighting and appliance panelboards and in that statement it does not limit this rule to be used as the main power [service entrance] conductors as you interpret. It specifically states between the main disconnect and the lighting and appliance branch circuit panelboards. It does not anywhere say main service rated panel.

My opinion that you asked for.

Sorry about being gone, WENT FISHING WAY DOWN SOUTH AT GUNTERSVILLE, Al. GOING BACK AS SOON AS I CAN TOO.

You guys did fine without me. Makes me feel not needed huh. I am sure you missed my opinions so freely provided in great numbers though whether right or wrong. Just like to talk a lot.

Good Luck

Wg
 
  #38  
Old 05-16-01, 05:33 PM
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wg;
good post, you just keep talkin' and we'll keep learnin'!

FYI, there are some interesting ROP's. it seems that this has been an ongiong concern.

a few inparticular;
( about pg 65)
Log # 4200, read the substantiation, it would seem the CMP is almost venting it's frustration here.....claiming the "99 wording schizophrenic"

Log# 3908 , moves to exempt 336-26 in 310-15(b)(6)
http://www.nfpa.org/Codes/National_E...f/A280-310.PDF

%$&##!! I wish i could learn to post an url

 
  #39  
Old 05-16-01, 08:28 PM
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Smile

Wirenuts,

You remind me of a guy named Sparky, ...
Hmm, or is it ..

All you have to do to copy a url is to right click up in the Address box and select 'copy' then you 'paste' into the Forum.

The correct url you were trying to post before is:

http://www.electrical-contractor.net...2_NEC_ROPs.htm

(You had an apostrophe in ROP's)

Bill
 
  #40  
Old 05-16-01, 10:29 PM
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Question

It seems to me that 310-15(b)(6) states "...120/240-volt, 3-wire, single phase..." does that eliminate SER or does the equipment grounding conductor count as a "wire"? Am I splitting hairs here?

 
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