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#190137 - 06/24/05 08:14 PM Wire size for 100 amp load center
permda Offline
newbie

Registered: 11/27/04
Posts: 15
What size wire should I use to connect a 100amp load center to the existing breaker box? Thanks!

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#190138 - 06/24/05 08:56 PM Re: Wire size for 100 amp load center
ront02769 Offline
Carpal Tunnel

Registered: 03/03/04
Posts: 10118
Loc: New England
what size breaker will it be connected to in the main box? that will determine wire requirements. the size of the load center is actually immaterial in this setting.

ront

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#190139 - 06/24/05 10:35 PM Re: Wire size for 100 amp load center
MaintenanceRoger Offline
member

Registered: 12/25/04
Posts: 189
Loc: Kansas
I think we need to explore what you are wanting to operate from this 100 amp sub-panel. Can you tell us what you need this sub-panel to supply power to? This is very critical in determining if you need to have 100 amp capability or not. If you will only need 60 amp capability then costs are much cheaper. The fact that it is a 100 amp load center does not mean you must run 100 amp feeders. The other thing to consider is demand load on the main panel from which you are going to feed this sub-panel. So if you are going to run a arc welder or something with a big current draw out of that sub-panel then you really need to make sure your main panel has the spare load capability to support it.

Sorry Handyman meant to reply to poster


Edited by MaintenanceRoger (06/24/05 10:40 PM)
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#190140 - 06/24/05 11:36 PM Re: Wire size for 100 amp load center
dora Offline
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To be on the safe side, if the load center has a 100 amp breaker you can install # 2 wire for both hot (black) conductors and the neutral (white) conductor. Then install a # 4 cable for the ground.
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#190141 - 06/25/05 11:15 AM Re: Wire size for 100 amp load center
permda Offline
newbie

Registered: 11/27/04
Posts: 15
This will be for a basement. The original breaker box is full. There will be lights, TV, stereo, small fridge and chest freezer. There will also be a pump for the bathroom waste. I would think the 60 amp breaker in the main panel would be enough. What do you think?

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#190142 - 06/25/05 12:18 PM Re: Wire size for 100 amp load center
MaintenanceRoger Offline
member

Registered: 12/25/04
Posts: 189
Loc: Kansas
Yep I would say 60 amps would do well. I would dedicate one circuit to the sewage pump. I am assuming here that you have an MLO panel (main lug only). If your not using conduit my first choice would be to run SER copper cable #4/3 awg with ground or 2nd choice SER #3/3 awg aluminum with ground cable. At the sub-panel make sure that the neutral bar and ground bar are kept seperate and not bonded to each other. You may have to purchase a grounding bar kit for your sub-panel in order to comply with this arrangement. If you run more than six circuits from that sub-panel I believe you must provide a disconnecting means at the sub-panel itself. If it is a main breaker type panel and you already have that 100 amp breaker installed then that will serve as the disconnnecting means.
Also it would be a good idea to have someone do a demand load calculation on the main panel to insure you have the capacity to serve this new sub-panel.


Edited by MaintenanceRoger (06/25/05 12:22 PM)
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#190143 - 06/25/05 07:12 PM Re: Wire size for 100 amp load center
monocline Offline
Handyman

Registered: 02/06/03
Posts: 656
My understanding is that the six switch rule is for the service or feeder disconnecting means where the conductors enter a building (service entrance). In addition to the service disconnect, each panelboard (for lighting and appliances) is required to have overcurrent protection by not more than two breakers or fuses. This overcurrent protection device (OCPD) cannot exceed the rating of the panelboard and in most cases the OCPD cannot exceed the ampacity of the conductors it protects.

The main breaker in the main panel usually serves as both.

Since this is sub panel in the same structure as the main, the service entrance disconnect should already be in place where the service enters the building.

Since this sub panel will be fed from a breaker (OCPD) in the main panel, the sub panel has overcurrent protection.

6 AWG copper should be big enough for 60 amps.

I am not an electrician, verify everything with your inspector.

Bryan

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#190144 - 06/25/05 11:18 PM Re: Wire size for 100 amp load center
MaintenanceRoger Offline
member

Registered: 12/25/04
Posts: 189
Loc: Kansas
"6 AWG copper should be big enough for 60 amps"

My understanding is since we dont know the temperature ratings of the terminations on the supply end and load end and also we are dealing with 100 amps or less we must use the sixty degree column for ampacity making 6 awg good for 55 amps not 60. By using a copper SER cable 4-4-4-6 we cover several bases for just a few pennies more. I would agree however that dropping down to a 50 amp breaker and SER 6 awg copper cable would probably serve his needs.

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#190145 - 06/26/05 06:28 AM Re: Wire size for 100 amp load center
SpeedyPetey Offline
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Registered: 12/20/03
Posts: 5393
Loc: NY State
Roger,

55 amps is the ampacity of #6cu at the lowest temperature rating. Since no 55 amp breaker is made it is code compliant to use a 60.

Unless we are talking 200'+ of feeder length #4 really would be unnecessary overkill.
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#190146 - 06/26/05 10:05 AM Re: Wire size for 100 amp load center
MaintenanceRoger Offline
member

Registered: 12/25/04
Posts: 189
Loc: Kansas
Yes Speedy, that is correct. I had forgotten that detail that you move to the next standard breaker. Now let me ask you this....would the #4 SER be good for 100 amps if we used table 310.15(b)6? That would make the feeder cable capable of 100 amps equal to the rating of the sub-panel. I understand this is an AHJ call and I'm not sure how common it is approved. Just thinking if you used #4 SER then you could still have the 60 amp and be good for 100 amp if your AHJ allowed 310.15(b)6.
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#190147 - 06/26/05 10:34 AM Re: Wire size for 100 amp load center
SpeedyPetey Offline
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Registered: 12/20/03
Posts: 5393
Loc: NY State
Yes, that is the way we do it in my area. We do use that table.
For instance if we run a 100 amp sub-panel from a 200 amp service we use #2al SER.
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#190148 - 06/26/05 11:01 AM Re: Wire size for 100 amp load center
MaintenanceRoger Offline
member

Registered: 12/25/04
Posts: 189
Loc: Kansas
Ok, thanks Speedy and of course I was refering to copper SER cable but the distinction does need to be made.
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#190149 - 06/26/05 02:07 PM Re: Wire size for 100 amp load center
monocline Offline
Handyman

Registered: 02/06/03
Posts: 656
permda,

Check with your AHJ before using table 310.15(B)(6) for this sub panel. While some inspectors will allow it, the overwhelming majority of the opinions I have read agree that table 310.15(B)(6) would not apply to a sub panel such as this.

Section 310.15(B)(6) covers service conductors and feeder conductors that serve as the main power feeder TO A dwelling unit.

Is this basement a separate dwelling unit?

Bryan

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#190150 - 06/26/05 02:42 PM Re: Wire size for 100 amp load center
SpeedyPetey Offline
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Registered: 12/20/03
Posts: 5393
Loc: NY State
Which is why I said "in my area".
This has been hashed out many times on these forums. We can cut & paste and quote code all day. It will not change how anyone feels.

I would also disagree that it is "overwhelming" in favor of not allowing it. IMO it is about even. Luckily my area has the common sense to allow it.
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#190151 - 06/26/05 03:45 PM Re: Wire size for 100 amp load center
reorange Offline
Super Handyman

Registered: 02/19/05
Posts: 2016
Loc: CENTRAL NEW YORK
#2 AL SER is always accepted by all of the inspection agencies in this area for a residential 100 amp feeder to a subpanel.

None of the electrical supply houses around here even carry #1 or 1/0 AL SER. Next size up is 2/0 which is accepted for 150 amp.




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#190152 - 06/26/05 06:05 PM Re: Wire size for 100 amp load center
sarabellum Offline


Registered: 08/07/04
Posts: 306
Hi. Permda
General advice given over the internet to DIY,
Should prudently follow the NEC very closely.
How things are done ‘locally’ is often of little help to DIY given that it may add to the confusion level.
DIY are always advised to learn from the NEC advice ,but consult with local rules.

Load centers can be protected by breakers of any rating that is equal to or less than the [load center] panel rating,and the wire feeder size choosen accordingly.

Makers of load centers do not make a unique size for each rating imaginable for every wire and breaker size.

The new load center ampacity has to be determined by the demand load needs ,but also from a very practical limitation of the existing service size.
You cannot purchase a $200 item when you only have $50 in your pocket.

If your main panel available ampacity will stand the added load of the new load center
,then that defines your upper limit possible,tho not necessarily likely needed .

For simple basic feeders ,wire sizes for feeders are mainly dependent on whether cable or conduit is the choosen route.
For safety reasons from NEC experience…Cable ampacity is generally a bit more restrictive for the same wire size ran in the more protective conduit.

The ampacity for circuits ran via cable give you the least amps per wire $$$ .
Old load center and breaker terminals also had to follow this same least amps per $$$ as if all wiring were in cable,and is due to the temperature rating of the old type terminals.

New breakers and load centers have teminals rated a knotch higher than this ,so if you are running the most common feeders at 100amps and less ..or..1gage and lower building wire etc ,you now get more amp bang for your buck
Always verify the small print tho!
A casual trip through your local DIY store will echo this…so see for yourself if the newer breakers and panel lugs etc are rated 70ºC on the tiny piece of paper! THEY ARE

Service feeder sizes ..[not sub-panel] for dwellings [not any commercial] are usually permitted a size smaller than what you find in most ampacity tables used by DIY and hardware stores etc.
This is in the NEC at T310.15.B.6 and is most often only advised or permitted as the service feeder for single family homes etc….sub feeders in apartment houses…sub feeder to mobile homes ….and also sub feeders from a central location on rural area farms and the like.
If your application is beyond this …’main power feeder’ then most definitely DONNOT follow any advice to the contrary..no matter where it comes from.
Install the larger size wiring!
Always consult with your local code authority.
All wiring tables also have to be adjusted for temperature and bundling and length of run

These feeders are generally either in open air or otherwise do not travel within a structure where they may add to any safety issue.

There are many local codes and PoCo requirements that DONOT allow or otherwise permit you to take advantage of this one size smaller service feeder size and the associated cost savings.
The rule is NOT ‘one size smaller’ ..it just works out that way.
See Table 310.15.B.6
A visit to some of the pro electrical sites will veirfy this.

……………………………………………………………………………………..

It is customary, in most places to run 6gage copper for 60amp feeders whether via cable or conduit/THHN wiring.
60amp feeders while 6 gage copper ,only need a 10gage copper ground.
Number 6 and smaller wires cannot be remarked for use as neutrals and grounds.
DIY are usually advised to avoid aluminum wiring when possible due to the various problems and issues…tho circuits 30amps and especially the larger feeders are not an issue when installed properly and terminated where aluminum is permitted.

NOTE:
Although 6gage cable is rated only55amps,we commonly install the ‘’next size-up’’ standard breaker,..[60amps]…since there is no 55amp standard breaker…40-45-50-60-70 etc
…………………………………………………………………………….
I also agree with monoclines experience…
‘’While some inspectors will allow it, the overwhelming majority of the opinions I have read agree that table 310.15(B)(6) would not apply to a sub panel such as this’’.

The NEC is quite clear on this issue in 310.15

especially in 310.15.B.6 prior to the Table 310.15.B.6,where it states that ..
"for dwelling units,conductors as listed in T310.15.B.6 shall be permitted as 120/240v 3 wire ,single phase service entrance conductors,service lateral conductors ,and feeder conductors THAT SERVE THE MAIN POWER FEEDER to a dwelling unit,....

...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)..."


This is stated very well and does not include or allow any sub feeders ,to benefit from this rule...and the intent of the NEC is also obvious to limit the application of this reduced service feeder rule.

There are those who love to test the limit of the rules ,even tho their own resources and experience does not equal that of those who write these codes!
…………………………………………………………………………………

The six breaker rule [aka six movements of the hand…] is an old rule that still applies in new installations for ,so-called sub-panels,where a main breaker disco is required.,and if no more than 6 circuits… no single disco is required.
This pops up when you run a small feeder to a sub panel in an out building separate from the main structure.
Over 6 circuits..you need some form of disconnect..commonly a main breaker installed in the remote sub panel.

NOTE: Sub panels that are still in the main structure,will not need any main breaker ..or even a disco ..other than the breaker that protects the feeder!
Remember you MUST run 4 wires and isolate the neutral from ground!
I always advise 4wire regargless of inside or outside to a second building.

New installed lighting and appliance Panel boards used as service equipment ,in residential ,are NOT permitted to use the 6 breaker rule…tho older EXISTING ones can stay as is.
……………………………………………………………………………………………

I hope this helps you to realize that a lot of the opinions that state …”THIS IS THE WAY WE DO IT HERE…” can be misleading and a cause for confusion although it helps the poster to support their own argument.

Follow the NEC and your local AHJ…[authority having jurisdiction]..{aka inspectors!

sara

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#190153 - 06/26/05 06:21 PM Re: Wire size for 100 amp load center
SpeedyPetey Offline
Carpal Tunnel

Registered: 12/20/03
Posts: 5393
Loc: NY State
I had a feeeling you'd be around to disaggree with most and aggree with your old buddy.

First off, this is not a cut and dry issue that some small local AHJs decided to disregard. This is a contentious issue, with strong arguments for both sides. There is contrary wording to allow or disallow this sections use in this case. Even your friend monocline has stated the fact that the wording could be taken either way.
There IS NO widely agreed upon decision. So don't play the fact that your statements are the only ones which are correct.



In reply to:

I also agree with monoclines experience…



It is not his experience, as he says, he read it on the internet. He's not an electrician, remember? Or at least that is what he reminds us in almost every post.
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#190154 - 06/26/05 09:15 PM Re: Wire size for 100 amp load center
MaintenanceRoger Offline
member

Registered: 12/25/04
Posts: 189
Loc: Kansas
Well I suppose the mistake that started all this was I should have opened a new thread to discuss T310.15(b)6. My questions were directed to Speedy and not the poster. Who is probably confused as heck after all this. Anyway I dont see how "permda" would have confused that discussion with his project.
As for the correct NEC approach I believe he was made aware of the demand load concern for the main panel. The neutral and gound in the sub-panel concern was discussed. Even the possibility that a ground bar kit may be needed. Even panel types were discussed. I advised the use of an SER type cable because it is IMO the most common cable used by pro's for a non-conduit application to feed a sub-panel. The home centers give you an NM-b cable. As for trying to go into temperature termination requirements with a DIYer I never do. 90% of the time I qoute the 60 degee column. You may be surprised how many electricians I meet on the job that dont have a clear understanding of that issue. I readily admit I errored on the breaker issue and wire size to meet NEC minimums for 60 amps. But if he goes ahead and installs #4 SER he gives himself a little room for the future and he wouldnt be in violation of any code. Mentioning the six disconnect rule was actually an after thought and was posed as an "i'm not sure" bit of information.
The big issue here appears to be T310.15(b)6. I dont believe any body advised Permda to apply this table to his project. This was just a question I asked because we use it here in my counties juristiction when feeding sub-panels. I was posing a question to see if they applied this table as we do where Speedy is located. It really IMO is logical if your using the right cable. SER (service entrance rated round type) is compliant with T310.15(b)6. Of course you need AHJ approval to use it for sub-panel applications.
I see no reason for this self serving long monologue in response to a discussion I was having with this other professional and not the poster.


Edited by MaintenanceRoger (06/26/05 11:15 PM)
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#190155 - 06/28/05 02:01 AM Re: Wire size for 100 amp load center
monocline Offline
Handyman

Registered: 02/06/03
Posts: 656
Speedy,

In reply to:

He's not an electrician, remember? Or at least that is what he reminds us in almost every post.




I mention that I’m not an electrician to make it clear that I’m not qualified to advise or instruct in this topic. If I do offer advice I try to keep very general in nature, such as “check with your AJH”, the intent of my posts is to ask questions or offer comments that might aid in clarification, or just for general discussion.




In reply to:

Even your friend monocline has stated the fact that the wording could be taken either way.




I never said that, I merely expressed how I was confused by some of the wording in that section. But, the main reason for my confusion was your arguments. When I have trouble understanding something I will research it. So, I asked, and read, a number of sources and section 310.15(B)(6) seems clear to me now.




In reply to:

It is not his experience,…




While I did not use that term, it is certainly an appropriate one to describe my actions and findings.




In reply to:

We can cut & paste and quote code all day.




The wording in my posts to this thread is mine, I have not copied anything.


Bryan
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#190156 - 06/28/05 04:54 AM Re: Wire size for 100 amp load center
SpeedyPetey Offline
Carpal Tunnel

Registered: 12/20/03
Posts: 5393
Loc: NY State
Posted by monocline, 04/15/05 04:08 AM, regarding the exact same topic:

In reply to:

Thanks speedy,

I guess I wasn’t sure how to interpret the intent of this section because, as reorange said, there are different opinions on this subject, and a couple of points in the section seem inconsistent.

For example, the title sentence “120/240 Volt, 3 wire, Single Phase Dwelling Services and Feeders.” And, the next sentence both seem to imply that it only applies to 3 wire feeds. It then goes on to specify (for feeders) that the feeders serve as the main power feeder to a dwelling unit. (Which would not apply to a sub panel in the same structure.)

But then it says, the feeder can be with or without an EGC. So, now it applies to 3 or 4 wire feeds (not just to 3 wire as the first two sentences seem to imply).
Then it says, “for application of this section” and gives a definition for a feeder, which is consistent with the definition in article 100, but not with the sentence just prior which specifies the feeder serve as the main power feeder to a dwelling.

No wonder there is disagreement,

Bryan

BTW, the AHJ in my area takes the same interpretation as you mentioned.





I guess the inspectors in your area are not part of that "overwhelming majority" mentioned earlier.

This is why I didn't want to string this topic along again and that this has been hashed out many times and it will not change anyone's mind. I guess you and DenisB (sarabellum) feel differently.
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#190157 - 06/28/05 11:11 AM Re: Wire size for 100 amp load center
monocline Offline
Handyman

Registered: 02/06/03
Posts: 656
Petey,

Yours:
In reply to:

You'll get some to say the #2 should not be on a 100 amp breaker. This is due to the ampacity charts used to determine breaker size for branch circuits. I disagree. This cable is considered a feeder and I use the ampacity charts for feeders ( 310.15(B)(6) ).





Mine:
In reply to:

I have a question on interpreting section 310.15(B)(6).

Do feeders covered in this section have to serve as the main power feeder to a dwelling unit?

Would this feeder from the main to a “sub” fit this description?





Yours:
In reply to:

Look at the definition of "feeder".
Verbatim it says:
"The conductors between the service equipment and the FINAL branch-circuit overcurrent device".
Also is says these the main power feeders are the feeders between the main disconnect and the lighting and appliance branch circuit panelboard(s).


Sounds like the cable feeding a sub-panel to me.
Wouldn't a sub-panel likely contain the final overcurrent devices?





If you read my post prior to the one you displayed, you’ll notice it was your statement that led me to question my interpretation of 310.15(B)(6). So, I called my AHJ and was told that table 310.15(B)(6) could be used for any sub panel.
I mistakenly took that to mean he interpreted that section to allow it. In a later conversation with the AJH, I learned that he did not interpret the NEC to permit table 310.15(B)(6) for any sub panel feeder, but based on his assessment of the loads served by the panel, he might. I failed to revisit that thread to make that clarification.

Bryan
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#190158 - 06/28/05 11:32 AM Re: Wire size for 100 amp load center
SpeedyPetey Offline
Carpal Tunnel

Registered: 12/20/03
Posts: 5393
Loc: NY State
In reply to:

So, I called my AHJ and was told that table 310.15(B)(6) could be used for any sub panel.



In reply to:

In a later conversation with the AJH, I learned that he did not interpret the NEC to permit table 310.15(B)(6) for any sub panel feeder,




So which one is it??? Sounds like someone who can't make up their mind.


This is taking up too much of this thread with the OP probably frustrated by now. Sorry about that.
I am bowing out and agreeing to disagree.
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#190159 - 06/28/05 12:51 PM Re: Wire size for 100 amp load center
monocline Offline
Handyman

Registered: 02/06/03
Posts: 656
Petey,

If you take sentences in their entirety you might understand more.

Bryan

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#190160 - 06/28/05 02:25 PM Re: Wire size for 100 amp load center
SpeedyPetey Offline
Carpal Tunnel

Registered: 12/20/03
Posts: 5393
Loc: NY State
I can understand sentences perfectly. I can even figure out paragraphs. I can surmise that the last one you wrote is completely contradictory.
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