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100 Amp Detached Building Sub Panel Wire Gauge?

BentBierz

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Hello,

Very sorry for the long post and will probably lose a lot of readers a quarter of the way through...maybe after this opening sentence! ;)

This is for the electricians on the forum. I pretend to be one as I have done a lot of electrical work (both in the CG on boats and structures and in my own homes).

BLUF (As they say in the Coast Guard...Bottom Line UpFront): New 100 Amp sub panels installed (with 100 amp breakers at main panel) in each of two detached buildings (one building approx. 50' run from main panel, the other approx. 90' away). Electrician installed the following:

- #2 aluminum wire for the hots
- #4 copper wire for the neutral
- #4 aluminum wire for the ground

At a minimum, I believe the hots are at least one size too small, maybe two sizes for the 90' run.


My story...our electrician was sub'd out by the general contractor (GC) who was hired to erect a new steel building from clearing the land, roughing in plumbing, pouring the slab and running 100 amp sub panels to the new building as well as to an existing steel building...both buildings are non-commercial.

My problems began when I was running new branch circuits inside the new building well after the final check had been written to the GC. We had a BBQ several weekends ago and an electrician friend of one of my buddies was invited over. With a beer in hand and the smell of brisket coming out of the smoker, the electrician said, "Hey, lets go take a look at what your doing in there." He headed for the open sub panel and took one look and said that the panel was wired wrong i.e. it was 3-wire instead of 4-wire. Thus began 3 nights of internet research trying to determine if he was right. What I found was that since the writing of the 2008 NEC 4-wire is now code for detached sub panels; 3-wire is still allowed in existing buildings.

I had three different email exchanges back-and-forth with the GC who was playing middle man between me and the electrician. In each of my emails I provided him with multiple sites as well as pictures/diagrams from reputable electrical sources all showing that 4-wire should have been installed. He contacted the electrician who responded back that "the customer is confused; the 3-wire with neutrals and grounds bonded together and with a ground rod he installed was actually safer than a 4-wire system." As I had provided him with so much "evidence" and all I got back, twice, was that I was confused I decided to contact our county (as well as the city that is only a mile from our property) to get their take. BTW...out in the country so we were not required to pull permits or get inspections...HUGE regret at this point for making that decision. Both the city and county came back and said 4-wire was required. I forwarded the emails on to the GC and also said that at this point I had no expectation that the electrician was going to budge and that I would be filing a complaint against his license. Miracle of all miracles, the next day I get an email from the GC asking when the electrician could come out and "make it right."

The electrician & helper showed up yesterday and pulled all of the existing wire out of the PVC conduit. They added the fourth wire (#4 copper neutral -- assuming they used copper because it would be easier to pull than equivalent rated aluminum) and pulled the wires back through the conduit to each building and hooked everything up including adding a ground bar to each sub panel to separate the neutrals and grounds.

Unfortunately, I just had knee replacement surgery and was at PT when they came to do the work. By the time I got back they were already heading off the property. I stopped them and they explained what they had done and headed out. Later on that day I decided to go out to one of the buildings to pull a panel cover off and see what they had done. What I saw was three different physical sizes of wire but none of the wires had visible markings on them in the short lengths I could see. I sent the GC an email and said, "Hey, for my records what size wire did the electricians run?" His reply, "#2/#4/#4--all 100amp wire".

i have found NO sites, calculators, charts, etc. online that says these wire sizes are sufficient for 100 amp panels...certainly not the hots. So, very long story, short...is there something I am missing here or does the GC need to sub out more competent electricians and come do my job right.
 
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terabitdan

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What size are the breakers in the main panel for these feeds? 2-2-4-4 AL is good for 90 amps.


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mm08822

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It has to be #2 copper if 100a cb in main panel. If alum needs to be 90a.
What was spec'd out?
Do you also have two grd rods connected to ground bar?
A pic of both panels with cover off would be good to post. Try and get wire size and insulation type on conductors.
 

wyliesdiesels

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I havent read it all will read the rest later.

To give u a quick answer: if the contract was for 100a, then the contract wasnt fulfilled since #2 al is limited to 90a in that application.

@90' voltage drop isnt an issue so no need for larger conductors due to distance.

And yes the 3-wire feeder is completely wrong.
 
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BentBierz

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It has to be #2 copper if 100a cb in main panel. If alum needs to be 90a.
What was spec'd out?
Do you also have two grd rods connected to ground bar?
A pic of both panels with cover off would be good to post. Try and get wire size and insulation type on conductors.

The job was spec'd for 100 amp service to each building.

There is only one ground rod installed per panel...that is something else I had found during my research i.e. without testing, it is highly recommended that 2 ground rods at least six feet apart should be installed.
 
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BentBierz

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I havent read it all will read the rest later.

To give u a quick answer: if the contract was for 100a, then the contract wasnt fulfilled since #2 al is limited to 90a in that application.

@90' voltage drop isnt an issue so no need for larger conductors due to distance.

And yes the 3-wire feeder is completely wrong.

Thanks...wasn't too certain about the 90' so glad to hear the problem isn't worse for that building.
 

terabitdan

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Is the ground rod embedded in the foundation or driven into the ground? If it's in the foundation that's a Ufer ground and you should be fine, if not, they typically have to prove a low enough resistance to ground, IIRC 25 ohms, for it to be approved.

In the original quote did they specify the wiring size? 100 amp service and 100 amp panel could be interpreted as two different things in the specifications. Either way they installed is wrong.

If it were me I'd tell the electrician to either do it to full code compliance for 100 amps, which means upgraded wiring, fixing the ground rod or refund your money so you can hire someone who will.


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justsam

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A reasoned solution may be to just change out the panel breakers feeding the subs to 90 Amp as opposed to 100. Of course if you contracted for 100 Amp service you have every right to insist on it if you feel so inclined. The statement from your GC about 2/2/4/4 being 100 Amp wire would be correct if it were Cu, which he may not have known, the electrician ran Al.

Have you installed breakers and or receptacles yet? You must have either GFCI breakers or head end receptacles in a garage.
 
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BentBierz

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Is the ground rod embedded in the foundation or driven into the ground? If it's in the foundation that's a Ufer ground and you should be fine, if not, they typically have to prove a low enough resistance to ground, IIRC 25 ohms, for it to be approved.

In the original quote did they specify the wiring size? 100 amp service and 100 amp panel could be interpreted as two different things in the specifications. Either way they installed is wrong.

If it were me I'd tell the electrician to either do it to full code compliance for 100 amps, which means upgraded wiring, fixing the ground rod or refund your money so you can hire someone who will.


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Thanks for the comments. The ground rod is outside the building driven into the earth and not part of the concrete foundation. No wiring size was specified in the initial contract.

This is definitely a case of me knowing a hell of a lot more about distribution panels, sub panels, amp ratings, 3-wire vs. 4-wire, bonding neutrals & grounds, etc. than I knew just a few short months ago. In other words, back when we contracted to have this building erected I didn't really know what I didn't know.
 
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BentBierz

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A reasoned solution may be to just change out the panel breakers feeding the subs to 90 Amp as opposed to 100. Of course if you contracted for 100 Amp service you have every right to insist on it if you feel so inclined. The statement from your GC about 2/2/4/4 being 100 Amp wire would be correct if it were Cu, which he may not have known, the electrician ran Al.

Have you installed breakers and or receptacles yet? You must have either GFCI breakers or head end receptacles in a garage.

The reality is that in this building I will never load up the panel whether it was 100 amps or 90 amps. I used the following Mike Holt spreadsheet (http://www.mikeholt.com/documents/calculations/formulas/ResidentialLoadCalculations.xls) and loaded it up with stuff I will never have in the building e.g. laundry, electric stove, huge A/C, three refrigerators (ok...I will definitely have at least 2 fridges with one of them being a True glass front commercial beer fridge), etc. and I never got over 80 amps (and, of course, it would never all be running at the same time). In my older building at some point I plan to put in some 240V equipement e.g. table saw, band saw, air compressor, etc. but the majority of the time I would be working by myself with only so many pieces of machinery running at one time.

My issue at this point...the principal of the matter!!!
 

Norcal

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According to the Mike Holt site Texas is on the 2014 NEC, ask the installer to show you in the NEC where any of the wiring he installed is rated for 100A.
 

alfredeneuman

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He's treating the outbuilding as if it's a SERVICE from the utility, which it isn't. It's a FEEDER. :willy_nil :eyecrazy:

There's an exception in the Code where #2AL can be used for a 100amp SERVICE, but that doesn't apply in this case.

(Only 3 wires are run from the utility to the customer with a SERVICE)
 

justsam

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I fully understand the notion of principal! You want what you specified, and paid for! It is even more frustrating when the electrician is the supposed professional and you wind up educating him, at no charge.

I am also of the school where reasonable people come up with reasonable solutions to issues, since you really do not appear to need the full 100 Amps. Perhaps something like determine the Time and Materials to make it right, and ask for half that amount to be kicked back. I personally would try to avoid having this hack professional come back to my property.
 

justsam

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Speedy Petey

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That is an interesting outcome! Sounds to me like that electrician under that AHJ will continue to install subpanels that are non-compliant with NEC given the same circumstances. At least the OP in that case solved it all by going to 90 Amp breakers, it appears at his expense.
It is STILL very common to see. I see it all the time.

IMO this is one of most ******* stupid parts of the code. There is ZERO justification for allowing #2AL for a 100A service to a whole dwelling, but not to a 100A sub-panel in/around a dwelling.
If anyone has a logical explanation, I'm all ears.
 

wyliesdiesels

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It is STILL very common to see. I see it all the time.

IMO this is one of most ******* stupid parts of the code. There is ZERO justification for allowing #2AL for a 100A service to a whole dwelling, but not to a 100A sub-panel in/around a dwelling.
If anyone has a logical explanation, I'm all ears.

Its not just #2 al if u look at table 310.15(b)(7) (which was removed). #4 cu is fine for 100a to dwelling but not a feeder or non dwelling building.

Same with 200a and 2/0 cu or 4/0.
 

wyliesdiesels

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OK now that Ive read the whole thing it sounds like he still ran 3 wires.

Did he unbond the neutral bars?

Can u take some pics of panel with covers off and post them here. It would be easier than trying to understand a description.
 

AntonLargiader

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I wonder if there's a disconnect in what people think of, and speak of, as 100A service. I understand specifications... I'm talking more like saying 110V whether it's 110 or 120.

90A seems to be a very common way to wire 100A panels because it's a sweet spot cost-wise. So I'm thinking of a guy asking for a 100A panel somewhere, and the response can go a few different ways.

1) "Sure. That's $4/foot for the cable; we can do that."

2) "Sure. It's actually 90A and not 100, and it's $1.75 per foot. Lemme know if you really need 100."

I bet a lot of people would be fine with 90 if it were presented that way up front. Seems like these two recent cases would be. If it's being asked for as 100A just because it's a round number, or because panels are sold as 100 rather than as 90, a little bit of education would go a long way. OP; what was behind your request for 100 specifically?

No one who's new to this is going to ask for 90A service. The GC may not even realize the difference, and he is between the electrician and the customer.

At least the OP in that case solved it all by going to 90 Amp breakers, it appears at his expense.

$30~60 for the breaker FWIW.
 
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BentBierz

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He's treating the outbuilding as if it's a SERVICE from the utility, which it isn't. It's a FEEDER. :willy_nil :eyecrazy:

There's an exception in the Code where #2AL can be used for a 100amp SERVICE, but that doesn't apply in this case.

(Only 3 wires are run from the utility to the customer with a SERVICE)

I completely agree with you! I found an electrician forum discussion yesterday morning from a few years ago. What appeared to be very seasoned, knowlegable and articulate electricians debated your very point with some of them saying there are many cities/counties/etc. (don't remember if an entire state) that allow the use of NEC 310.15(B)(7) for branch circuits instead of requiring the use of NEC 310.15(B)(16). There was one electrican with thousands of forum posts who said he would run #2 aluminum to a 100 amp sub panel all day long and not look back. He didn't specifiy what his locality required. Several pages of responses after his didn't take him to task for his comment!?!
 
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BentBierz

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I fully understand the notion of principal! You want what you specified, and paid for! It is even more frustrating when the electrician is the supposed professional and you wind up educating him, at no charge.

I am also of the school where reasonable people come up with reasonable solutions to issues, since you really do not appear to need the full 100 Amps. Perhaps something like determine the Time and Materials to make it right, and ask for half that amount to be kicked back. I personally would try to avoid having this hack professional come back to my property.

Yeah...my wife asked if I felt the installation was safe and I told her yes but not to code unless I changed out the breakers to 90 amp. She didn't skip a beat and said, "change out the damn breakers and move on...I don't want that a$$h*le back here again.!" Gotta love a military wife! ;)
 
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BentBierz

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Same thing just happened to someone else; read that thread for some perspective:

http://www.garagejournal.com/forum/showthread.php?t=355330

Just read through that thread...not real certain why it didn't show up when I put in my search words. At any rate, same situation as me. The only difference, whether or not the electrician knew what he was talking about as debated on the thread, is that he at least seemed willing to work with the customer. My electrician just kept insisting that I was confused i.e. he is the electrician; I am the idiot.
 
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BentBierz

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It is STILL very common to see. I see it all the time.

IMO this is one of most ******* stupid parts of the code. There is ZERO justification for allowing #2AL for a 100A service to a whole dwelling, but not to a 100A sub-panel in/around a dwelling.
If anyone has a logical explanation, I'm all ears.

I have been searching for this logic as well primarily to help convince myself that, code compliant or not, it is safe.

What I don't understand is why if this is such a heavily debated, and often misinterpreted, section of the code why it hasn't been made clear these many revisions later? I was a Coast Guard commercial vessel inspector for the last 18 years of my active duty career (first 14 was at search and rescue stations and on cutters). I now am the Chief, Domestic Vessel Inspections (as a civilian) in Houston, TX. We have a ton of CG regulations related to domestic and international shipping. I am not going to pretend for a moment that our regs are all crystal clear but what the CG is good at is coming out with supplementary information to the field and industry to explain, in plain-speak, subjects that have become a hot potato because of unclear verbiage and/or complaints from industry of inconsistent application across the country by CG inspectors. I don't have to look to hard to see parallels here!
 
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BentBierz

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OK now that Ive read the whole thing it sounds like he still ran 3 wires.

Did he unbond the neutral bars?

Can u take some pics of panel with covers off and post them here. It would be easier than trying to understand a description.

I can send a picture later but, yes, he definitely unbond the neutral bars. My GE panel didn't come with a ground bar, just a neutral, and he had to install the ground bar.
 
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BentBierz

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I wonder if there's a disconnect in what people think of, and speak of, as 100A service. I understand specifications... I'm talking more like saying 110V whether it's 110 or 120.

90A seems to be a very common way to wire 100A panels because it's a sweet spot cost-wise. So I'm thinking of a guy asking for a 100A panel somewhere, and the response can go a few different ways.

1) "Sure. That's $4/foot for the cable; we can do that."

2) "Sure. It's actually 90A and not 100, and it's $1.75 per foot. Lemme know if you really need 100."

I bet a lot of people would be fine with 90 if it were presented that way up front. Seems like these two recent cases would be. If it's being asked for as 100A just because it's a round number, or because panels are sold as 100 rather than as 90, a little bit of education would go a long way. OP; what was behind your request for 100 specifically?

No one who's new to this is going to ask for 90A service. The GC may not even realize the difference, and he is between the electrician and the customer.



$30~60 for the breaker FWIW.

GC asked me if I wanted 50 amp or 100 amp to the buildings. I asked him what the price difference was and it wasn't significant so I went bigger so I would have no future-proof regrets later. At the same time, I didn't do any load calcs like I have recently so did not really know how much 100 amp capacity actually gives you.

I don't regret going 100 amp but I do regret going through a GC where you are counting on him to provide reputable concrete people, plumbers, building erectors and electricians. This is my first time working with a GC. I have no occasion to build another steel building but if I did, I would be the GC and interview/hire all of the other trades.
 

AntonLargiader

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GC asked me if I wanted 50 amp or 100 amp

Yeah, those seem like kind of arbitrary numbers to me at least. I read many more references to 60A than 50A. You can go 60A on #6 copper, at least for short distances.

but I do regret going through a GC where you are counting on him to provide ...

After our renovation, I kind of feel the same way. Way too much stuff got done poorly because I wasn't there every minute and talking to the subs myself. If I need to be there the whole time, why have a GC?

In some areas you have to pass a test to be your own GC.
 

justsam

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I have been searching for this logic as well primarily to help convince myself that, code compliant or not, it is safe.

What I don't understand is why if this is such a heavily debated, and often misinterpreted, section of the code why it hasn't been made clear these many revisions later? I was a Coast Guard commercial vessel inspector for the last 18 years of my active duty career (first 14 was at search and rescue stations and on cutters). I now am the Chief, Domestic Vessel Inspections (as a civilian) in Houston, TX. We have a ton of CG regulations related to domestic and international shipping. I am not going to pretend for a moment that our regs are all crystal clear but what the CG is good at is coming out with supplementary information to the field and industry to explain, in plain-speak, subjects that have become a hot potato because of unclear verbiage and/or complaints from industry of inconsistent application across the country by CG inspectors. I don't have to look to hard to see parallels here!

I'm not sure there is an issue with clarity on the part of the NEC. I am a LONG way from being a professional electrician, (anything below 2GHz is DC to me), but I have read the NEC portions that several of the professional electricians have posted here. Seems pretty clear to me.

One may or may not agree with the NEC but that is true of several things, take AFCI for example. Many here disagree with them but it is pretty clear in the NEC they are not optional or subject to interpretation.
 

lakeroadster

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"change out the damn breakers and move on...I don't want that a$$h*le back here again.!"

Great quote. Seems like your wife and my wife must be twins from different mothers :lol:

This 100 Amp vs 90 Amp topic is a recurring theme. I had a similar adventure, but caught it before the wire was buried. The county said #2 was fine, in the end I upgraded to the larger wire for 100 Amp service. More here:Can You Review the 100 Amp Service For My Pole Barn?
 
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BentBierz

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Yeah, those seem like kind of arbitrary numbers to me at least. I read many more references to 60A than 50A. You can go 60A on #6 copper, at least for short distances.



After our renovation, I kind of feel the same way. Way too much stuff got done poorly because I wasn't there every minute and talking to the subs myself. If I need to be there the whole time, why have a GC?

In some areas you have to pass a test to be your own GC.

I'm in the county and don't need to pull permits (septic is the only thing I think requires a permit) so I am able to be my own GC (at least I believe that to be true). I have pretty much been my own GC in the past when we did a kitchen gut/remodel. I coordinated the electrician (did a lot of it myself i.e. can lights in the ceiliing, 120 V under-cabinet lighting, island outlets, etc.), plumber, cabinet installers and tilesetters. What made that easier was there didn't have to be the same level of coordinating schedules as putting up a steel building. Still...seen it done once now and would do myself next time.
 
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BentBierz

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I'm not sure there is an issue with clarity on the part of the NEC. I am a LONG way from being a professional electrician, (anything below 2GHz is DC to me), but I have read the NEC portions that several of the professional electricians have posted here. Seems pretty clear to me.

One may or may not agree with the NEC but that is true of several things, take AFCI for example. Many here disagree with them but it is pretty clear in the NEC they are not optional or subject to interpretation.

I was on the Mike Holt forum this morning searching around and it was amazing the number of electricians on many issues (primarily when there was a code change) who pretty much said, "Ain't gonna do it...there was nothing wrong with how I did it for the past 30 years so how wrong could it be on my next job." In fairness to the majority of electricians, there were many more that replied to him with, "Dude, do it to code and charge accordingly...why would you do it any other way?" as well as, "Hope for your sake that your inspector agrees with you."
 
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