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  1. #1
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    Default Multi-wire circuits

    Sparkies. I generally feel comfortable with electrical, but am still a bit shy with multi-wire circuits.

    House is just completed new construction, single family home. Service entrance panel and meter at exterior rear is grounded and bonded. Main distribution panel is inside, so this panel has isolated neutral and grounding bus bars.

    A multi-wire circuit that appears to be servicing the second floor, including one or more bedroom areas, and a bathroom (with GFCI receptacle). The wire bundle has 5 conductors:
    - black
    - red
    - white
    - white with red stripe
    - bare copper

    Two breakers:
    #1 is general purpose 20-amp breaker
    #2 is a 20-amp AFCI breaker

    - Red conductor (hot) goes to the top general purpose breaker
    (labeled as "Up Bath (outlets)"
    - Black conductor (hot) goes to the AFCI breaker
    (labeled as "Upstairs")
    (the AFCI breaker also has the normal 2nd neutral to the neutral bus bar)

    - White grounded conductor (neutral) goes to left neutral bus bar
    - White w\ red stripe grounded conductor goes to the right neutral bus bar
    - Bare copper grounding conductor goes to a separate grounding bus bar.

    COMMENT. Approved by local state elec inspector. No obvious 'grouping' of these wires, other than they can be seen originating from the same sheathed conductor bundle at the top of the panel.

    Sorry, I didn't get a good picture of the whole panel with the cover off. see pics.

    Does this make any sense, and any comments?

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  2. #2
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    Default Re: Multi-wire circuits

    Here is a simplified diagram to illustrate

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    Default Re: Multi-wire circuits

    Well, you don't have what's called a multiwire branch circuit. You have 2 separate circuits in one cable, cable type is 12-2-2. The white with red stripe is simply the neutral for the red wire and the white wire is the neutral for the black. The wire savings here is only one ground is needed in the cable rather than the 2 that would be needed with 2 separate cables.

    This gets treated like 2 runs of NM-B going to the same box. No big deal. Grouping isn't necessary and neither is a breaker handle tie because it's pretty obvious there are 2 neutrals and 2 hots, and one neutral is marked to go with the red wire.


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    Default Re: Multi-wire circuits

    Quote Originally Posted by Bill Kriegh View Post
    You have 2 separate circuits in one cable, cable type is 12-2-2. The white with red stripe is simply the neutral for the red wire and the white wire is the neutral for the black. The wire savings here is only one ground is needed in the cable rather than the 2 that would be needed with 2 separate cables.

    This gets treated like 2 runs of NM-B going to the same box.

    Bill,

    I have not run across 12-2-2w/g yet - that cable would require derating as there are more than three current-carrying conductors in a raceway or cable ... interesting that they make a cable which requires derating right out of the box.

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    Default Re: Multi-wire circuits

    Don't think I have seen this before. Thanks for the explanation. It makes more sense now.

    Usually I notice a single red hot conductor, and make some effort to determine that it's not part of an improper 240-volt circuit (common in older homes). Then I got confused when I began tracing two neutrals, and only found one grounding wire. Multi-wire circuits are not very common except in some 1970's homes, hardly ever see multi-wire circuits in newer homes around here.


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    Default Re: Multi-wire circuits

    Quote Originally Posted by Jerry Peck View Post
    Bill,

    I have not run across 12-2-2w/g yet - that cable would require derating as there are more than three current-carrying conductors in a raceway or cable ... interesting that they make a cable which requires derating right out of the box.
    Wait 'till the requirements for a neutral in a switchbox hit..........


  7. #7
    Roger Frazee's Avatar
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    Default Re: Multi-wire circuits

    Southwire Cable sort of reinvented the older 12/4 nm when afci became code. 12/2/2 g was their answer for the home run to bedrooms since afci will not work on a multiwire shared neutral. This lets you have another circuit for smokes or bathrooms and a circuit for the afci requirements for the bedroom.

    It also has other useful purposes in some 3 way light configurations and 3 way pilot lights that require a neutral..


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    Default Re: Multi-wire circuits

    Quote Originally Posted by Jerry Peck View Post
    Bill,

    I have not run across 12-2-2w/g yet - that cable would require derating as there are more than three current-carrying conductors in a raceway or cable ... interesting that they make a cable which requires derating right out of the box.
    This is not an issue. Once you apply the 80% deration for 4 current carrying conductors you are still allowed to install the conductors on a 20 ampere breaker legally.

    Article 240 restricts #12 to a 20 ampere overcurrent device BUT ........
    We derate from the 90 degree column of 310.16- as the cable in question has thhn conductors. #12 THHN is rated for 30 amperes.
    30 amps x 80% = 24 amps.


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    Default Re: Multi-wire circuits

    Quote Originally Posted by Bill Kriegh View Post
    Wait 'till the requirements for a neutral in a switchbox hit..........
    Use plastic boxes and run 2 - 2 wire cables - Legal as per NEC


  10. #10
    Roger Frazee's Avatar
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    Default Re: Multi-wire circuits

    COMMENT. Approved by local state elec inspector. No obvious 'grouping' of these wires, other than they can be seen originating from the same sheathed conductor bundle at the top of the panel.
    Terry

    In answer to your comment .. last paragraph in 210.4 .. your multiwire is an nm-b cable and therefore (D) of 210.4 would apply .. attn to the exception.

    (D) Grouping. The ungrounded and grounded conductors
    of each multiwire branch circuit shall be grouped by wire
    ties or similar means in at least one location within the
    panelboard or other point of origination.
    Exception: The requirement for grouping shall not apply if
    the circuit enters from a cable or raceway unique to the
    circuit that makes the grouping obvious.

    EDIT TO ADD:
    This section of code would be correct if we had an multiwire branch circuit. In this case we do not have a multiwire as this is a 12/2/2 g nm-b cable serving two separate circuits . There is no neutral conductor to carry unbalanced current, therefore no multiwire is in use..

    Last edited by Roger Frazee; 03-14-2011 at 08:01 AM. Reason: Claification that the cable is not a multiwire in this thread

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    Default Re: Multi-wire circuits

    Quote Originally Posted by ken horak View Post
    This is not an issue. Once you apply the 80% deration for 4 current carrying conductors you are still allowed to install the conductors on a 20 ampere breaker legally.
    Not if any part of that NM cable goes through the attic, in the exterior wall outside the thermal envelope,

    The 12 AWG NM-B cable has conductor insulation rated for 90 degree C, and the first derating is for ambient, followed by derating for lack of maintaining spacing.

    12-2-2 w/g NM-B has a rating of 30 amps before derating.

    Let's presume that any part of that circuit goes into an attic or exterior wall with a temperature 110 degrees F, which is a VERY easy temperature to reach, and which has a derating multiplier of 0.82, or 30 amps X 0.82 = 24.6 amps derated for ambient, then 24.6 times the derating multiplier for lack of maintaining spacing, or 24.6 X 0.80 = 19.68 amps, which is *not allowed* on a 20 amp breaker if it feeds a multioutlet circuit.

    If it only feeds a single outlet, yes, the overcurrent device rating of the next higher standard size can be used.

    Now derate for a more normal attic temperature of 130 degrees using its multiplier of 0.71 ... (you end up with a rating of 30 X 0.71 = 21.3 X 0.80 = 17.04 amps.

    Now derate that for the common practice of installing two NM cables together without maintaining spacing ... the new 12-2-2 w/g would be *8* current carrying conductors, the lack of maintaining spacing derating multiplier goes from 0.80 to 0.70 ... not going to work on a 20 amp breaker.

    Seems to me that you forgot to include derating for ambient (you did not mention it, and you only used the derating for lack of maintaining spacing) - that is something I find to be very common with electricians who are trying to justify a non-compliant installation.

    The best practice is to derate for ambient *first*, then derate for lack of maintaining spacing.

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  12. #12
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    Default Re: Multi-wire circuits

    Quote Originally Posted by Terry Beck View Post
    Two breakers:
    #1 is general purpose 20-amp breaker
    #2 is a 20-amp AFCI breaker

    - Red conductor (hot) goes to the top general purpose breaker
    (labeled as "Up Bath (outlets)"
    - Black conductor (hot) goes to the AFCI breaker
    (labeled as "Upstairs")
    (the AFCI breaker also has the normal 2nd neutral to the neutral bus bar)

    - White grounded conductor (neutral) goes to left neutral bus bar
    - White w\ red stripe grounded conductor goes to the right neutral bus bar
    - Bare copper grounding conductor goes to a separate grounding bus bar.

    AFAIK the neutral for the AFCI circuit should go to the breaker not the neutral bus bar.


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    Default Re: Multi-wire circuits

    [quote=Jerry Peck;16197 Now derate that for the common practice of installing two NM cables together without maintaining spacing ... the new [COLOR=red]12-2-2 w/g would be *8* current carrying conductors[/COLOR], /quote]

    NO sir- it is not.

    Worse case is it's 2 cables with 4 current carrying conductors. You would derate based on each cable. That makes it a 80% deration as opposed to your claim of using 70 %

    12-2-2 is Two single circuits. Not a multiwire circuit.
    Seeing hows it's not a multiwire circuit, we do Not use multipole circuit breakers. We use single pole circuit breakers. Thus its 2 single circuits.
    Thus we have 2 current carrying conductors with 2 neutral conductors.


    A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted whan applying the provisions of 310.15(B)(2)(a).

    Now lets look at your 100 degree attic comment.
    You are using the deration percentage for one. You chose to use .71 from the 60 degree column.You should have used .91 from the 90 degree column. This is a common mistake. Read article 334.80 and you will learn that you derate from the 90 degree column but use the 60 degree column for the maximum amperage.
    30 amps x .91 = 27.3

    I'm not going to argue this as I can throw enough "what if's" into to the equation that one would need to use #6 to wire 15 amp circuits.

    The OP question was answered : it is a 12-2-2 cable. It is not a multiwire circuit.


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    Default Re: Multi-wire circuits

    Quote Originally Posted by ken horak View Post
    NO sir- it is not.

    Worse case is it's 2 cables with 4 current carrying conductors. You would derate based on each cable.
    Ken,

    NO sir - that is incorrect.

    If there are two 12-2-2 cables bundled together YOU COUNT THE NUMBER OF CONDUCTORS ... or you can count the number of cables THEN MULTIPLY BY 2 ... the answer WILL STILL BE *8* current carrying conductors bundled or lack of maintaining spacing.

    This is basic stuff, Ken, when you pull four circuits into a raceway and each circuit has two current carrying conductors - how many current carrying conductors are in that raceway?

    If you answer *4* ... you are incorrect.

    If you answer anything other than *4* you will need to explain why you do the math differently than with cables, and you will be incorrect ...

    ... until you come to the answer of "there are *8* current carrying conductors in that raceway, just like there are *8* current carrying conductors in two 12-2-2 cables".

    I eagerly await your math.

    Jerry Peck, Construction / Litigation Consultant
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    Default Re: Multi-wire circuits

    Quote Originally Posted by ken horak View Post
    Now lets look at your 100 degree attic comment.
    You are using the deration percentage for one. You chose to use .71 from the 60 degree column.You should have used .91 from the 90 degree column.
    Ken,

    Incorrect, that is not the mistake I made (however, I did make a mistake, but that was in writing 110 degrees and looking up 115 degrees and writing 130 degrees and looking up 135 degrees.

    Your reference to the 60 degree columns has numbers which do not match up with anything I wrote.

    Regardless of my 110/115 and 130/135 mistake, the derating at 110 ends up being 20.88 (which just barely makes the 20 amp breaker, but it does) and the derating at 130 ends up being 18.24 WHICH STILL DOES NOT MAKE a 20 amp circuit. See the NEC table section I posted below.

    Thank you for catching my error, only next time do it without making your own error in your own direction - which is also a common mistake electricians make.

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    Default Re: Multi-wire circuits

    After reading this I think Ken meant that a 12-2-2 would be the same a 2 12-2s conductor count-wise.

    All answers based on unamended National Electrical codes.

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    Default Re: Multi-wire circuits

    Quote Originally Posted by Jim Port View Post
    After reading this I think Ken meant that a 12-2-2 would be the same a 2 12-2s conductor count-wise.
    Jim,

    Ken may have meant that, however, that is why I tried to be clear in what I was posting by stating: (bold and underlining are mine)
    Quote Originally Posted by Jerry Peck View Post
    Now derate that for the common practice of installing two NM cables together without maintaining spacing ... the new 12-2-2 w/g would be *8* current carrying conductors, the lack of maintaining spacing derating multiplier goes from 0.80 to 0.70 ... not going to work on a 20 amp breaker.
    Those two 12-2-2 NM cables are still just two cables, and there is a practice throughout (it seems) the industry where two NM cables are bundled together such as to not maintain spacing, and doing that with 12- NM cables which run through an attic can still lead to problems when using the next higher ambient temperature for the attic (135 degrees F), and installing two 12-2-2 NM cables just makes that problem worse.

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    Default Re: Multi-wire circuits

    I might not have been as clear as I could. My take on Kens statement was that running a 12-2-2, instead of 2 12-2's was the same conductor count-wise. Not 2 12-2-2 run together.

    All answers based on unamended National Electrical codes.

  19. #19
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    Default Re: Multi-wire circuits

    I'd love to enter the fray so thought I would post this article from my files on the subject. I have found it to be an excellent summary of the correct considerations when adjusting for ambient and bundling of cables. Please note the article is from Mike Holt enterprises. No intention to promote the business site.

    :: Welcome to Power Panel Pros:: Your Residential Experts

    Just a nice graphic also from Mike Holt.The way I would look at Jerry's example above is the right diagram .. only make 1 and 2 grounded conductors. So 1234 are grounded conductors serving ungrounded conductors 5678. If I have (two) 12/2/2 g nm-b cables (operating at 120 volts no MWBC) bundled in an attic I essentially have one cable with 8 current carrying conductors as shown with an ampacity correction factor of .70. The ambient correction factor at 110F would be .87 And an the calculation would be (if none of the exceptions apply) 30x.87x.70 = 18.27 amps. I have two choices downsize to 15 amp breaker or increase wire size to 10 awg and place on 20 amp breaker. This of course for a general multi-outlet receptacle branch circuit .. other considerations would be individual branch circuits or circuits where you could pin down the calculated load.



    Last edited by Roger Frazee; 03-11-2011 at 04:24 PM. Reason: edit to clarify number of grounded conductors

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    Default Re: Multi-wire circuits

    Jerry lets address your math first .........



    You said you count THE NUMBER OF CONDUCTORS .Well you were on the right path but then you weave off the path again.
    You count the NUMBER OF CURRENT CARRYING CONDUCTORS.
    Lets start over here.......
    The OP shows a 120/240 volt single phase panel, that has a 12-2-2 NM Cable. This 12-2-2 NM Cable feeds and bedroom and a bathroom.
    The 12-2-2 cable has 2 hots and 2 neutrals. Thus this 12-2-2 cable is not a multiwire ciruits, Rather they are 2 single circuits. Yes this clarification is important. Heres why ...................
    Lets look at NEC article 310.15(B)(4)(a) -it states:
    A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be counted when applying the provisions of 310.15(B)(2)(a).

    So these 2 SINGLE circuits do not share a neutral,thus the neutral of each circuit ONLY CARRIES THE UNBALANCED CURRENT from other conductors OF THE SAME CIRCUIT.

    Also we already determined that these 2 separate circuits feed a bedroom and a bathroom, So I'm sure it's safe to say they do not deal with non-linear loads either.

    So to review:
    We have 2 separate circuits, that do not share neutrals, are not part of a 4wire 3 phase wye system ( remember they on a 120/240 single phase system),and do not have non-linear loads. This means the neutrals are NOT current carrying . Thus those two 12-2-2 cabel you are speaking of DO NOT HAVE 8 current carrying but rather ONLY 4 current carrying conductors.


  21. #21
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    Default Re: Multi-wire circuits

    Ken

    I'm not following at all. How can the grounded legs not be current carrying if each grounded leg in the 12/2/2 g cable serves a single hot wire as per OP's description? There can only be unbalanced current in a 3 wire 120/240 volt system if two ungrounded conductors share a neutral and the ungrounded conductors are on opposite hot legs. If I run 4 ungrounded conductors for 120 volt loads and none are multiwire circuits then I would have 4 grounded legs also that carry the current in each circuit. There is no unbalanced current on a neutral because there are no neutrals.

    Please read your text more closely ...

    A neutral conductor (singular) that carries only the unbalanced current from other conductors (plural) of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(2)(a).


    Last edited by Roger Frazee; 03-25-2011 at 08:59 PM. Reason: clarified number of ungrounded conductors in 3 wire system

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    Default Re: Multi-wire circuits

    Quote Originally Posted by Roger Frazee View Post
    Ken

    I'm not following at all. How can the grounded legs not be current carrying if each grounded leg in the 12/2/2 g cable serves a single hot wire as per OP's description? There can only be unbalanced current in a 3 wire 120/240 volt system if two or more ungrounded conductors share a neutral and the ungrounded conductors are on opposite hot legs. If I run 4 ungrounded conductors for 120 volt loads and none are multiwire circuits then I would have 4 grounded legs also that carry the current in each circuit. There is no unbalanced current on a neutral because there are no neutrals.

    Please read your text more closely ...
    I had the same question as I read Ken's post, Roger covered it quite well.

    Roger, I suspect that Ken is making another mistake many electricians make (had one make the same mistake today too): they all commonly refer to the white grounded conductor as the "neutral" on a 120 volt circuit, and they are still thinking "neutral" when doing those calculations, when in fact the white conductor is not a neutral, it is a "grounded conductor" which carries the same exact current as is carried by the "ungrounded" conductor.

    After 15 minutes of talking with that electrician he said: "So, I count the neutral in these installations?"

    After some hesitation I said: "No, you count the GROUNDED CONDUCTOR in these installations."

    I still think he did not grasp that we were not talking about a "neutral" conductor ... in his mind he "knows" that "white is neutral" ... whether it is or not.

    Like this morning talking to a garage door installer who I have talked with several times (about trying to get *one* garage door installed properly): He kept asking how many anchors I wanted - that he would do what I wanted; I kept repeating that what I wanted was him to read the engineering and install the anchors as stated in the engineering as that is what I am looking at; he eventually came back with 'I'll put the anchors every 6 inches if you want me to.'; to which I replied (yet again) 'I want you to read the engineering and put the anchors in according to what the engineering states.'

    I think I lost that installer when I said "read the engineering" - I really think that he has no idea what he is looking at or for in that engineering, even though he is a "garage door installer" and should KNOW how to install the garage doors (i.e., install the garage door 'in accordance with the engineering' as different garage doors have different engineering).

    Jerry Peck, Construction / Litigation Consultant
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  23. #23
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    Default Re: Multi-wire circuits

    Hi Jerry

    I think you may be right and I find myself making the same mistake at times. It really isn't harmful in conversation but it causes mistakes just like using the term sub-panel does at times...if that is a good comparison.

    Generally using the term neutral is common for the grounded conductor in a 2 conductor with ground 120 volt circuit however you have to stay focused on the definition of terms when things start to get technical as you can easily find yourself making rather simple mistakes. I've made my share that's for sure.....

    As for derating bundled cables or ambient adjustments I didn't do much of it .. if any.. in residential work. We did quite a bit of derating in commercial due to almost everything was conduit. At any rate it is a simple process that can at times be challenging depending on the complexity of the installation.

    However just like my hardheaded eventual acceptance of afci being a good thing I have become more interested in ambient adjustments realizing ambient needs to be checked out as does bundling .. IMO this an area residential wireman need to pay more attention to and make adjustments for bundling and ambient. There really is no excuse for bundling IMO but if you do you need to calculate if any issue is going to come of it.

    I would venture to say that it will take both ambient and bundling to make a difference in my area but we have some hot weather here in Kansas just like you guys in the South and attics here IMO will have an ambient adjustment in almost every case now that I am forcing myself to consider it.. Throw in bundling it just might make a difference. My opinion is it is overlooked in many cases. Obviously it is not a serious issue in residential in so much as causing millions of fires but it may cause a few and it may cause some insulation failures ... so ...if your going to go by code then you need to treat them all with the same degree of importance and not pick and choose..

    Last edited by Roger Frazee; 03-11-2011 at 08:05 PM.

  24. #24
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    Default Re: Multi-wire circuits

    Quote Originally Posted by chris mcintyre View Post
    AFAIK the neutral for the AFCI circuit should go to the breaker not the neutral bus bar.
    Chris

    I read right over your post. That is a good catch and your absolutely correct.


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