AFCI on Multiwire Circuit

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Old 04-05-10, 11:09 AM
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AFCI on Multiwire Circuit

We have an older house with older wiring (1955 BX--has the bonding strip for grounding).

I have been thinking, lately, that I would really like to AFCI protect this circuit. With the wiring agin and not knowing exactly what's happened to it over the years it seems that, and afci would give us a little more peace of mind.

The problem is, that the main circuit (which feeds the living room and bedrooms) has a common neutral with the furnace circuit. The furnace is on a separate circuit, but these two share the same neutral.

So the only way that I can AFCI this house circuit is if I use a 2-pole breaker--meaning that a trip will take out the furnace. I'm not entirely comfortable with the idea that the furnace could be taken out (i.e. if something happens in the winter, while we are gone)

So my question--in your opinion: is the increased safety factor on the wiring great enough to overcome the potential risk of a false trip, in the winter while we are gone, causing problems with freezing, etc.

The other question is, I think the code requires the furnace to have its own circuit (for good reason) but is there any code that would not allow me to have a common trip with another circuit (thus defeating one of the reasons for the furnace having its own circuit)

Thanks
 
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Old 04-05-10, 12:36 PM
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I have been thinking, lately, that I would really like to AFCI protect this circuit.
My recommendation is to pull a new circuit for the furnace and use a standard 15A breaker for it. I would not put the furnace on AFCI.

You can then cap off the existing hot that went to furnace and install a single-pole AFCI breaker for the other one that feeds the living room and bedrooms. This effectively turns it into a regular branch circuit instead of a multiwire.

The cost of installing the new furnace circuit will be less than a double-pole AFCI breaker anyway. Also what is the make and age of your panel? If it is contemporary to the BX wiring, you probably can't get an AFCI for it.

is there any code that would not allow me to have a common trip with another circuit
I believe your installation would be considered legal as-is, but it is not typical. Usually the furnace is on a stand-alone circuit, even in older homes.
 
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Old 04-05-10, 12:57 PM
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Breaker or Fuse Box?

If Breaker Box;
Back then that may of been ok but today by code it should be on a dedicated circuit. It should be ok because its "grandfathered" in. A AFCI in my opinion may not be the best option here because the fact that it is two circuits it will be more subjective to trip and I think you will be hitting the reset button more than you want to. The best option would be to run a new circuit (dedicated) and put the other circuit on a AFCI or just leave it how it is. Another option you may consider would be to go with a GFCI breaker. The GFCI will protect your equipment and should not be as "touchy" as the AFCI. Though code does call for a AFCI breaker.. I am not sure to what degree here though because its not a complete upgrade.

My recommendation is to pull a new circuit for the furnace and use a standard 15A breaker for it
it maybe on a 20amp circuit... I would double check that.
 
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Old 04-05-10, 01:01 PM
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Panel

The panel is newish--it does accept Cutler Hammer BR breakers, I already have one AFCI in there.

I've had the thought about running a new furnace wire. its just a lot of wire--the panel is in the garage and the furnace in the basement. The garage used to be connected only by a breezeway (so the basement doesn't go all the way to the garage)

Thanks for the replies, other thoughts out there?
 

Last edited by mlibbs; 04-05-10 at 02:33 PM.
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Old 04-05-10, 01:20 PM
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IIRC there is no code that required the furnace to be on a dedicated circuit. It is however a requirement of many manufactures so therefore it would be required. Kind of a round about way but I just wanted to be clear to what is code required and what is not.

I feel Ibpooks suggestion is the best one. Just for food for thought: There is miles of BX cable out there in use with no issues. In fact one of the exceptions to not have to protect a circuit between the panel and the first device with an AFCI is to use steel BX cable. IMO you do not have a safety issue.
 
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Old 04-05-10, 01:54 PM
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Really? Is article 422.12 not referencing a deticated circuit? I may be wrong but that's how I read it.
 
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Old 04-05-10, 02:07 PM
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I agree that 422.12 states an individual circuit.

422.12 Central Heating Equipment.
Central heating equipment other than electric space-heating equipment
shall be supplied by an individual branch circuit.
 

Last edited by Tolyn Ironhand; 04-05-10 at 03:48 PM. Reason: just a little cleanup ;)
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Old 04-05-10, 02:31 PM
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Central Heating

Yeah, thats my understanding too. My question was, then, would having separate circuit (which is what I have) but a common trip with another circuit, defeat the code intent of a separate circuit.

It probably would. I think you guys have talked me out of worrying about it. Since it isn't new wiring, I don't technically need AFCI and since its BX it is completely encased and should keep it relatively safe (even if the cloth/rubber insulation in the boxes is starting to show its age).

If I really wanted to pursue this (or if I wanted to add to the circuit and have to bring it up to today's standards) I probably should recircuit the furnace to give it its own neutral.

Unfortunately our old house has been just giving me a lot of anxiet (mostly anxiety that is not rational). I've been just trying to find simple ways to make it safer.

Thanks
 
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Old 04-05-10, 02:39 PM
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Again back then it was ok (up to code). In todays world technically yes it should have its own neutral according to code. Though in my opinion you are fine and it is still grandfathered in.
You may want a 2nd opinion from one of the pros below me though.
Good luckBeer 4U2
 
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Old 04-05-10, 02:56 PM
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Originally Posted by mlibbs View Post
defeat the code intent of a separate circuit.
I don't think so. The electrical code writers are really only interested in preventing fires, so I believe the dedicated circuit requirement is to prevent overload on the circuit and have enough capacity for the blower start-up and ignitor. They don't really care about the reliability as that is left to the discretion of the engineer/installer. Your MWBC provides the full 15A to the furnace so there is no overload issue.

I don't technically need AFCI and since its BX it is completely encased and should keep it relatively safe
I would not worry about the BX that has a bonding strap.

I probably should recircuit the furnace to give it its own neutral.
...and it's own hot and ground too. I think that's what you meant, but you can't just add a neutral for it.

I've been just trying to find simple ways to make it safer.
After the neutral split in the circuit, you could add a GFCI receptacle to provide downstream protection. The GFCI cannot be added on the shared neutral section, but it can be used in the section that does not have a shared neutral. GFCI catches the a lot of faults that AFCI also would have caught.
 
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Old 04-05-10, 03:37 PM
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One thing thou that I would want you to check was to be sure that the furnace and bedroom breakers are on opposite legs of the panel if they are on the same leg you neutral could be overloaded and your breakers probably would never trip because neutrals arn't fused.

OK so in a newer panel if the breakers are side by side that usually means they are on different legs pull the cover and look with your eyes if your unsure(throw the main if it makes you feel safer) If you still cant tell you should test with a meter so see if you get 240 volts across the 2 breakers(living room and furnace)
 
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Old 04-05-10, 03:43 PM
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Originally Posted by SilverTattoo View Post
Really? Is article 422.12 not referencing a deticated circuit? I may be wrong but that's how I read it.
I stand corrected. Thank you for pointing that out.
 
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Old 04-05-10, 03:46 PM
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They are on opposite legs

Yes, they are on opposite legs. The panel is a modern style and layout (with alternating legs in each column). For instance 240V loads are fed from a standard 2 pole (handle-tied) breaker.
 
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Old 04-05-10, 03:48 PM
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I stand corrected. Thank you for pointing that out.
Thats not a problem.Beer 4U2
 
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Old 04-05-10, 05:10 PM
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After the neutral split in the circuit, you could add a GFCI receptacle to provide downstream protection. The GFCI cannot be added on the shared neutral section, but it can be used in the section that does not have a shared neutral. GFCI catches the a lot of faults that AFCI also would have caught.
Yes this is true. I was suggesting putting the GFCI breaker on the circuit AFTER running a new circuit for the furnace. He stated it was the living room and bedroom sharing that circuit in which a GFCI breaker would in my opinion be a better way(if a new circuit was ran to furnace..) due to a AFCI on a living room circuit and bedroom with vacuum cleaners and such I think isn't a great idea. That could be a pain in the rear.. The AFCI are way more sensitive than the GFCIS... This is one of the problems with the 08 codes and the AFCI requirements. Lots of people are removing them and placing "normal" breakers in place of them due to nuisance tripping.
 
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Old 04-06-10, 12:31 PM
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Followup Question

Ok I have a followup question.

If I need to do a repair or re-route part of an existing circuit (or in this case, hire an electrician to do so) does the circuit then need to have AFCI protection installed (and tamper resistant outlets for that matter).

There will be no new receptacles, Just looking to cut out an old piece of cable, and splice in new piece of cable to correct a problem in an attic run that feeds recepts in the living room and bedrooms.
 
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Old 04-06-10, 12:43 PM
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Generally you can do repairs of existing circuits without triggering the requirement for upgrade.
 
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Old 04-06-10, 12:44 PM
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Originally Posted by mlibbs View Post
Ok I have a followup question.

If I need to do a repair or re-route part of an existing circuit (or in this case, hire an electrician to do so) does the circuit then need to have AFCI protection installed (and tamper resistant outlets for that matter).

There will be no new receptacles, Just looking to cut out an old piece of cable, and splice in new piece of cable to correct a problem in an attic run that feeds recepts in the living room and bedrooms.
Technically he doesn't need to bring anything up to code that he isn't replacing. I would recommend, however, that the breaker be upgraded to AFCI anyway. AFCI prevents the majority of electrical fires. The safety benefit far outweighs the small cost of replacement.
 
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Old 04-06-10, 03:11 PM
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also maybe think about splitting the living room and bedrooms onto there own circuits or a new mwbc if you can find a wire in the place where they meet in the attic.
 
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Old 04-06-10, 10:50 PM
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While this topic is up ill toss this out there cuz ive been wondering. What is the "requirement" on a service upgrade as far as a percentage of house being remodeled or a percentage of new circuits ran etc...?? I know this will very per county and state but I have never got a definite answer.
Thanks
 
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Old 04-07-10, 09:20 AM
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Originally Posted by SilverTattoo View Post
What is the "requirement" on a service upgrade as far as a percentage of house being remodeled or a percentage of new circuits ran etc...??
It's entirely up to the local authority, so there's no standard. In one area I worked you could essentially do everything short of bulldozing the house before you had to upgrade the electrical and literally the next street over which was technically a different city required AFCI breakers during a panel change, interconnected smokes on all floors and GFCI receptacles in appropriate locations as a condition of pulling any electrical permit. That added about $600 (plus some drywall repairs) to an otherwise run-of-the-mill service change.
 
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Old 04-07-10, 11:17 AM
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Ok thanks a lot for the answer. That was on the same page to what I was thinking.

P.S> and ouch on the extra $600!! would it not be so much easier if all the counties within a state or all states for that matter stay on the same code requirements.. If you work outside of your county you already know what I am saying.
 

Last edited by SilverTattoo; 04-07-10 at 11:50 AM.
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Old 04-07-10, 12:32 PM
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NFPA could step in too

Or it would be nice if the NEC was written in such a way to not allow inspectors (who seem to function largely at their own whim) the freedom to interpret. I know in our city (Madison, WI) there's even been instances of one city inspector enforcing things differently than others in the same city.

I wonder what the NEC handbook (published by the NFPA) has to say about these items it--

The NEC handbook was an indespensible resource when I was an engineer because it includes the code language and then elaborates on the intent (and what isn't intended).

I always enjoyed faxing snippets of this to AHJ's who had interpreted things differently than the writers of the NEC.
 
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Old 04-07-10, 11:51 PM
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I agree with you on that one 100% It leave a lot of speculation and assumption with the inspector having the final say of his interpretation.
 
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Old 04-08-10, 12:37 AM
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I posted this in one other forum but, this one seems more fitting.

Electriciansparadise -- Multiwire Branch Circuits

Thoughts on this? Scroll down a little for the article. Its worth reading.
 
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Old 04-08-10, 06:13 AM
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Originally Posted by SilverTattoo View Post
I posted this in one other forum but, this one seems more fitting.

Electriciansparadise -- Multiwire Branch Circuits

Thoughts on this? Scroll down a little for the article. Its worth reading.
I think, for multiwire branch circuits, he misses the most significant disadvantage. When someone fiddles with the neutral who doesn't know what they're doing you can fry a whole lot of stuff, not to mention hurting yourself on a white wire which you thought was deactivated. I think multiwires are great, but I'd like to see them banned in homes where homeowners will likely cause personal or property damage through lack of knowledge.
 
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Old 04-08-10, 10:40 AM
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I think he did a good job on it. The 2 pole breaker vrs 2 single pole breakers is the main thing here. Also this is code compliant with the NEC 08. I have never really been a big fan of a multiwire circuit myself because if you lose the neutral it will take down a minimal of 2 circuits (commercial its more) and it can be very dangerous in residential, especially with all the DIYers.
 

Last edited by SilverTattoo; 04-08-10 at 11:08 AM.
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Old 04-08-10, 11:35 AM
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just a random question...can a mwbc be made with a 3 phase breaker and say 12-4g in a conduit for example


240 3phase to neutral that is...
 
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Old 04-08-10, 11:53 AM
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Well im not sure what a 3 phase breaker is. I know what a 3 phase service is. And you wouldnt run 12/4g in a conduit you would run thhn in a conduit. So im not sure what you are asking?
 
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Old 04-08-10, 12:12 PM
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Originally Posted by SilverTattoo View Post
Well im not sure what a 3 phase breaker is. I know what a 3 phase service is. And you wouldnt run 12/4g in a conduit you would run thhn in a conduit. So im not sure what you are asking?
I was saying 4 wires and a ground not a cable... and a 3 phase breaker is liek a double pole except its 3 pole
 
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Old 04-08-10, 12:15 PM
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That would be the main breaker on a 3 phase system which would not apply, unless if a 3 phase motor or something like that. I am still not sure what you are asking.
 
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Old 04-08-10, 12:19 PM
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it wouldn't have to be the main breaker.. 3 phase motors have to have a 3 pole breaker......or anything else requiring all 3 phases...

I work with 480 volts 3 phase systems...I know a little bit about what I'm talking about.

i was simply asking a moderator if it would be possible in theory to make a MWBC out of a 3 pole 20 amp breaker using 240 volt 3 phase 120v to neutral........................
 
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Old 04-08-10, 12:23 PM
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Im not sure man. A moderator could prob give you a better answer.
 
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Old 04-08-10, 12:27 PM
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it wouldnt have to be single phase its the same thing as a 20 amp double pole but is 20 amp 3 pole

do you even know how 3 phase works?? when you tap any hot of 240 3 phase you get 120 volts to neutral.............. to get 240 volts you hit any 2 hots to get 240 3 phase you need all 3 of the hot taps
 
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Old 04-08-10, 12:53 PM
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Do you know how it works? 3 phase is high voltage 3legs of 120volts. You can not get 120v circuit from a 3 phase panel! 120v is single phase... BRB = 120/208(single phase) BOY is 277/480(3 phase)
 

Last edited by SilverTattoo; 04-08-10 at 12:59 PM. Reason: my brane is not here today..
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Old 04-08-10, 01:48 PM
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Originally Posted by SilverTattoo View Post
Do you know how it works? 3 phase is high voltage 3legs of 120volts. You can not get 120v circuit from a 3 phase panel! 120v is single phase... BRB = 120/208(single phase) BOY is 277/480(3 phase)
Wow, I'm not sure what you 2 are talking about. You can get 120v out of a 3 phase system. A 120/208 3 phase system you get 120v from each leg to the neutral, 208 single phase using 2 legs & 208 3 phase using 3 legs. A lot of businesses use this because you get more power and you can run MWBC circuits. MWBC circuits you can put in a 3 phase breaker or 3 single pole breakers. On this circuit you have a wire to each breaker or leg and only use 1 neutral & a ground. They use these a lot for lighting and general receptacles. This saves them money on wire. You were also talking about 120/240 3 phase and in this system you most of the time have only 2 phases that you can get 120v out of from hot to neutral and the other phase is what we call the high leg which you'll get around 200v to neutral. So ask me your question and I'll try to figure it out.

Thanks!
Jim

 
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Old 04-08-10, 02:02 PM
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Hey thanks Jim for the reply. I was and am still a little stumped on this one. Ok I know BOY is 277/480 3 phase (brown, orange, yellow. Technically 120/240 is called "single phase" not two or double phase. Now with 120/208 is where I get confused. I know it as 3 phase BRB (Black, Red, Blue) but, with that being said some argue its single phase..?? So on the 120/208 is where I need to do my homework. I get the single pole 120 circuits but I really dont get the 208 part because I really have not done a whole lot with 208v. I should probably start new forum for this because this was not the original question and I do appologize for that.

Thanks
 
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Old 04-08-10, 02:23 PM
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Thanks jim... thats all I was curious about was if mwbc were used in 3 phase systems too.......

and the other part...I was talking about 240 volt 3 phase the cousin to 208...NOT 240 volt single phase


No real question...just wondering


ok so now I see the problem your saying you can only get 120v off the 2 stable legs and not the wild leg of 240 Delta
 
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Old 04-08-10, 02:51 PM
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I posted a tread on this subject so if you wanted to post there rather than here because its not relevant in anyway to the OPS question.
 

Last edited by SilverTattoo; 04-08-10 at 04:06 PM.
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