Arc Fault Breaker/Shared neutral circuits

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mudcat555

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Mentor, Ohio (25 miles east of Cleveland)
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Retired Electrician IBEW Local 38
The inspector wants an arc fault breaker installed on a particular circuit. The circuit involved is sharing a neutral with another circuit. Can I install a single pole arc fault on just this circuit, or do I need to install a two pole arc fault and catch both circuits because of the shared neutral?
 
mudcat555 said:
The inspector wants an arc fault breaker installed on a particular circuit. The circuit involved is sharing a neutral with another circuit. Can I install a single pole arc fault on just this circuit, or do I need to install a two pole arc fault and catch both circuits because of the shared neutral?
Click to expand...

A single pole AFCI will not work on a multy wire branch circuit, you could use a two pole AFCI and it may work, but it really depends on the load involved .

With AFCI's in play multy wire branch circuits are not such a good idea in my opinion.
 
how much is load on the two circuits? What is amp on each ckt15 or 20 .you could Junction the two circuit in your main panels( only if the manufacturers permits using the panel as a junctIon box)
 
Arc fault breakers/shared neutral

Arc fault breakers/shared neutral

I don't know the loads yet, as I only spoke with the customer over the phone. One circuit is lighting, and the other is the furnace feed. I don't want to combine the furnace feed with the other circuit as it should have been on its own circuit from the beginning. Arc fault rules are still a little hazy.
 
mudcat555 said:
I don't know the loads yet, as I only spoke with the customer over the phone. One circuit is lighting, and the other is the furnace feed. I don't want to combine the furnace feed with the other circuit as it should have been on its own circuit from the beginning. Arc fault rules are still a little hazy.
Click to expand...

Some of the newer AFCI breakers do not have GF (ground fault) detection. The GF part of the AFCI is why you can't share neutrals. Same as with GFCI receptacle/breaker.
So you can either.....
Run a new circuit with its own neutral
Use a 2-pole AFCI
Use an AFCI without GF protection
 
Little Bill said:
Some of the newer AFCI breakers do not have GF (ground fault) detection. The GF part of the AFCI is why you can't share neutrals. Same as with GFCI receptacle/breaker.
So you can either.....
Run a new circuit with its own neutral
Use a 2-pole AFCI
Use an AFCI without GF protection
Click to expand...

I don't know how the AFCI works without the GFI circuit but I would not want to put a single pole AFCI breaker on a MWBC. I would think you could be introducing problems.
 
Sierrasparky said:
I don't know how the AFCI works without the GFI circuit but I would not want to put a single pole AFCI breaker on a MWBC. I would think you could be introducing problems.
Click to expand...
The basic GE single pole combination-type arc fault circuit breaker has been providing arc fault protection without any ground fault sensing for over three years now. It works just fine on a multiwire branch circuit with, OR WITHOUT, the load neutral of the multiwire branch circuit conntected to the GE AFCI breaker.

You can read GE's shared neutral application documentation by clicking here.
 
MWBC CAFCI Common Trip

MWBC CAFCI Common Trip

al hildenbrand said:
The basic GE single pole combination-type arc fault circuit breaker has been providing arc fault protection without any ground fault sensing for over three years now. It works just fine on a multiwire branch circuit with, OR WITHOUT, the load neutral of the multiwire branch circuit conntected to the GE AFCI breaker.

You can read GE's shared neutral application documentation by clicking here.
Click to expand...

Hi Al,
Thanks for the link about GE's AFCI technology. Even though the NEC is based on safety as a rule, the use of MWBC protection being compliant with a common trip handle or tie-bar [210.7] does have it's safety setback in two ways. The main safety concern of shared neutrals is more of a sensitive function in arc fault protection shutting down two circuits at the same time. This is applicable to safety in commercial contracts depending on the local facility requirements. (I.e. NFPA Life Safety 101).

Repair efficiency and safety of getting lighting and critical equipment back on the line together become a problematical hazard when simultaneous circuit shutdown becomes a home emergency situation. The second aspect of circuit transient coupling is a factor when the branch cabling is split into bundled pairs, then low level AFCI detection becomes a factor in distance runs. Just saying, that is one of the safety reasons for the 50ft and 70ft restrictions in the NEC 2014 [210.12(A)(1-6)] length constraints.

Noted in the reply comments, as mentioned, that more than one AFCI circuit is affected in some cases. Saving copper is nice, but labor costs in fixing the problem is doubled when two BC's take time in troubleshooting a re-occuring failure when various loads interact from combined EMI coupling. Eliminating MWBC's in NM cabling is almost a fact of economics now...IMO, rbj
 
gndrod said:
This is applicable to safety in commercial contracts depending on the local facility requirements. (I.e. NFPA Life Safety 101).
.
.
Eliminating MWBC's in NM cabling is almost a fact of economics now...IMO, rbj
Click to expand...
The Opening Post doesn't specify that this is a commercial contract. It's most likely a residential setting.

The OP does indicate that this is an existing MWBC. One does not have the economics of new wiring to trade off against the perceived risk of a handle-tied pair of single pole breakers. It is simply more economic to install a single pole AFCI IF, and I do say IF, the existing panel will accept the GE full sized CAFCI, than it is to alter most common existing residential MWBCs.

My point in offering the information, here in this thread, is that the GE CAFCI single pole works great on an existing multiwire branch circuit, in my personal experience.
 
al hildenbrand said:
The basic GE single pole combination-type arc fault circuit breaker has been providing arc fault protection without any ground fault sensing for over three years now. It works just fine on a multiwire branch circuit with, OR WITHOUT, the load neutral of the multiwire branch circuit conntected to the GE AFCI breaker.

You can read GE's shared neutral application documentation by clicking here.
Click to expand...

Al interesting read. The way I read this is that you need both circuits connected to AFCI and only 1 wire need connected to the breaker nuetral. I don't know if this was accident or purposeful.
If they left the nuetral off of both then I would not have any questions.
 
al hildenbrand said:
The Opening Post doesn't specify that this is a commercial contract. It's most likely a residential setting.


My point in offering the information, here in this thread, is that the GE CAFCI single pole works great on an existing multiwire branch circuit, in my personal experience.
Click to expand...

I agree.
 
Sierrasparky said:
The way I read this is that you need both circuits connected to AFCI and only 1 wire need connected to the breaker nuetral. I don't know if this was accident or purposeful.
If they left the nuetral off of both then I would not have any questions.
Click to expand...
One single pole breaker may be conventional, and the other may be CAFCI. What they are showing you is that the return current on the neutral is not part of the AFCI sensing of the CAFCI electronics. GE MOD 3 CAFCI are pure AFCI, no ground fault sensing, no current matching between hot and neutral. They work just fine.

Given that GE will not divulge the actual circuitry, the only thing I think that will help you is to go put a GE CAFCI and a conventional GE breaker together on a multiwire branch circuit and try it out. You'll find they work as advertised.
 
AFCI

AFCI

Los Angeles area. IBEW local 11. Inside wireman, Cable splicer.

Been following the forum for years and here I am looking for specific information.

I am doing some sidework for residential remodels and the AFCI requirements are interesting. All circuits except the bathroom and the kitchen areas (I am being general here) shall be AFCI protected on all outlets. Plugs and lighting.

From what I have been seeing:

Normally 14/2 is used on the lighting and 12/2 is used on the outlets. The circuit interrupters that are typically used are 15 amp for the 14/2 and 20 amp for the 12/2.

And

I understand that if the grounded conductor is shared by two ungrounded conductors that both must disconnect at the same time and I understand that both must be AFCI protected. From this information I was thunking:

It is not going to be a matter of one AFCI breaker in a panel but rather 2 or 4 AFCI breakers per panel. 2 AFCI's if you are going to use all 12/2 for everything and have all the required circuits on just two breakers. 4 AFCI (2 at 20 amps for the outlets and 2 at 15 amps for the lighting). If a circuit that requires an AFCI shares a grounded conductor with an circuit that does not I still have to have both on a AFCI circuit interrupter with handles that are tied together.

Even if you do not share the grounded conductor you are not going to wire all the required lighting and outlets to only 1 or 2 AFCI's no matter how small the place. That would be lame that all the lights or half the lights go out when a breaker trips. Splitting the load up further means more AFCI breakers at what around 35 per pop.

Is my thinking correct or am I missing something.
 
I would advise you to not share a neutral with an afci. Unless you use GE afci breakers you would need a dp afci not 2 sp breakers with handle ties- that will not work.
 
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