MWBC with AFCI

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RustyShackleford

Senior Member
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electrical engineer
For multi-wire branch circuits (MWBC) that share a neutral between two opposite-phase hots ...

It appears that prior to 2008 it was acceptable to use two single-pole breakers with a handle tie. Do I understand correctly that afterwards a double-pole (common trip) breaker is required ?

Now throw in a need for AFCI protection (as required either by code or by preference). Is THE only option an AFCI common-trip double-pole breaker ?
 
There is a duplicate discussion started by a different user

MWBC

I was thinking of providing a neutral for each 120V branch circuit in lieu handle ties or 2-pole breakers. I am not sure which method best in terms of cost. Your thought are most welcome. Thank you
forums.mikeholt.com
 
RustyShackleford said:
For multi-wire branch circuits (MWBC) that share a neutral between two opposite-phase hots ...

It appears that prior to 2008 it was acceptable to use two single-pole breakers with a handle tie. Do I understand correctly that afterwards a double-pole (common trip) breaker is required ?

Now throw in a need for AFCI protection (as required either by code or by preference). Is THE only option an AFCI common-trip double-pole breaker ?
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Common trip is not a requirement , 210.4(b) requires a single throw to disconnect all ungrounded conductors, says nothing about a common trip , 240.15(b)(1) permits the use of .two single pole breakers with identified handle tie
Two single pole afci breakers with no gfp ( GE, siemnens) will accomplish this
 
Ok, so in conclusion, if you have access to single-pole AFCI breakers that have only a hot connection (the neutral goes straight to the neutral busbar), it's fine to use two of them with a handle tie. And even for non-xFCI MWBC circuits, two single-pole breakers with handle tie is acceptable. In either case, having neither a handle tie nor a double-pole breaker is unacceptable (but found in many older installs).
 
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RustyShackleford said:
Ok, so in conclusion, if you have access to single-pole AFCI breakers that have only a hot connection (the neutral goes straight to the neutral busbar), it's fine to use two of them with a handle tie.
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Some of the AFCIs that don't have any GF detection still have a neutral terminal you can use for the branch circuit wiring; this keeps you from needing twice as many neutral bar terminals.

Cheers, Wayne
 
The issue are:
1.AFCI breakers USE the neutral, but do not SENSE the neutral. It only SENSES the current in the hot line to look for patterns in the current flow that indicate there is an arcing fault taking place (setting aside the inaccuracy for this discussion). So an AFCI “only” breaker works fine with an MWBC, it’s just a 120V circuit as far as it is concerned. You can get (now) a 2 pole AFCI, but you don’t always need it for MWBCs. If you are only using it for two 120V circuits. You can use two AFCI breakers and a handle tie. (See #5).

2. A GFCI breaker DOES sense the neutral current in a circuit. So it CANNOT work on an MWBC, because the neutral current will not match the hot line current, since the neutral is shared with the other hot and it will trip.

3. So a DUAL FUNCTION AFCI/GFCI breaker cannot work on an MWBC either, because of the GFCI functionality. Note that contrary to common misinterpretation, a “Combination” AFCI (CAFCI) is NOT a combination of AFCI & GFCI, it means it is a combination of Series AFCI & Parallel AFCI. Neither of those sense the neutral current. So for an MWBC circuit that needs GFCI protection and must have AFCI now, you have to use AFCI breakers and GFCI outlets.

4. SOME brands** of AFCI & CAFCI breakers might ADD what is essentially an EQUIPMENT GFI function (GFPE), set for 30mA trip. It’s NOT a requirement, so apparently they just threw it in. THOSE type of AFCIs would not work in an MWBC, for the same reason above.

5. An MWBC does not “normally” need common trip, only handle ties. BUT, you CAN feed a 240V single load off of an MWBC, and if you do AND the load is 120/240 inside, you MUST have a common trip, not just handle ties. THAT is when you need the true 2 pole AFCI breaker, which is why they sell them now.

** I do not know who does or does not, I’m basing this on what I have heard, as I have never encountered it personally. It might be that some people have ASSUMED that there was GFPE built in, but I don’t know. I brought this up because IF true, they wouldn’t work on an MWBC and that would be why.
 
Jraef said:
4. SOME brands** of AFCI & CAFCI breakers might ADD what is essentially an EQUIPMENT GFI function (GFPE), set for 30mA trip. It’s NOT a requirement, so apparently they just threw it in. THOSE type of AFCIs would not work in an MWBC, for the same reason above.
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That is true today. The first ~10 -12 years of AFCI requirements in the code all AFCI breakers had a ground fault component to them and needed it to pass listing requirements. I don't know what changes were made to be able to meet listing requirements without GFP component, but the ones that have dropped the GFP can be handle tied to use for MWBC's.

I'm pretty sure GE was the first to not have GFP and then maybe Siemens. I don't think Eaton has taken it out but they possibly may have some models with and some without. Square D still has GFP in all their AFCI's last I knew.
 
kwired said:
That is true today. The first ~10 -12 years of AFCI requirements in the code all AFCI breakers had a ground fault component to them and needed it to pass listing requirements. I don't know what changes were made to be able to meet listing requirements without GFP component, but the ones that have dropped the GFP can be handle tied to use for MWBC's.

I'm pretty sure GE was the first to not have GFP and then maybe Siemens. I don't think Eaton has taken it out but they possibly may have some models with and some without. Square D still has GFP in all their AFCI's last I knew.
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Makes sense. The big change that happened was that the original AFCIs were only “series”, in that they only detected an arc across a broken wire or a bad connection. Then later it became a requirement to add “parallel” AFCI protection, meaning detecting an arc from line to another line or line to ground/neutral. So I can imagine that some chose to just use GFPE for the L-G/N protection as a “quick solution” rather than take the time to gather the data on what those arc signatures actually looked like. Given all the painful lessons on the early AFCIs with nuisance tripping, it makes a lot of sense that this is what some of them did.
 
Jraef said:
Makes sense. The big change that happened was that the original AFCIs were only “series”, in that they only detected an arc across a broken wire or a bad connection. Then later it became a requirement to add “parallel” AFCI protection, meaning detecting an arc from line to another line or line to ground/neutral. So I can imagine that some chose to just use GFPE for the L-G/N protection as a “quick solution” rather than take the time to gather the data on what those arc signatures actually looked like. Given all the painful lessons on the early AFCIs with nuisance tripping, it makes a lot of sense that this is what some of them did.
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I think you have that backwards. The original branch circuit and feeder AFCIs only detected parallel arcing faults, the combination type that we are now required to use added series arc fault detection.
The original AFCIs all had GFP because the only way the manufacturers could get the AFCI to pass all of the performance tests required bu UL 1699 was to add GFP. Even after the code required the use of combination type AFCIs that detect both series and parallel arcs, they all still had GFP.
GE was the first to find a way to pass all of the required performace tests without the GFP. They did that about 12 years ago. Over time other manufacturers have also removed the GFP from their AFCIs, but not all. I believe that SquareD still uses GFP in their AFCIs.
 
if it says on a afci or gfci 2 pole breaker 'to be used on 120/240V circuits' can I use it on 120/208V single phase circuits? in NYC we only have 120/208 single phase for residential.
 
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