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I am wondering if a multi wire shared neutral circuit is considered a 240v circuit.

I ask because in the code it says that AFCI breakers are not required on 240v circuits. A 240 20amp breaker costs 16 dollars, a 240v 20amp afci costs 216 dollars. Is a 220 breaker safe on a multi wire circuit, will it trip as quickly as two single breakers?

I would just add a new wire and split the circuits (my first choice) but my attic is full of insulation and I can't see how I can remove enough to put a raceway on the rafter tops. The problem is I have a 4 in 12 roof and have added insulation 4 times over the years to keep up with new standards so I have insulation batts going two directions and one layer of blown in insulation. Does anyone have any tricks on how to work around the insulation?

Would I have to de rate the wires in conduit in the attic?

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You have a lot of questions. Usually you'll get better answers if you ask one cohesive question at a time. –  Jay Bazuzi Apr 19 '12 at 16:27
What are you feeding? –  Jay Bazuzi Apr 19 '12 at 16:36
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2 Answers

First of all, what you are describing is not a true 240VAC circuit. You are actually creating two 120 volt branches, not a single use 240vac circuit. You cannot protect opposing legs of two 120vac branches with a double pole breaker. You must use two single pole breakers, one per leg. In your case, if the areas being serviced are 120vac legs that require AFCI, then you must install single pole AFCI breakers as required by code.

The 240vac rule is referring to single use circuits such as air conditioners, heaters etc that require 240vac source voltage.

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Well, but if the shared-neutral circuit is to code, it would use a double-pole breaker (to make sure both of the "hots" that could energize any part of the circuit are cold). You couldn't use two single-poles in this instance because that's a shock hazard. I still think you're right in that using this as a dual-120V circuit instead of a single-purpose 220V does not entitle OP to the 220V exception, making AFCI a requirement if two single-poles controlling separate runs to the same area would have to be AFCI. –  KeithS Apr 19 '12 at 14:29
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You could use a subpanel.

I'm assuming you have a series of 120V receptacles on this circuit, at 20A or 15A.

You would keep the 20A 2-pole breaker on the main panel, and the 12/3+G cable.

At the other end, put a very small load center. You can use a "main lug" type, 2-circuit, 4-space. They're under $20. For example: http://www.amazon.com/dp/B000BPOROY/

Then you can put AFCI breakers in this subpanel to branch out as needed.

I haven't gotten a reliable answer as to whether 20A subpanels are allowed in code (30A may be a minimum), but the electrical theory is sound.

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Do you secretly own a business that makes subpanels? :P –  Steven Apr 19 '12 at 16:38
Looks like I can use a 220 breaker but it has to be an afci breaker. I cannot see spending $600 for three breakers so I will fight the insulation to split the circuits. On the existing wiring can I just cut the red wire flush at each end of the run and at the panel and use the cable as one circuit, or do I have to pull the old wire out. –  Dave Apr 19 '12 at 17:28
@Dave: Maybe you misunderstand me: the feeder breaker in the main panel doesn't need to be AFCI. The branch circuit breakers in the subpanel only need to be 120V, so they won't be so expensive. –  Jay Bazuzi Apr 20 '12 at 4:50
You might like to know that professionals hate the term subpanel. –  Philip Ngai Jul 24 '12 at 17:50
@PhilipNgai: No one will mistake me for a professional! ha ha –  Jay Bazuzi Jul 24 '12 at 21:11
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