California rules apply
A question has come up regarding AFCI requirements when a service upgrade takes place. If branch circuits are extended or altered for a length of conductor greater than 6', AFCI is required. In the past no 6' exception existed. I would allow an extension from the old service to the new location if the conductors were in a ridgid or intermediate conduit that was outside the building. No underfloor or attic runs.
Now the code code will allow six feet I am loathe to allow more than 6,' conduit outdoors or not. The question is the problem with the existing wiring being hardly compatible with AFCI technology. Specifically with regards to shared neutrals. Two pole AFCI breakers will take care of the shared neutral.
The conductors from the old service location to the new spot generate plenty of corrections. Keep in mind that the 6' is 6' of wire not 6' between enclosures.
210.12 Arc-Fault Circuit-Interrupter Protection
(A) Definition: Arc-Fault Circuit Interrupter (AFCI): A device intended to provide protection from the effects of arc faults by recognizing characteristics unique to arcing and by functioning to de-energize the circuit when an arc fault is detected.
(B) (A) Dwelling Units. All 120-volt, single-phase, 15- and 20-ampere branch circuits supplying outlets or devices installed in dwelling unit kitchens, family rooms, dining rooms, living rooms, parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, hallways, laundry areas, or similar rooms or areas shall be protected by any of the means described in 210.12(A)(1) through (6):
Arc-Fault Circuit-Interrupter Protection. Where a receptacle outlet is supplied by a branch circuit that requires arc-fault circuit interrupter protection as specified elsewhere in this Code, a replacement receptacle at this outlet shall be one of the following
(1) A listed outlet branch circuit type arc-fault circuit interrupter receptacle
(2) A receptacle protected by a listed outlet branch circuit type arc-fault circuit interrupter type receptacle
(3) A receptacle protected by a listed combination type arc-fault circuit interrupter type circuit breaker
This requirement became effective January 1, 2014.
Exception No. 1: If RMC, IMC, EMT, Type MC, or steel armored Type AC cables meeting the requirements of 250.118 and metal outlet and junction boxes are installed for the portion of the branch circuit between the branch-circuit overcurrent device and the first outlet, it shall be permitted to install an outlet branch-circuit type AFCI at the first outlet to provide protection for the remaining portion of the branch circuit.
Exception No. 2: Where a listed metal or nonmetallic conduit or tubing is encased in not less than 50 mm (2 in.) of concrete for the portion of the branch circuit between the branch-circuit overcurrent device and the first outlet, it shall be permitted to install an outlet branch-circuit type AFCI at the first outlet to provide protection for the remaining portion of the branch circuit.
Exception No. 3: Where an individual branch circuit to a fire alarm system installed in accordance with 760.41(B) or 760.121(B) is installed in RMC, IMC, EMT, or steel-sheathed cable, Type AC or Type MC, meeting the requirements of 250.118, with metal outlet and junction boxes, AFCI protection shall be permitted to be omitted.
(B) Branch Circuit Extensions or Modifications
Dwelling Units. In any of the areas specified in 210.12(A), where branch-circuit wiring is modified, replaced, or extended, the branch circuit shall be protected by one of the following:
(1) A listed combination-type AFCI located at the origin of the branch circuit
(2) A listed outlet branch-circuit type AFCI located at the first receptacle outlet of the existing branch circuit.
Exception: AFCI protection shall not be required where the extension of the existing conductors is not more than 1.8 m (6 ft) and does not include any additional outlets or devices.
This IAEI article on AFCI helps a lot.