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GFCI or AFCI?


Neal Lewis
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I was told recently by an electrician that installing a GFCI outlet as a safety retrofit for knob and tube wiring is code. Of course I know that is total BS (not to mention the insurance aspect). The electrician was trying to cover his butt because he missed a couple of runs of K&T during a whole house renovation.

AFCI devices aren't even required in Jersey, yet. But, I would think that as a temporary fix only, the AFCI device would be better protection than a GFCI? (I know there are combination breakers out there).

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Originally posted by Neal Lewis

I was told recently by an electrician that installing a GFCI outlet as a safety retrofit for knob and tube wiring is code. Of course I know that is total BS (not to mention the insurance aspect). The electrician was trying to cover his butt because he missed a couple of runs of K&T during a whole house renovation.

The NEC allows K&T to remain in service. There's no requirement to remove it or to protect it with anything other than a garden-variety circuit breaker. If you want to place a 3-slot receptacle on a K&T circuit (which is, by definition, ungrounded), you can do so if it's GFCI protected. Perhaps that's what he was saying.

AFCI devices aren't even required in Jersey, yet. But, I would think that as a temporary fix only, the AFCI device would be better protection than a GFCI? (I know there are combination breakers out there).

The present generation of AFCI devices -- the branch circuit AFCIs that take the form of circuit breakers in panels -- only protect against parallel arcs, not series arcs (well, mostly). A parallel arc occurs when one wire arcs to another or to ground. One nice thing about K&T systems is that the wires in a circuit are spaced apart -- the likelyhood of a parallel arc is less than it would be with cable.

An AFCI breaker will only provide protection of arcs in the K&T circuit in the very rare instance where you have an arc between two wires or between a wire and a grounded surface. In most cases, such an arc would trip a regular breaker; the AFCI would just trip at a lower threshold.

A GFCI device would protect the circuit against all ground faults, whether they were arcing or not.

Each device is useful in different ways. Neither is a "fix" for old K&T.

- Jim Katen, Oregon

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Originally posted by charlie

According to Square D, AFCIs do provide some additional protection for knob & tube where the connections are made (junction boxes---where parallel arcing can happen and where a lot of the deterioration/overheating takes place)

Perhaps, but why would that protection be specific to K&T?

And how rare would you guess it is to have an arc that's strong enough to trip an AFCI breaker and not trip a conventional breaker?

If there's deteriorated K&T wiring at a junction box, I'd be loath to recommend an AFCI as the solution.

- Jim Katen, Oregon

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