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I have this old fuse box in the attic (pictured, basically acting as a junction box) and 6 wires total running into it, 4 are knob and tube, so two are hot, two neutral, they all go dead when one 15 A breaker is tripped in the basement, and the other two wires are more Romex-like wires with hot, neutral, ground all bundled together, they are run on a 15A breaker and a 20A breaker in the basement. I wanna connect the knob and tubes to the Arc-fault breaker I got in the new square D sub-panel which will go in he attic, the knob and tube aren't in the basement for me to put the arc fault breaker in down there and connect to them

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    An AFCI is a good idea but you don't need to move the arc-fault breaker closer to the knob-and-tube. Fitting it down in the service panel you have will suffice. The modern cable in between won't matter. So there is no benefit to fitting the arc-fault in the attic. What will matter is a breaker panel in an attic will have some serious accessibility issues, and may not be permitted. – Harper May 7 '17 at 18:38
  • This part where you said "they all go dead when one 15 A breaker is tripped in the basement" worries me. – Kris May 8 '17 at 13:20
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I agree with Harper and RME. You do not need a sub-panel.

A junction box will work just fine especially in an attic even if it is a walk-in attic.

The National Electrical Code requires, with few exceptions, that a conductor be protected at the point where it receives its supply.

The fact that you can shut all of this off with three breakers in the basement means that they are already protected.

If you wish to add AFCI protection then just replace the breaker or breakers in question with an AFCI breaker. Much easier and cheaper.

Good luck and stay safe!

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I think you will not want to crawl into your attic every time you need to reset an arc fault.

My question is why put in a subpanel at all? It looks like your fuse box was at least part of the original service panel when it was installed. Form what your are describing all circuits are now be protected by breakers in you new circuit breaker panel. If what I just said is true, then I would just replace the fuse box with a junction box. Do not tie all the neutrals together identify them with each circuit.

They now manufacture arc fault devices just like GFCI's. What I would do is put them at the head device of your circuit and feed the remaining downstream. Thinking ahead I'm not sure you can get the arc fault to work properly on knob and tube wiring. We know is not grounded and the old system may need to be replaced before proper operation is accomplished. I don't know because I've never tried it.

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