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Replace old subpanel or not?

Posted by kisu (My Page) on
Tue, Feb 19, 13 at 20:46

Friend had old main panel with 100amps and no spare spaces. Getting everything upgraded - new main panel with 200amps already done last month.

House is 100 years old with mixed knob& tube wiring and new NM romex wiring in parts that were remodeled. For example the kitchen was remodeled 4 years ago and since it was right above the service room in the basement, they ran all new wiring to the panel.

Currently, remodeling entire 3rd floor and contractor is putting a 16 space sub-panel on the 3rd floor to connect all the lights and breakers for the entire 3rd floor to include 2 bathrooms on 3rd.

In the Kitchen on the 2nd floor is a very old Square D QO 125 amp sub-panel with 8 breakers - I think it's fed from a 60amp breaker from main panel, but I don't remember size.

All of the wiring coming into this old subpanel is old knob and tube type. None of it is for the kitchen. Some of the breakers power the other old rooms on the 2nd floor which have old plaster covered walls. Some of it originally also powered the 3rd floor but is no longer needed, so on the 3rd floor the contractor cut the old K&T lines and put junction boxes wherever he cut the knob& tube wire since it is still live due to possibly going to other locations on the 2nd floor and basement and the Attic room on the roof. The entire 3rd floor was newly wired with NM (romex).

Back to the old 8 space subpanel in the kitchen. It is old - and it is working fine. Should it be replaced with a new one? They would simply be swapping out the old box with a new one and not adding any arc fault switches since it is all knob and tube wiring and has shared neutrals. Or should they just leave the existing old panel as is.

Cost is not an issue here since the panel is less than $50.
They will not be changing any of the knob and tube wiring feeding it since it goes to areas on the 2nd floor that were not remodeled at all so that will all stay.

Follow-Up Postings:

RE: Replace old subpanel or not?

Square D QO is still in production and well respected. The breakers are current and modern. Only replace the box if there are safety issues such as visible corrosion, burn or arc marks, or pitting or arcing where the breakers contact the power bus. You could probably get arc fault breakers to fit this existing box. They would be safer, but expensive. And nuisance tripping may drive you crazy. Or you could look at it this way. If an arc fault keeps tripping maybe that circuit needs to be replaced.

RE: Replace old subpanel or not?

If by "shared neutrals" you mean MultiWire Branch Circuits, some companies do make double pole arc faults for MWBCs. I don't know if square d makes them though.

RE: Replace old subpanel or not?

Square D does NOT make any 2 pole arc faults. They're not that common even in other lines. CH does not make them any more. GE does.

RE: Replace old subpanel or not?

Note, that it is unlikely that there are any MWBC with k&T. If you have any "sharing" of neutrals it most likely is NOT going to be solved by a two pole AFCI.

RE: Replace old subpanel or not?

Yes - it is all old style K&T going to this subpanel so no way to add Arc Faults. Box looks clean and since it is in an interior kitchen wall it does not appear bad from a quick look. It's maybe about 65 years old.

RE: Replace old subpanel or not?

Even touching K&T can cause prob\lems.

Some of the wire is rubber insulated directly over bare copper (lead plating the copper was added later to protect the rubber).

The rubber and copper react to make the rubber insulation extremely brittle.
The slightest flexing (even vibration) often results in the insulation falling away in large pieces.

RE: Replace old subpanel or not?

Are you saying there is K&T as the FEEDER of the subpanel? HOLD THE PHONE. This is a different story. Something is very wrong here. Replace that feeder.

RE: Replace old subpanel or not?

no no no - sorry for the bad explanation. The feeder wires coming from below this subpanel and going straight to the main service are not K& T. They appear to be about 6AWG-
definitely fatter than 8awg - I'll have to look again. The main service panel has a 40A breaker to this sub-panel. And yes sometimes they had issues if a vacuum is plugged in and couple other things.

But now that the entire 3rd floor is being taken off the load of this subpanel, I am thinking they won't have an issue anymore when construction is done.

You have convinced me that the old subpanel in the Kitchen should just stay or they risk cracking the old K&T lines coming into it when doing a swap to a new model panel.

But should we keep the 40A breaker in the main service? If they continue to have problems when plugging a vacuum on the 2nd floor (where most of the plugs are still 2-prong and fed by K&T lines) - should we change it to a 50A breaker?


RE: Replace old subpanel or not?

You can size the breaker for the feeder to the largest that is applicable to the remote panel and the feeder conductors themselves. If you've really got #6 going there you could up the feeder breaker to 60A provided the subpanel can handle it.


...which would be unlikely that it wasn't.

One thing I'd be sure to check on some ancient K&T based panel is whether that subpanel is properly wired as a sub panel. Normally things things were left over main panels repurposed as subs when wiring got updated. Now if the only thing coming off that panel is K&T, there's not really an issue (K&T doesn't usually have equipment grounds). However if someone hacked in a piece of NM or something with a ground, you better make sure that ground is *NOT* just tied to the neutral conductor.

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