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Knob and Tube � are these ganged fuses or fused neutrals?

Posted by DutchColonial (My Page) on
Sun, Aug 17, 14 at 9:52

Summary: I�m starting to think we�re going to need to replace all our knob & tube because of likely damage due to too much amperage running through it. But I need help understanding how the system is wired with all these two-fuse circuits I�m finding.

The long story: The 1902 house we just bought has a 60-amp fuse panel in the basement, another subpanel (which was probably originally the main panel) in the back entryway, and both knob & tube and newer wiring found throughout the house. Our original thought was to rewire everything circuit by circuit as we renovate (we are advanced DIYers with some experience in this area, but not with an old house) and to have a professional install a new 200-amp breaker box. But after talking with neighbors and reading more online, we started feeling okay about keeping the knob and tube, especially since all of the K&T outlets are on interior walls, so we don't have to worry about insulation added around it. We would just add a couple new circuits of grounded outlets on the exterior walls.

I just made a circuit map of the whole house by unscrewing one fuse at a time and checking which outlets turned off. The theory that the back-entrance panel used to be the main panel definitely makes sense, as almost all the ceiling lights in the house, as well as most of the 2nd-story outlets, as well as the garage, are all controlled by fuses on that subpanel. Since we have seen K&T on some of those circuits, we're assuming that all of those circuits are K&T (except for the garage).

The rest of the circuits (1st-floor outlets, basement outlets, large appliance circuits) come straight from the larger fuse box in the basement, and these all have newer wiring (not brand new, just not K&T).

But here�s the thing I�m worried about� the back-entrance subpanel is controlled by two fuses on the main panel, a 30A and a 12.5A fuse that are next to each other. Then, the subpanel has 6 fuses. (I was assuming and hoping that each fuse protected it�s own nice little circuit, but no.) Two of the fuses (30A and 30A) work in tandem to control the rest of the sub panel. Then, the remaining four control just two circuits� a 15A and a 30A control the main floor ceiling lights, a yard light, and a porch light, and a 30A and a 25A control everything upstairs and the garage.

First, why all the fuses that work together but aren�t matched in amperage? Just bad fuse replacement by previous homeowners?

Second, how much amperage has been running through this K&T system? I�m pretty sure the answer is "way too much." But on the circuit that is on a 15A and 30A, which is on a 30A and a 30A, which is on a 30A and a 12.5A, has it all been getting 45A? 42.5A? Or 30A? Or 12.5A? I read somewhere else that sometimes they used to fuse the neutrals. Is there a possibility that on these double-fused circuits, one of the fuses is for the hot and one for the neutral?

Let me know if I should post pictures of the panels. Thanks!


Follow-Up Postings:

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RE: Knob and Tube � are these ganged fuses or fused neutrals?

Almost certainly the mismatched fuses were supposed to be the same value and someone changed one over time with whatever they had handy.

It's not uncommon to have a few pullouts for larger things (typically stoves) and then one feeding a bunch of screwin fuses for smaller lighting circuits.

Indeed K&T installations often did fuse the neutrals. This we learned over years to be a bad idea and it's no longer permitted.

It's pretty much impossible to tell just based on your observations just what the loads and ampacities are.

Someone trained can inspect the visible parts of the K&T for integrity of insulation. Frankly, if it's bad due to overheating or just plain age I'd get rid of it. The problem with the stuff in the walls is you can't inspect it easily. Frankly if you're willing to learn a bit about doing electrical work and you have an electrician to set up a new modern panel for you, you can probably do a lot of the grunt work yourself. If you have accessible attics and basements it's not even hard.

As for doing it incrementally, well the argument is it's last the last 75 years or however old the house is, it might be ok for a bit, but I'd certainly NOT keep it in any room I've restored.


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RE: Knob and Tube � are these ganged fuses or fused neutrals?

So your wiring is only 100+ years old. It's not just the wire, or lack of grounds, that insulation is shot - brittle.

As an engineer (not a electrician) I'd get that stuff out of there in a heartbeat.


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RE: Knob and Tube � are these ganged fuses or fused neutrals?

The last guy couldn't figure it out either, that's why there's a subpanel - he left the old service in place because he was confused to heck.

Find the smallest-value fuses you can. Replace all of them with that. 12.5A isn't a normal common value anymore, so I'm going to bet that one is the oldest. You should probably try to find more 12.5's and use them in all of the spaces.

The problem you're going to find with your incremental replacement is that all of those circuits are going to tie into each other in ways that make no sense to you. A 'hot' wire is going to go up in the wall in one room but the neutral to the same circuit is going to be going the opposite direction in a totally different room. You're going to run a new circuit to one room and when you're pretty sure you're done with all of the k&t wire that goes to the old circuit you're very sure you followed, you're going to unhook that wire and three things you thought were completely unrelated are going to stop working.

Been there. Done that.

I enjoy the challenge, because this is a bit of a hobby to me - and the history of it is as interesting to me as the work, but if you're a "normal" person with a real life... I feel for you.


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RE: Knob and Tube � are these ganged fuses or fused neutrals?

I wouldn't say that we are normal people. We are the kind of people with the over-active imaginations that it takes to buy a fixer-upper. Thanks for the tips! I'll get the amperage of the fuse lowered right away.


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