# Knob and Tube amperage.



## Sparkee

What is the amperage rating for a knob and tube system in a home? I'm finding entire systems being protected by a 30 amp screw-in fuse, is that kosher?


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## crazymurph

The fuse size is determined by the wire size, as you know. My guess is that the wire is #12 or #14 and shold be fused at 20 or 15 amps. The HO put the 30 amp fuse in because he was contantly blowing the fuses. IMO the old K&T shold be fused at no more than 15 amps. If the HO doesn't like it, TFS. Mr HO needs a new service and some rewiring. Take out his 30 amp fuses and install 15 amp fusestats. That way he is limited to 15 amps, and those suckers are hard to get out


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## egads

Well, having worked on some of those old K&T places, I can say that they often have a main out by the meter with 30 amp fuses. That protects the #10 feeding a group of circuits with more fuses on the back porch or laundry room. Most of those old houses only have one 20 amp circuit, #12 wire run to what was a iron plug. The rest of the house is usually #14 protected by 15 amp fuses. So, like crazy says, if you cannot talk them into rewiring, you should use a fusestat sized to the wire to keep folks from upping the fuses to keep them from blowing. 

I bet Sparky is seeing these in rental units with long time owners. It is impossible to close escrow on any property in most areas of CA on property with fuses. It's not a law, it just that you cannot obtain property insurance. No insurance, no loan funding.


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## Sparkee

egads said:


> Well, having worked on some of those old K&T places, I can say that they often have a main out by the meter with 30 amp fuses. That protects the #10 feeding a group of circuits with more fuses on the back porch or laundry room. Most of those old houses only have one 20 amp circuit, #12 wire run to what was a iron plug. The rest of the house is usually #14 protected by 15 amp fuses. So, like crazy says, if you cannot talk them into rewiring, you should use a fusestat sized to the wire to keep folks from upping the fuses to keep them from blowing.
> 
> I bet Sparky is seeing these in rental units with long time owners. It is impossible to close escrow on any property in most areas of CA on property with fuses. It's not a law, it just that you cannot obtain property insurance. No insurance, no loan funding.


 Your right, it's a rental unit but it was vacant for a year after having a long time renter. It was a 30 amp screw in fuse before I changed it out to a 100 amp panel and the 20 amp breaker was blowing when they used those damned space heaters (since then they got the furnace working). I'm glad the breaker is doing it's job especial since you guys say it's only 12 gauge wire. I'm waiting for the O.K. from the owner to change out the interior wiring today.


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## nolabama

I have seen stupid loads put on Knob and Tube. You would think that 80 years with a penny under a fuse would damage the wire, but it seems to take a licking and keep on ticking.


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## 480sparky

K&T was usually type R conductors. Type R had an NEC ampacity of 15amps for 14 and 20amps for 12.


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## s.kelly

I have an old meter base/disco out in the garage that for some reason I think was 30 amp. It was gutted and being used as a junction box when I got it. If I remember right tere was a bit of old wire left that looked like it was probably #10 and at the time it seemed likely it was the main wiring into the house based on the area. Rest of the circuts were what looked to be a #14 like you would expect.I guess there was some sort of fuse panel somewhere once upon a time,but someone had already done a hack remodel or two. Some scarry stuff in that one.


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## MDShunk

You guys are gonna hate me for this, but I have two standards of practice when working on existing K&T systems:

1) When I'm working in an Edison base fusebox I find that is overfused, I automatically install type S adapters and install the proper sized type S fuses

2) When I'm doing a service upgrade or panel change on an installation that has existing K&T, I put the #12 K&T on 15 amp breakers and put the #14 K&T on 10 amp breakers (even though 10 amp branch circuits are technically illegal).

To date, neither of these practices has gotten me in any hot water. Indeed, I'd say I've gotten a great deal of work from it. 

I'll add this also (without shame). When I find full boxes of spare 30 amp fuses on top of a fuse box that is generally overfused, I steal them. They go right in the trash can at my shop.


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## 480sparky

MDShunk said:


> You guys are gonna hate me for this, ...


We won't hate you for that Marc. We'll just hate you. :laughing:




MDShunk said:


> ......but I have two standards of practice when working on existing K&T systems:
> 
> 1) When I'm working in an Edison base fusebox I find that is overfused, I automatically install type S adapters and install the proper sized type S fuses.


SOP for me as well.



MDShunk said:


> ..........2) When I'm doing a service upgrade or panel change on an installation that has existing K&T, I put the #12 K&T on 15 amp breakers and put the #14 K&T on 10 amp breakers (even though 10 amp branch circuits are technically illegal)..


I don't go as far a 10a breakers. Just all the old, original circuits go to 15a breakers no matter what size they were on to start with.


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## AFOREMA1

Sparkee said:


> Your right, it's a rental unit but it was vacant for a year after having a long time renter. It was a 30 amp screw in fuse before I changed it out to a 100 amp panel and the 20 amp breaker was blowing when they used those damned space heaters (since then they got the furnace working). I'm glad the breaker is doing it's job especial since you guys say it's only 12 gauge wire. I'm waiting for the O.K. from the owner to change out the interior wiring today.


Sparkee you change the main panel out too a 100a panel without removing the old K&T and installing romex? If so How did you pass inspection? If we alter a K&T system here it needs to be completely removed and wires brought up to code.


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## 480sparky

AFOREMA1 said:


> Sparkee you change the main panel out too a 100a panel without removing the old K&T and installing romex? If so How did you pass inspection? If we alter a K&T system here it needs to be completely removed and wires brought up to code.


Requiring that would be up the the local AHJ. Yours does. His doesn't.


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## Murphy

not to mention side jobs dont ever get inspected


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## Sparkee

AFOREMA1 said:


> Sparkee you change the main panel out too a 100a panel without removing the old K&T and installing romex? If so How did you pass inspection? If we alter a K&T system here it needs to be completely removed and wires brought up to code.


I wish it was like that here. The inspector never even looked inside the house. He just wanted to see if it was grounded and bonded (ground rod and water pipe) and that the circuits were labeled. The owner just gave me permission today to rewire all the outlets so better late then never.


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## sparky.jp

Nobody has mentioned the fact that conductors in free space can radiate a lot more heat then they can when in conduit or NM. Compare the ampacities in Table 310.17 to those in Table 310.16. From 310.17, a 12-gauge wire is good for 30 amps at 60 deg. C rating (yes, I know, you can't do this per 240.4(D), but from strictly a safety perspective, it doesn't worry me a bit).

Also, if the splices were soldered properly as was the practice, these connections are much more reliable than any wirenut junctions (lower electrical resistance).

I'd much rather have 1920s K&T in my house than 1940s asphalt-insulated NM cable on which the wire insulation inside the J-boxes disintegrates when you touch it or move it, especially in the ceiling fixture boxes (like the house I grew up in)!

There is still K&T going to the outbuildings on my grandparents' farm. Shoot, even if all of the insulation falls off the wires, there is still 6" of spacing between the hot and neutral (and of course no ground to short out to). I still remember the exposed two-pole knife switch with the tiny black bakelite knob that my grandfather used to turn on the back pasture well pump. Standing on wet muddy ground of course! Oh, how far the safety pendelum has swung! Now I need a TR receptacle for my garbage disposal???:blink:


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## Sparkee

sparky.jp said:


> Nobody has mentioned the fact that conductors in free space can radiate a lot more heat then they can when in conduit or NM. Compare the ampacities in Table 310.17 to those in Table 310.16. From 310.17, a 12-gauge wire is good for 30 amps at 60 deg. C rating (yes, I know, you can't do this per 240.4(D), but from strictly a safety perspective, it doesn't worry me a bit).
> 
> Also, if the splices were soldered properly as was the practice, these connections are much more reliable than any wirenut junctions (lower electrical resistance).
> 
> I'd much rather have 1920s K&T in my house than 1940s asphalt-insulated NM cable on which the wire insulation inside the J-boxes disintegrates when you touch it or move it, especially in the ceiling fixture boxes (like the house I grew up in)!
> 
> There is still K&T going to the outbuildings on my grandparents' farm. Shoot, even if all of the insulation falls off the wires, there is still 6" of spacing between the hot and neutral (and of course no ground to short out to). I still remember the exposed two-pole knife switch with the tiny black bakelite knob that my grandfather used to turn on the back pasture well pump. Standing on wet muddy ground of course! Oh, how far the safety pendelum has swung! Now I need a TR receptacle for my garbage disposal???:blink:


What you said about wire in free air makes total sense. This is the first job where I've really studied this K & B stuff instead of just dismissing it and I got to say there really was an art to it and it was built to last. I mistakenly thought one circuit was feeding the whole house but there was a jumper on top of the 2 pole disconnect so it was two 30 amp circuits feeding the house and I'm assuming it was one for outlets and one for lights. Still I've already started replacing it with romex and grounded outlets so bye bye knob and tube.


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## XCasper

Florida's "Citizens Insurance" program (state funded; where you go when no one else will insure you) requires all live K&T be removed and I'm told requires that the service be less than 20 years old.


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## MDShunk

XCasper said:


> Florida's "Citizens Insurance" program (state funded; where you go when no one else will insure you) requires all live K&T be removed and I'm told requires that the service be less than 20 years old.


I'd say that of all the "completely remove the K&T" jobs that I've done, a good 90% of them were an insurance company requirement. Insurance companies have developed a habit of actually visitin a home they are about to insure, especially right after it has been sold and the new owner goes to buy insurance. If the people don't remove the K&T, they simply can't buy insurance from that company.

As has been pointed out (and in my professional opinion, I agree), old cloth braided romex was 20 times more dangerous than K&T. I'd take K&T over cloth NM cable any day of the week.


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## Dennis Alwon

One of the big problems with K&T is that the wiring method was designed before insulation was around. It was not intended to be installed in insulation. Every house that has added the insulation in the attics or walls have now created a new hazard. 

K&T by itself is not that bad a system. It is what we have done to it that has caused the problems.


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## Archania

In CA here, I recently did a 200A service upgrade in an old 20's house. Just hooked up all the old 14ga k&t circuits to the new 15 amp breakers. They don't care what kind of wiring is in there, just as long as its properly fused. Kindof ridiculous to say that if you are just switching out the service you should have to rewire the whole house...
And yea, all the fuses that were in the old box were 30amp time delay fuses!


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## Sparkee

Archania said:


> In CA here, I recently did a 200A service upgrade in an old 20's house. Just hooked up all the old 14ga k&t circuits to the new 15 amp breakers. They don't care what kind of wiring is in there, just as long as its properly fused. Kindof ridiculous to say that if you are just switching out the service you should have to rewire the whole house...
> And yea, all the fuses that were in the old box were 30amp time delay fuses!


I would say it's because you want it to be grounded and with the Knob and Tube there is no ground


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## XCasper

MDShunk said:


> I'd take K&T over cloth NM cable any day of the week.


 
Yup, yup. But have you ever run into the house with live K&T that then has both cloth NM and then later modern NM spliced in? In the attic is one thing but I have found these buried in walls before. Hense why I tend to agree with the all out K&T ban. There is no way to guarantee it hasn't been tampered with.

Of course the same can be said about flying splices of NM-B in a wall but I'm sure none of us have ever found that before.


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