Working with knob and tube
March 12, 2005 12:54 PM   Subscribe

My house still has some knob and tube wiring. I am concerned, in particular, with the ceiling light fixture in my home office where I just installed a new light. Neither wire was marked as hot or neutral, and my voltmeter read the same thing no matter which way I touched the wires. So I figured I had a fifty-fifty chance of getting it right. Will anything bad happen if I got the lamp wires connected to the wrong source wires? How can I tell which wire is which in the future?
posted by booth to Home & Garden (12 answers total)
knob and tube?!
it will work the same either way, the only difference is from a safety point of view - you should switch live (or both), rather than neutral, so that when it's "off", you lower the chances of getting a shock if you stick your finger into where the bulb goes.
so if you just added a light fitting, and not a switch, there's no need to worry.
if you want to tell the difference, measure the voltage difference between a wire and earth. live should be 120V (in the USA), neutral will probably be less.
i guess you tried putting the red doodah on live and the black one on neutral, and then switching them round? that won't work because house electrics are alternating current - positive and negative switch back and forth 50 times a second.
ps i guess "hot" is american english for "live", so translate as necessary.
posted by andrew cooke at 1:10 PM on March 12, 2005

To a light fixture, hot and neutral are meaningless -- a light just puts a load across the wires. In a proper fixture, hot is on the center pin, neutral is on the socket, but since code demands the socket be insulated, it really doesn't matter. Light and Space Heaters are the two electrical appliances that are basically impossible to wire in wrong. (Motors would turn the wrong way, and when you're dealing with neutral and ground, miswires can create real safety hazards.) Note that even today, floor and desk lamps have two prong, not three prong, plugs.

Your voltmeter read the same because it is the same = ~120V AC. If you had the tools, you could have told that you'd switched wires by the phase change.

In cases of properly installed k&t wiring, the hot line is colored black, the neutral, white. If not coded, the only way to tell it to walk the wire back to the fuse block. The hot line will be the fused line, the neutral connects directly to the neutral bar. You'll have three bars, the hot bar for the left fuses, the neutral, and the hot bar for the right fuses.

In improperly installed K&T, the neutrals will also be fused. This is a bad thing, and needs to be looked at by a pro.

If you have any 220V wiring, they'll be wired from the left to right bars directly.

Note: Most K&T maxes out at 60A total for the *house* -- be careful about new loads. Many K&T installs don't have a master fuse, and it really isn't safe to trust that the correct fuses are still installed. I saw one box that had 12 15A fuses installed. This is bizzarro wrong -- many houses built today have only 150A feeds, and this box was set to draw 180A. Turns out the feeder was rated for 60A.

Electricians can, and will, use K&T with modern installations, by using a sub-panel. But, they can't do it unless the K&T wiring meets modern code regs. If installed by a pro, K&T probably will -- back in the day, they were afraid of electricity, and they tended to do a very thorough job. It's when you get Joe Amatuer hacking on the wires that K&T can get unsafe.

The big problem with K&T remains, though -- it was made in a day where much less electricity was used. So, you must watch how much of a load you place on the circuits, and *never* overrate or bypass a fuse. If you keep blowing 7.5A fuses, it means you've got too much on that circuit. Don't install a 15A fuse, find out what's overloading.

On preview: Andrew Cooke: Hot=Live, Neutral=Neutral, Ground=Earth. Two cultures seperated by a common language.
posted by eriko at 1:13 PM on March 12, 2005

There is a tool that can easily ID the hot wire, like this Non-Contact AC Voltage Detector. I was using one just today and they not only does it ID the hot wire but can do so through the insulation and also tell you if a circuit is live in general -- when touched to the outside of Romex cable, for example.

I was just adding an outlet to my shop today and used this Complete Guide to Home Wiring: A Comprehensive Manual, from Basic Repairs to Advanced Projects. It is a pretty thorough reference and does cover some knob and tube issues.
posted by Dick Paris at 2:54 PM on March 12, 2005

knob and tube?
posted by bonaldi at 2:54 PM on March 12, 2005

(Motors would turn the wrong way...)

No, not on AC.
posted by kindall at 3:21 PM on March 12, 2005

eriko : Note that even today, floor and desk lamps have two prong, not three prong, plugs.

eriko's got it right. Note that the reason we can get away with two prong plugs on double insulated equipment and lamps is because the plug is polarized (one prong is bigger than the other).

Knob and tube wiring or post and tube. Basically the hot and neutral wires are ran separately using ceramic insulators. Where the wires go thru a stud they are insulated from the stud with a ceramic tube which has one end enlarged to stop it from going all the way thru the hole in the stud. Where the wires run along a stud they are fastened to the stud using a knob/post. A knob consists of a ceramic post about the size of a spool of thread and a retainer about the size of a beer bottle cap both with holes in the centre. The two pieces are held together with a nail that is driven into the stud. The wire is placed into a grove formed into the end of the post before setting the nail.
posted by Mitheral at 3:25 PM on March 12, 2005

Thanks to all. (It's tough to pick a "best answer" from this lot.) FYI, I did not add anything to the system - I just replaced a light fixture, clipping the old one off and wiring a new one in its place.

Mitheral, thanks for the explanation. I assumed that people knew what knob and tube was. Every house I've lived in has had it. I likes the old houses.

Dick Paris: Thanks for the book recommendation. I have stood in the home repair aisle at Borders without finding any in-depth discussions of how to work with k&t.
posted by booth at 6:40 PM on March 12, 2005

eriko: Your voltmeter read the same because it is the same = ~120V AC. If you had the tools, you could have told that you'd switched wires by the phase change.

Okay, I don't mean to be picky, but it would be pretty hard telling anything from a phase change without a reference to check it against...

Dick Paris's non contact volt meter idea is great though, I got one the other day for about $10 and it is way cool!

Mitheral, do you have any specific knowledge about the reason for polarized two prong plugs? There is a lot of misinformation around, like this site:

You've probably noticed that plugs often have one prong that is wider that the other. This is done so that it can be properly aligned with a receptacle. The smaller narrower pin is for the "hot" black wire and the larger wider one is for the "neutral" white wire. This is very important when it comes to wiring new appliance plugs or receptacles as it assures that the current flow is proper through that device. Of particular note are certain devices or appliances that have circuitry, such as, coffee makers, alarm clocks, computers, etc. If the hot and neutral wires are reversed, damage can occur to these appliances. Do not "clip" polarized plugs to fit into a two (2) prong receptacle or a receptacle not designed for polarized plugs.

That is dead wrong of course. What the hell does "the current flow is proper through the device" mean anyway...

Of course, plugs are polarized for a reason... The only reason I can think of is that safety approval agencies allow neutral connected metal to be single insulated while live connected metal must be double insulated.
posted by Chuckles at 7:24 PM on March 12, 2005

One advantage to wiring hot to the bulb/fixture tip instead of the ring is that you won't get a shock when replacing a dead bulb with the socket "live" if you accidently touch the ring of the bulb. I believe this is why 2-wire lamps have polarized plugs, and fixtures have definite hot (black) and neutral (white) wires.
posted by ZenMasterThis at 8:25 PM on March 12, 2005

more generally, neutral is supposed to be ground, more or less, so if you've a two wire system like knob and whatever this is called, you end up using neutral as earth for some extra safety. hence the need to have sockets that preserve the distinction between live and neutral.
posted by andrew cooke at 4:13 AM on March 13, 2005

It's kind of like Boston and Seattle. If you live in Boston (hot,) then Seattle is a long way off, if you live in Seattle (hot,) then Boston is a long way off. The distance between the two (voltage) is the same measured from B to S or from S to B. Home is where the hot is.

Or is this just so much BS.
posted by leafwoman at 1:38 PM on March 13, 2005

The smaller narrower pin is for the "hot" black wire and the larger wider one is for the "neutral" white wire.

This part is right the rest is just gibberish. At least this time it is not dangerous gibberish.

ZenMasterThis and andrew cooke have the basic concept right. In a perfect world where everything was done to code; we didn't have 30-100 year old wiring hanging around; equipment never malfunctioned and no one ever made a mistake you could safely walk up to any neutral conductor and grab on to it. It would be no more dangerous than touching your kitchen tap. Because of this safety regulations allow some neutral conductors to be exposed, like the socket of a light fixture. Good thing to because other wise light bulbs and fixtures would be much more expensive.

Getting to your question: If we allowed people to plug two prong cords in either direction 50% of the time the hot wire would be connected to the exposed conductor and their would be a chance for a shock every time someone changed a light bulb. So the polarized plug forces neutral to be connected to the exposed conductor. You may notice that some three prong plugs are not polarized. The third prong in this case acts to orient the neutral. Polarization is as you surmised strictly a safety feature; the equipment could care less and will not be harmed in anyway. In fact I'd bet 1 in every couple hundred devices are plugged in wrong either because the device has been assembled wrong or the wall socket is wired wrong. Extreme example: I've done service work in an knob and tube house where the mains were reversed. Of course they had no polarized outlets either so it was 50-50 anyways.

Note that this explanation is applicable to single phase 120V North American power only. Other places and some commercial settings often have totally different systems. Even in NA don't go around grabbing hold of live wires; mistakes and outright idiocy are frighteningly common. You never can tell if your neighbour is stealing power to run his grow op and has screwed up feeding 100V to your neutral.

The reason it doesn't matter to the equipment is your house power is AC. Positive and negative change places 120 times a second (60hz). Hooking up DC equipment backwards will damage a lot of equipment unless you have a pure resistive load. Everyday extreme example: reversing a set of jumper cables can cause a car battery to explode spraying hot sulfuric acid all over.

Also a note on grounds. Although in theory[1] neutral and ground are the same thing, practice is totally different and the two should never be connected together except in special conditions. If you don't know what those conditions are you shouldn't be connecting the two together. I've seen as much as 70V to earth on a "ground" that was floating so its also a good idea to not assume a ground conductor is actually dead. It's so bad there is a whole set of special equipment to create isolated grounds. If you've ever seen an orange wall socket (or more recently a normal socket with an orange triangular sticker, sigh) you've seen an isolated ground.

[1] you know what they say about theory and practice: 'the difference between theory and practice is, in theory, theory and practice are the same; in practice, they're different.'
posted by Mitheral at 8:18 AM on March 14, 2005

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