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How can i tell the "black" wire from the "white" wire?

I have an old house that is 100 years old. Much of the wiring is still the cloth covered copper wiring. I need to know if there is a way to tell the "black" wire from the "white" wire without buring the house down first. Thanx

8 Replies

Re-design (author)2009-09-28

Go to home depot and get a powered "hot wire" detector. You don't have to touch anything. when it gets close to the hot wire it will beep. Much saver. I wouldn't trust a neon light or anything that puts ME in the circuit to discover the hot wire. I don't care how long they've been doing it.

seandogue (author)Re-design2009-09-28

yup. what he said. There is likely no earth ground nearby at most of the locations where you'll be testing, unless they're near a water line or near the main electrical distribution panel. And the water line isn't a 100% sure thing for earth ground either. Being a bit of a busy body, I'd urge you to consider replacing it as soon as possible, at least for your ac outlets if not for the entire home. the end-of-life on that insulation came up years ago, and knob and tube also provides no earth ground connection in almost all older homes. The only thing that keeps it from flaming is that knob and tube has a reasonable separation distance...except where it connects to something (the fly in the ointment). The last time I dug some out, the clothe insulation literally fell off the wire. I also live in a pre-1900 home, and I have two circuits left to gut. In fact, I'm glad you reminded me to get off my lazy butt.

MahavishnuMan (author)seandogue2009-09-28

Living in central Pennsylvania, we see a lot of that as this area was settled during the early Victorian era. While the architecture is lovely, the wiring is atrocious. The volunteer fire department is stretched to its limits answering calls to house fires, particularly during the summer and winter months when window unit A/C's and electric heaters are most frequently used. The cause of almost all these fires: ancient electrical wiring. It's difficult for the codes department to enforce the laws requiring landlords to change this wiring before these fires start, primarily because most are completely clueless as to how old the wiring is when they rent (or even that grounded plugs are a necessity for safe electrical usage - I see the ground prong busted out of countless power strips when I work on computers and home theater setups around here). Not to mention the vast majority of landlords around here are happy to own tinderbox slums. Hey - as long as it's insured, they make out anyhow. If you're a homeowner, take action immediately and buy the piece of mind that comes from knowing your wiring is safe to use. If you rent, alert your landlord immediately, and if s/he refuses or delays to replace the wiring, get the authorities involved. You may well be saving your life and property.

seandogue (author)MahavishnuMan2009-09-29

Knob and tube wiring was once state of the art. Knob an tube is not in itself intrinsically dangerous. In fact, in some ways it is considerably safer than Romex, due to the separation distance between the hot and return wire. The problem is that the insulation on 80-100 year old wiring has exceeded its useful life, and it also provides no ground path, which is necessary for operating many modern pieces of equipment. It worked just fine for 60 or 70 years before the new standards were implemented, and people still cause electrical fires with modern wiring by using overrated fuses and attaching too many loads to single outlets. There's no need to play alarmist games. Most knob and tube is safe, as long as it's left alone. Like asbestos sheathing in heating systems, it is only when it is disturbed that it becomes problematic. That disturbance often occurs when new homeowners decide to start replacing switches and outlets, thinking that it will make things better. or hide the fact that they don't have earth grounds. Sloppy re-wiring the outlets and switches is what can cause the localized shorts. But worst is morons (that would be users, not landlords) who apply overrated fuses or circuit breaker to distribution boxes so they can operate large loads thru single outlets. As I said, I would urge the author to consider replacement, starting with runs to outlets, if for no other reason than to add earth ground. If he/she can afford to replace the entire electrical system, so much the better, but economic realities being of issue in today's world, where the top 10% is taking home 110% the income of average citizens, it's becoming more and more difficult to do things "the right way". BTW, the number one cause of electrical fires, as far as I know, is space heaters.

MahavishnuMan (author)seandogue2009-09-29

I assure you I'm not trying to be an alarmist and I agree that there are a fair number of these fires caused by "user error". I also agree that it is an expensive repair (and perhaps too expensive for most to afford out-of-pocket, particularly in this day and age). However, while there may be a fair distance between the conductors, the degradation of the insulator is a matter that should be taken seriously. There are many things that could go wrong. I'm not saying they definitely will, I'm saying they could - and do. An appliance could malfunction, causing the wire to overheat and ignite what's left of the cloth insulation. The house settling over 100 years time could, somewhere, bring the conductors close enough together to allow arcing. Water from a leaky century-old pipe could short the wiring in the walls. Yeah, the scenarios seem far-fetched, but it happens. All I'm saying is that no one wants to live in a place where something could go wrong. We drive cars with air bags, we throw out milk when it's past the sell-by date (even if it's good for the next couple of days), and we make sure our babies don't sleep on their bellies. If I bought a house with ancient wiring, I'd borrow enough to replace it before I moved in; I'd do the same if the water heater wasn't installed correctly, or if I had a loose railing on the porch. And yes, the landlords around here are purposely neglectful of codes. I've had to call shenanigans on a few of them (not for wiring, thankfully).

orksecurity (author)Re-design2009-09-28

Probably a better answer, for someone who has to ask the question. Much less risk of accidentally contacting something they shouldn't.

framistan (author)2009-09-28

Go get a NEON voltage bulb at radioshack. They sell them 2 ways. individually, and as a pocket sized little "probe." This sounds dangerous, but it is NOT DANGEROUS if you have a NEON bulb. What you do is hold onto ONE wire of the neon bulb(or neon probelight). Touch the OTHER wire to the UNKNOWN wire. (touch the COPPER part of course, not the insulation). You will see the neon bulb light-up DIMLY when you touch one wire, but NOT the other wire. The one that LIGHTS up the neon bulb is "HOT" or "black" the other one is white. If it is OUTSIDE, and sunshiny, you will not be able to see the neon bulb light up. you will need to wait for the sun to go down. This works because the neon bulb will light up with only MICROamps of current, and your body acts as a kind of GROUND. Do NOT try this with any other kind of bulb, or you may get shocked! NEON ONLY. Of course I am assuming the house is connected to the grid, (electricity connected). Sometimes these neon probes are sold as a kind of SCREWDRIVER probe.The TOP of the screwdriver has a little metal rivet you are SUPPOSED to touch with your finger, while touching the wire with the screwdriver blade. Be sure it is a 110 volt detector, not a 12 volt CAR screwdriver-probe! Be carefull and ask questions at radio shack, to be sure they sell you the right thing. Your safety is your own responsibility, dont blame me if you get shocked. .............. framistan

orksecurity (author)framistan2009-09-28

The probe I'm used to doesn't put the user in circuit. One contact goes the the wire being tested, the other goes to a known ground (typically the outlet box is grounded -- or should be! -- for safety). The line which lights up is hot; the line which doesn't is neutral.