I am wondering if there is a standard practice for verifying that existing installed AC electrical wiring in conduits, especially older buildings, is good and safe? My thoughts were some kind of AC hipot or DC insulation resistance test from each wire to conduit. Any thoughts?
Testing existing electrical circuits
- Thread starterLPdan
- Start date
JD
Well-Known Member
Not without disconnecting each wire at both ends. (Very laborious.) I can't recall any modern THHN type conduit wire insulation problems. The problems I have run into are the conduits that go down, as in through a concrete slab. Often the slab may crack over the years and crack the conduit as well. If you have a high water table, the conduits can flood. Still takes years, but the insulation will saturate. I've pulled my fair share of wire here at the church and found "black copper." Building is from 1989 and we have cracks and water. Often, I can get a resistance reading between the wire and the conduit. Pretty high, but there. Then it's time to pull a new line.
In this case, it's about 12 circuits, all in the same panel. It would not be that much work to disconnect, as the loads are being changed out anyway. Do you find that a simple DC resistance check is pretty reliable? This is all above panel into ceiling, so no concern about flooding.
JD
Well-Known Member
DC resistance only checks for water saturation. Plastic wire in conduit lasts a LONG time. Unless there was a fire in the building that may have damaged the wire in the conduit. Hi-Pot testers usually test in the 2Kv+ region, but THHN as well as most other conduit wire is normally rated at 600vac. The test itself could break down insulation. Look for thermal damage and any signs that the plastic insulation has become dry and brittle. (not common.) Now if it's cloth covered rubber insulation, I would worry! Generally, that wire has not been used since the early 1950s.
BillConnerFASTC
Well-Known Member
@BillConnerFASTC When I began my provincially licensed electrical installation and maintenance IBEW apprenticeship in June of 1967, we were still pulling TW above grade and pulling or direct burying TWU below grade and below the water table. I'm not recalling THHN until well after the completion of my five year apprenticeship when I became involved with wiring automated AC servo drives.Anybody know when THHN was first introduced ? John talks as if its "new fangled" but I have a feeling some could be getting quite old. May still be fine - but maybe not "modern".
Toodleoo!
Ron Hebbard
Anybody know when THHN was first introduced ? John talks as if its "new fangled" but I have a feeling some could be getting quite old. May still be fine - but maybe not "modern".
My copy of Practical Electrical Wiring, vintage 1973 or so, indicates THHN, so 45+ years at least.
Still modern enough as it’s still in use.
JonCarter
Well-Known Member
If you're serious buy, rent or borrow a megger insulation resistance tester. These will test insulation at higher voltages than an ohmmeter (typically 500V for low voltage wiring; higher for MV & HV stuff) and provide resistance readings in the megohms. Disconnect both ends of each conductor to be tested.
For 10 and 12ga 120v branch circuits in conduit it is generally more cost effective to just replace the wire if it is suspected to be defective. A mega-ohm meter will only give you a number, not a pass/fail, you still need to interpret it. They work best when you have a "before" test to compare it to. It doesn't hurt to do it but I don't think the information you are going to gain will be worth the time required.
JD
Well-Known Member
Kind of what I do. When it's a wet conduit (fish tape comes back wet), I simply label and disconnect all the wires, strip, twist, and solder them on new spools then yank away. Once the solder lump makes it to the other end I cut it off and reconnect everything. You can then cut off sections of the scrap heap and see the level of damage. Probably cheaper and faster than buying a tester. You can usually tell which conduits have trouble by smelling the ends. Sounds crude, but there is a very specific odor. Not sure how long THHN lives underwater. What's in there has been there since 1989. None of the replacements have had any failures. Hard to say what year a crack may have occurred. There is probably in excess of 1000 feet of conduit in the slabs. Most all of the church is slab construction.
Having dealt with maintenance of a few old buildings, I've never seen modern, plastic insulated wire fail unless the conduit was wet inside for a long period of time.
Varnished cambric wire, which was used until the mid to late 1950s, with a woven cotton jacket over a rubber layer, can fail in places where it is exposed to heat. It will work reliably if left alone, but the insulation gets extremely brittle and fragile. For example, in a J box over an incandescent light fixture, the insulation will fall off when it is disturbed to change the fixture or paint. More than once, I have had a 5 minute job turn into a 5 hour job pulling new wire for a string of lights. The same wire in a receptacle box is perfectly fine for service for a 100 years.
Varnished cambric wire, which was used until the mid to late 1950s, with a woven cotton jacket over a rubber layer, can fail in places where it is exposed to heat. It will work reliably if left alone, but the insulation gets extremely brittle and fragile. For example, in a J box over an incandescent light fixture, the insulation will fall off when it is disturbed to change the fixture or paint. More than once, I have had a 5 minute job turn into a 5 hour job pulling new wire for a string of lights. The same wire in a receptacle box is perfectly fine for service for a 100 years.
What FM says. I too am in an old building (1955) so pre THHN. Most failures we see are at the device ends or at connections for taps. In general if there’s any suspicion, NOBODY will re-use. They might as well re-pipe as well, just cause pulling out the old crap to re-use can be a headache and time consuming. And they’ve no guarantee on what they can’t see in terms of the condition of the original conduits.
Our building is easy in one respect, if it’s mounted in the wall, it’s original. If it’s surface, it’s been added and can be followed.
Randy Worden
Member
As a recently retired, California Licensed electrical contractor who made a living working on buildings over 100 years old, here’s how my consultation with a client would go...
Are there any symptoms that make you think that there is something wrong with the wiring in the conduit? Unless you have some reason to doubt the condition of the wires, you would have thousands of better ways to spend your budget (fixture maintenance, replace electrics cables that are always moving and bending, etc)
1) Examples of evidence that there may a potential issue worth troubleshooting: circuit breakers tripping with no load attached, more than one circuit turning on/off at once, conduit significantly hotter in one spot than the rest of the run, smoke venting from the conduit, etc.
2) Wires in conduit vascillate beteeen hot and cold during normal use, which often causes water accumulation due to condensation. Depending on slope, air flow and pressure, environment, and other factors, it is not unusual to pull a bundle of wet wire out of a conduit run - The water will fade the dye in the insulation causing it to loose it’s color, but is rarely the sole cause of any physical damage to the insulation.
3) The age of the wire, surprisingly, is of little concern. As long as it is undisturbed and not otherwise abused (e.g. chronically overloaded, additional wires pulled at a later date, etc) even the oldest wire will be fine inside the conduit run. I have pulled 100year old wire, whose insulation had completely fallen apart in the junction box. But, the portion of the cloth-covered, rubber-insulated, wax-coated, & colored wire that was inside the conduit looked brand new.
So, again, unless you have an evidence-based reason to think there’s a problem, spend your limited time and money on things that have genuine wear & tear.
All the best,
Randy
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