Engineering Forum: Articles and discussions by participants of the Edison Tech Center

Commentary on Wire Insulation

ETC resident collector Rick Delair talks to us about the extensive history of wire insulation, its weaknesses and the long path we've taken from the days of Thomas Edison.

The breakdown of PVC - perhaps the most common home wire insulation:

When oils or fats, such as the fatty acids in our skin, and fats from meat etc, solvate and remove the plasticizer in PVC, the PVC doesn't get dissolved, but rather is left dry, stiff and brittle. Heat exposure dies the same thing, causing the plasticizers to get more volatile as they lose viscosity, and actually evaporate. The same thing happens when a vinyl dash in a car cracks, it is plasticizer loss from the hot car interior in summer. In a car the vinyl dash deposits on the windshield, especially near the top, as a grayish, oily film. Contact with the earth will migrate plasticizer from vinyl too, so special direct burial PVC jacketed cables like type UF (Underground Feeder) cable like that used to feed yard lights, sheds etc, needs a linear phthalate plasticizer to prevent ground contact from removing plasticizer and making the cable stuff and brittle.

I have seen UF cable from the 1950's with neoprene insulated wires (no outer braid!) in an outer black neoprene jacket, and this is also good. The transitional type of UF had type TW vinyl linear phthalate plasticized wires with the black neoprene jacket. At one time, the NEC (National Electrical Code) even allowed type S hard service cord as an early UF cable! These usually used the Goodyear type black natural rubber insulation on the wires, as well as the outer jacket and you had to use a minimum of #14 wire for the same reasons as regular wiring. I have pulled type S hard service cord out of the ground after as much as 75 years and it was still in good condition of the grade of rubber was good. In many other cases it lasted a year. If it had a neoprene jacket it did well. I have some GE "Vulkene" insulated control cable feeding my museum trailer, with #14 wires I doubled up for 30 amps capacity (it is a 7 conductor). With the Vulkene on the wires(that being cross-linked polyethylene insulation)(XLPE) ironically carrying the type RW designation "Rubber/Water resistant" even though it is NOT rubber! Today, it is type XHHW. The outer jacket id "GEoPRENE" which is synthetic neoprene rubber, has been in ground contact over 10 years, on the surface, and in the weather, and is like the day I put it there other than being really filthy! And the jacket specifies direct burial, overhead and outdoor use! And also oil resistant! Cloth braiding: The reason most rubber insulated wire has an outer cloth braid is because the cloth protects the relatively fragile rubber insulation. On building wire-like Type R, RW, RHW, RHH, etc, the braids vary. The oldest, type R, was only good for 60 centigrade total, both wire temp due to I/R losses with heavy loads, AND ambients-such as a hot attic, or the wires in a junction box that a "drum" type incandescent fixture is mounted on, inside fluorescent fixture channels near ballasts, and other heat generating apparatus, used a black or red or both (2 layers) rubber compound, mostly it was black though, applied directly to the copper conductor. Because rubber would tend to stick to bare red copper, the conductor was typically "tinned" with solder, giving the solid or stranded wire a silvery color. The tinning also made soldering easier, as it helped get a good joint. On the odd type that used un tinned copper, a cotton thread wrapping was applied over the wire before the rubber was applied-this was actually super common on flexible, finely stranded wires like flexible cords and fixture wires (solid fixture wires were usually tinned as above). Over this rubber compound was applied 1 or sometimes 2 braids of mercenized (or "mercerized") cotton threads, and the whole wire was f dipped in a "special moisture resisting and fore retardant compound, forming a smooth, slick sheath for easy pulling through rigid and flexible conduits".

In the old days up to the early '30's, most type R was black and black only. Once polarity indication was required, white also came out. Anything other than black or white (produced by thread color and sometimes also the compound applied to it) was painted, like red, blue, etc. Type R was actually made well into the 1960's, but was not used much after the early-mid fifties as a general rule. Type RH had a better rubber compound, and was rated for 75 centigrade. Type RW was the first water resistant type R, and used an even better rubber compound, with a fiberglass braid. The colors were all paints. This type was used mainly for underground feeder conduits and in areas where wetness and moisture prevailed.

Type R was fine in wet locations as openwork on insulators (close relative of Knob & Tube) wiring though. The fiberglass braid was waterproof as was the rubber compound. It was also common, starting on the late '40's after the war, to just spiral wrap the fiberglass threads around the rubber, as a spiral "braid", and though it was fine in the BX cable or conduit it was used in, in the boxes it unraveled and left the rubber exposed, and looked dangerous, though it wasn't. The neoprene compound used was usually high grade, and could survive just fine with the braid frayed off.

Wet and dry ratings vary:

I have some BX armored cable made just after the war by GE that has NO braid on the wires! There is a piece of it in use in my shop out back, installed in 2000 by yours truly! It is actually kind of rare. and the compound was good-I have some I left out doors for 10 years and the rubber isn't cracked at all! RHW was good for 75 C, and was both heat and water resistant, though under wet applications it has to be downgraded to 60 C. (Modern type THHN/THWN is good for 90 C. dry and 75 C wet, the 2 "H's" mean 90 C and one "H" is 75 C. I'd venture to say it'd be fine at 90 C even wet!) RHHW, of course is 90 C rated wet or dry, the 2 H's and one "W" in the type initials tell that tale! The higher temp type R wires use the fiberglass braid or wrapped braid, and a 90 C neoprene rubber (synthetic) compound.

Vulkene and neoprene insulation

Please note that the GE "Vulkene" and other makes of cross-linked polyethylene insulated wire often still carry the RHW/RHHW nomenclature! I have a bunch of this stuff, and the reason it carries a "rubber" wire type is because it came out as a direct replacement for RHW and RHHW during the era when these types still prevailed. It usually also carries the correct XHHW nomenclature, except the really early version.

Also, GE made some RW/RHW rubber insulated wire probably in the 1950's and maybe into the early '60's that is insulated with 2 layers of neoprene rubber. One black layer of this is quite thick for standard 600 volt building wire and a second over covering that obviously takes the place of an outer braid, in black, white, and colors, I have these plus maybe a little red. It is rare, and this stuff is extra special as it came from the now gone WestVaco paper mill in Mechanicville, NY. I don't have a lot--it was in a conduit feeding outlets in cast iron "F" type boxes with weather tight covers in the "wet end" next to the actual paper press--#1 to be exact, and it was the ONLY installation of it in the entire building as well! I saved the wire, conduit (aluminum!) and boxes, and used some and still have some.

The double insulated neoprene cable is thick. It is #12 solid, and has the same diameter as modern THHN/THWN #8! They pulled a black and white through 3/4" conduit, as I think it would be a tight fit in 1/2", though I did get it through that size plenty of times. Forget getting 3 through 1/2" though-go to 3/4 for 3 wires-it is that thick! Good stuff though, but I think the thickness worked against wide acceptance of it, especially seeing as type T, (rare today) TW, etc came out around the same era and was just as good in wet locations! This GE stuff is 75 C rated.

I also should mention that I have some lead-sheathed type R for wet locations that came from the old Bouquet Creamery we tore down on Nott street back in '94. I may have both 1 and 2 conductor versions of this, and yes it has an extruded lead sheath over the regular type R wire--way too rare and cool to ever scrap! In closing, the double braid type R was typically used and often required by code in iron conduits, but not everywhere as single braid was also allowed in some places. I have specimens in great condition protected all those years in the piles to prove that. In fact, if I find double braid type R it is a special treat, as it is rare here. In unlined conduit (just plain iron pipe essentially) some localities required double braid. Especially in big cities like NYC. In lined conduit, an iron pipe with a fiber (cotton based paper) lining, single braid was always permitted, though double braid could also be used.

"Twin wire", of which I have specimens in spectacular shape, was 2 single braid regular type R wires side by side in an overall braid, and looks kind of like modern romex, the older cloth type anyway. It was of course considered equal to double braid, so good for both lined and unlined conduits in all installations. Though you could actually run it like romex, that is a big no no--it is for conduit ONLY, either rigid or "Greenfield" flexible conduit, and never to be used on insulators either. Because of this fact, I have always thought romex to be garbage. It should ONLY be in conduit like twin wire, but electricians love it because it is quick to install. It is a sitting duck for rodent damage and other damage, and is like wiring with lamp cord. Utter crap if you ask me, and I truly believe many houses have burned because of mouse damage etc to romex!

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