After Knob and tube and "Rag" style wire there was another wire type that came into common use - BX wire (although it had been around since 1899 it was most commonly in use under the trademark "BX" from the early 1920's up until it was replaced by "Modern" Grounded Armored Cable "AC"). Two (or more) cloth covered rubber coated nickel/tin clad copper conductors (Nearly identical to the knob and tube style conductors) were run through a spiral of wire armor (Sometimes referred to as "Greenfield" WITH NO GROUND WIRE OR TAB. This is the primary difference between the BX and "Grounded" AC which came after 1938/39.
Source Underwriters Laboratories: Armored cable (AC) was first listed in 1899 for the Sprague Electric Co. of New York, and was originally called “Greenfield Flexible Steel-Armored Conductors,” after one of its inventors, Harry Greenfield. There were originally two experimental versions of this product, one called “AX” and the other “BX,” with the “X” standing for “experimental.” The “BX” version became the one that eventually got produced, and hence the name “BX” stuck, which also became the registered trade name of armored cable for General Electric, who later acquired Sprague Electric. Armored Cable first appeared in the NEC in 1903.
Differences between BX (left) and AC (right): (Image found on 6 websites)
Why was this ground wire so important? Well to put it simply, the metal spiral covering of BX or Greenfield wire was good at protection against physical wire damage - BUT when electrical current passes through it (such as when the wire shorts out internally or something is shorted out to the outer casing) it had a tendency to heat up - much like a toaster element. This potential "worst case" heating up of BX wire was blamed for fires and eventually led to the replacement of BX wire with "Grounded" Armored Cable (AC). (The ground "tab" in early AC and later the ground wire in it gave short circuits a safe ground path and nearly eliminated the issue sometimes referred to as BX "toaster wire" - The description an old retired electrician in my area called the old BX wire he installed as a kid.)
The wire has several limitations:
It cannot be bent sharply. If bent too much, the outer coil of the wire can separate, expose the conductors, or even "pinch" and damage the conductors inside it.
It cannot be used in damp/wet locations. The wire is not water tight, and water entry into and/or corrosion on this type of wire can compromise the wire's safety - CORRODED BX or AC cable should be replaced for safety.
It cannot be buried in grade or masonry without protection. The method of the time was to put this wire in watertight metal conduit whenever it needed to pass through the ground or masonry. IF YOU HAVE BX OR AC WIRE BURIED IN OLD METAL CONDUIT - IT LIKELY NEEDS REPLACEMENT - SINCE THE METAL CONDUIT HAS LIKELY RUSTED OUT MANY YEARS AGO! Again GFCI and AFCI breaker protection on such wire is STRONGLY advised if rewiring is not a current option for you.
BX wire should NOT be used to feed grounded outlets or circuits. THIS IS VERY COMMON AND OFTEN GOES UNDETECTED as homes have their outlets replaced and updated - and RARELY does anyone (including many electricians i have encountered over the years) check for the presence of such ungrounded BX wire before updating the outlets to grounded 3-slot outlets. Again GFCI and AFCI breaker protection on such wire is STRONGLY advised if rewiring is not a current option for you.
This wire is another form of wire which has lived past it's anticipated safe lifespan. The inner wires can be brittle - especially if overheated during their life and short circuits are more possible now than when the wire was new.
BUT - I have old ungrounded BX wire in my home - Do I need to rewire? That question can only be properly answered by a licensed electrician after a complete evaluation of it. The simple answer is "Yes - If you want to eliminate it's Risk Factors" or "Maybe". I would recommend rewiring it if possible - but if not the addition of Combination GFCI/AFCI Circuit breakers on all circuits where the wire is in use would give the maximum amount of protection that is possible without rewiring it. (any current leaking into the outer metal coil due to age or damage would likely trip the GFCI and shut down the circuit, and Short Circuits of any kind would likely be sensed by the AFCI - again shutting down the circuit)
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