Len,
You can use BX wiring in a 10 gauge. [............]
"BX" cable??? Is that a typo or a mis-identification? "BX" is a very old cable type (pre WWII).
From the Middle Atlantic paper:
Armor Clad – Designated (AC) by the NEC, and sometimes called “BX”, its original manufacturer?s trade name. While it is the least expensive, is the least desirable for AV systems due to the fact that there is no supplemental grounding conductor (wire). The metal jacket, along with its aluminum bonding strip, is the safety grounding conductor and is detrimental to AV performance due to its higher comparative impedance than a solid piece of copper wire. Without a supplemental grounding conductor, the ground impedance and integrity is dependent on the length of the sheath and all the connectors and fittings (in series). BX cannot be used with isolated ground receptacles.