I have been in the "trade" for 35 years, American Wire Gauge has not changed
it that time. 12 is 12 and 14 is 14. What has changed in romex is that
some manufactures A LONG TIME AGO made romex with 12 or 14 conductors and a
reduced ground ~16awg. These cables were produced before I started buying
Guages of wire, like same of metal, do not change, they are standards.
The use of #12 Cu wire is prohibited under the Ont electrical code due to
the added stresses the thicker wire places on the terminals and screws of
Prob the same where you live. This is one instance where overdoing is
underdoing with a result in No Insurance Coverage.
When #12 or #10 is used on a branch circuit, the device must be rated
commercial and applicable for the wire used.
The responses to your question include a lot of
nonsense and extraneous discussion. The answer is
that wire gauges are the same, unless you go back
a long way. Like someone said, put a gauge on
both wires. It could be that one of the wires is
labeled wrong. The gauge will tell the story.
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