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It depends _a lot_ on the audience to not use the correct units. So, it is
always better to use the correct units.
Despite the attempted Friday humor, the SI is clear:
12 VAC or 12 volts AC (as in Alternating Current)
I've seen 12 VAC, but don't consider myself an authority on this unit. An
electrician I'm not.
12 VAC is usual (and kind of acceptable) in English speaking area. If I am
not too wrong, the SI uses "12 V~"
40 amp or 40 Amp
Unfortunately, good ol' Andre Ampere wasn't a Dead White _English_ Male (nor
was his colleague Coulomb, whose own eponymous units go into making amperes
<g>), so evidently he (they) missed out on the capitalization of the unit
named after him. I've never seen cap-A Amp.
The SI unit abbreviation for electrical current is "A".
"Amp" is very often used as abbreviation for "Amplifier".
So, for example a household fuse is labelled "230 V 6A"
BTW, the abbreviation for Coulomb is "C".
Hope, this can help.
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