Ask 'Dr. Spa' Q & A Message Board

Absolutely not! #12 wire will is not rated for the amperage required. If you were to use the #12 (if your electrical requirement is for #6 or #8), you would have a fire. As a "rule of thumb":

#12 for 20A circuits
#10 for 30A circuits
#8 for 40A circuits
#6 for 50A circuits
#4 for 60A circuits
#2 for 100A circuits

There are other "variables/factors" to consider: length of run (voltage drop), conduit, wire type (Romex, THHN, etc), etc. THHN/THWN is what is recommended for hot tub installations. So if, for example, you have a 40A hot tub that calls for #8AWG, that is the minimum - and often it is advisable to upsize one size (to #6), etc. If your hot tub is 50A you would use #6 (or possibly #4).

: Any expert--- can I use #12 wire doubled in lieu of #6 or #8. I already
: have the wire in conduit and cannot re-wire with the larger gauge.