Derate like this:
Table 310-16: #12 THHN cu @ 90 degrees = 30 amps.
Table 310-15(b)(2)(a): Number of current carrying conductors: 21-30 = 45%
Multiply 30 amps x .45 = 13.5 Amps
This is for 30 degrees C Ambient Temperature. If higher than 30 C, adjust maximum amperage of 310-16 [30 amps for #12 THHN cu] by the corection factor, then derate for 21-30 conductors.
If the loads are LCL, then derate the maximum amperage [30 amps for #12 THHN cu] by 80%, making it 24 amps - then apply temperature corrections [if any], then finally derate for 21-30 current carrying conductors.
The conductors in the raceway [conduit] are the ones being derated. Box fill is another area.
Steve had already pointed out the corrected value of 13.5 amps - using 15 amp OCPD, so this message is just to explain how he came up with that figure.
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!