Do I need to derate a wire if it is stranded? Or will a #12 wire have the same ampacity whether or not it is stranded? Is this affected at all by AC versus DC?
Gauge is defined by cross-sectional area, not outside dimension, so the stranded #12 wire has the same per-length resistance as the solid #12 wire. Ampacity is more complicated (it depends on type of insulation, what other wires are nearby, and other details), but whether it's solid or stranded again does not matter (or at least not significantly so) for this.
AC vs DC also has no impact, unless your "stranded" wire is actually litz wire. As litz wire is very expensive and has to be specially ordered, it almost certainly is not--you would know if you had it.
By the term 'derate', you must mean to reduce a manufacturer's specified ratings to compensate for some aspect of its application.
The wire manufacturer's specifications will give you the current carrying capability, be it stranded wire or solid core. You do not have to derate the values in that specification - the manufacturer is giving you finished values. You just have to interpret them correctly.
Both AC and DC ratings are usually given for the max. current and voltage.
The max. current may be specified at different temperature rises, as higher current will dissipate more lost power in the cable because of its resistance and heat the wire.
The max. voltage comes from the max. breakdown voltage of the wire insulation, if it is insulated.
It will be very close to identical current rating for a stranded and a solid core cable with the same wire guage. The stranded is simply more flexible and suited for uses where it will be moved around.
IF the stranded cable was Litz wire it would have insulated strands woven in a special way making it able to carry more high frequency AC-current, as it lowers the "Skin effect". But for DC there will be no difference.
Yes, the current capability of a wire will decrease as the number of strands increases.
This short article on AlphaWire's website gives a good explanation:
- Wire gauge is defined by conductor diameter.
- Ampacity is defined by the max. current of wire for a specified temperature such as; 60, 75, 90'C which depends on the electrical/thermal insulation of the wire.
- Max. current rating however is usually done at 30'C
The thermal resistance rises sharply with the number of strands in the core, unlike the electrical resistance, so Ampacity is maximum for a single core.
e.g. wire with 1kV PVC insulated wire from 1 core to >=43 stranded cores
AWG D(mm) D(in) area(mm2) R Cu(ohm/km) 1 3 4-6 7-24 25 - 42 >=43 cores 12 2.1 0.081 3.3 5.2 34 20 16 14 12 10 Amps ambient temperature 31 - 40 'C: correction factor = 0.82 ambient temperature 41 - 45 'C: correction factor = 0.71 ambient temperature 45 - 50 'C: correction factor = 0.58
More graphs and details for AWG12