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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?

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  • \$\begingroup\$ @ElEctric - that is incorrect unless each strand is insulated and they are also woven appropriately, known as Litz wire. Even Litz wire is not effective above a few hundred kHz to low MHz frequencies. \$\endgroup\$ Commented Nov 7, 2019 at 18:51
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    \$\begingroup\$ Current rating is a better term than ampacity. \$\endgroup\$ Commented Nov 7, 2019 at 19:32
  • \$\begingroup\$ @LeonHeller I agree, now that you mention it. "Ampacity" to me seems like it should be the maximum current a conductor can carry before melting under specific physical conditions. "Current rating" makes more sense, since it involves safety. I don't actually want anywhere close to the maximum current in my home wiring: while copper at <1000°C might not melt, it would be Very Bad™ for my home. \$\endgroup\$
    – JYelton
    Commented Nov 7, 2019 at 19:39
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    \$\begingroup\$ "Ampacity" is the standard and accepted term as far as I can see. The national electric code says "ampacity." Electricians say "ampacity." The vendor tables say "ampacity." Why not just go with the flow? \$\endgroup\$
    – user57037
    Commented Nov 11, 2019 at 8:40

5 Answers 5

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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.

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  • \$\begingroup\$ Doesn't ampacity rating only consider insulation insofar as tolerable temperature rise (e.g. free air versus in a bundle)? I wouldn't think insulation affects ampacity aside from this aspect. \$\endgroup\$
    – JYelton
    Commented Nov 7, 2019 at 19:23
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    \$\begingroup\$ @JYelton Yes, that is exactly the way in which insulation affects it. I'd consider that a fairly important consideration, if your insulation has a low tolerance for heat (some types do break down when exposed to heat over time, so it's not just melting point that matters). \$\endgroup\$
    – Hearth
    Commented Nov 7, 2019 at 19:36
  • \$\begingroup\$ After considering Leon's comment on the OP question, if we define "ampacity" and "current rating" separately, then insulation shouldn't affect how much current a conductor can carry (ampacity), but it certainly affects its safe operating conditions (current rating). \$\endgroup\$
    – JYelton
    Commented Nov 7, 2019 at 19:42
  • \$\begingroup\$ @JYelton In my experience, "ampacity" and "current rating" are synonyms, and I am using them as such here. Do you think I should change my wording? \$\endgroup\$
    – Hearth
    Commented Nov 7, 2019 at 19:48
  • \$\begingroup\$ Not necessarily. I agree with you that they are synonyms. I am not sure what word is appropriate for "absolute maximum current before destruction/melting" of a conductor. But obviously this is not usually the term we care about here (maybe moreso on the physics SE). I was just overthinking, as usual. \$\endgroup\$
    – JYelton
    Commented Nov 7, 2019 at 19:50
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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.

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    \$\begingroup\$ This is not really correct. The manufacturer doesn't tell you how much current the wire can carry. The ampacity of a wire in a structure is determined by building codes and depends on several things including insulation temperature and whether the wire is in conduit and even conduit fill factor. There are also road vehicle ampacity tables and boat ampacity tables. But it is true that the the number of strands doesn't matter. (As far as I know). \$\endgroup\$
    – user57037
    Commented Nov 7, 2019 at 19:01
  • \$\begingroup\$ For example in boats, wire that goes through the engine compartment has a lower ampacity than the exact same wire outside the engine compartment, per ABYC. You could say that the apmpacity is de-rated when wire passes through engine compartment. \$\endgroup\$
    – user57037
    Commented Nov 7, 2019 at 19:04
  • \$\begingroup\$ @mkeith, yes, I see what you mean - I've answered a different question to that asked :-) I'll delete this answer when a comment from you tells me you've read this... \$\endgroup\$
    – TonyM
    Commented Nov 7, 2019 at 19:44
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    \$\begingroup\$ Tony I'd perhaps consider editing or amending your answer instead of deleting it. Seeing the error (and the correction) could be pretty useful. \$\endgroup\$
    – JYelton
    Commented Nov 7, 2019 at 19:46
  • \$\begingroup\$ @JYelton, I think writing right answers is a better legacy but I'm too busy to fix it :-) \$\endgroup\$
    – TonyM
    Commented Nov 7, 2019 at 19:50
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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.

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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:

http://www.alphawire.com/en/Company/Blog/2015/June/Helpful%20Tips%20for%20Cable%20Ratings

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    \$\begingroup\$ Safe current capability of a cable decreases with more conductors, but current capability of a wire would increase with more strands of a given cross-sectional area. I think you've confused "strands" with "conductors" here. AlphaWire's chart is showing a multiplier factor for number of conductors in a cable which themselves may be solid or stranded. A cable can have one or more conductors, and a conductor (wire) can have one or more strands. In a single 12 AWG stranded wire, there can be 50 or more strands. \$\endgroup\$
    – JYelton
    Commented Nov 7, 2019 at 19:33
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  • 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

REF

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

https://www.engineeringtoolbox.com/amps-wire-gauge-d_730.html

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