Ampacity vs. cross sectional area

ggunn
ggunn Solar Expert Posts: 1,973 ✭✭✭
in Advanced Solar Electric Technical Forum #1
This is probably better posted on the Mike Holt forum, but it's been off line for a while.

But, anyway, my question is this: why doesn't wire ampacity track with cross sectional area/resistance? Copper AWG#6 has half the area and twice the resistance of AWG#3, but to get twice the ampacity you have to go to AWG#1. That doesn't seem logical.
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #2
    Re: Ampacity vs. cross sectional area

    http://en.wikipedia.org/wiki/Skin_effect

    Skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor with the current density being largest near the surface of the conductor, decreasing at greater depths. In other words, the electric current flows mainly at the "skin" of the conductor, at an average depth called the skin depth.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,631 admin
    #3
    Re: Ampacity vs. cross sectional area

    My assumption has been is that the area of the wire goes up with the square of the diameter, but the surface area only goes up linearly... So while resistance does go down as wire diameter goes up by the square, the ability to dissipate heat only went up linearly.

    So, to dissipate 2x heat, you would need the wire surface area to double, which would be more than 2x the cross sectional area of the wire.

    The other possibility is that we have heating = I^2*R... So, if you double the current and 1/2 the resistance, you still end up with 2x the amount of heat (watts) per length of wire. Again, needing more surface area to limit heat rise in the wiring.

    So, we probably have both effects forcing the use of more copper than a simple ratio of cross sectional area to current would suggest.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,631 admin
    #4
    Re: Ampacity vs. cross sectional area

    I am guessing that skin depth is not going to be a huge issue until you hit higher currents/larger wiring... From this chart, 100% skin depth for solid 0000 (4/0) copper wire is 125 Hz.

    Using stranded cable would address this to some degree too.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #5
    Re: Ampacity vs. cross sectional area
    solar_dave wrote: »
    http://en.wikipedia.org/wiki/Skin_effect

    Skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor with the current density being largest near the surface of the conductor, decreasing at greater depths. In other words, the electric current flows mainly at the "skin" of the conductor, at an average depth called the skin depth.
    That doesn't explain it. The skin effect depends on frequency and the article you cite says that at 60Hz the skin depth is about 8.5 mm. The radius of #6 wire is 2.06mm and of #3 is 2.91mm, both well below that.

    But anyway, the NEC makes no distinction between AC and DC ampacities.
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #6
    Re: Ampacity vs. cross sectional area
    ggunn wrote: »
    That doesn't explain it. The skin effect depends on frequency and the article you cite says that at 60Hz the skin depth is about 8.5 mm. The radius of #6 wire is 2.06mm and of #3 is 2.91mm, both well below that.

    But anyway, the NEC makes no distinction between AC and DC ampacities.

    BB explanation of heat dissipation make more sense.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #7
    Re: Ampacity vs. cross sectional area
    solar_dave wrote: »
    BB explanation of heat dissipation make more sense.

    Yes it does, seeing as ampacity tables are all about the heat tolerance of insulation. I have learned something new, so now I can go home. ;)
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #8
    Re: Ampacity vs. cross sectional area
    ggunn wrote: »
    Yes it does, seeing as ampacity tables are all about the heat tolerance of insulation. I have learned something new, so now I can go home. ;)

    Many of us have learned something. I wouldn't have known about "skin depth" vrs frequency if you hadn't brought it up. So now I can go home too :D
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #9
    Re: Ampacity vs. cross sectional area

    skin effect shouldn't bother most of us with any real concern for what we deal with. even what that website shows isn't that finite as beyond what they spec it only starts to add a bit of lowered capacity. it doesn't just penetrate so far and stop. most don't concern themselves unless getting into ultrasonic or even radio frequencies and very high power levels. the higher the frequency and power then the more concern there would be and for utility power ranges nobody should really care to know about skin effects.
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #10
    Re: Ampacity vs. cross sectional area

    NEC is based on heat generation and ability to dissipate heat. Surface area and total mass modify the limits. In rough numbers, NEC is based on approximately 2 watts of IR loss per foot of wire.

    This is modified for ability to dissipate heat. Like what kind of environment the wire is in, conduit type and wire/cable insulation. For example, the thick rubber covered SOOW cable like used to connect a portable generator is derated because of its multi-layer of insulation that reduces wire ablility to dissipate heat. Number of current carrying wires in a conduit will derate max current because there are more heat generation.

    The original reason was that elevated heating on the insulation will degrade it over time, making it more brittle and subject to cracking. New insulating materials are much better at resisting degradation due to heat.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #11
    Re: Ampacity vs. cross sectional area
    RCinFLA wrote: »
    NEC is based on heat generation and ability to dissipate heat. Surface area and total mass modify the limits. In rough numbers, NEC is based on approximately 2 watts of IR loss per foot of wire.

    This is modified for ability to dissipate heat. Like what kind of environment the wire is in, conduit type and wire/cable insulation. For example, the thick rubber covered SOOW cable like used to connect a portable generator is derated because of its multi-layer of insulation that reduces wire ablility to dissipate heat. Number of current carrying wires in a conduit will derate max current because there are more heat generation.

    The original reason was that elevated heating on the insulation will degrade it over time, making it more brittle and subject to cracking. New insulating materials are much better at resisting degradation due to heat.

    you bring up good points. having a high outer exposure area is good in terms of thermal dissipation and would be a common cure in solving for skin effect too, but they are 2 different things. the insulation and its type influence the heat dissipation by adding thermal dissipation resistance to the wire. high outer exposure area helps to disperse the heat being generated within a wire as it is better able to conduct that heat to the ambient air around it whereas skin effect is the depth into the wire the power is without attenuation and that is frequency dependent. thermal heat dissipation will still be present with ac or dc unlike skin effect only being present during ac and its effects will increase with increases in frequency.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #12
    Re: Ampacity vs. cross sectional area
    RCinFLA wrote: »
    NEC is based on heat generation and ability to dissipate heat. Surface area and total mass modify the limits. In rough numbers, NEC is based on approximately 2 watts of IR loss per foot of wire.

    This is modified for ability to dissipate heat. Like what kind of environment the wire is in, conduit type and wire/cable insulation. For example, the thick rubber covered SOOW cable like used to connect a portable generator is derated because of its multi-layer of insulation that reduces wire ablility to dissipate heat. Number of current carrying wires in a conduit will derate max current because there are more heat generation.

    The original reason was that elevated heating on the insulation will degrade it over time, making it more brittle and subject to cracking. New insulating materials are much better at resisting degradation due to heat.
    All true, but my original question was about the un-derated ampacity numbers in T310.16. The simple answer is that it is about heat dissipation for the protection of insulation, not just conductance.
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  • SirSparks
    SirSparks Solar Expert Posts: 43 ✭
    #13
    Re: Ampacity vs. cross sectional area

    "NEC is based" On Insurance companies and their ability to avoid paying a claim.
    All good points made here but Skin effect at 60 Hz is irrelevant as is thermal conductivity of insulation type (It's already been derated for this). What IS NOT irrelevant is maximum temperature for the Insulation type or the terminations.

    Basically the NEC is full of BS, for example #12 THHN copper is good for 25 amps (according to NEC). Just that you can only ever use it to 20 amps (accordfing to NEC); Now what kind of BS is that ? Why not just say TWENTY AMPS. And it is NOT a matter of multiple conductor derating in a raceway either !
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #14
    Re: Ampacity vs. cross sectional area

    I don't believe NEC ratings have anything to do with insurance companies at all. But the Amperage capacity of wire varies quite a lot according to its usage and installation. Most of this has to do with its ability to dissipate heat. Naturally different kinds of insulation will affect that rating, either in or out of conduit. There is also a difference between peak current handling ability and continuous use, which is why standard 12 AWG has a 25 Amp rating but is limited to 20 Amp maximum circuits: peak vs. continuous.

    You will find many different charts in this world giving limits and recommendations for wire size in applications. You will find they differ. Mostly they err on the conservative side, because no one wants to be responsible for that one-in-a-million chance that some odd confluence of factors comes together and someone's house burns down because just at that moment the 12 AWG could not handle its maximum 41 Amps.
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