What is the ampacity of 14 gauge silver wire?

Question

There is no information I can find (charts/specifications) that gives the ampacity for single core silver wire. But I know this can be calculated, I've found this property list for silver from a [metals supplier][1]

Thermal Conductivity: 428 W/m * K at 20 oC 356 W/m * K at 450 oC

Electrical Properties:

Electrical Conductivity: 108.4% IACS for extremely pure silver

Electrical Resistivity: 14.7 nΩ * m at 0 oC

Temperature Coefficient: from 0 to 100 oC, 0.0041 per K

Cold working of silver considerably increases resistivity: 5% for 90% reduction.

Any ideas why this information is not available anywhere? I'd love to know how this is calculated and what it would be for 14 gauge (2.5mm wire).

Many thanks!

Answer 1

In general the current rating of a wire is limited by the wire's ability to dissipate heat to its surroundings without getting too hot. That in turn depends on the maximum safe wire temperature and the thermal resistance to ambient. Unfortunately figuring out exactly what the thermal resistance to ambient is for a given installation method is nontrivial.

Unfortunately standards don't normally say how they came up with the tabulated figures but one can assume they were determined from experimental data.

We can come up with a figure for a silver wire by assuming that the acceptable heat dissipation for a silver wire of a given size is the same as that for a copper wire of the same size. We divide the resistivity of silver by that of copper and then take the square root (remember \$P=I^2R\$).

Using numbers from this table tells us that a silver wire should have about 1.03 times the current carrying capacity of a copper wire of the same size installed under the same conditions.

This is small enough that other factors (exact installation method, material purity, how the material was worked into a wire etc) are likely to outweigh the difference between silver and copper.

Answer 2

Other than material and gauge, ampacity is limited by insulation temperature rating, maximum ambient temperature, bundling (how many other wires, and how much current are they carrying) and by altitude and the anticipated installation conditions. A PTFE-insulated wire that is allowed to run at 180°C will carry a lot more current safely than one that is only allowed to run at 105°C, especially if the ambient temperature is high (the ratio increases as the maximum ambient temperature increases and becomes infinite at the ambient equal to the lower rated wire).

So basically just use tables for copper of equivalent insulation under the equivalent conditions and you'll be "about right" and slightly on the safe side. Silver wire may not be approved for residential electrical wiring, aircraft use, or other specific applications, however, so keep whatever standards you may have to meet in mind.

If it's speaker wire there are no standards as far as I'm aware, nor do you typically have ampacity issues. We used AWG8 silver-plated copper conductors deep inside very expensive plastic molds (think 2l pop bottles- but at the preform stage, not the blow molding). The wire was MIL-spec not UL.

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Bare wire is another thing altogether. The "fusing current" for silver is about 75% of that of copper of equivalent diameter, based on the ratio of fusing constants.

Answer 3

You won't find an answer easily because it depends on many other factors as well. The ones listed for copper are usually standards for use in construction.

The current carrying capability of a wire depends on, amongst other things:

1. The operating conditions of the wire
2. The allowed maximum voltage drop across the wire for a given current
3. The maximum temperature the wire is allowed to reach because of either safety concerns (it might cause damage or risk starting a fire if it gets too hot) or the temperature the insulation can handle without melting.

There may be additional factors, such as the environment of the cable. For example, if the cable is to be used in a large harness with many others, it will be able to carry less current as it will have a harder time getting rid of heat because of the other cables (that might also be getting warm). Hence, the current that cable may carry could be significantly lower than what it can carry when suspended in the air on its own.

Long story short: You can't find any information because it depends, and in addition, I have never really heard of anyone using solid silver as conductor.

Answer 4

Maximum current for a wire is primarily limited by overheating and power loss concerns. Both of these are proportional to the resistance of the wire. Heating is further affected by insulation, airflow, and so on around the wire.

For a simple estimation, you can use any table for copper wire you like, and multiply the ampacity by \$\sqrt{1.084}\$ since silver (by your data) has 108.4% the conductivity of copper, and power loss is proportional to the square of current (\$P=I^2R\$). This is assuming silver wire of identical geometry, insulation, and environment.

Answer 5

audiophools...yeah as in their silver cables are silver plated copper. Which is much sillier than gold plated cable contacts.

Silver tarnishes. While tarnish protects from further corrosion similar to oxides on aluminum...silver tarnish is not nearly as conductive as untarnished silver.

Gold at least has the virtue of yielding a zero corrosion contact surface even with careless handling over decades (unless frequently enough plugged and unplugged that gold wears off). Though somewhat pointless if interior mating of female connectors is not also gold and is corroded (which many audiophools overlook).

Of course if you properly handle cheap chrome or other common connectors they won't have significant corrosion deposits either. I.E. Avoid touching with finger oils and other corrosives and polish off finger oils at least once a year/decade (depending on metal and corrosive).

Answer 6

It seems to be that while silver is a better heat and electrical conductor, it's lower fusing constant (1900 vs 2530 for copper) make it act more like a fuse and less like a conductor. Curiously, solder is 325.5. That's the number of Amps to fuse a 1cm diameter wire in still air

• reference: http://www.jpier.org/PIERM/pierm31/15.13051311.pdf

So, since it is 8.1% more conductive and fuses at 24.1% less current then the answer is really that while is more efficient at carrying current, it can actually carry less for a given thickness. I imagine a superconductor that heats itself up in use, above the magic temperature... poof! Pure silver does not oxidize (like gold) and become more resistive, but once alloyed with copper or nickel (and others) it does oxidize, get higher resistance, and depending on the alloy, the change can be rapid.

• reference: http://www.jewelry24seven.com/metal_facts.htm).

• A long list materials resistivity can be found here.
  

For Copper you will find it changes its resistance once annealed.

• How it is calculated from a safety and standards viewpoint?
  https://www.copper.org/consumers/copperhome/HomePlan/safety/Lvng_cde_rdcs_rsks.html.
• How I would adopt use of silver in place of copper? Allow 1.3315 times the diameter that you would use if it were copper. That's the actual value and meaning of fusing constant 2530/1900.
• Direct answer the ampacity of pure silver 14 gauge wire is (1900/2530)*the ampacity of 14 gauge copper wire.
• 0.750 * coppers' ampacity.

Answer 7

Lots of good answers. Most of which point toward the answer having no practical use outside an academic test. Pure silver is expensive and much softer than common copper wiring. Without reference I believe it introduces the same bi-metal issues as mixing aluminum and copper wiring. Thus silver is undesirable from engineering and theft standpoints.

Some of those engineering points become even more complex as you attempt to alleviate them. As was mentioned both alloying and working silver cause phase and density changes in silver plus the other included metals.

In reality standard amperage carrying capacity of copper and other wire in industry is based on a number assumptions: standard alloy, insulating materail, ambient temperature and humidity, minimum air space and circulation, safety margins. The most common of these standards are building codes for interior wiring where wire temperatures must not melt the insulation nor ignite paper or wood. I cannot remember if the US safety margin was 30% or 50% -- but I do remember that it was quite generous to allow for startup surge currents of electric motors and such.

A slightly different capacity standard is sometimes used in enclosed electronics. There ignition temperatures of paper/wood is replaced by softening/melting temperatures of wire itself...although the melting point of wire insulating material usually remains the governing factor except inside vacuum tubes.

So for that all important academic test or EXOTIC electronics application...I would say the question is missing a plethora of qualifying environmental criteria (ambient operating environment: temperature range and anything connected to cooling: such as surrounding gas/liquid composition and flow volume, other thermal sources, thicknesses of surrounding thermal conducting media {like part of heat sink}, etc etc)

Ultimately only a very few remote exotic environments (mostly space or deep sea) or microchips would justify even momentary consideration of silver as an alternative to just using a more over-engineered version of the standard copper circuits. Last I heard even micro-chips were considering copper more and more for chip to substrate connections as a replacement for gold assuming that corrosion issues could be overcome. And at those fine diameters I don't think silver has much advantage over copper as the protective tarnish becomes relatively thick to the application and plays hell with resistance calculations.