Current rating on AWG cables

[joniengr081]

Is the current rating of AWG cables dependent on the insulation as well. Which factors are involved in determining current rating.

I guess AWG copper wire resistance has nothing to do with insulation and voltage drop.

[digger]

Is the current rating of AWG cables dependent on the insulation as well. Which factors are involved in determining current rating.

I guess AWG copper wire resistance has nothing to do with insulation and voltage drop.

resistance definitely has to do with voltage drop.

the insulation does not affect voltage drop, but it does affect the ampacity because better insulation can tolerate higher temps.

to summarize, wire gauge (or resistance per unit length) affects ampacity because it affects voltage drop and heat generated.
insulation type also affects ampacity beacuse you can heat the wire up more (run more current through it) and still maintain integrity with better insulation.

finally, the wire's environment also affects ampacity. if it's a high temp environment (like a conduit packed with other hot wires), you have to de-rate the ampacity.

[joniengr081]

Sorry, I mean voltage withstand.

[joniengr081]

Thanks for reply. This means better insulation wire can have higher amps. But the voltage drop is based a on wire resistance.

[Kurets]

I recommend using the MIL standard wire ampacity, and de-rate to 50%. While you may get away with less, in general more copper is better.

https://www.circuitcalculator.com/wordpress/wp-content/uploads/2007/12/mil_std_975_wire.htm

[themadhippy]

This means better insulation wire can have higher amps.

No ,in fact it could be less as the heat cant escape as easily

[CatalinaWOW]

This means better insulation wire can have higher amps.

No ,in fact it could be less as the heat cant escape as easily

This depends on what you mean by better.  Higher voltage rating often means thicker insulation, which tends to mean higher thermal resistance.  But that relation isn't rock solid.

For hobby applications the best answer is heavily derated.  If you can't do that for some reason you have to go through data sheets on the wire you propose using.  All of the data you want often won't be available from low cost venders.  Then you either take a risk or spend the money for a well documented wire.

[joniengr081]

I understand that wires with thicker insulation will have less amps as the thermal resistance will increase with thicker insulation and heat can not escape easily. This will reduce the current rating of the wire.

[EEVblog]

Is the current rating of AWG cables dependent on the insulation as well. Which factors are involved in determining current rating.

Using the AS3000 standard as reference, cables are "derated" in current capacity by type and how they are mounted.
e.g. enclosed would be inside a conduit. And then it matter is the conduit is in free air or under ground.
And that's just assuming PVC sheath and take no account of any other application specific issues.

[calzap]

For electrical insulation of wires of a particular composition and diameter, thicker electrical insulation will mean thicker thermal insulation too.  This does not necessarily mean the wire ampacity has to be derated.  For most applications, wire ampacity is limited by the ability of the electrical insulation to withstand heat and ability the surroundings for the intended application to withstand heat.  For example, ampacity rating of mineral-insulated wire (type MI) is higher than that of thermoplastic wire (type THW) of the same conductor diameter because mineral insulation can withstand higher temperatures than thermoplastic insulation.  However, minimum thickness standard of mineral insulation is slightly greater than the minimum thickness standard for thermoplastic insulation of wire of the same gauge and voltage rating.

I’ve seen considerable variation in thickness of thermoplastic insulation (i.e. departure from standard).  This has always been in the upward direction.  A common place to see this is on zip cord versus hookup wire.   Insulation on zip cord is often thicker than the minimum standard to provide toughness, ease of handling and as part of the “bridge” between wires in the cord.  I think there is no worry that the small increase in thermal insulation from the thicker insulation necessitates decreasing ampacity rating.  As long as currents are within the usual acceptable range for the wire diameter and type of insulation, the insulation will not suffer thermal damage if it's somewhat thicker than minimum required.

It's not heating of the metal conductor itself that is usually a concern in assigning ampacity.  It’s heating of the insulation and surroundings like insulation of adjacent wires, wood framing, etc. that’s important.

Mike

[CatalinaWOW]

For electrical insulation of wires of a particular composition and diameter, thicker electrical insulation will mean thicker thermal insulation too.  This does not necessarily mean the wire ampacity has to be derated.  For most applications, wire ampacity is limited by the ability of the electrical insulation to withstand heat and ability the surroundings for the intended application to withstand heat.  For example, ampacity rating of mineral-insulated wire (type MI) is higher than that of thermoplastic wire (type THW) of the same conductor diameter because mineral insulation can withstand higher temperatures than thermoplastic insulation.  However, minimum thickness standard of mineral insulation is slightly greater than the minimum thickness standard for thermoplastic insulation of wire of the same gauge and voltage rating.

I’ve seen considerable variation in thickness of thermoplastic insulation (i.e. departure from standard).  This has always been in the upward direction.  A common place to see this is on zip cord versus hookup wire.   Insulation on zip cord is often thicker than the minimum standard to provide toughness, ease of handling and as part of the “bridge” between wires in the cord.  I think there is no worry that the small increase in thermal insulation from the thicker insulation necessitates decreasing ampacity rating.  As long as currents are within the usual acceptable range for the wire diameter and type of insulation, the insulation will not suffer thermal damage if it's somewhat thicker than minimum required.

It's not heating of the metal conductor itself that is usually a concern in assigning ampacity.  It’s heating of the insulation and surroundings like insulation of adjacent wires, wood framing, etc. that’s important.

Mike

Said better than I did.  Everything about the wire, its insulation and its environment matters (did you note the temperature and humidity at which the ratings apply?).  If you use a wire at far below its nominal capability none of this is particularly important.  Which is another way of saying de-rate heavily.   But if you are trying to push right up to the limits then you must pay attention to everything.

If you don't feel capable of evaluating all of that on paper, empirical methods can also be used.  Build your gadget and operate it in an area where fires, fumes, splatters of molten metal and all of the other nasty problems won't cause more damage than you care to deal with. Operate it such that the current is as high as it will ever get and leave it on in that condition for an hour or more.  If your wires don't get hot (infrared or even the finger test), if no nasty fumes occur, if the insulation doesn't change color or pull away from the wire then you are probably OK.  May want to increase wire "rated" ampacity a bit for margin.  Or try again with a smaller wire size if that would be beneficial for space, cost or ease of use reasons.  If you fail the empirical test, obviously you try again with a larger wire size.

[calzap]

Increasing thickness of insulation on a conductor of a particular diameter and carrying a particular current won’t increase the temperature of the conductor or its insulation as much as one might think at first.  It’s a cylindrical heat source encased in a cylindrical thermal insulator.  Increasing the thickness of the insulation also increases the radiation surface for cooling.  However, there can be complicating factors like coiling or adjacent wires.

I’ll leave it to thermal engineers to explain more.

Mike

[Geoff-AU]

Is the current rating of AWG cables dependent on the insulation as well. Which factors are involved in determining current rating.

https://en.wikipedia.org/wiki/American_wire_gauge#Tables_of_AWG_wire_sizes

8AWG cable has a nominal capacity (at 30C ambient temp) of 50A at a 75C rating, the rating being how hot you're prepared to let the insulation and other adjacent objects get.

You can also see that the fusing current of 8AWG is 472A.  That's the limit of the copper.

Kuret's link about the MIL-STD gives rating based on 70C ambient, hard vacuum, and for 200C rated wire with PFTE insulation.  Obviously targeted for space work.

I guess AWG copper wire resistance has nothing to do with insulation and voltage drop.

Current rating is a thermal rating only (insulation-limited)

Your application determines how much voltage drop you're willing to accept, and therefore the minimum wire size to use to keep resistance within reason.