Resistance is Futile

[kwired]

I am not expert by any means so I may learn something too but I don't believe there really is an AC resistance, if it is AC it is impedance and it seems every detail of circuit can change the impedance. From the raceway type to the layout of the conductors.

But I think most of us electricians do like George, treat it like DC and call it good enough. Engineers would go deeper and make it right.

I will not claim be an expert on this either, but I believe that with AC RMS voltage with little or no harmonic distortion you can treat it like DC at the same voltage - as long as you have resistance loads, throw in some capacitive or inductive loads and things get more complex, although impedance is still measured in ohms and can be plugged into the same formulas as resistance - and be close enough anyway in most cases.

The tricky part of impedance is it is a different value when not energized then it is energized, and this gives you that "inrush" current when first energizing.

 

[GoldDigger]

Not so much a different value when energized unless you are looking specifically at ferromagnetism with hysteresis.
But rather that the waveform at turn on is not a pure fundamental frequency but lots of high harmonics related to the step change in the waveform.

Tapatalk!

 

[Little Bill]

That's how I see after Jon's response.

Thanks Jon believe it or not that made sense to me

This is what I was actually looking for




So hypothetically it may be possible to run 621 amps on the 12 awg if it is the wire is the only resistance? Is it physically possible to have that or someplace near it in the real world? Just curious-- has nothing to do with anything I am working on or plan to do...

It would have to be as Jon said, you would have to have a short circuit. Because if the resistance is only that of the wire, you couldn't have any other load as that would increase the resistance. That would decrease the current, nullifying your calculations that gave you the current to start with.

 

[iwire]

So hypothetically it may be possible to run 621 amps on the 12 awg if it is the wire is the only resistance?

Sure, if you could figure out how to keep it from melting the copper and you kept increasing the voltage to overcome voltage drop.

But in most cases the bare copper would become a fuse well before 621 amps.

Again I do not believe the resistance of a conductor alone can be used to determine its current carrying capacity. To many other factors come into play. Ambient temperature and how hot you are willing to run the copper would be the main ones I think.

 

[Smart $]

Do not forget the wire temperature is constrained to 75?C. Resistance change is roughly proportional to temperature change, while these both change roughly proportional to the current amount squared if heat dissipation also remains unchanged. The wire and ambient environment would have to be super cooled to run that much current through a wire and maintain both the wire's resistance and temperature.

 

[kwired]

When we size conductors using art 310 we are limiting them to 60 or 75 deg for termination temp, and usually 90 deg for insulation temp.

But 310 is more about keeping the insulation intact then it is about what the conductor itself is capable of.

Those conductors can physically carry much more current then 310 allows us to design them for before they would be physically compromised, and as mentioned temperature does come into play. You likely can operate a conductor at a higher temp in either a vacuum or in an inert gas then you can at standard atmosphere conditions as well.

Other things to consider is look at how they make fuse links. They have small cross section areas but only in limited sections - the rest of the link is larger cross section to act as a heat sink - this gives it time delay characteristics so the short duration surges of current do not instantly blow the fuse, but long term overload eventually heats up the heat sink enough that allows the smaller link to melt at a carefully engineered current/time value.

 

[charlie b]

But 310 is more about keeping the insulation intact then it is about what the conductor itself is capable of.

This is a key point, and it answers some of the questions that are being presented in this thread. We establish ampacity limits to protect the conductor's insulation system. Too much current will cause degradation, even melting, of the insulation. The electrons will thereafter escape from their prison (i.e., you get a short circuit to any nearby metal, such as the conduit). The reason we have three different columns (60, 75, 90) in the ampacity tables is that the insulation systems of the higher rated conductors can handle a higher temperature than those of the lower rated conductors.

 

[ggunn]

Sure, if you could figure out how to keep it from melting the copper and you kept increasing the voltage to overcome voltage drop.

But in most cases the bare copper would become a fuse well before 621 amps.

Again I do not believe the resistance of a conductor alone can be used to determine its current carrying capacity. To many other factors come into play. Ambient temperature and how hot you are willing to run the copper would be the main ones I think.

Current carrying capacity of a conductor (ampacity) is a function of its insulation, not of the metal itself. But you knew that, right?

 

[mike_kilroy]

There ARE uses of that DC resistance measurement!

There ARE uses of that DC resistance measurement!

... Again Table 8 also has DC resistance-- how can you use that ??? ....

Well, say you are in the army, in France, in WWII, your under fire, and the Colonel comes up to you as the radio/electronics man in the trench next to his, and frantically tells you the phone line to the outpost is out and he HAS to call for reinforcements. After a blank look, you tell him you will get right on it, so you whip out your ohmeter and measure that line the 2 guys strung 3 miles over the country side 3 days before and the two wires SHORTED! You grab your book with table 8, check resistance of the #16 wire used, and do some quick math: the short is 2,456.5 feet downwind (accounting for down and back so double the distance). You tell the colonel the short it 0.0388 miles from the trench; he sends guys out under fire, they find it, fix it, and YOU saved the day.

You can read a real life more accurate case of this as my dad recounts it here:[h=3]Yorlik's Messerschmitt[/h] http://www.kilroywashere.org/003-Pages/03-0-HarmsWay.html

 

[iwire]

Current carrying capacity of a conductor (ampacity) is a function of its insulation, not of the metal itself. But you knew that, right?

We have already established we are talking about bare copper.

 

[GoldDigger]

We have already established we are talking about bare copper.

I guess in that case we only need to worry about the termination temp rating. :angel:

Tapatalk!

 

[ggunn]

We have already established we are talking about bare copper.

Then the Table 8 or Table 9 numbers won't answer the question. All they tell you is what the resistance/impedance is for 1000' or a km of a certain gauge conductor, not whether or not a conductor will vaporize immediately if you put a particular voltage across a particular length of it. The Tables and Ohm's Law will tell you how much current will flow, but whether or not the wire can stand it is another question entirely.

You can use Table 8 to calculate how much current will flow though a two inch length of #18 copper wire if you were to put 1000VDC across it, but you'd better have a very fast ammeter and a flash suit if you want to measure it empirically.