technical question about AWG chart

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Old 11-24-20, 09:53 AM
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Question technical question about AWG chart


Can someone explain, why the "max Amp" column seems to be so low? ie. AWG 8 has a max current rating of 24 Amps. I understand this is related to DC, however, I wouldn't think at 60Hz the resistance of the wire would be much different, or the current handling capabilities would be greatly affected. If I remember correctly, 8AWG is good for a 40 amp circuit.
Thanks in advance.
 
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Old 11-24-20, 10:42 AM
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I think you're using a non-standard chart for amperage. Another one I found online looks much more reasonable.
Notice that 14ga wire is rated at 20A - which is what's considered typical, although NEC limits to 15A in residential wiring.



In the chart you provided, I see Ampacity Notes:
The current ratings shown in the table are for power transmission and have been determined using the rule of 1 amp per 700 circular mils, which is a very conservative rating.
So this sounds like more of a physics/engineering determination. Not really sure why it's so far off what we consider typical.
 
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Old 11-25-20, 09:39 AM
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Yes, that was why I was asking the question. I would think, even at the physics/engineering level the amounts would be closer in value.
A solid copper conductor of X diameter, at the same temperature, should have the same max ampere rating (or close to it) on any chart you pull up. The circular area of the conductor is the same, the make up of the conductor is the same.....
How is the maximum amperes for a copper conductor calculated?

I'm not overly familiar with the chart you posted, but believe it has to do with the amp rating of terminations used on 14 AWG conductors. I could be wrong here, as I am not a electrician, deal more in electronics. However, from what I was able to gather, when connecting to a circuit breaker, you have to take into account the temperature rating of that CB and use the appropriate termination technique. The higher the temp rating of the CB requires a better type of termination, which results in a higher ampere value. Please correct me if I am wrong.
 
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Old 11-25-20, 02:21 PM
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Maximum current is called "fusing current". That is the absolute max before the wire, well, fuses.
Less than that is used when you are not designing a wire fuse, and have to worry with temperature. When you insulate a wire, that adds a significant derating current factor, in order to keep the insulation happy. So, in use rated current now becomes a complex factor that accounts for AWG, insulation jacket, bundling, ambient temperature, solar loading, and even run length.

regarding that chart; I've never seen one with those numbers. What is the context? or, who published it?
That max frequency/skin depth number set leads one to think this got pulled out of a physics book..
 
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Old 11-27-20, 11:42 AM
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Where did the chart come from? Good question. Here is the background. I am an instructor and received this chart as part of a presentation being given to students. The previous instructor must not of dwelled on the meaning of each column, or didn't see how far off the values were. I didn't have the full document, so went here to find it online:
https://studylib.net/doc/25291894/am...tor-size-table

Zorfdt I now see where you got the quote.

So, now, I believe you all are correct, this is a chart out of a physics book and is nothing more than using calculations to come up with a base number for each column, based on the diameter or CM of a copper conductor.

At the bottom of the document, in addition to the previously mentioned quote, is this:

"For reference, the National Electrical Code (NEC) notes the following ampacity for copper wire at 30 Celsius: 14 AWG - maximum of 20 Amps in free air, maximum of 15 Amps as part of a 3 conductor cable;..."

That 20A rating far exceeds the 5.9A value in the chart.

So, this tells me: 14AWG copper conductor, at 30 C or 86 F, in free air environment, will fuse (melt, liquefy) at 20A. This would be true, no matter the length of the conductor. With these specs, you could create a fuse using 14AWG copper conductor, either within a fuse cartridge of free air?
Is this a calculated value or one determined by testing?
Not trying to get super deep into this, just want to better understand where the values come from.

Thanks again.
 
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Old 11-27-20, 12:56 PM
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The answer is: That Table Is From A Solar Power Installer.




That GENERALLY means 1) DC power, 2) low voltage 3) outdoor installation exposed to the elements, and 4) wide temperature swings during operation.

That array of limitations requires VERY conservative numbers for 'wire diameter per amp' as you've seen.

 
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Old 11-27-20, 03:32 PM
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"So, this tells me: 14AWG copper conductor, at 30 C or 86 F, in free air environment, will fuse (melt, liquefy) at 20A. This would be true, no matter the length of the conductor. With these specs, you could create a fuse using 14AWG copper conductor, either within a fuse cartridge of free air?"

The "fusing current" for 14awg copper is 166A. But this isn't enough information to design a fuse, since it lacks a time variable.
 
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Old 11-27-20, 04:36 PM
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It helps to read it in context- two different measurements.

If you have a solar panel providing sustained DC power to a battery, the screw connections at the power bus will melt with a sustained current around 20 amps.

If you have an AC wire, you need a pulse of 166 amps to melt the wire.
 
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Old 11-28-20, 08:41 AM
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Hi all –

The table linked below includes “Maximum Amps for Chassis Wiring” and states 14 gauge = 32. Quite a bit of difference from the “Maximum amps for power transmission” for 14 gauge = 5.9 found on the same row. It just looks like maybe the instructor pulled out a table (post #1) which only focused on power transmission for some reason.

Maybe he was just trying to make a point that maximums vary depending on wire size and didn’t want to get into differences like amps for Chassis Wiring, NEC amps for 3-wires in a cable, etc. ,and so the table he used was good enough to make the point. Just a wild guess, don’t know whether that makes sense!

https://www.powerstream.com/Wire_Size.htm

The table linked below states that its’ values are the same for AC (
 
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Old 11-30-20, 09:08 AM
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Maybe he was just trying to make a point that maximums vary depending on wire size and didn’t want to get into differences like amps for Chassis Wiring, NEC amps for 3-wires in a cable, etc. ,and so the table he used was good enough to make the point.
I'm beginning to feel this same way!!!

This is an introduction type class, so just trying to get the point across that different conductors have various properties. Normal perspective is the larger the conductor the higher current capacity. I thought I understood it all pretty good, but you all have made me question this...

The "fusing current" for 14awg copper is 166A. But this isn't enough information to design a fuse, since it lacks a time variable.
When you say "fusing current", do you mean the point a copper conductor will become..... liquid? melt? When I hear "fuse" I think of "fusing" optical fibers together....
Also on this thought: Does this mean 14 AWG solid wire used in house wiring would need to have 166A of current, for a determined period of time, before they would melt? At what point, would there be a chance of fire, outside a junction box?

Dang it, asked a question, and now have so many more! May just have to do a lot more reading....

Thanks all for the replies and conversation. You have all helped me better understand how much I don't understand!!!
 
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Old 11-30-20, 12:19 PM
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Hi Trying –

I’m no expert for sure, but I think the Fusing Current is the current which will start the melting process. Say it starts at some point in time (say t1), then there is a later point in time (say t2) where there is no more current flow through the wire. I think what happens is that there is actually some current flow through the wire even while it is melting. But eventually the current stops after a certain amount of wire melting. And I guess obviously when the wire breaks or falls there wouldn’t be any current.

I think this assumes a wire hanging out in free air. I think those times in the table on the Fusing Current columns represents t2-t1 for different currents. But I’m not certain, although that seems to make sense.

Luke M is post#7 stated
The "fusing current" for 14awg copper is 166A. But this isn't enough information to design a fuse, since it lacks a time variable
If I understand then, that statement would be correct and the information under Fusing Current columns in the table linked below would be the answer. But I’m not 100% sure about all that.

https://en.wikibooks.org/wiki/Engine...can_Wire_Gauge
 
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Old 11-30-20, 01:44 PM
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>>

Discussing the fusing current of 166 amps for 14 gauge wire is completely out in left field, completely off topic, completely irrelevant, regarding the original question of why the chart lists 5.9 amps for the current rating of 14 gauge wire while "everybody" uses 15 amps as the current rating.
 
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Old 11-30-20, 07:36 PM
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zoesdad [QUOTE]The table linked below states that its’ values are the same[color=#222222] for AC (
 
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Old 12-01-20, 10:55 AM
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"Does this mean 14 AWG solid wire used in house wiring would need to have 166A of current, for a determined period of time, before they would melt? At what point, would there be a chance of fire, outside a junction box?"

Yes. Note that circuit breakers are designed to trip "instantly" at high current (around 5x their rated value, so >75A for a 15A/14awg circuit).
 
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Old 12-01-20, 11:14 AM
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Copper melts at 1984 degrees F. I'll let you research how hot wood, and even PVC insulation needs to get to flame. But, its less than 1984!

Also, no breaker acts "instantly". A magnetic breaker needs to pull back a pawl and then let the spring pull back a contact. This all takes time, around 20 milliseconds +/-. This is in the area of 10 x to 1000 x the rated hold current.
 
 

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