# American Wire Gauge



## Shawncfer (Jul 21, 2010)

Simple question for yall.

So How do I determine what size awg I need for so many amps of power?

I found this chart: American Wire Gauge table and AWG Electrical Current Load Limits with skin depth frequencies

And it has two parts listed, one says "Maximum amps for chassis wiring" and the other says "Maximum amps for power transmission" So which one do I need??


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## gafftaper (Jul 21, 2010)

Please don't take this personal, but electricity is an area where the act of asking the question suggests, you need to hire a professional to do the work. While we love to help, you are stepping into territory that can be very dangerous. Giving a simple blind answer without knowing what you are doing can potentially be fatal to you or others. So, please give us a little context. We'll try to help you or steer you to a professional who can do the work safely for you.


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## Chris15 (Jul 21, 2010)

Unfortunately the relationship is not a simple one, it depends on a number of factors, which determine how well the heat produced by the copper conductors can be dissipated.
So what's the application you are trying to work out cable size for?


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## derekleffew (Jul 21, 2010)

Shawncfer , the chart you found is not a particularly good one. Most will point you to (in the US) NEC Table 310-16, Table 310-17, Table 400.5(B), and others as applicable. 

As you can see from reading the tables and from what others have said, the ampacity of a conductor is determined by many factors, including but not limited to, conductor material, type, insulation, number of current carrying conductors, ambient temperature, free air or conduit, diversity, derating.

The answer is just not as simple as "12 AWG can carry 20 amps" or "4/0 is good for 400A," even though both of those statements have been historically regarded as gospel, and are still being printed in modern lighting textbooks. They _might_ be true, but then again they might not.


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## Shawncfer (Jul 21, 2010)

gafftaper said:


> Please don't take this personal, but electricity is an area where the act of asking the question suggests, you need to hire a professional to do the work. While we love to help, you are stepping into territory that can be very dangerous. Giving a simple blind answer without knowing what you are doing can potentially be fatal to you or others. So, please give us a little context. We'll try to help you or steer you to a professional who can do the work safely for you.



Im not doing anywork myself. Im a student who wants to learn. I know I don't know what I'm doing and I'm not going to go out and pretend like I do.


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## dcollins (Jul 21, 2010)

Well then let me talk about some of the things that affect the size wire you might need!

For one, the NEC differentiates between constant loads and temporary loads. Permanent loads are defined as any load that lasts for more than three hours - our lighting loads are considered permanent. Any permanent load gets an 80% derating - that is, if a wire can handle 100A temporarily, then we can only use 80% of that, or 80A, permanently.

There's also a derating based on the number of current carrying conductors connected within a single conduit. This counts the hots and the neutral, but not the ground, and also (I think?) applies to socopex. I don't remember the exact numbers, but it's based on the number of conductors and functions the same way as the above. This can be as much as a 50% derating, turning a 100A wire into a 50A wire.

There's another factor to take into account, which is heat. If you're running wires in a 120 degree sauna, you need to use wires that can handle that. NEC also has correction values for ambient temperature, but air circulation is another factor, etc.


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## Chris15 (Jul 21, 2010)

So since we're talking about the expansion of knowledge here, let's get down to the fundamental physics to start with... (all of this is metric and 240v based...)

We know that any material at room temperature will have a resistance that is governed by it's resistivity, it's cross sectional are, the length and the temperature. R = rho*l/A, where rho is resistivity, l length and A the cross sectional area of the conductor.
Resistivity is temperature dependant and so R = R0[alpha (T-T0) + 1] where R0 is the resistance at the reference temperature T0 and alpha is the temperature coefficient, (which technically is also temperature dependent...)

The physical effect of that resistance is the conversion of electrical energy to heat. The ability to dissipate that heat effectively is what underpins the whole process of cable selection.

Let's take for example 2.5mm2 twin and earth TPS cable as is used for wiring power in walls here.
It's normally either V75 or V90. The TPS designation is Thermo Plastic Sheath, the type of outer jacket. A V75 cable is designed for a maximum conductor temperature of 75 degrees C under normal conditions, under short circuit it can cope with more for a short period.

So basically you need to make sure that the conductor temperature will be kept under 75 degrees or bad things start to happen... Airflow is what allows the heat to be dissipated and that's where many of the variables come into play...
So with that in mind, let's look at some numbers.
In free air and not touching a surface, the cable has a current capacity of 27A. If it touches the surface it's mounted to that comes down to 26A, that's because there isn't airflow on one side now... Now if I go and place the cable in a conduit that's in free air, the current capacity comes down again, to 22A for a flat cable. This is because in conduit, the air is trapped and can't move and dissipate heat as effectively... Now if I go and surround the cable partially in thermal insulation, say with one side touching a surface and the other touching an insulation batt, the capacity is down to 18A. Surround it completely in thermal insulation and you're looking at 13A, about half of the current capacity of the cable in free air... Now if you go and bury that cable direct (assuming the cable is rated for that), you get 40A capacity, 31A if in conduit. That's because the standards assume soil temperature to be 25 degrees C rather than air temperature of 40 degrees C. Because the soil is cooler, it's better able to dissipate heat and so you have a higher continuous current rating.

The conductor material makes a difference too because the material changes the resistivity. For instance, a 16mm2 cable in free air with a copper conductor has a current capacity of 85A. The same size aluminium conductor has only got a capacity of 66A. 

The insulation material also makes a difference. If instead of the V75 insulation, I had X90 insulation, I would get 34A capacity out of a 2.5mm twin and earth, if it were say X-HF-110, I get 41A capacity. 41A is a fair bit more than 26A and I haven't changed the amount of copper, just the insulation around it...

Ambient temperature I mentioned in passing when talking about buried cable... Remember that you need to base the sums on the maximum ambient temperature, which is why the Australian Standards are based on a 40 degree ambient... Let's throw some numbers around, going back to our V75 cable for this. With an ambient temp of 40, the rating factor is 1. If that comes down to 20, you have 1.28x the capacity. If that goes up to 50 degrees, the cable's down to 0.82x capacity, at 60 degrees it's 0.60x, at 70 degrees it's a mere 0.37x the capacity.

Now if we consider the bunching of circuits. If we look at cables bunched on a surface or enclosed (say multiple wires in a conduit), then we get these derating factors: 1 circuit, 1.00, 2 ccts, 0.80, 3ccts, 0.70, 5ccts, 0.60, 10 ccts, 0.48, 20 or more circuits, 0.38. That means our original 2.5mm cable is down to a capacity of 8.36A if in a group of 20 or more circuits, which is less than the standard 10A capacity of a normal dimmer and so you would need to use a cable >2.5mm2... Earthing conductors, lightly loaded neutrals (something which one cannot assume for entertainment applications) don't get counted in these sums...

You can uprate a bunched set of cables IF and only if you can show that they can never all be fully loaded simultaneously. I would be very hesitant to do this in a theatrical application...

Be aware that all the numbers mentioned so far are for fixed wiring. The principles are equally applicable to flexible cordage, but the standards set a lower capacity to keep the conductor temperature to 60 degrees to keep the surface temperature of the cables down... 2.5mm2 flex is rated for 20A max. Derate as needed for bunching (say in Socapex style cables) and other factors mentioned...

Add to that that sometimes the voltage drop will be the governing factor in cable selection, not the current capacity, as we try to keep the voltage at the point of usage within 5% of that at the point of supply.

I haven't and I won't talk about fault current ratings, it's not needed for most people...

All numbers come from AS3008.1.1:1998, Copyright Standards Australia / Standards New Zealand. The US NEC will have different numbers and you need to ultimately use the code in force in your jurisdiction...

I hope I've given enough background to allow you to now ask more specific questions...


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## JD (Jul 21, 2010)

dcollins said:


> and also (I think?) applies to socopex.



Socopex is considered an "engineered" cable assembly. A good thing, or we would all be dragging around #8 "widowmaker" Socopex!


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## gafftaper (Jul 22, 2010)

JD said:


> Socopex is considered an "engineered" cable assembly. A good thing, or we would all be dragging around #8 "widowmaker" Socopex!



"Go get the pallet jack, I need another 25' of socopex."


So why is it okay for a socopex or a twofer to break the rules?


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## derekleffew (Jul 22, 2010)

gafftaper said:


> ... So why is it okay for a socopex or a twofer to break the rules?


I don't recall anyone saying anything about twofers. To what "rules" are you referring? The exceptions to the Extra Hard Usage jacket requirement in NEC 520.69(c) have been explained many times.

As for Socapex and other multi-conductor cable, see Borderlight cable and Load diversity. (Table 520.44.)

Perhaps the attached _Protocol_ Spring 2002 article, _Breaking Convention: Industry use of Portable Cords and Cables_, by Ken Vannice (AKA the other SteveTerry), will prove enlightening.View attachment estacablessp02.pdf


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## gafftaper (Jul 22, 2010)

Yes the SJ vs. SO jacket on a two-fer is what I was referring too. 

Just trying to stimulate further discussion.


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