Why do some wire gauge ampacity charts state vastly different current ratings?

Question

This is a question that I've had for a while but have somehow managed to go without asking. Looking at all the wire gauge ampacity charts online, some of them appear to provide vastly different ratings for the same wire gauge. I understand this may be due to different standards, temperature, conductor types, strand count, etc.

However, lets looking for the current rating associated with 10AWG copper conductor at a low operating temperature (60c):

Chart 1: 15 amps

Chart 2: 30 amps

I've only cited two sources, however, I've come across many other charts that provide this 15 to 30 amp difference. What gives?

Answer 1

Notice that your first link gives two ampacity values for AWG 10 wire. For power transmission applications, the rating is 15 A. For chassis wiring applications, the rating is 55 A. These ratings (and the third one at your other link) are based on different assumptions of convective cooling available and allowances for wire self-heating.

For example, the first link says

The Maximum Amps for Power Transmission uses the 700 circular mils per amp rule, which is very very conservative. The Maximum Amps for Chassis Wiring is also a conservative rating, but is meant for wiring in air, and not in a bundle.

You should use a rating consistent with how you're using the wire and the cooling environment around the wire in your application. You should also consider resistive voltage drop along the wire if you find one of the higher ratings appropriate for your thermal environment.

Answer 2

The temperature a wire will reach when carrying a certain amount of current depends upon the gauge, the ambient air temperature, and the amount of thermal insulation between the wire and the air. Passing a certain amount of current through a piece of bare copper wire which is surrounded on all sides by air will not cause its temperature to rise nearly as much as as passing that same current through a like-gauge wire which is buried under 4 inches of fiberglass insulation. The difference between the 55A rating in one table and the 10A rating in the other is likely because of this.

The 55A and 30A figures assume the limiting factor is heat dissipation. The 15A rating takes into account another factor: any power that gets converted into heat won't be usefully delivered to its destination. If one were trying to send 2400W of power over a cable, one wouldn't want to lose hundreds of watts in the cable even if it could dissipate that much heat safely. While the required gauge to usefully send a certain amount of current with a certain fraction of loss will depend upon the voltage and transmission distance, there are many situations where it will exceed the gauge that would required to avoid overheating.

Answer 3

The type of insulation used determines the maximum ampacity of a wire. In reality, this maximum must also be derated for safety (normal load should be 80% of rated ampacity), ambient temperature, armoured sheaths, raceways or adjacent power conductors.

The first site is more:

As you might guess, the rated ampacities are just a rule of thumb.

Long power runs, assume 15A for #10.

The Maximum Amps for Power Transmission uses the 700 circular mils per amp rule, which is very very conservative.

Short power runs, assume 55A.

The Maximum Amps for Chassis Wiring is also a conservative rating, but is meant for wiring in air, and not in a bundle.

They protect their butt by:

NOTE: For installations that need to conform to the National Electrical Code, you must use their guidelines. Contact your local electrician to find out what is legal!

If you figure a maximum of 3% voltage drop to a feeder, this means a specific maximum distance for #10.

At 120V, 3% means 3.6V. At 30A, this equates to a feeder resistance of \$0.12\Omega\$ or feeder length of 120ft or 60ft from panel. 30A with a load 60+ft from panel violates the 3% maximum voltage drop. Too much current for the insulation, which would go through irreversible deterioration and possible fire, death and destruction.

You could do the math and figure out the correct size of wire for the wire distance or using their rule of thumb and half the current. To get 30A, double the area or #7.

Answer 4

One meter of 8 gauge wire at 300K emitted as a black body approximately 9.185 W of energy. σST^4 - Stefan-Boltzmann law. The same one meter wire will consume 9.185 W at approximately 67.77 A. I^2*R Joule–Lenz law.

So, at 68 A, 8 gauge wire will be in equilibrium in consumption/emittance at 300K (27C or 81F). 68^2 * 0.002 (I^2R) is approximately equal to 5.6710^-8 * 0.02 * 300^4 (σST^4).

This means, that 8 gauge wire on the open air will withstand 68 A without heating above 300K. Any one could easily calculate this equilibrium for any given gauge and temperature

Used constants: Resistance of 8 gauge copper stranded wire - 0.002 ohm per meter. Outer diameter of 8 gauge wire - 6.25mm, surface area of 1 meter - 0.02 sq.m. Stefan-Boltzmann constant - 5.6710^-8 Wm^-2*K^-4

Answer 5

The two charts are based on different criteria. The first chart uses a rule which says that a wire should carry no more than 1 amp per 700 circular mils of cross-sectional area. The second chart gives the maximum current for several different temperature increase. (The environmental conditions are not specified.)

You'll notice that the first chart says 15 amps for "power transmission" (long runs in enclosed spaces) but 55 amps for "chassis wiring". So there's a wide range depending on the thermal conditions.

I suggest staying closer to the low end unless you know what you're doing.

Answer 6

Probably more importantly;

The first chart is a listing for the COPPER wire itself and if you notice, they also list it as 2600Hz with regard to skin effect. This is a chart for engineers who are going to make cable to be used in high frequency applications, not use it in the real world. it even says that in the beginning: "This data is useful for high frequency AC engineering." details matter when reading things on the net.

The second chart is based on the National Electric Code for insulated wire, as in what you would buy and install in an every day application.