Cable Ampacity Calculator

Wareagle,

Could be the due to the calculator being based on some standard and having a "Correction" build in to meet the applicable standard for that country.

The table we use in Australia (AS\NZ 3008) has conditions built in based on the installation so 70mm may support 220A in air, but be derated if installed in conduit.

Likewise volatge drop over a long run may require a step up in size to supply the voltage at the load end or next DB.

So the raw calcs may say 68mm, but 70 is required.

Could also be that the calc is literal and the "numbers" say 68mm, but the next commercially available cable size is 70mm.

Just my thoughts,

Regards,
Sapper

My interest is what is this answer of 3.52e+3. I have never seen ampacity in the form.

Sapper, My chart had the same ampacity as you are showing. I was trying to make the calculator answer equal the same but could not.

This is an echo of the old Fortran format for scientific numbers. In this case "e" is not Euler's number but signifies the power of ten exponent. So 3.52e+3 is actually 3,520.

Wareagle,

That's what I figured...was only trying to identify the why of the disparity.

Apparently we're on parrallel tracks, but my focus of the explanation was not what you were after.

Regards,
Sapper

Consider the source. The disparity is based upon using a MIL-STD that is not appropriate for most of our earthbound work. Clicking on the link from the calculator yields the following:

Reference information for the Wire Parameter Calculator at

http://circuitcalculator.com/wordpress/2007/09/20/wire-parameter-calculator/

Wire And Cable Derating Criteria From MIL-STD-975

Notes:

1. Derated current ratings are based on an ambient temperature of 70°C or less in a hard vacuum of 1E-6 torr.

2. The derated current ratings are for 200°C rated wire, such as Teflon Ô insulated (Type PTFE) wire, in a hard vacuum of 1E-6 torr.

a. For 150°C wire, use 80% of value shown in Table 16.

b. For 135°C wire, use 70% of value shown in Table 16.

c. For 105°C wire, use 50% of value shown in Table 16.

3. The current rating for bundles or cables are based on bundles of 15 or more wires. For smaller bundles, the allowable current shall be determined by IBW = ISW x (29-N)/28 where N = number of wires, IBW = current, bundled wire and ISW = current, single wire.

Digging further...

MIL-STD-975M (NASA), MILITARY STANDARD: NASA STANDARD ELECTRICAL, ELECTRONIC, AND ELECTROMECHNICAL (EEE) PARTS LIST (5 AUG 1994)., This standard is the technical baseline for standardization of EEE parts. The intent of the effort tis to focus part selection on EEE parts used in the design and construction of space flight hardware as well as mission-essential ground support equipment (GSE). The overall objective is to provide the designer with acceptable parts and specifications for procuring those parts for spaceflight missions. The result of this effort should provide the designer with EEE parts, including the necessary criteria pertaining to use, choice, and applications...

Hardly stuff that should be used by electrical engineers or electricians let alone novices.

I trust it isn't simply the difference between 70mm (a diameter) and 70mm² (an area)?

Does the program have a way to display the formulas and logic tree being used?

Once you get to large currents you have to consider skin effect. Electrons all have the same charge, so the repel each other, and migrate to the outside of the cable. Thus most of the current flows at the perimeter and very little at the centre. This is why in high current busbar systems the centre is dispensed with. Busbars, like cables are rated based on an allowable temperature rise. Sub stations normally use hollow tube busbars. Cooling gas/fluid often flows within the tubes to further increase the current capacity. For larger currents, (typically power station generator to main step up transformer) connections use one of three formats; each phase having a tubular 'O' or toe- to- toe channel '[]' or toe-to-toe angle '<>' configuration.

Sorry, skin effect is a function of frequency, not current magnitude (although it does manifest its effect in varying current density)....

From Wikipedia, the free encyclopedia

Skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor with the current density being largest near the surface of the conductor, decreasing at greater depths. The electric current flows mainly at the "skin" of the conductor, between the outer surface and a level called the skin depth. The skin effect causes the effective resistance of the conductor to increase at higher frequencies where the skin depth is smaller, thus reducing the effective cross-section of the conductor. The skin effect is due to opposing eddy currents induced by the changing magnetic field resulting from the alternating current. At 60 Hz in copper, the skin depth is about 8.5 mm. At high frequencies the skin depth becomes much smaller. Increased AC resistance due to the skin effect can be mitigated by using specially woven litz wire. Because the interior of a large conductor carries so little of the current, tubular conductors such as pipe can be used to save weight and cost.

Thanks for your help. I spent a lot of time trying to understand why Euler's number was used in the calculation.