ampacity

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hardworkingstiff

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Anyone here have the scientific reason as to why you have a doubling of circular mils from 250MCM to 500MCM but only have a 1.5 increase in amperage rating?

Thanks.

[ July 20, 2005, 09:43 PM: Message edited by: hardworkingstiff ]
 
Re: ampacity

I came up with a guess that the 500mcm will dissipate less heat than two 250mcm conductors.
 
Re: ampacity

I agree with BP, Inf and Iwire. Skin effect
results are greater in the larger conductors.
In the smaller conductors up to about 4/0 skin effect is negligible. Look at the following example:
300 amps R500 = .029/1000 R250 = .054/1000
watts = 300 x 300 x .029 = 2610 for #500
watts = 150 x 150 x .054 = 1215 for 1 #250

[ July 20, 2005, 10:58 PM: Message edited by: bob ]
 
Re: ampacity

skin effect. With AC eddy currents tend to make interior of conductors less efficient, Skin effect is the effect of the eddy currents tending to push electrons to the outer srface of the conductor, thus making a larger wire less efficient as it's size grows. That is why we parallel wires, because larger and larger wires can carry less and less amps per cm.

As I understand it.
Read this some place but cant find it at this moment
 
Re: ampacity

Skin effect is a function of diameter, spacing, and frequency. At 60Hz, skin effect is totally neglible (less than 1%) for wire sizes below 300kCMIL in open air.

Standard concentric stranded conductors are considered as solid conductors when calculating skin effect.

Based on 3 single copper conductor cables installed in metallic conduit at 60Hz and 65C, the skin effect ratios are:

2/0 = 1.01
3/0 = 1.01
4/0 = 1.02
250 = 1.03
300 = 1.04
350 = 1.05
500 = 1.10
600 = 1.14
750 = 1.22
 
Re: ampacity

Even DC currents tend to travel on the outter surface of a conductor. The electrons repel each other with the result that they try to get as far away from each other as they can. They do this by traveling more on the outside of the wire than the inside. (I have really simplified the physics, so I expect someone to point out that the electrons don't really repel each other. But that is an easy way to visualize this.)

Skin effect is an separate effect that only deals with AC. So skin effect may be negligible, but most of the 60hz current still travels on the outter surface of the wire.

Result is that doubling the area of the wire does not double the ampacity. And it would not half the voltage drop for the same current.

The best geometry for carrying current is a flat bar with a large surface area to cross sectional area ratio. Like a bus bar.

Steve
 
Re: ampacity

Ronald had the answer. The surface area (circumference) does not increase as fast as the cross section (circular mils). The cross section carries the current and generates the heat. The surface area gets rid of the heat.

Circumference = pi x 2 x radius = pi x diameter.
Area = pi x radius squared.

Double the diameter of the cable and the cross section area goes up 4 times. Surface area only doubles.

So you have 4 times as much copper but only twice as much jacket area to let out the heat.

500 kcmil has twice the copper of 250 kcmil but only 41% more surface area (circumference). 250 copper Class B compact diameter = 0.520", 500 diameter = 0.736".

As Jim pointed out, skin effect also increases the effective resistance, (about 7%) so there is more heat to get out of a smaller area.

Bob Wilson, P.E.
Edited to add conductor diameters.

[ July 21, 2005, 12:11 PM: Message edited by: rcwilson ]
 
Re: ampacity

hardworkingstiff,

No, the concentric stranding of standard circuit conductors is soley for ease of bending and handling. For calculation purposes this stranded wire (larger than #8) is treated the same as solid.
 
Re: ampacity

I also think that the answer is heat, but to calculate the temperature, one must factor in both area and the heat generated(current squared x resistance)

[ July 21, 2005, 03:22 PM: Message edited by: cosmos ]
 
Re: ampacity

I meant to put this link up last night but didn't get around to it.

Skin Effect

I'm becoming disappointed in what used to be my favorite reference site. It seems like it's getting more and more shallow all the time. If I was a conspiracy theororist. :D

Anyway, you can find the math for skin effect here and it's within the grasp of a lot of the more math oriented folks here.

Jim Dungar has the parameters. But I'm going to maybe reveal some of my ignorance here, I don't know about the spacing. I'm not sure about 1% but I haven't calculated anything either. Skin effect is far less of an interest at 60 Hz. than it is at radio frequencies but it's still in the game.
 
Re: ampacity

For the inquiring minds.

My reference book was:
Standard Handbook for Electrical Engineers - Tenth Edition
editors: Fink and Carroll
publisher: McGraw-Hill
copyright: 1969
 
Re: ampacity

I haven't really looked at skin effect before but I'm gonna take a stab at trying to relate it to the two conductor sizes with the little bit of info I have. This is just based on the web page I posted so I wont be any closer than what's there.

The math is calculated in terms of skin depth. From what I gather it works the same for any size piece of conductor. That is the ability of the coductor to carry current diminishes with depth in the same proportion regardless of thickness.

Skin depth is the depth below the surface of a conductor where the current is about .37 of the current at the surface.

Taking a figure from a little chart that's provided,

Copper has a skin depth of 8.57 mm. at 60 hz.
That means that current is 2.7 times less at 8.57 mm. from the surface.

Here are the diameters of the conductors mentioned:

250 kcmil 14.61 mm.
500 kcmil 20.65 mm.

What I don't have is any information on what kind of curve skin effect follows with depth. So I can't say a whole lot more than this:

All of the 250 kcmil conductor is above the skin depth (the point where current is 2.7 times less)

8 1/3% of the 500 kcmil conductor is below the skin depth.

I don't have enough information to turn this into percentages for current carrying ability. I've come up with only enough information to show that skin effect is in fact a signifigant concern. Just really rough guessing based on some numbers I did it's looks like it's better than 10% in this case. It might be a lot better than 10% but I really can't tell from what I have.

Edit: Typo's

[ July 22, 2005, 08:31 PM: Message edited by: physis ]
 
Re: ampacity

Did I do this right? If I extend skin effect as a straight line linear function I get:

0 conductance at 11.74 mm. from the surface.

So at 60 hz. a copper conductor 23.48 mm. in diameter would carry only 50% of the material's ability without skin effect.

I think Bryan's right. Skin effect.

Edit: I'm not sying this is what happens, I'm just using an arbitrary function to guess.

[ July 22, 2005, 01:55 PM: Message edited by: physis ]
 
Re: ampacity

Skin effect is not a straight line function.

Skin effect is caused by the unequal inductance in a conductor, that is why spacing (proximity to it's return conductor) and frequency are factors.
 
Re: ampacity

I was typing my edit in as you were posting.

Yes, you are indeed correct. Inductance is part of the equation.
 
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