ampacity

Status
Not open for further replies.
Re: ampacity

I missed rcwilson's post. What he says is also true. The ratio of surface area to cross sectional area decreases with increasing size.

So it's both heat and skin effect.

Edit: I forgot that you posted this Jim.

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
Click to expand...
There's a gigantic difference between what I come up with and the figures you posted. Even though I'm completely approximating I can't unerstand this much of a difference.

Besides me being wrong, could it be because of the difference in inductance when the conductors are ran together? :confused:

Edit: I get a difference of 70% for cross sectional area to circumference between 250 kcmil and 500 kcmil. That's pretty significant but I don't think it explains the ampacity difference by itself.

Edit again: To say it more rightly, The differece is that 500 kcmil has 70% the circumferance to cross sectional area ratio that 250 kcmil has.

[ July 22, 2005, 07:32 PM: Message edited by: physis ]
 
Re: ampacity

I'm sorry, I know I'm babbling on and on. Sometimes I get a bur in my butt and everybody suffers.

What is the skin effect ratio? Skin effect to conductivity? Skin effect to what?
 
Re: ampacity

When I commented on the circle I didn't think and still don't that the heat or skin effect has anything to do with it,although it might?

But just because you double the cross section of a circle doesn't mean you double its volume.Thats all I meant.

Ronald ;)
 
Re: ampacity

Whether you like it or not you're right Ronald. :D

Edit: And I'm convinced it's both skin effect and surface area to volume ratio.

[ July 22, 2005, 08:26 PM: Message edited by: physis ]
 
Re: ampacity

Like I said, I got a burr. :D

But that's how I've always learned stuff. Beat a subject up until it surrenders. :cool:
 
Re: ampacity

The skin effect ratio is very simply the difference (increase) in AC resistance versus DC resistance.

So for a copper 500kCMIL cable in free air the AC resistance will be 1.07 times the DC resistance (which is calculated based on the area and material of the conductor and is the value usually shown in tables). In conduit with other conductors the AC resistance increases to 1.1 times.
 
Re: ampacity

Well that's ball park. (I think it's shy) But that is 10% and not one. I'm not so much disagreeing with you Jim as I am pushing on the numbers. :)

Edit: I mean ball park with what I come up with.

[ July 23, 2005, 02:25 AM: Message edited by: physis ]
 
Re: ampacity

Remember I squared R. 500kcmil has 1/2 the R of 250 kcmil so will have sqrt(2) times the ampacity, about 140%. The rest of the difference has to do with skin effect and other issues.

The fundamental issue is that the allowable heat dissipation from a conduit or cable is roughly constant for all wire sizes and counts. You can only get so much heat from a conduit. The rest is I squared R!
 
Re: ampacity

Don't forget if we are talking about KCM or MCM wire sizes like 500KCM, that is the cross sectional area of the conductor. So if we ignore heating effects and skin effects, we would expect a 500KCM to carry twice as much current as a 250KCM.

Physics: Notice your "skin depth" also applies at DC and is different from the "skin effect" Jim mentioned that is only for AC.

Steve
 
Re: ampacity

By Steve:

Physics: Notice your "skin depth" also applies at DC and is different from the "skin effect" Jim mentioned that is only for AC.
Click to expand...
I don't have much data on it. DC's off the chart for me. :D

Edit: Wait a minute Steve, skin depth is a parameter of skin effect.

[ July 28, 2005, 02:17 PM: Message edited by: physis ]
 
Re: ampacity

Originally posted by hardworkingstiff:
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.
Click to expand...
Stiff,

My book says that skin effect causes R to increase only about 1% in a 250MCM stranded cable at 60Hz. I would guess that the thermal resistance from the center of the cable to the outer layers reduces the dissipation relative to the dissipation of two cables of half the area.
 
Re: ampacity

It says that a 3rd year student is not smart enough to understand the arithmetic.

No mention of any other size and no tables.

Also mentions "proximity effect".
 
Re: ampacity

Jim Dungar posted a list in this thread that I also reposted. It shows 1.10 for 500MCM.

There is a question as to how to apply the table because skin effect is dependant on inductance and (as I've recently realized I need to be aware of) proximity or spacing changes inductance. The values listed are for conductors in a conduit.

I'll requote his post.

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
Click to expand...
 
Re: ampacity

Come on everyone. Go back to basic basics. Heat loss = I^2 * R. Heat loss is the limiting factor. Cut R in half (250MCM --> 500MCM) and I is allowed to increase by SQRT(2)x not 2x.
Per 2002 Ugly's
250kcmil Cu 75C RHW in duct = 255A
500kcmil Cu 75c RHW in duct = 380A
DC resistance of 250kcmil CU 0.0515 ohm/kft
DC resistance of 500kcmil CU 0.0258 ohm/kft
250kcmil heat = 3.3 W/ft
500kcmil heat = 3.7 W/ft
The 11% difference is due to skin effect and other issues.
If you run the I^2*R calculation for several wire sizes, they all are pretty close to the same W/ft.
 
Re: ampacity

Beanland,

You too Sam,

Look at it this way:

For a constant current, the power dissipated in the cable would be halved by doubling the cross sectional area of the cable.

But, the surface area of the wire has been increased not by 2, but by a factor of 1.4. So the thermal resistance between the cable and air is decreased not to 0.5 but to 0.7 of its previous value.

It would appear that the ampacity is limited by the thermal resistance. I think that at 60Hz, the skin effect would be negligible.

Now if you double the area by stringing a parallel cable some distance from the first, you have doubled the ampacity.

Rattus

[ July 28, 2005, 07:35 PM: Message edited by: rattus ]
 
Re: ampacity

beanland, I guess I didn't realize it was such a slam dunk. :D

Edit: I haven't had a chance to read your post yet Rattus. It wasn't there when my post first landed.

[ July 28, 2005, 08:10 PM: Message edited by: physis ]
 
Re: ampacity

If you want to mess with it, try putting together a surface to volume table for different conductors.

The surface area won't go up as fast as the volume (volume = pi * r^e, surface = 2 * pi * r). The ampacity will follow roughly that ratio with some adjustment for skin effect.
 
Status
Not open for further replies.
Top