Withstand current.

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wocolt

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Bennie
You asked for the withstand current for NO.8, it goes by amp/cir.mil. ie, No.8 is 16510 cm /42.25 comes out to be a little more than 390 amps for 5 seconds.
This is according to the ICEA publication.
This is in reference to a 'thread already closed', but thought i'd answer it.

WmC
 
Re: Withstand current.

About 12 years ago I wrote a BASICA program to calculate the Insulated Cable Withstand Current which I believe is reasonably accurate.

For 8 AWG at 5 seconds ( 300 HERTZ ) I get 390.8981 amperes.
 
Re: Withstand current.

I am learning here, when you say 300 hertz I imagine you mean 300 cycles at 60 hertz.

And when we use the term "withstand" do we mean before the conductor opens or before the insulation fails or is damaged?
 
Re: Withstand current.

Bob, I understand there is more than on type of "withstand" conversation. It can be voltage or amperage.

As far as the amperage "withstand" numbers, these are dealing with insulation damage. with # 8 75 deg insulation.

The following figures are based on 60 cycles which would not match the total time durations of 300 cycles


1/8 cycle 19200 amps

1/4 cycle 13500 amps

1/2 cycle 9600 amps

1 cycle 6800 amps

2 cycle 4800 amps

3 cycle 390 amps

Roger

[ September 27, 2003, 06:39 PM: Message edited by: roger ]
 
Re: Withstand current.

Thanks Roger, This is interesting to me, it seems there would be to many variables to this to figure.

For one wouldn't the ambient temp and the conductor temp before the "event" happens make a difference as to how long it takes to raise the conductor temp to the point of damage.

Another question is an event as short as 1/8 cycle a possibility, or just a math exercise?

What I mean is will an OCPD open that fast even with a bolted short?
 
Re: Withstand current.

Roger,

I believe a digit was left off of the 3 cycle value.

The program calculated for cycles;

1/8 19150.02

1/4 13541.11

1/2 9575.01

1 6770.554

2 4787.505

3 3908.981

and that copper wire melts at 1083? C.

iwire has a point.

I just doubt that a few degrees ambient would make much difference under high Available Short Circuit Current (ASCC).

If the 8 AWG is exposed to 40,000 ASCC, the time would be 2.865025 -02 ( between 1/32nd and 1/64th of a cycle ).

gwz2
 
Re: Withstand current.

Glenn, you are right, I did miss a digit. :)

Roger
 
Re: Withstand current.

Roger:
I got this for sizing a grounding electrode conductor. The point to the whole exercise is not to exceed the max rating, especially if bare is used the 5 second time limit is in the safe zone where damage would not occur if the fault was with in this scope. That is minimum damage to surrounding wires, where a meltdown would deteriorate from a line to ground fault to a full blown line to line fault.
In fact they recommend up sizing the cable once a 100 + feet is reached to satisfy the voltage allowed by these people which is 40 volts, this rule of thumb, seems to be satisfactory. The example they use was on a 100 amp service so on a line to ground fault that cable should carry a max of 40 volts during the fault the larger cable of course would carry less.(other factors notwithstanding, such as length, environment, corrosion and of course Terminations)
This goes along with the I(squared)T rating, the IEEE recommends a 1200% rating to ensure tripping the breaker in 2-5 seconds.(especially, adjustable breakers)
This is as I understand it, to clear the fault as quickly as possible Is the sole reason behing all of this is it Not ?

WmColt
 
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