Cable Short Circuit Withstand Capacity
Cable Short Circuit Withstand Capacity
(OP)
Can someone please explain what does the graphs at the following link mean? These graphs indicate that the 8 AWG cable can withstand only "6 kA" short circuit current at one cycle. But in actual practice we use 8 AWG cable all the time for 50 HP motors where the available short circuit current is far more than 6 kA.
htt p://www.ge neralcable .com/NR/rd onlyres/D6 0F2C04-B58 0-4C2C-A4D C-07B638A1 DA02/0/Spe c_F125.pdf
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RE: Cable Short Circuit Withstand Capacity
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If we learn from our mistakes I'm getting a great education!
RE: Cable Short Circuit Withstand Capacity
The graph that shows 6kA for one cycle is an aluminium conductor with a thermoset insulation rated for 105 deg C, what is your conductor material, insulation material and continous temperature rating as these will make a difference.
desertfox
RE: Cable Short Circuit Withstand Capacity
For example, if a motor is connected to 2000 kVA transformer at 480 V secondary, the available short circuit current will be about 45 kA. The minimum size cable that will withstand 45 kA on page 2 is #1. So what if we have to connect a 1 HP motor to this transformer. Shall we have to use a minimum of #1 cable for the 1 HP motor? Hard to believe.
RE: Cable Short Circuit Withstand Capacity
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RE: Cable Short Circuit Withstand Capacity
RE: Cable Short Circuit Withstand Capacity
Have a look at this site:-
http:
it might help
desertfox
RE: Cable Short Circuit Withstand Capacity
Yes. Convert 50 kA to source impedance in ohms. You would know the voltage. (Look up basic formulas, if needed.) Add the impedance of the source to cable impedance in ohms and divide the total impedance by phase voltage (L-N). OR use a software.
At 480V, 50 feet of #8 cable brings the 50 kA below 9 kA and 40 feet brings it below 7 kA. A quick check also shows that an ordinary 40A MCCB will open fast enough at 5.7 kA and below, to protect #8 conductor. However even a 100A, class J fuse will protect the #8 conductor for 0.9 seconds.
There a few other things to consider. 50 HP motor at what voltage? . Per NEC (in conjunction with 110.14), #8 is generally only good for 40A. Even many local AHJ requires using 60 deg C column for sizes #1 and below. I personally do it as a routine.
Yes, you do need to consider short circuit withstand rating of the cables while sizing it. Especially, when you are tapping a #8 wire right at a 50 kA bus, as you describe or use current limiting fuse/breakers. I would think this is very rare. In most applications, NEC conductor sizing is found to be adequate for thermal withstand rating but there can be exceptions and you need to check it when in doubt.
Hope this helps.
Rafiq Bulsara
http://www.srengineersct.com
RE: Cable Short Circuit Withstand Capacity
If the conductor is protected by a fuse or molded-case circuit breaker sized in accordance with the NEC, it's probably OK. It's always good to verify by plotting the time-current curve for the cable damage on the same graph as your overcurrent protective device trip characteristics. The are a number of computer program that can do that for you.
Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.
RE: Cable Short Circuit Withstand Capacity
Second: one cycle fault clearing it is not possible. You need at least 70 milliseconds.
So, for 4.3 KA and 0.07 sec 8AWG will be fair enough. If the cable length will be less than 100 ft it will be a problem, indeed.
RE: Cable Short Circuit Withstand Capacity
I agree that 8 AWG cable will be ok. I just want to understand the reason why the cable is rated for 4.3 kA and not for 50 kA.
RE: Cable Short Circuit Withstand Capacity
RE: Cable Short Circuit Withstand Capacity
RE: Cable Short Circuit Withstand Capacity
RE: Cable Short Circuit Withstand Capacity
RE: Cable Short Circuit Withstand Capacity
RE: Cable Short Circuit Withstand Capacity