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Re: How long can a circcuit be? #71555 11/03/06 02:41 PM
Joined: Jul 2002
Posts: 8,277
Trumpy Offline
Mike, are you sure about the 250000 Amps??

That should have read 15kA, not sure how that slipped under the radar.
Just got a new laptop and the keyboard is VERY different to my normal computer keyboard.
Fault currents are very high at work, due to the size of the mains installed and the proximity of transformers.
The PoCo used more tranny's than usual with this area of town, to maintain the voltage at a respectable level, given the loads involved.

Work Gear for Electricians and the Trades
Re: How long can a circcuit be? #71556 11/03/06 07:38 PM
Joined: Nov 2000
Posts: 2,148
resqcapt19 Offline
I don't want to say that it's impossible for there to be conditions that could possibly put 5000A into a #14 cable, but even if you hooked that #14 cable to an ideal 120V 0-impedance source, if the romex exceeded 4', resistance in that cable is too high to allow current levels to reach 5000'.

My numbers say that 8' (4' of NM) of 14 will permit 4777 amps to flow if the system can supply it. Yes that is less than 5000 amps, but the 1/2 cycle damage point for #14 is 2384 amps.
In reality, transformer and feeder impedance drop current levels WELL below that point.

So there is never a need for a small breaker with an AIC greater than 5,000 amps?

[This message has been edited by resqcapt19 (edited 11-03-2006).]

Re: How long can a circcuit be? #71557 11/05/06 05:13 PM
Joined: Mar 2004
Posts: 98
OreElect Offline
Just for fun, try doing a voltage drop calculation on a 150 foot run of 14 with a 12 amp load.

Re: How long can a circcuit be? #71558 11/07/06 09:26 AM
Joined: Mar 2005
Posts: 1,213
SteveFehr Offline
So there is never a need for a small breaker with an AIC greater than 5,000 amps?
Well, that depends greatly on the distribution system and the size of the transformer. In this example, yeah, 5KAIC would be all that's required. But we could easily come up with another example (say, a 15A breaker on a 400A panel bus-fed from a 3000A switchboard/1MVA transformer) feeding a receptacle 6" from the panel. In THIS case, there is potential to exceed 5KAIC and damage the cable.

But in a typical residence, shorting out the receptacle? Not likely at all. The engineer really has to calculate all this and specify the KAIC rating of the breakers if it's expected to be an issue.

[This message has been edited by SteveFehr (edited 11-07-2006).]

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