ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
Recent Posts
Tiny Homes and the NEC
by gfretwell - 12/06/22 01:43 AM
EMC Glands on Motors
by Trumpy - 12/01/22 06:12 AM
Ground Rods: Installation and Hook-Up
by Trumpy - 12/01/22 05:54 AM
Happy Thanksgiving all!
by gfretwell - 11/30/22 05:55 PM
Colt Firearms Switchbox
by NORCAL - 11/29/22 01:04 AM
New in the Gallery:
240/208 to a house
240/208 to a house
by wa2ise, October 9
Now you know.
Now you know.
by Tom_Horne, September 7
Who's Online Now
0 members (), 102 guests, and 11 robots.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
Page 4 of 4 1 2 3 4
Joined: Jul 2002
Posts: 8,432
Likes: 3
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.

Joined: Nov 2000
Posts: 2,148
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).]

Joined: Mar 2004
Posts: 98
Just for fun, try doing a voltage drop calculation on a 150 foot run of 14 with a 12 amp load.

Joined: Mar 2005
Posts: 1,213
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).]

Page 4 of 4 1 2 3 4

Link Copied to Clipboard

Tools for Electricians
Tools for Electricians

* * * * * * *
2023 National Electrical Code (NEC)
2023 NEC Now Available!
* * * * * * *

2020 Master Electrician Exam Preparation Combos
2020 NEC Electrician
Exam Prep Combos:
Master / Journeyman


Member Spotlight
Washington...Not DC
Posts: 240
Joined: March 2005
Top Posters(30 Days)
Trumpy 9
Popular Topics(Views)
300,580 Are you busy
230,266 Re: Forum
215,050 Need opinion
Powered by UBB.threads™ PHP Forum Software 7.7.5