Conductor Withstand Ratings

[josephkendall]

I have a situation where the conductors that I am trying to qualify have the possiblity of seeing locked rotor current for a maximum of 100 seconds. Does anyone know where to find short term withstand ratings for copper conductors?

 

[TWW]

The Okonite Company have an "Engineering Data For Copper and Aluminum Conductor Electrical Cables" Bulletin 721.1 With Section 4 discussed and plots the allowable Short Circuit Currents for insulated copper conductors + the equation. It is probably now located on there web site.

 

[Shortstub]

Also look into any edition of "Standard Handbook for Electrical Engineers".

One point often overlooked when determining long-time temperature exposure effects on cable, is that the connectors are more likely to be overstressed, than are the cables.

Another point of concern is the the increase in length of long cables.

 

[TWW]

I noticed from the IEEE STd 141, the same formula is used as was shown in the Olkonite Sheet, However the IEEE has the caviot that this formula is only good to appproximately 10 sec. Sorry

 

[jghrist]

The formula is only good for short time periods because it assumes no heat loss from the conductor. It would be conservative (maybe too much so for 100 sec) for longer time periods because there will be heat loss.

 

[Shortstub]

JosephKendall,

An excellent treatise on the subject was presented in the April '79 issue of EC&M (Electrical Construction & Maintenance) magazine. It was presented by Dr. K.D. Srivastava of University of Waterloo, in Canada.

Four graphs show that a straight-line relationship between time and current, plotted on a log-log scale, also can be found for short-time overload... defined as being from 10 to 1,000 seconds.

Thus, if you have access to the typical Short-Circuit stress curves, then, by extending the time axis, you can determine the over-load thermal capability line for the cable & current condition you are concerned about.

 

[Shortstub]

BTW, the four graphs represented conductor materials of copper & aluminum, and typical NEC-code listed PVC insulations such as TW or THW, and XLPE insulation such as XHHW.

A rather complex temp-rise formula is also presented.

 

[meihm]

You may find the article at the end of this link (

http://www.usawire-cable.com/techpapers/tempratings.htm)

of interest. It deals with three different time frames: Normal; emergency overload; and fault conditions.

Since the emergency overload is "long duration", this may be what you seek.