Upsizing equipment grounding conductor based on availabl

[wbbowman]

What is the figure 250-177 talking about in the Mike Holt book Understanding The NEC - Volume 1?

I do not know where the 250 degree termination comes from and how that relates to 7000 A of fault current.

I am most curious as to where the 1.56 multiplier comes from and how the mulitiplier turns amps into circular mils.

If someone knows, please advise.

William Bruce Bowman, PE

 

[jtester]

Re: Upsizing equipment grounding conductor based on availabl

I don't have access to that figure, and am hoping one of the moderators can post it.
Jim T

 

[physis]

Re: Upsizing equipment grounding conductor based on availabl

Without having that book I don't know how to respond.

There's not enough information in your question to answer.

 

[bphgravity]

Re: Upsizing equipment grounding conductor based on availabl

What I believe you are refering too is specific withstand ratings of conductors.

Insulation damage will occur at around 150? C for a 75? C rated conductor. The annealing level occures around 250?, and complete vaporization occurs around 1085?.

Using the 1-ampere for 5-seconds for every 42.25-cm withstand rating and the formula I?(t), one can determine how much current a conductor can withstand in a specified amount of time.

What I think Mike has done, is use a ground-fault calculator that reverses the formula and tells you what size conductor is needed at the specified trip rate of a particular sized OCD.

So for the example you are talking about, a 7,000A fault would generate heat that exceeds 250? for #10 awg so a cm increase of a factor of 1.56 for a 1/2 cycle trip breaker provides you a conductor that will not have a temperature raise of 250?.

This can be proved by performing the I?(t) formula using a 1/2 cycle OCD. #8 is good for around 9,600-amperes in 1/2 cycle. #10 is only good for about 6,000-amperes. So under a 7,000-ampere fault and a 1/2 cycle OCD, #10 is not sufficent, hence the note.

[ February 10, 2005, 03:06 PM: Message edited by: bphgravity ]

 

[BAHTAH]

Re: Upsizing equipment grounding conductor based on availabl

The 250 degree value Mike states in his book may come from the IEEE committee report in "A Guide to Safety in AC Substation Grounding." There is also information available from the "Insulated Cable Engineers Association" publication P32-382. Soars Book on Grounding has a portion devoted to "Bolted Connections" that covers this subject very well.