Arc Flash Calculation

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What are you talking about?

TonyS

I am talking about safety to people working on or near energized conductors and thus potentially exposed to the heat and explosive force of an intense entirely-accidental electrical short circuit. Perhaps you have seen some posted videos of people being enveloped in a blast of this sort and blown as much as 1-3 meters away while also having their clothing lit on fire.

Sorry about an apparently confusing word. "Tap" is a term used in the electrical trade in the USA (and the National Electrical Code or NEC) to describe a splice from a (large) feeder conductor to a smaller conductor, called a "tap conductor". In my question, the feeding circuit is protected by an 800A circuit breaker with an instantaneous trip setting of 3300A. The taps are going to individual combination starter enclosures for motors typically 2-10Hp in size. The #12AWG wire has an ampacity of 30A, but the NEC permits this tap under its 10-ft (3m) exception.

--John M.

Could it be that the tap has a higher impedance and resistance thereby limiting the maximum level of prospective short circuit current that can develop in the event of a fault?

JOAT,

Yes, it certainly does. The calculations for a short (7.5m or 25ft) tap show the theoretical Isc to drop from the >20kA at its beginning to about 5kA at the other end. However, I am wondering why the engineer's calculation didn't start with the >20kA (the capacity of the feeder circuit of paralleled 500MCM cables) at the point of the tap and instead used 2800 or 2820A at this point. I wonder if it has something to do with the maximum fusing current that a #12AWG wire can carry as it vaporizes almost instantaneously.

--John M.

However, I am wondering why the engineer's calculation didn't start with the >20kA (the capacity of the feeder circuit of paralleled 500MCM cables)

I am not sure either, possibly there is something limiting the current before the feeder, the calculation design program (if any) required this lower value at the start not end of the feeder, or a mistake was made.

Check this

Arc Flash Calculation

Appreciate your reference to a site, and I have bookmarked it. The links in that site are all copies of the original calculator I use, from the Pacific Northwest National Laboratory (formerly Hanford National Laboratory). It is available directly at:www.pnl.gov/contracts/esh-procedures/forms/sp00e230.xls.

Unfortunately I have not found why the original calculations used such a low initial value for the short circuit current when they calculated the available fault current at the end of the #12 tap conductors. I have looked up the fusing (melting) current for this wire and find it to be over 5000A for time intervals of 0.04 seconds or less. The voltage drop on 20' of #12AWG wire at 5000A is approx. 200V, so that is within the available circuit voltage. I am beginning to think that the engineers who did this 2007 study made an error. However, I am still looking at additional reference materials.

Thanks--John M.