110.24 - 2011 NEC

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pete m.

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Ohio
To enforce this requirement correctly would the label need to contain the Peak value or the RMS value of available fault current?

I assume the Peak value would have to be used due to the verbiage of "maximum"

Pete
 

jumper

Senior Member
I believe Vrms would be fine, Vp or Vp-p might be acceptable if marked, but since everything else in the NEC is Vrms I would use Vrms.

Voltage (of a circuit). The greatest root-mean-square (rms)
(effective) difference of potential between any two conductors
of the circuit concerned
 

pete m.

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Location
Ohio
I believe the intent is to use the RMS value. Everything we do is in terms of RMS.

Thanks for the replies... and Charlie to quote you just a little bit I believe you wrote "...it says what it says..." :)

I do know what you are saying but the word maximum seems to imply peak as opposed to rms. Perhaps I should ask NFPA for a clarification?

Pete
 

rbalex

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I believe Jumper's response in post 2 is sufficient to establish the intent. (It cited the definition in Art 100)

Recently, I've seen way too many references to a single sentence or phrase without considering the entire context. True, "It says what it says" - but in context. Otherwise, the Code would have to be a series of extremely long, run-on sentences if we insisted each sentence had to include all relevant factors internally.
 

jim dungar

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The peak value would be absolutely useless.
All of the reasons for using this fault current (i.e. comparison to an SCCR) are based on RMS values.

Now as far as what is "maximum (RMS)", that is not as easy to answer from the wording in the NEC
 

pete m.

Senior Member
Location
Ohio
Below is the rationale behind the word "maximum":1-115 Log #605 NEC-P01 Final Action: Accept
(110.24)
_______________________________________________________________
Submitter: Donald R. Cook, Pelham, AL
Comment on Proposal No: 1-183

"The word ?maximum? was added in an attempt to clarify the requirements as an equipment rating consideration and not for use in conjunction with arc flash hazard analysis."


Don't get me wrong. It makes sense to me that the intent is to use the RMS value as opposed to peak. I suppose it would have been nice if the CMP saw fit to use something like "RMS" as opposed to "maximum".

Then again I may be the only person who deals with the NEC that would take the word maximum and relate it to the peak value.

Pete
 

charlie b

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"The word ?maximum? was added in an attempt to clarify the requirements as an equipment rating consideration and not for use in conjunction with arc flash hazard analysis."
Equipment ratings are expressed in terms of RMS. Also, the second sentence of 110.24(A) speaks of the calculation used to determine the maximum available fault current. Such calculations are always performed in terms of RMS current. The idea is to show the operator or maintenance person that the available fault current is no higher than the equipment's rating, evidence that the system was designed and installed safely. Bringing a peak value into the process would confuse, not clarify, the situation.

The article speaks of maximum "current." But current is expressed as RMS, so there is no reason we have to look at the current's waveform, and try to find the highest value that it attains. We look at the RMS values, and find the highest value that that can attain. Besides, during a fault condition, the waveform is not a pure sinusoid. You can't calculate the peak value by multiplying the RMS value by the square root of two.

Regarding any application of "Charlie's Rule," the general nature of the NEC is that in the absence of explicit definitions, we are to reach back into industry standard usages.

Finally, regarding the NFPA's response to this particular proposal, arc flash analyses do not have their results expressed in terms of current (neither peak nor RMS nor any other current value). The results are in terms of energy released from the onset of the event until the overcurrent device terminates the event.
 
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