Nfpa 70e CALCULATION

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brother

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Im curious as to who would be responsible in the calculation of the what incident energy/hrc is at a panel or motor control center. ?? this is also suppose to let use know what ppe we are suppose to have on. I always thought that the engineers did this, but at a recent job it looks like they are going to have the electricians do this. I personally dont know how to do that yet, but im learning. what do you think?? doesnt one have to know what the maxium short circuit/AIC current is ??
 
There is a ton of information required to perform an arc flash hazard analysis. As an electrician I would not feel qualified to perform the necessary calculations.

I know many electrical engineers that will not perform these calculations.

As far as 70E is concerned, the calculations must be performed by a "qualified person".

Chris
 
brother said:
... doesnt one have to know what the maxium short circuit/AIC current is ??
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No.

You need to know what the short circuit current (SCA) is at the location where you are trying to calculate your PPE requirements.You also need to know the clearing time of your protective device. Without these two values any calculation is meaningless.

Any trained individual is allowed to perform the calculations.

FWIW<
AIC is not a measurement of fault current; it is a measurement of interuppting capacity. By simply knowing a circuit is #10AWG NM cable you cannot tell how many amps are in it. But knowing how much a circuit has to carry, you can determine what capacity (AWG) NM cable is required.
 
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Why do a calc. Why not just use the tables at 130. They are more conservative but usually it is for safe condition check in support of lock and tag. If this is for hot work then yes I would do a calc.
 
Jake,
I find that the notes on the bottom of the table almost always make the table useless, or require enough information to determine compliance, like available fault current and clearing time, that I might as well do the calculation anyway.
 
brother said:
Well thanks for the info!! I was wondering about the tables, if they already have it listed on whats needed, then why do the calculation?? I guess we would just follow the table.
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To use the tables you need to know the available fault current and the clearing time of the protective device, so you have almost done the analysis already, if you have that info you might as well do the calcs.

Rons comment about the tables was right on, they are not very useful and I am willing to bet they are misused 95% of the time because poeple dont understand them and think they are an easy shortcut.
 
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