Ishara777 wrote:

I have two 60 KW generators 208V running parallel to power some equipment using 118KW load. Will IEEE 1584 applies to arc flash hazard calculations?

It is my understanding that arc flash analysis is not required for voltages 208V generated by less than 125 KVA transformers. How about Generators with 150KVA load?

The passage which is more of a passing reference than an actual "rule" which states that for cases with a single transformer not more than 125 kVA in size with 208 V or less that it "need not be considered" presumably because the incident energy is less than 1.2 cal/cm2 although this was not stated in the standard. Internal data in the joint IEEE/NFPA working group as well as published data show that there are cases where incident energy can exceed 1.2 cal/cm2 below this limit so the revised standard is likely to have a lower limit. It is also not useful for conditions such as any circuit fed by a source other than a single 125 kVA or smaller transformer.

Second, IEEE 1584 contains 3 calculations. The first is a theoretical calculation based on maximum power transfer that can apply to anything but it gives a value that is higher than will ever occur in reality, often much higher. The second is a set of equations that work with a specific set of equipment (fuses and breakers) so it can only be applied with that type of equipment so it doesn't see much use. The third is the empirical model which is based on laboratory tests (apply a specific voltage, current, arc gap, etc., and measure the result) where the model is curve fit to the data. This model has only a single data point at 208 VAC (the rest of the tests failed to produce a stable arc), so the validity at 208 VAC is questionable at best. But if we simply accept that the model works to 208 VAC then it can work but we have to supply the inputs (system voltage, available fault currents, arc gap, etc.). This data comes from some other source of information and IEEE 1584 itself does not specify the source. Typically practitioners use an IEC method which essentially uses Ohm's Law because it uses fewer short cuts and simplifications compared to for instance the most commonly used ANSI method.

So to state whether or not IEEE 1584 "applies" to a specific set of equipment is not easy to answer. It can be applied to anything, especially with the theoretical model. However to say that the results are actually valid and meaningful is a much different judgement call. With anything below 250 VAC it is clear that arc flash does exist but numerical modeling does not appear to be valid. I've penned a paper on this forum on the subject of arcs below 250 VAC summarizing the available literature and I believe one of the software vendors (ETAP) has published something very similar.