IEEE 1584 has an exception

Equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger low impedance transformer in its immediate power supply. This is interpreted as no arc flash calculations are necessary and assumes the incident energy would not be as significant (although an arc flash hazard still exists)

IEEE 1584 is presently under revision. New language may replace the 125 kVA cut off with a value of short circuit current.

Do you think overall, a short circuit cut off would be easier to use than a transformer kVA cut off?

• Yes
• No

I voted yes because I think in the big picture, it will be easier to exclude equipment on the end of long circuits with low short circuit currents that otherwise might not have been excluded.

However, just looking at a one line for transformer sized and omiting the circuit is nice and easy.

I voted no because it would require extending the system model to 120/208 secondaries at all transformers to determine if the fault current cutoff is met.

If the fault current at the transformer secondary is higher than the cutoff, then you need to gather data on 120/208 V feeder breakers and cables and model the feeders.

You bring up a great point jghrist! How do you know what data to collect until you have run the model. hummm....

I have adopted a cut off for short circuit studies using a 45 kVA transformer as the cut off for 480-208Y/120V circuits. That size limits the secondary current to less than 10 kA (lowest device rating) with an impedance down around 1.3% Maybe something similar can be done here.

I wonder if it would be suitable to use both, meaning it either has to have a 125kva transformer on the line side OR short circuit cut-off is met

I think that when IEEE publishes the short circuit limitation, the 125 kVA limitation will go away. You won't be able to use both.

No Arc Flash is more related to Joules, it is the magnitude of the energy not just the current, and the magnitude of energy is better expressed in the form of kVA.

Yes an ARC flash exists but at the lower voltage the cal/cm2 is not making it below 125 kVA unless you get into prolonged periods of time of the arc event.
An overall short circuit cutoff would be good only if one voltage is factored, they are trying to account for all circuits below 240. Also they are considering the transformer impedance into the equation. I would say the biggest problem at the lower voltage and the lower kVA is to do with the clearing time of the protection device. At the lower capacity it may not immediately trip or in the case of a fuse melt, and prolong the time the short circuit exists.

Either way is good for me

It is standard for me to model down to 15kVA 3-phase anyways for selective coordination circuits. As long as it will be a selectable option in arc-flash options of the software we use, I think we will be fine. I do have a problem however that currently software generates a Cat 0 for these conditions while NFPA 70E publishes a hazard risk category of 1 for the same equipment.