Here is the situation:

It is a municipal system, very rural, and has long single phase runs with low fault current. For example: 2400 delta single phase lateral ends up with about 92A fault current at the end of the run. The run is 7.5 miles with AAAC Ames and a 40K fuse on the txf secondary. The only fusing is on the secondary side of a stepdown transformer (13.2kv primary). Trying to use ArcPro and the fault current is too low (<200A lower limit). IEEE 1584 is for 3 phase faults but did try it and got an absurd value.

Any ideas on how to figure the incident energy?

Try ArcMaster. You can download free demo version of the program online at ARCAD store (just look for Download AFA V5.0 Demo Free link on the product page). The program calculation procedure is well documented and is available online at http://arcadvisor.com/faq/single-phase-arc

Does Lee give a crazy answer? The result will be extreme to the high side but it's good for low risk cases.

Kva of the txf?

I have put it aside for a little while to contemplate ways to calculate.

The kVA of the transformer is 100

The 100KVA 13200/2400V transformer would limit fault current down to 480A on its secondary assuming 5% transformer impedance and infinite buss. Adding the 7.5 miles run with AAAC Ames impedance would reduce the fault down to approx 420A SC. The 92A fault current at the end of the run just doesn't seem right. What's the available fault current value on the substation transformer secondary?

It is a very rural system and the primary system voltage is 13.2kV and there are some single phase stepdowns to 2.4kV. This particular stepdown is a 100kVA with an impedance of 3%. Primary is 7620V with the secondary being 2.4kV phase to phase.

SLG fault current on the primary side is 981 A.

981A SLG fault current on the 7620V primary of 100KVA %3Z single phase 7620/2400V transformer translates to approx 960A on secondary of the transformer. I don't quite understand how you've arrived at the 92A value.

Sorry I should been more clear as the low fault current is at the end of the circuit from that transformer. From the txf to end of line, it is 7.5 miles of 77.47 Ames AAAC with gmd of 2.5ft.

indeed adding 7.5 miles of AAAC Ames to secondary of the 100KVA single phase transformer with 960A available fault current on its 2400V secondary would introduce approx 13Ohm impedance (based on 0.31Ohm per 1000ft AAAC Ames cable resistance and 0.1Ohm per 1000ft inductive reactance) resulting in approx 155A SC current at the end of the circuit from that transformer.

ok, just to be clear this is a phase to phase fault.