Disconnect sizing question

[Noway2]

I am working on a project to replace a steam meter panel (enclosure) and the project sponsor wants it to be built to UL508A. He is also telling me that the max available fault current at the building transformers where these will be installed is 65KA and he wants the panel to have a rating greater than that. There are two things / questions that are causing me some hesitation and confusion about this requirement.

Background and description:
The steam meter will consist of a PLC, HMI, power supply, possibly a HART to (Modbus ?) converter and connect to a Rosemount V-Cone transmitter via a 4-20mA + HART line. The panel will be powered by 120Vac and will have a GFCI service outlet inside the cabinet so that the technicians can plug in their HART communicator. The PLC will also communicate to a SCADA system via standard Ethernet.

Questions / request for sanity check.
1) As the panels are 120Vac, and I am going to assume that the buildings are either 208Vac or 480Vac 3-phase that this pretty much precludes the meter being potentially subject to (receiving) that level of fault current because I would expect this meter to be downstream of a distribution panel and likely a transformer (except for possibly 120/208 wye) and don't think that it would ever be "service entrance" equipment, but who knows. Making a panel like this, Service Entrance would create a lot of expense requiring a suitable disconnect and door latching mechanism.

2) If this equipment is NOT going to be service entrance would it not be a violation to have a larger sized protection device down stream of a smaller protection device, as this circuit would become a feeder rather than a branch circuit. The power requirements of the PLC system would be minimal, call it a couple of amps maximum (and that is likely at the DC power supply level). Does anyone even make service entrance rated devices for circuits that small or would this necessitate a larger entrance circuit and then a transformer to step down the voltage and / or capacity?

 

[Gr8blu]

Maybe a return question: Does the sponsor think the building load will increase at some point - say to where a higher fault current capacity (and new transformer) is required? If so, by how much? That might be where the new panel needs to be rated so the panels don't require replacement after the fact.

 

[cherryg]

The value of 65kA appears to be grossly overestimated. Typically to derive 208/120V, you will need a small distribution Transformer. Lets say the Transformer is 500kVA (we need to know this rating) and the % impedance of this Transformer is 5. The calculated nominal short ckt value is 27kA and if you consider the negative tolerance, it is 30kA. Please revisit the kVA rating of the source Transformer, and it's % impedance.

 

[davidbeach]

Not so fast down playing the fault current level. Go with what the utility says. If that 500kVA is comprised of three 167kVA cans they might have impedances closer to 2%.