Wolfgang,
Let me try to enhance what Reno has previously posted.
Rated Full-Load Amperes (FLA) on the 240/120V Secondary Side of a 1 Phase 50 KVA Transformer will be 208 Amps.
The typical Short Circuit Amperes (SCA) available at the Terminals of a 1 Phase 50 KVA, 240V 3.0 % Z Transformer is apx. 6,250 Amps.
With 50% of the KVA rating as Motor Loads, the Motor Contribution increases the SCA to 6,874 Amps.
The Over Current Protection Devices (OCPD) may be rated at 10KAIC throughout the System.
Disclaimer:
The Serving Utility should be contacted, and a Fault Level Quote should be requested.
The Minimum SCA at the Service Disconnect will be quoted by the Utility, which should be used for the Base Fault Level to coordinate downstream devices' Fault Ratings (Inrush Capacity / Ratings) from.
Normally, the Building Department will request a Fault Quote from the Serving Utility, to be submitted by the Electrical Contractor.
The Service Feeders between the Transformer and the Line Side of the Meter _MIGHT_ be the Electrical Contractor's responsibility to install.
Some PoCo's require the EC to install both the Ducts and the Feeders; others only require the EC to install Ducts.
In both cases, the Utility's Service Planners / Engineers will specify requirements for Duct size & type; Feeder size, type, etc., and Burial depth + encasement.
The Service Planners will also verify if the Proposed Service Equipment is Compliant per their ESRs (Electrical Service Requirements), and will identify the location to install the Service Equipment at (known as "Meter Spotting")
The Service Equipment will most likely be at least 225 Amps, more likely a 400 Amp Service will be installed (Load Calcs and Panel Schedules will determine the Minimum Service Capacity).
In my area, the quoted SCA for a Single Phase 240/120V Service, with Capacity of 600 Amps and less, would be quoted no higher than 42,000 Amps.
Typically, the same Utility will quote a 225 Amp Service fed from a 1 Phase 50 KVA Transformer at 18,000 Amps, and 22,000 Amps for a 400 Amp Service.
Service Disconnects at the Service Equipment - via Main Circuit Breaker:
If the Service size is 225 Amps, the Main Circuit Breaker will likely be a Frame rated at least 22KAIC @ 240VAC, with a Time-Trip current Curve that extends at least 10X longer than would be found with 100 Amp Frames (types used for Branch Circuits).
Verification of Trip-Current Characteristics may be obtain via Manufacturer's Specifications ("Cut-Sheets").
To confirm proper selective Coordination, verify that the Trip-Current curves for smaller Frames do not overlap with the Larger, upstream frames.
As to Grounding:
A Grounding Electrode System (GES) will need to be created at the Building.
This may consist of a connection to the Cold Water line, where it enters the Building - supplemented by at least one additional Electrode.
Typically, the Supplemental Electrode is Two Driven Ground Rods, separated no less than 6 Feet apart, connected together with a #6 Copper Conductor, then connected to the GES via #6 Copper Conductor.
The Cold Water line will require a Grounding Electrode Conductor (GEC) sized per the Service Feeder Conductors.
If the Service Feeders are 4/0 to 250MCM AL, the GEC would be #4 Copper.
I will stop here, so queries can be made to this information.
-- Scott
