Fault Level, Switchgear Rating and LT Cable Schedule
The LT fault-level cascade, the per-equipment breaker schedule and the per-feeder cable schedule: inputs, method and gotchas.
What these calculators are for
Three calculators for an LT distribution system: the short-circuit fault
level at each panel, a breaker for every item of equipment, and a cable
schedule with derated current and voltage drop for every feeder.
Fault Level Calculation
One transformer feeds, in series, an LT isolator panel (by
busduct), a main LT panel (by LT cable), and a
capacitor panel fed from the main panel (by cable). Every
field has a default value, given below.
Inputs
Source fault mva — source fault level, MVA
(default 1000).
Base fault kva — base, kVA (default 1000).
Transformer rating kva (default 2500) and
Transformer impedance — per unit, so 0.0625
means 6.25 % (default 0.0625).
Busduct to the isolator panel: impedance (ohm/m,
default 0.0000162), count of runs (default 1) and
length (m, default 10).
LT cable to the main panel: impedance (ohm/m, default
0.00013), runs (default 10) and
length (m, default 30).
Cable to the capacitor panel: impedance (ohm/m,
default 0.00013), runs (default 5) and
length (m, default 30).
How it is calculated
Utility impedance = base kVA ÷ source…