QRM has many meanings, I agree with you Jim. Anyway, in my opinion, the o.p. is
referring to Quadrilateral Ring Main Unit. RMU (Ring Main Unit) is of two types. TRM (Triangular Ring Main Unit) & QRM (Quadrilateral Ring Main Unit). TRM is (2 Feeder+ 1 Transformer Feeder) and QRM is (2 Feeder + 2 Transformer Feeder). RMU is mainly used for redundancy of feeders.
So, a sketch it could explain more.
The short-circuit current will state the cable cross-section area. If you know the circuit breaker short-circuit rated current then you may use the NEC Table 240.92(B) Tap Conductor Short-Circuit Current Ratings.[for copper conductor]:
I = short-circuit current in amperes
A = conductor area in circular mils
t = time of short circuit in seconds (for times less than or equal to 10 seconds)
T1 = initial conductor temperature in degrees Celsius.
T2 = final conductor temperature in degrees Celsius.
T1=75oC usually T2=250oC for XLPE or EPR conductor insulation.
Let's say I=20000 A T2=250oC t=1 sec T1=75oC then A= A=26300 cmls [=135 mm^2].