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Engineering
Electrical Engineering
How to calculate the cable size between QRM and transformer
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[QUOTE="Babadag, post: 6015941, member: 407262"] 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. [ATTACH=full]227143[/ATTACH] 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]: A=I/SQRT(0.0297*LOG10((T2+234)/(T1+234))/t) where: 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]. [/QUOTE]
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How to calculate the cable size between QRM and transformer
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