How to calculate the cable size between QRM and transformer

In summary, the conversation was discussing cable size calculation between QRM and a 11kv to 400v transformer with a capacity of 1500 kva. The discussion included the importance of providing specific information, such as the type of cable, its placement, and the temperature rating of the insulation. It was also mentioned that a Quadrilateral Ring Main Unit (QRM) may be used for redundancy of feeders. A formula for determining the appropriate cable cross-section area based on short-circuit current and conductor temperature was provided. The conversation ended with appreciation for a thorough and helpful response.
  • #1
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cable size calculation
between QRM and 11kv to 400v transformer (1500 kva)
 
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  • #3
What's a QRM?Is your cable copper or aluminum, is it run in air or in conduit, and if buried what is thermal conductivity of Earth covering it?
What is temperature rating of your insulation ?

That's why we have electrical codes, so that one avoids 'beginner's mistakes' .
 
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  • #4
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.
upload_2018-6-21_9-17-16.png


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].
 

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  • #5
Babadag said:
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]:

What an excellent answer !
 
  • #6
Thank you Jim.
Thank you very much indeed !
 
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1. What factors should be considered when calculating the cable size between QRM and transformer?

The factors that should be considered when calculating the cable size between QRM and transformer include the voltage and current rating of the transformer, the distance between the QRM and transformer, the type of cable being used, and the expected load on the transformer.

2. How do I determine the appropriate cable size for my specific QRM and transformer setup?

To determine the appropriate cable size, you will need to use a cable sizing calculator or consult the manufacturer's specifications for the transformer. You will need to input the voltage and current ratings, the distance between the QRM and transformer, and the expected load to get an accurate cable size recommendation.

3. Can I use a smaller cable size than what is recommended for my QRM and transformer?

No, it is not recommended to use a smaller cable size than what is recommended. Using a smaller cable size can lead to overheating and potential damage to the equipment. It is important to follow the recommended cable size to ensure proper functioning and safety.

4. What happens if I use a larger cable size than what is recommended for my QRM and transformer?

Using a larger cable size than what is recommended will not necessarily cause any harm, but it can be a waste of resources and may lead to unnecessary costs. It is best to follow the recommended cable size for optimal performance and cost-effectiveness.

5. Can I use the same cable size for both the QRM to transformer and transformer to load connections?

It is not recommended to use the same cable size for both connections. The cable size for the QRM to transformer connection is typically smaller than the cable size for the transformer to load connection. This is because the QRM is usually located closer to the transformer, and the voltage drop is less significant over a shorter distance.

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