How i can do MV cables sizing calculation

[mustafa]

appreciate if i can receive full calculations regarding medium voltage cable sizing

 

[charlie b]

I don't understand the question. MV is no different than LV, in the concept of sizing conductors. First you calculate the load, then you select a conductor that has sufficient ampacity for that load, then you select an overcurrent device capable of protecting that conductor against overcurrent. If necessary, you insert a step to check for voltage drop, and adjust the conductor size accordingly. What is it that you are asking for?

 

[mustafa]

I don't understand the question. MV is no different than LV, in the concept of sizing conductors. First you calculate the load, then you select a conductor that has sufficient ampacity for that load, then you select an overcurrent device capable of protecting that conductor against overcurrent. If necessary, you insert a step to check for voltage drop, and adjust the conductor size accordingly. What is it that you are asking for?

but what about short circuit effect and SC time duration ?

 

[Julius Right]

NEC refers to conductor short-circuit current withstanding in art.240.92:
240.92 Location in Circuit. An overcurrent device shall be connected in each ungrounded circuit conductor as required.
(C)(1) Short-Circuit and Ground-Fault Protection
(3) “The conductors shall be considered to be protected if calculations, made under engineering supervision, determine that the system overcurrent devices will protect the conductors within recognized time vs. current limits for all short-circuit and ground-fault conditions.”
See Table 240.92 for calculation.

 

[Tony S]

240----- covers LV cables

 

[iwire]

240----- covers LV cables

Yes it does but it is not limited to that.

I. General

240.1 Scope. Parts I through VII of this article provide the general requirements for overcurrent protection and overcurrent protective devices not more than 600 volts, nominal. Part VIII covers overcurrent protection for those portions of supervised industrial installations operating at voltages of not more than 600 volts, nominal. Part IX covers overcurrent protection over 600 volts, nominal.

 

[Julius Right]

I agree with you, Tony. However, in IEEE 242/1994 a similar formula for any rated voltage it is shown:
(I/CM)tFac = 0.0297 log10 [(Tf + 234)/(To + 234)]
Fac=skin effect factor. No proximity factor considered.
[See 9.4.2.1 Temperature rise of phase, neutral, or insulated grounding conductors]
In IEEE-80/2000 CH.11.3.1 -but for grounding system of course:
I^2/A^2*tc=5.07/10^3*Tcap/ar/ror*ln((Ko+Tm)/(Ko+Ta)) [re-arranged for similarity]
BS7671 Regulation 434.5.2 - it is for low voltage systems also, I am afraid.
t=k^2*S^2/I^2
The same formula it is shown in DIN VDE 100 Supplement 5 for undefined voltage. k is a little different here [DIN VDE -100-540 Tb2-5].

 

[Julius Right]

See also the same equation in EPRI-EL-5036-V4 Eq.4-26.