Issues regarding the selection of power cables
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This post was last edited by myface on 2020-9-5 at 13:23. I thought the key aspect in selecting cables was determining their current-carrying capacity, but it turned out that aside from this capacity, there are many other factors to consider, and the compatibility with the motor’s power rating and the circuit breaker also poses questions. If we assume that the current-carrying capacity of the cable has already been determined, then for a 110kW motor with a rated current of 203A, a circuit breaker is selected; the rated current of this circuit breaker needs to be 250A, and Ir1 is set at 0.9In. So what roles do 1.05In and 1.3In in the operation of the circuit breaker play? How should the condition “I2≤1.45Iz” in 50054 be understood? Some sources state that I2=It=1.45In; is the current-carrying capacity therefore determined based on 1.45In? An article states that in IEC standards, 1.45Iz is intended for situations where the load varies periodically, and it is used to reduce the cross-sectional area of the cable. However, if 1.45In is a requirement that must be met by the circuit breaker, then Iz cannot be less than 1.45In; as a result, the cable chosen would have to be quite large. If the cable cross-section is determined based on this calculation, then why is there a 1.25 times factor specified for explosion-proof areas? “In explosive hazardous environments, when selecting the cross-sections of insulated wires and cables for Zones 1 and 2, the allowable current-carrying capacity of the conductors should be no less than 1.25 times the rated current of the fuse elements and the setting current of the long-time delay overcurrent release in circuit breakers.” " Does this mean Iz>=1.25*I2=1.25*1.45In? Or does it mean Iz>=1.25Ir1? If it’s the latter definition, then the previous selection is completely unnecessary, right? I’m completely lost after reading this. Is there anyone who can help clarify the relationship between these regulations or parameters? I would appreciate it if the experts could advise me on how this should be carried out in practice.
Uh, is it maybe because my question is too amateurish?:$
It depends on the power of the motor on site, dear
Thank you for the reply, but the premise is to determine the power first before making a choice~
Ir: Overload long-time delay trip current setting value. Tr: Overload long delay trip time. Ii: Setting value for the short-circuit instantaneous tripping current. Isd: Setting value for the short-circuit short-time delay trip current. Tsd: Setting value for the short-circuit short delay trip time.
As a fallback option, does anyone know how the actual current-carrying capacity of cables is determined? Is it based on 1.45In?
Just make the margin a bit larger. There is no exact standard for this. :lol:lol:lol.
The ICE60364 standard calculates it in roughly this way: a temperature coefficient of 0.91 is used for an ambient temperature of 40°C, and 0.7 is used when the cables are placed without any spacing between them; the product of these two values is 0.637. 203A/0.637=318A; the corresponding XLPE cable rating is 120. Of course, some people in China suggest that the cables should be thinner; this can be verified using the cable calculation software mentioned above.
I get In and Ir mixed up. . .
I’m sorry, big bro – posts from four years ago have actually been dug up again...