Sizing 4160V Motor Cables

[coloradog1]

I had a code question regarding sizing 4160V motor cables.

430.224 says that all conductors shall have an ampacity greater than the overload trip setting.

However, I cannot find any requirements regarding the setting of the overload for 4160V motors. 430.225 does not make any mention of the maximum settings for overload protection.

Is an overload setting generally assumed (such as 125%) in order to size the cables during the design phase?

Thanks for the help

 

[lansford]

At our 4KV pump station motors, the Overload pick up of the Multilin was set about 105% of FLA. The consulting firm noted that the cables were OK if their ampacity was above this number.
JIM

 

[7anoter4]

I don't think the cable rating is "solid" connected with motor rating. The cable cross section is stated
-in the medium voltage system- mainly by short-circuit current and clearance time setting.
Let's say your motor rated power is 1000 hp=746 kW. If efficacy is 0.9 and pf=0.9 then the
Irated=746/4.16/sqrt(3)/.9/.9= 127.8 A.
If the cable is buried in a soil of 1.k.m/w thermal resistance at 20oC and a depth of 0.7-1.2 m according to VDE-0298 a 3*35 sqr.mm 6/10kV it will be enough.
But if the short-circuit current I"k=20 kA the minimum cross-section for a copper conductor XLPE insulated for 0.1 sec short-circuit clearing time will be 48 sqr.mm. Then 3*50 has to be chosen.
The maximum admissible for this cable in the above conditions will be 210 A.[210/127.8=1.64].
An average maximum overload setting will be 1.25*127.8=160 A [this is to protect the motor and not the cable].