Sizing MV Cable into unit substation

[Nreda1996]

Please see below:



1. Per NEC 2017, Table 310.60(C)(79), One Circuit, 5001-35,000 volts, a single 500 kcmil has an ampacity of 450 A at 90 deg C and 485 A at 105 deg C.



2. For Two Circuits, in parallel, the aforementioned ampacities have to be multiplied by 0.88 (NEC 2017, Informative Annex B, B.5(a). So, the reduced ampacities are 396 A at 90 deg C and 426.8 A at 105 deg C



3. The load of the four (4) unit substations at 13.2 kV is 210 Amps (4800 kVA/(SQRT(3)x13.2kV) x 4 = 840 Amps.



4. We can’t specify two (2) conductors per phase, in 5” PVC duct (which is only rated for 90 deg C), as they just fall short (2 x 396 A = 792 A). Therefore, our recommendation is to use 5” RTRC (aka fiberglass duct), which will allow operating the 500 kcmil at the higher temperature of 105 deg C and ampacity of 426.8 A (2 x 426.8 A = 853.6 A).

Nick is the clients electrical engineer. I am the designing consultant.

The unit substations for the Paint shops have 5,000A M-T-M circuit breakers on the 480V bus.



Nick’s contention is that I should have used the HV amperes equivalent to the 5,000 A, i.e. 181.82 amps, to size the MV cable. This would then allow the use of the 5” PVC duct.



Your thoughts?

 

[Julius Right]

It is not clear what it is your schematic diagram

If M-T-M signifies Main-Tie-Main that means you have 2 groups of 2* 4800 kVA 13.2 /0.48 kV and 210 A MV Breakers and 5000 A 0.48 kV LV Breakers [one per transformer].

The rated current at 480 V it is 4800/√(3)/0.48=5773.5 A

If you know the actual load is 5000 A and no room for extension in the future you may reduce the cable ampacity.

But in my opinion the outside surface of 105 oC conductor rated temperature at rated load has to be less than 105oC [an accurate calculation could get even less than 90 oC]

If you look at the manufacturer catalogue-Caledonian for instance-for detail 1 NEC 2017, Table 310.60(C)(79) 485 A is permissible for 90oC insulation rated temperature even the cable is EPR 105 oC insulated cable.

 

[Julius Right]

According to my calculation for 2 cables in a duct bank of 2 ducts and distance as per NEC and earth and concrete RHO and temperature as per Annex B, cable dimensions [for 3*500 MCM copper conductor EPR 105oC insulated] as following:

conductor diameter 18.7 mm. ,insulated core 29.6 mm, 3 cores stranded diameter 68.26 mm and overall diameter 75.7 mm.

For 420 A the conductor temperature was 84 oC and the outside surface temperature about 60oC.