MV Cable Calculation - 310.60(C) Tables

[cah279]

We have an installation at a new manufacturing plant where the Utility Company is Designing & Constructing a 20MVA Substation (69/4.16kV). Our demarcation point where we will pick up power, will be at their rigid bus structure after their secondary CB and disconnect switch in the substation. The plan is to then run below grade from their rigid bus via duct bank approximately 40’ so we can get outside of their substation fence. The utility company will provide the duct bank and raceway system and we will provide all cabling. Once outside the fence, we will rise vertically with rigid conduit to an overhead cable tray system 20’ in the air, and then extend via cable tray 600’ or so to our electrical room. Inside the electrical room, we’ll have cable tray as well where we then terminate on the top of our 3000Amp Switchgear Lineup (3000Amp MCB). So, our installation is approximately 600’ of Cable Tray, (2) risers via 5” Rigid Conduit (one in the substation to their rigid bus, one outside the fence up to our cable tray system) and about 40’ of underground duct bank that they will install.

We are trying to order cable and I’m looking at the following cable (per client’s spec) - 3/C, 5kv, 115 Mil, NL-EPR, 133% , MV-105, Cu Conductor Cable.

I’m somewhat confused on which 310.60(C) Table I should be following. Initially I keep getting pulled into 310.60(C)(79) b/c of my underground 40’ of duct bank to get out of the substation, but my trip up is when they start talking about One, Three and Six Circuits. Obviously looking at (6) circuits decreases my ampacity a lot. I was initially looking at running (6) Sets of 750kcmil to get my full 3000Amp bus rating per the above cable which has a 530Amp capacity per cable at 90degC (assuming I have to follow 110.40 for temperature limitations despite specifying MV-105 Cable).

Any thoughts or guidance would be appreciated. If I am leaving out any detail, let me know.

Thanks

 

[Julius Right]

According to NEC 2017 art. 310.60 Conductors Rated 2001 to 35,000 Volts.

(1) Selection of Ampacity. Where more than one calculated or tabulated ampacity could apply for a given circuit length, the lowest value shall be used.

The lowest value is in the ductbank , I think.

In my opinion, you need 500 A [at least] per each of the 6 cables and according to 310.60(C)(79) detail 3 even for 1000 mcm it is only 420 A.

By-the- way, NEC 2020 art. 311.60(D) (3), Detail 3 it is 445 A and so you have 19.238 kVA only.

The insulation thickness for 133% it is 140 [and no 115 as for 100% only] according to UL1072. The ampacity as shown in NEC it is considered for a single grounding point [no circulated current through shield].

If you intend to employ 750 or 1000 kcml copper conductor EPR/XLPE 5 kV shielded single core cables you need a different ductbank construction-increasing the distance between ducts, limiting the covering depth, filling earth thermal resistance lower or other.

 

[Julius Right]

Sorry. My mistake! Insulation thickness for 133% 140 and for 100% 115 according to UL1072 it is for 8 kV rated cable. For 5 kV shielded cable 90 mils is enough for both 100% and 133%.

 

[Julius Right]

First, I see now, you intend to use the 3 conductors cables of 750 kcml [mcm] similar to Southwire 3/C CU 5kV 115 NL-EPR 133% TS PVC MV-105.

As per NEC 2017[ or 2020] 6 parallel cables 310.60(C)79 the ampacity will be 375 A

If you will employ 3 single core cables per duct 1/C 1000 mcm CU 5kV 115 NL-EPR 133% TS PVC MV-105 -see the attached sketch- you'll get 445 A [310.60(C)77 Det.3]

If the distance between ducts will be 12” instead of 7.5 as per NEC and the cover depth of 20” the ampacity of 1000 mcm will be 470 A.

 

[cah279]

Julius, thank you for your feedback. I believe you are confirming my thought process. If we were to utilize (6) sets of 3/C, 750 Cu, we would be limited to 375Amp b/c of the Duct Bank to get out of our substation, based on the detail of 310.60(C)(3) Detail 3. We were trying to keep the cable continuous from our gear to the substation and b/c of this short run, it'll force us to upsize or run more sets which will be expensive, that's provided we follow the duct bank detail as is. The other option is we work with the utility to see if we can keep that connection to the substation above grade and not have to go with a duct bank. Thanks again Julius.