Parallel Cables for Large Industrial Service

[Fnewman]

Industrial 4000 amp, 480 V, 3 phase service with no neutral load. HRG system to be connected to transformer neutral. Client would like to use multiple 3/C IAC (type MC) armored cables to connect an exterior utility transformer to the indoor switchgear with 4000 Amp main breaker. Cables to be installed in ladder tray, maintaining one diameter spacing between cables. Per table 310.15(B)(16) the ampacity of 500 MCM copper (90 C) would be 430 A, implying that 10 cables (or 11 if limited to 75 C) would seem to be adequate, before any derating. However, because the utility transformer will be a pad mount, some portion of the run will have to be underground (probably less than 10 feet, still in tray). Centerline of transformer and switchgear to be the same in order to make sure cables are the 'same' length. Total length of run probably less than 50 feet. What, if any, derating factors should apply?

 

[Julius Right]

In my opinion AC-according to UL4-conductor cross section is limited to 1 awg. Then your cable is MC type -according to UL1569. AC cable it is a small cable with 4 wires and an aluminum strip connected to the armor in order to use-together-as equipment grounding conductor.MC cable is provided with a special grounding wire-the cross section area of this conductor is equal to the ungrounded conductors.
According to 330.12 direct buried in the earth or embedded in concrete it has to be identified for direct burial. Up to cable tray- in open air- the cable has to be supported [all 6 fts].
Art.330.80 Ampacity (B) is referring only to Single Type MC Conductors Grouped Together.
For ampacity of multiconductor cable [MC] in cable tray and in open air the informative table Table B.310.15(B)(2)(3) it seems to be suitable. Table B.310.15(B)(2)(3).416 A for 500 MCM copper conductor.
Cable tray art.392.80 (c) Where multiconductor cables are installed in a single layer in uncovered trays, with a maintained spacing of not less than one cable diameter between cables, the ampacity shall not exceed the allowable ambient temperature-corrected ampacities of multiconductor cables, with not more than three insulated conductors rated 0 through 2000 volts in free air, in accordance with 310.15(C). Informational Note: See Table B.310.15(B)(2)(3).
Underground Table 310.15(B)(16) 380 A for 500 MCM copper 75oC insulation.
If you have to use (3) Adjustment Factors you may take 430 A for 90oC and check if at the end the
allowed will not be more than 380 A.
310.15 Ampacities for Conductors Rated 0–2000 Volts.(A) General.(2) Selection of Ampacity. Where more than one ampacity applies for a given circuit length, the lowest value shall be used.
For exception it has to be 10 ft. and no more than 10%.It seems to me no less than 10% it is available.
It seems to me any way the underground ampacity will be the required.

 

[Fnewman]

Although over a hundred people have seen this post, you are the only one who has taken time to provide any input - and very detailed as that. Thank you!

For clarification, the MC cable proposed is rated for direct burial, but only one end would need to be underground; the rest of the run to the switchgear would be overhead in cable tray. Likely the underground position could be limited to less than 10 feet.

 

[ces]

4000 amp underground feeder takes 7560 amps of wire using software calc

4000 amp underground feeder takes 7560 amps of wire using software calc

I have the very similar situation except the 4000A feeder is >10% of its length or 10 feet underground.
On this project I was required to run Neher-McGrath heat calcs, which I did using SKM Cable Ampacity module.

This is my first time doing this software calc. My question is: Do my results seem reasonable to you?
I did run this by SKM tech support in an online screen sharing, and they did not see anything of concern.

The results are (18) sets of 600KCM = 7,560A which is a 53% derating.
This is after adding an extra 12" separation of ducts in the center of the concrete ductbank, plus as the local soil thermal resistivity was 0.58 RHO verus 0.65 for concrete or slurry I used local soil above the ductbank. Without these two measures more copper was required.
Other factors are a Load Factor of 1.0 which is high - but this is a data center so these days it needs to be, and a Power Factor 0.90. Bottom line is for this 3000kW 480V generator we need 4006A @0.90PF which I round to 4000A due to the 4000A 100% rated breaker.
There has been discussion about using 0.95PF = 3800A. But the client has yet to commit to this power factor and the 5% reduction by itself does not reduce the wire size or duct quantity.

Any thoughts appreciated!
Gary

 

[ces]

Here are the calcs... View attachment Underground Raceway 1_Underground Raceway 5 - Copy.txt

 

[Julius Right]

On my calculation-Visual Basic 6 my software -according to Neher and McGrath for 5 feet duct bank depth-229 A and according to IEC 60287 for 1.5 m depth 225 A [temperature from 64 to 74 oC for all cables] .The gap between the bank ducts 7.5 inches only.

 

[Julius Right]

By the way, for 500 mcm at 2.5 feet depth only 214 A per cable.

 

[ces]

On my calculation-Visual Basic 6 my software -according to Neher and McGrath for 5 feet duct bank depth-229 A and according to IEC 60287 for 1.5 m depth 225 A [temperature from 64 to 74 oC for all cables] .The gap between the bank ducts 7.5 inches only.

Thank you kindly for this comparison. It is within 2-3% of my results. My gap, the on-center spacing, is also 7.5"
500kCM 214A at 2.5-feet sure is close to 225-229A at 5-feet. 19x500=9,500kCM v. 18x600=10,800kCM I will take another look at this.