One Really huge underground ductbank?

[designer82]

So we have an outdoor switchgear that must feed 12 MCC'S located indoors about 200' away.

The cables required are 3 sets 600mcm (3) 4" conduits + 1 spare 4" conduit to each MCC.

How is this done practically? Do you run 12 different concrete encased ductbanks or one really really wide concrete ductbank (we're talking like 20' wide)?

 

[d0nut]

If you put it all in one ductbank, you would stack the ducts rather than running them all in a single row. For your 48 conduits, you could do an 8x6 conduit ductbank for example. That would be somewhere around a 3'x4' ductbank depending on how much separation you wanted between the conduits.

 

[petersonra]

If you put it all in one ductbank, you would stack the ducts rather than running them all in a single row. For your 48 conduits, you could do an 8x6 conduit ductbank for example. That would be somewhere around a 3'x4' ductbank depending on how much separation you wanted between the conduits.

can you actually put them that close?

 

[cadpoint]

Why don't you begin with Annex B in NFPA 70.

Or use plastic pieces like this to create your runs.
Note top left the separation of the 2" pipes which where for low voltage/signaling. The plastic was placed three times and the rebar
was for rigidity, neither where designed but an applied answer. The owner & Engineer both signed off with this application
all in concrete with 3" on top with bare ground in concrete. These was 20' PVC pieces.

This was an Airfield application. 1500 V on rectifiers.
We did the same with 4", I just don't have any pictures of that.

 

[don_resqcapt19]

Someone needs to run the ampacity calculations using electrical engineering software for that type of installation. Will also need to know the Rho for the fill around the ducts to run the calculations. The mutual heating from the ducts can result in a substantial reduction in the conductor ampacity.

 

[texie]

Someone needs to run the ampacity calculations using electrical engineering software for that type of installation. Will also need to know the Rho for the fill around the ducts to run the calculations. The mutual heating from the ducts can result in a substantial reduction in the conductor ampacity.

I agree. This needs to be engineered.

 

[Julius Right]

If we take 600 MCM -low voltage insulated-diameter as per NEC Table 5 Dimensions of Insulated Conductors and Fixture Wires 4 wires present 4*1.113^2*pi/4=3.89 in^2 and according to NEC Articles 352 and 353 — Rigid PVC Conduit (PVC), Schedule 40, and HDPE Conduit (HDPE) the available conduit cross section of 4" 40% filling is 5.022 in^2.
So, you can run 4 wires in a conduit but you have to reduce the ampacity.
If there are 36+12 ducts [6*6 active] then the ampacity will be 313.6 A [90oC] but if there are only 12 [4*3] it will be only 263 A

 

[Julius Right]

I am sorry! I thought it is about 36 live conductors [one 3 cables per MCC] and not 3 groups of 3 cables for 12 [108 lives].If the ducts are located 4 in depth and 12 horizontal [48 ducts 3+1 cables per duct] in 90 RHO ground 20oC only 147 A will be the ampacity.[10" center line to center line distance].

 

[Tulsa Electrician]

Well I find this most interesting. Since the OP did not provide a load and with the calculation of 147 for ampacity per cable per raceway. This would provide an allowable ampacity 3 x 147=441 per set (3) phase conductors. Am I following this correctly?

I do have a question or two Mr.Julius Right. Since you had mentioned Article 352. The calculation reflects 352.10 (I) and 352.12 (D). I ask as I am attempting to learn and understand.

 

[Julius Right]

Thank you Tusla for the question. First of all I was Mr. Julius Right when I was boss, but now I am retired and you can only address me as, simply, Julius.
The Royal Society's motto 'Nullius in verba' is taken to mean 'take nobody's word for it'. It is an expression of the determination of Fellows to withstand the domination of authority and to verify all statements by an appeal to facts determined by experiment.
So, my calculation based on an old Visual Basic program-I did it myself 20 years ago-it is not a modern one based on ETAP or else. The advantage is that I can modify it as I like it.
Now, as an answer to your question. Actually, in our design, we used schedule 80 pvc duct for concrete duct bank and not art.352 schedule 40 as I indicated above. The available cross section it is less but still enough.
If we refer to the MCC actual load I think we always take the MCC rated as cable possible load. I think I can consider only 3 busy ducts instead of 4-since the 4thd is reserve.
Another possibility is to check the voltage drop. Let say 3% at 200 feet length 3 parallel cables at 3*480 V rated the voltage drop is 14.4 V then 14.4/1.73/0.04/0.2[see chpt.9 Tab.9 NEC]=1039 A and 346 A per cable.
I have to try other possibilities: increase the distance between ducts may I can get 200 A ,for instance.
The demand factor of 0.6 [60%] may be also.

 

[Tulsa Electrician]

Thank you Julius for your response.
I'm learning something new to me. I like to have a general understanding of what engineers design.
Thank you for taking the time to ansawer my question.