Engineering Question- Cost Analysys

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JFletcher

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Williamsburg, VA
We are doing a commercial project for an ice skating rink. The service to it is supplied by a 45kVA 480/208Y xfmr. However, due to future use/expansion, we are setting a 600A fused disco, and the prints call for 2 runs of 4 wires 500MCM (8 500s plus ground, which I *think* is 3 or 4/0) thru one 4" PVC conduit. My question is wouldnt it have been cheaper to run 2+ conduits and much smaller copper rather than jam it all in one pipe and eat the derating? If I am calculating correctly, 4 runs of 4 4/0 or 250 MCM thru 4 smaller pipes would suffice. Or one set of 1000MCM thru one pipe (specs call for copper so AL is out). I assume the engineer isnt a moron, so what am I missing?

Also, the project calls for 3 ground rods vs 2. Why? What is that extra rod gonna do? Not that I mind that, more materials, more labor, more money, just wondering if there is some benefit of which I am unaware for a third ground rod.
 
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Ingenieur

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480/208?
is the 'service' from a dist panel or actual utility xfmr?

45 kva seems small for lighting, refrig, snack bar, etc

Rated i at 45kva and 208/3 is 125 A
the conductor sizes make no sense
 
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Ingenieur

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Derating for 8 conductors is 70%
if you need an ampacity of 600 you need 600/0.7 ~ 860 A of conductor
v drop with this over-sizing should not be an issue

in 1 conduit 2 sets of 600 or 700
in 2 conduits 1 set in each of 300 or 350
3 sets 3/0

4 sets of 250 >1000 A
 
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topgone

Senior Member
JFletcher said:
We are doing a commercial project for an ice skating rink. The service to it is supplied by a 45kVA 480/208Y xfmr. However, due to future use/expansion, we are setting a 600A fused disco, and the prints call for 2 runs of 4 wires 500MCM (8 500s plus ground, which I *think* is 3 or 4/0) thru one 4" PVC conduit. My question is wouldnt it have been cheaper to run 2+ conduits and much smaller copper rather than jam it all in one pipe and eat the derating? If I am calculating correctly, 4 runs of 4 4/0 or 250 MCM thru 4 smaller pipes would suffice. Or one set of 1000MCM thru one pipe (specs call for copper so AL is out). I assume the engineer isnt a moron, so what am I missing?

Also, the project calls for 3 ground rods vs 2. Why? What is that extra rod gonna do? Not that I mind that, more materials, more labor, more money, just wondering if there is some benefit of which I am unaware for a third ground rod.
Click to expand...

There's one reason why additional ground rods are specified: one ground rod gives too high a ground resistance!

It has been proven that when adding an additional ground rod (two ground rods driven down), properly spaced from the first (a distance 1 rod length apart) will give you 60% ground resistance compared to driving one-ground rod. A third ground rod (3 ground rods driven down) will give you around 40% earth resistance compared to having only one ground rod driven.

You can check for ground resistance measurements done in the area to see if having 3 ground rods is worth the investment.
 
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JFletcher

Senior Member
Location
Williamsburg, VA
Not sure if the neutral is counted here or not for CCC. I got 608A with 2 sets of 4 500MCM (380x2/.8) or 532 if neutral is CCC (380x2/.7) in one pipe. The 45kVA xfmr is a secondary from a POCO 12740/480V (not sure of size, but probably 750kVA).

I'm basically asking why not run 2 or more conduits to kill derating vs all-in-one conduit. Plastic is cheaper than copper. The materials here are substantially more than labor. Why spec 8 500 MCM in one conduit vs another configuration that would supply similar amperage? (again, much of this is FUTURE USE, so there are no loads to calculate from... someone just decided they need 500-600A 480V in the future for ???)

The exact ampacity equivalents of other than 500MCM arent really relevant here.

I guess another way to phrase it would be "if you needed to get a true ~600A service, what Cu conductors and conduits would you use to get the lowest overall cost?" If it matters here, the trenching cost for extra pipe would be zero*; another co is on site trenching for monster insulated chiller lines, and there is more than enough room in that trench to run any plausible electrical conduit configuration needed.

* I realize the other company is eating it if they help us, but I just want a materials cost of 8 500 MCM in one pipe vs other configurations. Nothing exact even, just "the engineer is right", "it would cost a LOT more/less to run smaller conductors in two conduits", etc.

If whoever wishes to go into more detail, I'm all eyes.
 
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Ingenieur

Senior Member
Location
Earth
Buried or exposed conduit?
interior or exterior?
length?

on the surface 2 runs seems cheaper, almost 40% less Cu
8 x 500 ~ 4000
vs
8 x 300 ~ 2400 (2800 for 350 or 30% less)
 
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