Please check my ampacity & conduit fill calcs.

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Jon456

Jon456

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Colorado
We need to install some new service feeder into existing conduit. We have two parallel conduits, but may only fill one for now and keep the second in reserve as a spare. The goal is to get the maximum current capacity (up to 1,000A) using either one or both conduits. Also being considered is whether to select copper or aluminum wire. Final decision will be made by the facility owner based on cost of the wire and other factors.

The conduit is buried 4" Sch-40 PVC, and the run is 360 feet long. Our service is 480V 3-phase wye. The OCPD at the main switch gear uses a rating plug and can be set to 600, 700, 800, 900, 1000, or 1200 amps. Based on demand as reported by the utility, our current maximum load is 80kW (~130A @ 0.75pf), but we want to allow for future load increases. Nonlinear loads do not comprise major portion of our load, therefore NEC 310.15(B)(4)(c) does not apply. Thus the grounded feeder is not considered a current-carrying conductor (CCC).

Option 1: One conduit, 8 copper feeders/conduit, 600A service
Two parallel sets of (4) 400 MCM CU-XHHW-2 feeders.
One #1 CU-XHHW-2 EGC.
Six CCC in raceway, therefore derate to 80%: 380A * 0.8 = 304A/wire * 2 wires/phase = 608A total.
Conduit fill: (8 * 0.843") + (1 * 0.435") = 36.8%

Option 2: Two conduits, 8 aluminum feeders/conduit, 1,000A service
Four parallel sets of (4) 500 MCM compact-stranded AL-XHHW-2 feeders (two parallel sets per conduit).
Two parallel #2/0 CU-XHHW-2 EGC's.
Six CCC in each raceway, therefore derate to 80%: 350A * 0.8 = 280A/wire * 4 wires/phase = 1,120A total.
Conduit fill: (8 * 0.870") + (1 * 0.521") = 39.6%

Option 3: Two conduits, 8 copper feeders/conduit, 1,000A service
Four parallel sets of (4) 300 MCM CU-XHHW-2 feeders (two parallel sets per conduit).
Two parallel #2/0 CU-XHHW-2 EGC's.
Six CCC in each raceway, therefore derate to 80%: 320A * 0.8 = 256A/wire * 4 wires/phase = 1,024A total.
Conduit fill: (8 * 0.748") + (1 * 0.521") = 29.7%

Option 4: Two conduits, 4 copper feeders/conduit, 800A service
Two parallel sets of (4) 600 MCM CU-XHHW-2 feeders (one parallel set per conduit).
Two parallel #1/0 CU-XHHW-2 EGC's.
Three CCC in each raceway, therefore no derating: 420A/wire * 2 wires/phase = 840A total.
Conduit fill: (4 * 1.033") + (1 * 0.477") = 28.1%

Option 5: Two conduits, 4 copper feeders/conduit, 900A service
Two parallel sets of (4) 700 MCM CU-XHHW-2 feeders (one parallel set per conduit).
Two parallel #2/0 CU-XHHW-2 EGC's.
Three CCC in each raceway, therefore no derating: 460A/wire * 2 wires/phase = 920A total.
Conduit fill: (4 * 1.102") + (1 * 0.521") = 32.1%

Thanks!
 
augie47 said:
Services ? Why are you installing an EGC ?
Click to expand...
Sorry for the confusion. These are not service conductors. They are feeders between our service switchgear (where the meter and main disconnect/OCPD is located) and our facility's primary distribution panel.
 
I checked option 1 and it looks good. I wasn't sure of your method but I did it my way for the fill and it works at 40%
 
I don't seem to get the same wire area as you. '08 Code Pg 670 400 kcmil XHHW-2 area = 0.5782, Pg 680 500 XHHW Compact =.6082
Where are you getting your numbers ?
 
Dennis Alwon said:
I checked option 1 and it looks good. I wasn't sure of your method but I did it my way for the fill and it works at 40%
Click to expand...
Thanks Dennis. Here's my long-hand calculations for Option 1 (using Southwire's published specs for XHHW-2):

4" Sched 40 PVC: 3.998" nominal ID. Thus cross-sectional area = 12.554 sq in.

400 MCM CU-XHHW-2: OD = 843 mils. Area = 0.558 sq in.
#1 CU-XHHW-2: OD = 435 mils. Area = 0.149 sq in.

Total area of wire: (8 * 0.558) + (1 * 0.149) = 4.613 sq in.

Conduit fill: 4.613/12.554 = 0.3675 = 36.8%
 
Yep but I would use schedule 40 PVC and take the area for over 2 wires in Table 4 and get 5.022 sq. in

Then I would look up in Table 5

400mcm-xxhw- .5782 sq. in then x 8=4.626 sq. in

Table 250.122 calls for 1 copper for 600 amps equipment grounding conductor

1 xxhw= .1534

4.626+.1534=4.779 sq. in so we are good since it is less than 5.022

Just a different method of doing it.
 
augie47 said:
I don't seem to get the same wire area as you. '08 Code Pg 670 400 kcmil XHHW-2 area = 0.5782, Pg 680 500 XHHW Compact =.6082
Where are you getting your numbers ?
Click to expand...
I'm using the wire manufacturer's published specs for the greatest accuracy. For the copper wire, I'm using Southwire specs. For the aluminum, I'm using Stabiloy/Alcan specs (because we had Alcan conductors originally, and they are a little larger than Southwire's compact aluminum XHHW).

I will confirm the calculations once the wire is selected and the actual manufacturer is determined.
 
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