V-Drop Calc verification...

[sparkydon]

Hello fellow folks of the electrical world, glad to stumble upon this site...this is my first post so bear with me...

I am currently working on a project (residential) that has been my most challenging up to date. A 2600 sqft. home is being built a top a ridge where the utility main "Service Disconnect" is located 390ft. from my loadcenter in the house. The service is rated at 200A and PG&E (utility company) brought the service as close as their engineerng would permit. From the main, I am feeding 180ft to an external subpanel on a generator/pumphouse, and then the final 210ft. to the house's subpanel. There will be zero deviation in the trenching. Basically from point A-B-C.
I will bring the full 200A to the pumphouse, and then supply the house with 150A, which may be overkill for the existing load calc, but they plan to "add-on later"..When I did the calculations, I came up with 250MCM CU THHN would carry 150A for 390ft. (underground RNC) @ an allowable 2.99% voltage drop...

My questions are:
-Is that conductor size appropriate for the desired ampacity/length of run?

-Is there a more economical yet code compliant way to achieve these specs (parallelled aluminum conductors etc..)

-Should I be concerned with the fact that the utility maxxed out their service distance to begin with??

Thanks in advance for any input...
Don

 

[Snorks]

Hi sparkydon,

Assuming 68?F ambient, buried 36" or less, USE-2 Cable, Table 310.8...I came up with some different conductor size / VD% values.

using...
3/0 Cu with #3 Gnd - VD = 2.75%
4/0 Cu with #2 Gnd - VD = 2.22%
250 Cu with #1 Gnd - VD = 1.86%

VD computations are per IEEE Std 141 Exact Formula and all computations were with Volts software.

I hope this helps.

 

[sparkydon]

ruuh roh, raggy

ruuh roh, raggy

wow...I was way off..:-?

I did the calc. using the VD formula..must have done something wrong, cuz for how expensive 'Volts' is, they better not be wrong!!

Maybe I'll pull 4/0, since right now the price diff. between 3/0 and 4/0 is only like .27per ft. difference, and VD on my branch circuits is a concern already...

With USE-2, do you reccommend directly burying it? I have always piped it in RNC-DB, only because laying wires in a trench just makes me uneasy...

thanks, don

 

[Smart $]

...With USE-2, do you reccommend directly burying it? I have always piped it in RNC-DB, only because laying wires in a trench just makes me uneasy...

Without knowledge of other conditions which affect the choice?terrain, soil and backfill composition, likelihood of future excavations and type, monetary considerations, etc.?I cannot recommend one method over the other. However, I would like to point out installing USE-2 direct burial decreases voltage drop because there is better thermal transfer from conductor to ambient. Not sure whether Snorks did his computations with or without RNC conduit...

 

[sparkydon]

paving...

paving...

Excavation is a non issue, as the entirety of the run (390') will be under the paved driveway..
As for soil and backfill material, what are the determining factors that would cause you to lean one way or the other? This place is a top a ridge on the California coast, with very humid air, and damp, compact soil, basically clay 12" down.

So maybe snorks calcs were using USE-2 in DB, not in RNC.. I'd be curious to know, because my numbers were a bit different..

thanks again for your input.

 

[Snorks]

Hi sparkydon,

I thought that you were from So. Cal from the PG&E notation....
On a second note, I really miss So. Cal a lot! You're a lucky guy.

I used direct burial for the cable with a soil resistivity (RHO) of 90.
Moist "clay like" soil conditions should not change the RHO factor.

However sandy conditions will lower the RHO factor and might require upsizing the conductor or living with hotter conductors and less voltage.

As for Volts, it's better to go to their website, http://www.dolphins-software.com, to get the current price. For me, Volts has been a great help and cuts my computation time by at least 10 times.

 

[sparkydon]

kewl..

kewl..

Thanks for the RHO input and math, I will check with my GenCon on the results from the preliminary soils test.

I actually live in far North California, Humboldt Co. to be exact, but I used to live in San Luis Obispo.

BTW Snorks, how much do the VD percentages change when the same numbers are done using RNC? I wasn't knocking Volts, I was just suprised at the differential in our results..

 

[Snorks]

Hi sparkydon,

Wow, Humboldt county is only about 200 miles from me...

I computed your senerio again with Volts using RNC conduit and the results were the same. This makes sense as the soil and PVC conduit are similiar insulators in regards to heat.

 

[Smart $]

I computed your senerio again with Volts using RNC conduit and the results were the same. This makes sense as the soil and PVC conduit are similiar insulators in regards to heat.

Your Volts result does not make sense to me. While I do not posess the technical expertise to point to an error in such a result, superficial analysis says they should not be the same.

For example, compare ampacities using say 4/0 USE between Table B.310.7, column "1 Electrical Duct (Figure B.310.2, Detail 1); RHO 90 LF 100; Copper", and the equivalent in Table B.310.8 for "1 Cable (Figure B.310.2, Detail 5), which only lists "Rho 90 and 100% load factor".

The respective ampacities are 263 and 304. I can only attribute the difference in ampacity to thermal resistivity from conductor to ambient. Is there something I'm missing here?

 

[Snorks]

Hi Smart$,

Sparkydon only mentioned soil and not electrical ducts, therefore I computed using direct buried RNC with an ambient temperature of 68?F using 310.16.

 

[Smart $]

Hi Smart$,

Sparkydon only mentioned soil and not electrical ducts, therefore I computed using direct buried RNC with an ambient temperature of 68?F using 310.16.

Umm...buried RNC is an electrical duct, although such is defined for conductors rated 2001 to 35,000 volts [ref: 310.60(A)]. Note the Table B.310.7 that I referenced earlier in Annex B uses the term electrical duct yet it is for conductors rated 0 to 2000 volts and Figure B.310.2 backfill hatch designates earth or concrete.