voltage drop

[101010]

Please check my math . I have an underground service 250 feet long Connected load of 154 amps . 120/240 v. I'am not sure what ambient temp to use for and underground service (used 30c) came out with 3.1 % vd using 4/0 alum. Sound right? Did it using cm formula came up with 226,722 cmills which would be 250 mcm . Which is right?

 

[bob]

Please check my math . I have an underground service 250 feet long Connected load of 154 amps . 120/240 v. I'am not sure what ambient temp to use for and underground service (used 30c) came out with 3.1 % vd using 4/0 alum. Sound right? Did it using cm formula came up with 226,722 cmills which would be 250 mcm . Which is right?

I have 3.2% or 7.7 volts for 4/0AL. 2.74% and 6.6 volts for 250 AL.

 

[infinity]

I got 3.1% also with #4/0 AL.

 

[ramsy]

I have 3.2% or 7.7 volts for 4/0AL.

Same here, with default ambient at 30c & conductor @ 75c.

However, 154A shows conductor temp. rise won't go past 60c on 4/0al., so when I drop conductor operating temp. to 60c L-L V-drop changed to 7.3v or 3.05%.

Further, the references to underground ambients I've seen usually prefer 21-25C.

Not that it matters for Tbl. 310.15(b)(6), but for Tbl. 310.16, when I drop ambient down a notch, for underground, L-L V-drop changed to 6.8v or 2.82% @ 60c.

Unless this was a continuous load, I'd say 4/0al fits nicely, @60c with the more efficient I2R savings, room for 9 ccc's, and future expansion.