Upsizing feeder wire and GND for voltage drop

[PanelGuy1315]

I need help from wire sizing gurus. I've had this same question come up multiple times when designing a feeder to control panel.

Say a large MCP needs a 400A feed from a distribution panel 1000' away (distance inflated to make point). Without taking voltage drop into account we would typically size the feed to 2 sets of 3/0 with a #3 GND in each conduit.

When I adjust feeder wire size to keep voltage drop below 3%, I come up with 2 sets of 600kcmil. I also have read that I need to increase GND wire size by the same ratio as I increased the feeder wires.

How would you do this with the parallel feed? I went from 3/0 to 600kcmil, which I calculate as a 3.57 factor increase (by cir mils). Do I increase the #3 GND in each conduit by 3.57 factor? I'm coming up with needing a GND wire with 188,152 cirmils which would be a 4/0 wire.

Thanks for your input.

 

[augie47]

Looks correct

 

[LarryFine]

In parallel conduits, each EGC basically needs to be sized as if it was the only one for the entire feeder.

 

[PanelGuy1315]

In parallel conduits, each EGC basically needs to be sized as if it was the only one for the entire feeder.

Right, and that's what I have with the #3 EGC (sized for the 400A overcurrent protection) in each parallel conduit. But I also feel I need to upsize that #3 because I am upsizing the feeder conductors for voltage drop.