I need to install a 480v 3 phase panel 350' from the power source. I plan on running 2" emt to a main disconnect at the other end and setting a 480v delta to a 208 wye transformer and then setting a sub panel for 208/120v power. My calculations give me 3/0 cu or 4/0 al with a1.81% vd. Can you verify my calculations?

Just to level set you, this is a DIY, residential site, not a commercial building site.

That said, we need to know the amperage that you are going to deliver to the main disconnect switch, please.

Also, I'm just curious, why are you putting the main disconnect in front of the transformer rather than at the secondary feed into the service panel?

Just to level set you, this is a DIY, residential site, not a commercial building site.

That said, we need to know the amperage that you are going to deliver to the main disconnect switch, please.

Also, I'm just curious, why are you putting the main disconnect in front of the transformer rather than at the secondary feed into the service panel?


The main disc. Switch is a 225 amp feeding a 75kva transformer. I need a disconnect in front of the transformer because it is so far away from the power source. I would need a means to kill it close to the panel

Okay, but normally you would fuse protect the primary/secondary of the transformer and then feed the disconnect switch.

Just curious, but is the disconnect switch's purpose to shut down the transformer when the site is not being used, as in a vacation home?

My calculator (480v/200 amp - 2% voltage drop) comes up with 4/0 copper conductors type THWN-2. I would suggest using schedule 80 PVC because underground is a wet location. EMT may rust out on you.

The same reason (wet location) I would use a an insulation type that is listed for wet locations.

Using copper for the 350' run will be expensive!!

Okay, but normally you would fuse protect the primary/secondary of the transformer and then feed the disconnect switch.

Just curious, but is the disconnect switch's purpose to shut down the transformer when the site is not being used, as in a vacation home?

My calculator (480v/200 amp - 2% voltage drop) comes up with 4/0 copper conductors type THWN-2. I would suggest using schedule 80 PVC because underground is a wet location. EMT may rust out on you.

The same reason (wet location) I would use a an insulation type that is listed for wet locations.

Using copper for the 350' run will be expensive!!

Thank You. I came up with the same wire size. I wasn't sure if I did the calculation correctly so you have answered my question.