# Buck-Boost Transformer Question

#### Skubie

##### Member
I need 240V power for back-up battery equipment (~10 kWh) and only have a 120/208V supply. The charging/discharging rate would be 80A on the high side. Can I use a b/b transformer and will I be able to feed back power in the reverse direction?

Thanks,
Skubie

#### GoldDigger

##### Moderator
Staff member
Yes. From 120 to 240, not from 208 to 240.
Yes.

Tapatalk...

#### david luchini

##### Moderator
Staff member
Yes. From 120 to 240, not from 208 to 240.
Why not from 208 to 240?

Can I use a b/b transformer and will I be able to feed back power in the reverse direction?
With power flowing in either direction, I wonder if you would need to provide an OCPD on both sides in order to comply with 450.4(A)?

#### Ragin Cajun

##### Senior Member
Below is a buck-boost sizing program from SquareD.

You can get 236V or 250V from 208V, single phase.

RC

#### Skubie

##### Member
Below is a buck-boost sizing program from SquareD.
Thanks, RC - the sizing program didn't come through.

#### GoldDigger

##### Moderator
Staff member
Why not from 208 to 240?
You could, but the result would be 240 phase to phase and the equipment might well be designed for 120/240 with no more than 120 to ground.
As long as the equipment can handle a phase-to-phase voltage of 240, then you can use a boost transformer with the primary connected from phase to phase to produce it.
Rather than go into all of that, I just gave the more general answer that 120 to 240 would be OK (since that can produce 120/240 single phase 3-wire.)

#### david luchini

##### Moderator
Staff member
I just gave the more general answer that 120 to 240 would be OK (since that can produce 120/240 single phase 3-wire.)
I'm not sure that I follow...how do you get 120/240 three wire from 120V using a buck-boost transformer?

#### Smart \$

##### Esteemed Member
I need 240V power for back-up battery equipment (~10 kWh) and only have a 120/208V supply. The charging/discharging rate would be 80A on the high side. Can I use a b/b transformer and will I be able to feed back power in the reverse direction?

#### GoldDigger

##### Moderator
Staff member
I'm not sure that I follow...how do you get 120/240 three wire from 120V using a buck-boost transformer?
The boost transformer would have to be 120-120, not necessarily isolating, and would be wired up as an autotransformer with the center tap to neutral. Maybe you would not call that a boost transformer, but the difference is quantitative not qualitative.

#### Smart \$

##### Esteemed Member
The boost transformer would have to be 120-120, not necessarily isolating, and would be wired up as an autotransformer with the center tap to neutral. Maybe you would not call that a boost transformer, but the difference is quantitative not qualitative.
Including the necessary transformer size and cost...