Sizing Transformer

[rjlahr]

They are installing 6- 40A 3-ph 208 Roof top Package units there is a 45k transformer. Primary 480V sec. 208/120. it is fed with a 70 amp breaker & the panel has a 125A Main breaker. nothing is hooked in but a couple of convienience receptacles and a 20A for the Fire eq. in the electrical room. The FLA on the Package units are 32A. As it is now it seems that i can only run 3 maybe 4 of the six units. can that 45k trans. handle this load?

 

[bob]

Yes you can run 3 or 4 but what about the other 2 units?

 

[rjlahr]

Sizing Transformer

It sounds like I need to change it out to a 75K transformer fed with a 100 amps & change the secondary feeders out to 3/0 and install a 200 amp main breaker! I kept comming up with 39,900 watts when i calc'd the 6 units. Obviously im not doing somthing right. Tired, old & burned out.:-?

 

[augie47]

If you are coming up with 39,900, I don't think you are considering the three phase aspect. If the units are actually 3 phase, the FLA equivalent would be 11,520 per unit or 69+ kw for the 6.

 

[drbond24]

They are installing 6- 40A 3-ph 208 Roof top Package units there is a 45k transformer. Primary 480V sec. 208/120. it is fed with a 70 amp breaker & the panel has a 125A Main breaker. nothing is hooked in but a couple of convienience receptacles and a 20A for the Fire eq. in the electrical room. The FLA on the Package units are 32A. As it is now it seems that i can only run 3 maybe 4 of the six units. can that 45k trans. handle this load?

For a 3-phase load:

208 V x 32 A x 1.73 = 11,515 VA for each unit for a total of 69,090 VA or ~69 kVA.

If all units are running at the same time your tranformer will be overloaded. If they all need to run at the same time, a 75 kVA transformer is the way to go.

 

[rjlahr]

Thanks Guys!