Transformer Sizing

[john2mcc]

I have a customer that is requiring a 3 phase transformer feeding a panelboard. The panel has all single phase breakers and for this reason they are sizing the transformer using 120 volts instead of 208v. Is this correct or should they be sizing the transformer from the higher voltage since there is potential down the road for someone to install a 3 pole breaker? Also is there a code requirement on the sizing of the transformer?

 

[Volta]

Unless you skip every second and third row of breaker spaces, the load on the transformer will be 208 volts.

 

[augie47]

There is no specific Code requirement that I know of for sizing the transformer. Art 450 prescribes transformer protection, Art 250 transformer grounding and Art 240, conductor protection.
Details beyond the Code such as load diversity have to be taken into account. You need to look at the load in kw terms. Hopefully you can
distribute the 120v single phase loads somewhat equally between phases.
You do need to look at the load per phase so as not to overload one winding. If you have 15 kva worth of 120v load and it is balance between the phases, a 15 kva transformer should suffice, but if for some reason one phase is more heavily loaded you would need to factor that in and would need a larger transformer.
It might help to look at the three phase transformer as 3 single phase units and take care not to overload one.

 

[Ralph Michaels]

You haven't described what the loads on the panelboard will be (in either kVA or kW). So, I am assuming you want to size the transformer for the panelboard's rated capacity. Lets assume that the main breaker and/or panelboard bus is rated for 225 amps. You can do that as follows:

208V x 225A x 1.732 = 81 kVA

or

120V x 225A x 3 (phases) = 81 kVA

So, you could actually use either voltage, provided you use the right formula.

Typically for 208/120V, 3-phase, 225A panelboards, a 75 kVA transformer is used. Reason being is that the panel is typically not loaded to 100% all of the time (especially considering the main and branch breakers are thermal-magnetic and not rated for continuous loads). Because the NEC effectively requires that such panelboards be loaded to a maximum of 80% of their rating with continuous loads; 80% x 81 kVA = 65 kVA. A 75 kVA transformer will support this just fine. It will also support short-term overloads such as motor inrush; but typically a 225A panelboard is only partially loaded. Oversizing the transformer will only result in less efficiency during most of its use.

 

[Volta]

You haven't described what the loads on the panelboard will be (in either kVA or kW). So, I am assuming you want to size the transformer for the panelboard's rated capacity. Lets assume that the main breaker and/or panelboard bus is rated for 225 amps. You can do that as follows:

208V x 225A x 1.732 = 81 kVA

or

120V x 225A x 3 (phases) = 81 kVA

So, you could actually use either voltage, provided you use the right formula.

Typically for 208/120V, 3-phase, 225A panelboards, a 75 kVA transformer is used. Reason being is that the panel is typically not loaded to 100% all of the time (especially considering the main and branch breakers are thermal-magnetic and not rated for continuous loads). Because the NEC effectively requires that such panelboards be loaded to a maximum of 80% of their rating with continuous loads; 80% x 81 kVA = 65 kVA. A 75 kVA transformer will support this just fine. It will also support short-term overloads such as motor inrush; but typically a 225A panelboard is only partially loaded. Oversizing the transformer will only result in less efficiency during most of its use.

Well said.