What size transformer/primary OCPD

[nedster]

Hi all,

Here's my situation, I have a little shack that has a 100A 120/240V panel with a 60A main breaker. I am supposed to find a transformer 600V to 240/120V Single phase that would be sufficient for maximum loading of this panel. My options are 7.5, 10, 12 or 15KVA transformers. I think 12KVA is my best option. Also with this 60A breaker on the secondary, could I size my primary to 250% of primary current? Even though I don't think inrush currents would be that big of a problem in this case.

Thanks for all replys!

 

[kiloamp7]

Re: What size transformer/primary OCPD

If you can find a 12KVA xfmr with a 1-phase 3-wire 120/240V. secondary - okay.
Generally these xfmrs are 100% rated & a 12KVA would handle a continuous balanced load of 50A.
Generally the 60A M/B in your panel is 80% rated & can handle a continuous load of 48A.

With a 600V. primary this 12KVA xfmr would have a rated pri. full load current of 20A.
I would use at least 25A pri. OCP & since you would most likely be using a minimum of #10 wire for the primary, I would not hesitate to use 30A OCP.

Sure NEC will let you use even larger pri. OCP, but generally, larger than 30A would not be needed. Remember that the primary wire size starts to grow if you start increasing the pri. OCP.

 

[nedster]

Re: What size transformer/primary OCPD

Thanks for your help. Ya I don't know where I got this 12kVA Single phase transformer from, non of the catalogs that I was looking at list it, so where I got that number blows my mind. I guess I will have to go with a 15kVA since 10kVA I think is gonna be too small with a 60A on the secondary.

 

[kiloamp7]

Re: What size transformer/primary OCPD

Remember, the fact that a 60A M/B exists in the panelboard does not mean you have to use an xfmr that can deliver that much current.
But you said you wanted to set this up so that you would have the capacity available for maximum loading of the panelboard.
Do not know details of this "little shack", but you are probably aware that the practical sizing of a general purpose xfmr can normally be done like an Article 220 feeder calculation.

 

[nedster]

Re: What size transformer/primary OCPD

Thanks again for helping out. Well basically it's a "permit shack" inside a pulp mill. Really small, 4 or 5 fluorescent lights, Two 240V heaters, 4 receptacles, (possible future installation of air conditioning unit, possible addition of a microwave and coffie maker). I do have doubts about the need for a 15kVA transformer myself cause I think they'll never need the full capacity of that panel, but I presented the owners with calculations for both 15 and 10kVA transformers and they want the 15kVA one in. Now regardles of what they want , what do you think would've been the right unit for the loads I have. Thanks.

 

[kiloamp7]

Re: What size transformer/primary OCPD

Well that question cannot be answered without a fairly accurate description of the loads.
For example, "Two 240V. heaters" - quite a difference if they are each say, 2KW or each say, 5KW.
"4 or 5 fluorescent lights" - quite a difference if they are 4-ft. 1-lamp T8 or 8-ft. 2-lamp VHO.

Taking into consideration the labor & the different materials to accomplish this task, there is probably not much cost difference whether the xfmr is 15KVA or is 10KVA.

 

[nedster]

Re: What size transformer/primary OCPD

Ya you're right, but the project is out, this is not the first permit trailer on site either, most of them were close to a 120/240V source, so there was no need for a transformer, but on this particular installation the cable was ~600ft long so the voltage drop was to big for 240/120V and that's why I had to bring the power from a 600V MCC and go through a transformer. But, regardless, they have a transformer is big enough for the panel so it should be no problem supplying whatever loads they might have coming from that panel. Thanks for your participation in this, your opinions were gratly appriciated.

Ned