Transformer Calca

[irelandnewyork]

Hi Guys, Thanks in advance for the replies.
I have a 408/277 service supplying a 30 kva transformer supplying a 208/120 panel in a building which is giving them a 3 phase 100 amp panel basically. They need to add another 3 phase 100 amp panel beside the existing so i see 2 options,
1. Add another 30 kva transformer and duplicate the original installation
2. Take out the original 30 kva install a 75 kva up grade the primary feed and let the original 3 phase 100 stay, then duplicate the original installation for the new 100 amp 3 phase panel.
Any pros cons for the 2 options ? #1 seems to be the logical choice just looking for some advice we never have installed transformers before also for option # 2 can transformers be double tapped on the secondary side ?
My calcs are 30,000 / 480 / 1.732 = 36.08 amps = 40 amp breaker on the primary # 6 cables
30,000 /208 / 1.732 = 83 amps = 90 amp d/c on the secondary # 2 cables
This is what is originally installed although they have a 100 d/c on the secondary.
Any thoughts as i said i am leaning towards Option 1, also the run from transformer to 3 phase 100 amp panel is about 200'
Thanks again for all replys

 

[kingpb]

add another one

 

[Smart $]

Whether to use two 30kVA transformers or a single 75kVA is pretty much a design decision.... but the 75kVA will provide just over the full 200A combined panel ratings.

Do they truly need another 100A panel or do they just need more breaker spaces? Do you have the calculated load for existing and new?

Yes, you can double tap a transformer secondary.

Review the requirements of Table 450.3(B) and 240.21(C) or 240.92(C). In either of your two options, the distance from the secondary terminals to secondary conductor OCPD will be likely be limited to less than 200' unless the transformer and secondary conductors are outside. Do not confuse the concept of transformer secondary protection [450.3(B)] with secondary conductor protection [240.21(C) or 240.92(C)]. Where the transformer primary and secondary protection scheme is used, the OCPD will be the same device... but it does not have to be the panel MCB.

 

[irelandnewyork]

Whether to use two 30kVA transformers or a single 75kVA is pretty much a design decision.... but the 75kVA will provide just over the full 200A combined panel ratings.

Do they truly need another 100A panel or do they just need more breaker spaces? Do you have the calculated load for existing and new?

Yes, you can double tap a transformer secondary.

Review the requirements of Table 450.3(B) and 240.21(C) or 240.92(C). In either of your two options, the distance from the secondary terminals to secondary conductor OCPD will be likely be limited to less than 200' unless the transformer and secondary conductors are outside. Do not confuse the concept of transformer secondary protection [450.3(B)] with secondary conductor protection [240.21(C) or 240.92(C)]. Where the transformer primary and secondary protection scheme is used, the OCPD will be the same device... but it does not have to be the panel MCB.

Some changes and additional info.
The existing panel is outlets lights small office equipment and is nearly full at the moment i don't want to add additional loads.
The new proposed panel will be 90 amps continuous draw so it looks like i will need a 45kva transformer, also at the secondary transformer terminals there will be a 125 amp disconnect in the same room less than 25' away. Then the secondary feeders will run about 200' to the new 125 amp panel.
Thanks for the reply