75 kva transformer

[tim89s]

I have a 480 volt primary delta and a 120/208v wye 75 kva transformer. The secondary load calc came out to be 157 amps. I ran 3/0 to a 200 amp main on the secondary side. The primary side has a 100 amp breaker with #3's ran to the primary side of the transformer. Is this install OK? I am a little worried the #3's are to small but on the other hand I didn't use the full potential of the transformer. Is the 75 KVA a rating, meaning I don't have to use all of the potential that the transformer can put out. Such as, a 225 rated panelboard having 150 amp rated wire feeding it. By the way the 75 KVA transformer was a free so I didn't look into using a smaller transformer. Thanks in advance, Tim

 

[augie47]

In my opinion you should be o.k. You are not that far off from "normal".
"Inrush" current is the only thing that might cause you a problem but I have seen 75kva on a 100 work with no problem.

 

[tim89s]

Thank you for your reply. I was wondering how do you calculate your primary. Do you first get the load requirement on the secondary side and work back from there. I have seen in other posts concerning 75 KVA that #2 wire was used and ocpd was 125 amp on the primary side and #300 wire and 300 amp OCPD on the secondary side. Is the reason I can use #3 with 100 amp ocpd because not using the full potenial of the transformer? I just want to have a good answer for the inspector in case I get called on using #3's. Thank you again, Tim

 

[A/A Fuel GTX]

I have a 480 volt primary delta and a 120/208v wye 75 kva transformer. The secondary load calc came out to be 157 amps. I ran 3/0 to a 200 amp main on the secondary side. The primary side has a 100 amp breaker with #3's ran to the primary side of the transformer. Is this install OK? I am a little worried the #3's are to small but on the other hand I didn't use the full potential of the transformer. Is the 75 KVA a rating, meaning I don't have to use all of the potential that the transformer can put out. Such as, a 225 rated panelboard having 150 amp rated wire feeding it. By the way the 75 KVA transformer was a free so I didn't look into using a smaller transformer. Thanks in advance, Tim

I agree with Augie....I have fed 75KVA transformers with 100A OCPD's without any problems.

 

[pete m.]

Thank you for your reply. I was wondering how do you calculate your primary. Do you first get the load requirement on the secondary side and work back from there. I have seen in other posts concerning 75 KVA that #2 wire was used and ocpd was 125 amp on the primary side and #300 wire and 300 amp OCPD on the secondary side. Is the reason I can use #3 with 100 amp ocpd because not using the full potenial of the transformer? I just want to have a good answer for the inspector in case I get called on using #3's. Thank you again, Tim

If you wanted to go to extremes using table 450.3(B) you could size the OCPD on the primary of the transformer at 225 amps and the secondary OCPD at 300 amps. Keep in mind that this is for transformer protection only!

To calculate primary and secondary FLA for a three phase transformer divide the VA rating by the primary or secondary voltage times the square root of three.

Pete

 

[A/A Fuel GTX]

Is the reason I can use #3 with 100 amp ocpd because not using the full potenial of the transformer? I just want to have a good answer for the inspector in case I get called on using #3's. Thank you again, Tim

Table 310.16 tells you #3 CU can carry 100A. The 200A OCPD on the secondary won't allow the primary to become overloaded.

 

[DARUSA]

75 kva Transformer

75 kva Transformer

I'm curious with this installation.
Let see an exemple : if I need a 27 Or 50 kva transformer based on my load calculation but I have in spare a 75 kva transformer. Can I use the 75kva and wire it like a 27 or 50 kva transformer because I don't will use the full load of the transformer?

 

[A/A Fuel GTX]

I'm curious with this installation.
Let see an exemple : if I need a 27 Or 50 kva transformer based on my load calculation but I have in spare a 75 kva transformer. Can I use the 75kva and wire it like a 27 or 50 kva transformer because I don't will use the full load of the transformer?

I would say yes with proper conductor/OCPD sizing.

 

[augie47]

I'm curious with this installation.
Let see an exemple : if I need a 27 Or 50 kva transformer based on my load calculation but I have in spare a 75 kva transformer. Can I use the 75kva and wire it like a 27 or 50 kva transformer because I don't will use the full load of the transformer?

I see that on occasion where facilities have spare transformers. It's rare as most installs I see build for the maximum transformer output but I have seen situations where they expect no increase in load, have the transformer, and wish to keep install costs as low as possible. Sometimes the primary panel is limited as to the breaker it will accept.

 

[skeshesh]

The only problem would be the inrush current as pointed out, but a lot of newer breakers offer settings that can satisfy higher in-rush at lower ratings for steady state. You can check online with most of the mfg websites (sqd, cutlerhammer, etc.) to check out their time vs current performence curves. Then its as simple as superimposing the inrush of the xfmr on the curves to see if the breaker will trip. BTW I think the install you described will be okay.