Double check my 75KVA TXFM.

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gofish

Member
I wanted someone to double check my calculations.
75KVA 3ph. 480vac Pri. 90.2amps
208vac Sec. 208amps.
90.2 x 1.25 = 112.75amps. Primary OCPD 125amp fuse.
#1 AWG THHN 75c.

200amp Secodary OCPD
3/0 AWG THHN 75c.
Secondary feeding Branch circuit panelboard less than 10 feet.
 

gofish

Member
I noticed that when i looked at the stander main in a panel board OCPD that it was a 225amp. So i us the 225 amp for my sec. Conductor. Thanks. Also, for my info, I do not need to multply the sec. amps by125% right?
 

jim dungar

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Location
Wisconsin
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PE (Retired) - Power Systems
gofish said:
I noticed that when i looked at the stander main in a panel board OCPD that it was a 225amp. So i us the 225 amp for my sec. Conductor. Thanks. Also, for my info, I do not need to multply the sec. amps by125% right?

You do not need to. You are allowed to.

The code says the secondary cannot be greater than 125%, but it does not have to be.

For what it is worth: in this area the most common arrangement has been a 90A primary breaker feeding the 75kVA transformer with a 200A secondary breaker/panel. 30 years ago is was expensive to buy a 480V panel with a branch breaker larger than 100A (of course that does not explain using a 90A one) and the normal residential main was 200A so I think that is what they thought of for 120V circuits. I have noticed some designers using 125A primary and 225A secondary breakers now that they are less expensive especially with the newer EE transformers.
 
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