- Location
- Connecticut
- Occupation
- Engineer
Not so, with a 120 secondary voltage, the ratio would be .25.
I must say, I don't quite understand the point you are trying to make. A 45kVA 480 delta-240/120 delta (w/center tap) has a primary rated current of 54.1 Amps and a secondary rated current of 108.2 Amps. The secondary-primary ratio is 0.5, not 0.25.
Sure it does. The secondary conductor amps(150) x secondary to primary ratio (.25) =37.5. A 60 amp primary OCPD is more than 37.5
Again, the secondary-primary ratio for this transformer is 0.5, not 0.25, so your result is incorrect.
The primary Ocpd will always be higher than the secondary conductor amps at the ratio quoted.
The way i understand it, even with 2-wire -2-wire transformers, the secondary conductor amps times the ratio number is what the maximum OCPD is allowed to be set at and still be within the limits of article 450.3.
I don't believe this is correct. The secondary conductor amps times the secondary-primary ratio must be larger than the actual primary OCPD, not the theoretical maximum OCPD per 450.3. In my example, the theoretical maximum is 70A, but I used 60A. You could use 45A on the primary if the load didn't exceed that value. If the example had a 70A primary OCPD (maximum permitted by 450.3) then #1/0 secondary conductors would be protected using the ratio.
With a 120/208 secondary, the 120 volts ratio is .25 with a 480 volt primary voltage. The .25 ratio x secondary amps would be if allowed,what the maximum primary OCPD is allowed to be set at, and if it was set at the typical 125/250% per 450.3, it would be higher than the secondary conductor amperage and now be a tap conductor.
As I have pointed out above, the ratio for the 480-208/120V transformer is 0.43, not 0.25. But you are also misapplying 450.3 here. You wouldn't need to use the 125/250% from 450.3 if you are using primary protection only. You would use the 125% only column for primary protection only. 70A is the max primary OCPD for a 45kVA (480V primary) transformer using primary only protection. #1 Awg for a 240/120V secondary or #1/0 Awg for a 208/120V secondary would be properly protected by a 60A primary OCPD (using the ratio method you describe) and are therefore not tap conductors, and yet they would still be required by code to have and OCPD to protect the secondary conductors. Why would the need the additional secondary OCPD if they are not taps?
To be quite honest, to me it doesn't really matter what the conductors are called. They can be secondary conductors or they can be tap conductors. It is a good debate.
Rick
I agree its a good debate. The forum is a great learning tool.