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A single-phase (B-phase) 75 kVA overhead transformer serves 17 single-family residential homes via overhead services, as seen in the attached feeder map. The three-phase primary feeder has distribution system voltage of 13.2GndY/7.62 kV.
The closest upstream fuse to the lateral on B-phase is an 80T fuse link. The available list of fuses includes: 10T, 15T, 20T, 40T, 65T, 80T
I have searched up and down on a method to use to solve this problem, the only methods i can find use TCC graphs to determine values. That method requires calculating short circuit currents, i don't think there is enough info to do this.
So the last method i found pretty much said from the end of the line back to the main source the fuse values should increase to prevent un-necessary outages. The increase was suggested to be double.
Very unsure if this is correct, my answers are next to each question. I calculated the full load current of the 75kVA transformer to be 10A then doubled it to get 20T and used that to begin sizing the lateral at 40T which according to the rule of thumb coordinates with the upstream 80T fuse. Just seems wrong to do it this way, there has to be a better solution.
A single-phase (B-phase) 75 kVA overhead transformer serves 17 single-family residential homes via overhead services, as seen in the attached feeder map. The three-phase primary feeder has distribution system voltage of 13.2GndY/7.62 kV.
- What is the most appropriate fuse size that most closely protects the single-phase transformer? ---------20T
- What is the most appropriate lateral fuse size that most closely protects the lateral and coordinates with the downstream transformer fuse? -----40T
The closest upstream fuse to the lateral on B-phase is an 80T fuse link. The available list of fuses includes: 10T, 15T, 20T, 40T, 65T, 80T
I have searched up and down on a method to use to solve this problem, the only methods i can find use TCC graphs to determine values. That method requires calculating short circuit currents, i don't think there is enough info to do this.
So the last method i found pretty much said from the end of the line back to the main source the fuse values should increase to prevent un-necessary outages. The increase was suggested to be double.
Very unsure if this is correct, my answers are next to each question. I calculated the full load current of the 75kVA transformer to be 10A then doubled it to get 20T and used that to begin sizing the lateral at 40T which according to the rule of thumb coordinates with the upstream 80T fuse. Just seems wrong to do it this way, there has to be a better solution.
