Transformer feeder protection
Transformer feeder protection
(OP)
Hi
A 2MVA 13.8/ 480-277 KV transformer data sheet stipulates that:
1- primary cable size 2/0 copper Tech cable
2- Secondary cables are 750MCM 1-C copper cable 6 per phase.
- My question is; the primary current is 84Amp. Multiplying by 1.25% (NEC Standard) gives us 105Amp which is good for #3 AWG, why did the manufacturer used cable size 2/0 copper Tech cable?
- My next question is; The max allowable circuit breaker rating to protect the primary is 300% of primary current i.e 84x3= 252amp, do we have to choose a cable of 250 AWG?
THANK YOU
A 2MVA 13.8/ 480-277 KV transformer data sheet stipulates that:
1- primary cable size 2/0 copper Tech cable
2- Secondary cables are 750MCM 1-C copper cable 6 per phase.
- My question is; the primary current is 84Amp. Multiplying by 1.25% (NEC Standard) gives us 105Amp which is good for #3 AWG, why did the manufacturer used cable size 2/0 copper Tech cable?
- My next question is; The max allowable circuit breaker rating to protect the primary is 300% of primary current i.e 84x3= 252amp, do we have to choose a cable of 250 AWG?
THANK YOU






RE: Transformer feeder protection
2 - Again, I don't have a sheet of cable ampacities in front of me, but remember that the circuit breaker is only there to protect equipment in case of faults. It's continuous rating needs to be selected to carry expected load, and it's interrupt rating sized to break the available fault current. It sounds to me your 300A is a continuous rating.
RE: Transformer feeder protection
2. No.
If you are using Teck cable are you using the NEC or the CEC?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Transformer feeder protection
MV cable sizing also have to take mechanical strengths and short circuit thermal capacity in account and not just the load. Not many mfr will even make a MV cable smaller than #2.
2. No. MV protection is typically for short circuit only not for overload. Also see above.
Rafiq Bulsara
http://www.srengineersct.com
RE: Transformer feeder protection
Bill
--------------------
"Why not the best?"
Jimmy Carter