Transformer feeder protection

[raync]

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

 

[vandal06]

1 - I don't know what the ampacity of 2/0 is, but depending on the cooling system of the transformer you can get away with running it overloaded for periods of time. Hence the larger conductor size to allow for this.

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.

 

[waross]

1. My best guess: 2/0 was in stock. There may not be the same choice of sizes in 15 kV cable as in 600V or 1000V. If the needed cable is not in stock it may be much cheaper to use what is available rather than incur delays to the project waiting for the smaller cable.

2. No.

If you are using Teck cable are you using the NEC or the CEC?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

1. It may be that the primary bushing provided can accept up to #/2/0 cables, which is "standard" for separable connectors at 13.8 kV for this size. Other than that not sure why a transformer data sheet would "stipulate" a cable size, perhaps it was specified by the design engineer. Also this transformer may not be the only load on the primary circuit. There may be other loads on the same primary circuit or the loop.

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

 

[waross]

A transformer data sheet may specify 2/0 AWG as the maximum size cable that the lugs will accept but would probably not specify Teck cable. That may be a requirement of the installation contract drawings.

Bill
--------------------
"Why not the best?"
Jimmy Carter