13.2 kV transfomrer secondary cable sizing
13.2 kV transfomrer secondary cable sizing
(OP)
The secondary current of 5/6.25 MVA transfomer @ 13.2 kV is is 273 Amps. NEC table 310.78 (detail 2) permits to use 4/0 cable (90 deg C rated) up to 185 Amps for 100% loading.
Can I use two 3c4/0 cables for my application? Or should I multiply the secondary transformer currnet with 1.25 first and then select a bigger size per NEC 310.78 table?
The transformer primary is 34.5 kV protected with 1200 Amp breaker and the secondary side is protected wth 630 Amp (frame rating) breakers. I am using electronic protection relays for triping both breakers.
Thak you in advance for your help and advices...
Can I use two 3c4/0 cables for my application? Or should I multiply the secondary transformer currnet with 1.25 first and then select a bigger size per NEC 310.78 table?
The transformer primary is 34.5 kV protected with 1200 Amp breaker and the secondary side is protected wth 630 Amp (frame rating) breakers. I am using electronic protection relays for triping both breakers.
Thak you in advance for your help and advices...






RE: 13.2 kV transfomrer secondary cable sizing
Your protective devices should then be sized to protect the transformer and cables.
What are the settings of the protective relays?
RE: 13.2 kV transfomrer secondary cable sizing
Thanks for help again..
RE: 13.2 kV transfomrer secondary cable sizing
Since this is medium-voltage, the cable ampacity is not as big an issue (per NEC) as for low-voltage circuits. But it's still a big drag for everyone when the cable melts, so I'd set the secondary protection at 125% to 150% or so.
Just make sure you've taken into account any future transformer rating increase through addition of fans, etc.
You'll have to work through any necessary conductor derating if this is in an underground duct bank. There are way too many permutations to try to deal with that issue here.
RE: 13.2 kV transfomrer secondary cable sizing
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for conversion if you don't know how to transfer units :)