13.2 kV transfomrer secondary cable sizing 1

[cirtcele]

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...

 

[DanDel]

The size of your cables (and your transformer, for that matter) should be based on the size of the load.

Your protective devices should then be sized to protect the transformer and cables.

What are the settings of the protective relays?

 

[cirtcele]

I have not determined the protection setting yet. We are using Siemens 7UT protection relays. If I am following your advice then the maximum load cannot be bigger than the transformer rating. In this case, I am multiplying the transformer secondary current with 1.25 regardless of my protection device settings. Am I right?

Thanks for help again..

 

[dpc]

Sizing the feeder at 125% of the transformer's maximum forced cooled rating should be adequate. Settings for the protective devices must meet NEC Article 450 for transformer protection.

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.

 

[sslobodan]

You have big backup with two of such cables (watch for the cost) I would go with 95mm2 cable (I was answering a post to some other guy so I have table near me) Its value is around 350Amps for land laying so it would be enough unless you have some specific conditions. Calculate correction factor for laying and than multiply with amperage producer have in his table for that cable. You should also calculate voltage drop and watch for the maximum 1s overcurious permissible when you pick the cable. For current calculation go with max. transformer overload (25 percent in most cases, transformer supplier should tell you that data) than use correction factor on cable and than pick first above the value from the table. Circuit breaker should go on cable current calculation because he is there to protect the cable 1inch=25.4 millimeters

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