Transformer Secondary Cable Rating

[BigRedfishCatcher]

I have a 13.8KV-480/277V 300KVA 5.6% imp.transformer.The tranformer feeds a 25KA fault rated bus. Secondary bus main with 1600 amp frame with PU set at 400 amps, ST PU 2.4KA amps Inst. set at 4KA. The transformer secondary is connected in air AWN #1 AWG 2 per phase. The secondary cable is on about 2to 3 ft long. I understand the cable rating is fine for 400 amps but my concern is the fault rating of the cable. The bus size in the switchgear is 1/4 X 6 and so is the secondary pads on the transformer. Thanks for any help.

 

[mpparent]

Have you performed a coordination study to determine if the cable damage curve is above your CB curve?

Mike

 

[BigRedfishCatcher]

First thank you for your timely response.
No study to my knowledge has been performed. The switchgear package has many future and blank space for feed outs. At present the system is lightly loaded not more than 200 amps mostly lighting. My question is, should the cable be rated for the fault rating of the the switchgear? Because of any future load added to the switchgear there is a very good chance that the secoundary cables from the transformer may not be checked again.

 

[RalphChristie]

Quick check:

Secondary full load current of the trsf: 361A
Short Circuit Current at secondary terminals (assume infinite bus): 6.44kA

What it means is that the max. current during a fault at the transformer secondary (bolted three phase fault) will be 6.44kA. Actually it will be a bit lower due to the source impedance, but this value is a rather conservative call. How further away from the transformer such a fault occurs, how lower will the fault-current be. Your cable is not long - so roughly it must be able to handle 6.44kA during a fault. Based on the time it will take the protection to clear such a fault, you'll have to choose a appropriate size cable. Unfortunately I am not familiar with the cable gauge you are using, but you can contact the cable manufacturer and obtain the data (short circuit current rating of the cable) from them, to see if your cable will be able to handle such a current.

See also:

http://www.okonite.com/engineering/short-circuit-currents.html

 

[mpparent]

I chose a generic static trip breaker, and quickly modeled 2 sets of #1 cables in SKM. I used the settings you gave, and 1 cable has a .01 sec rating at about 60kA. At 1 sec, it's about 6kA. I am not familiar with "AWN" insulation (I used plain old THHN), so I couldn't choose an equivalent. It appears that 2 sets of #1 is ok, but without knowing some more specifics, it's hard to say. What kind of breaker do you have? Trip unit type? All adjustable settings?

Mike

 

[mpparent]

To close my point, we don't know your system topology, eg motors connected to the bus, which will change your fault current level as seen at the main switchboard you discuss. I would seriously look into a fault study along with a coordination study by a licensed EE so that you're covered.

Mike

 

[dpc]

You're probably aware of this, but **in general** the NEC limits paralleling of power conductors to sizes #1/0 and larger.

And maybe I'm missing something, but if you really mean two #1 AWG conductors in parallel, they are only good for 260 A at 75 deg C rating (NEC 310.16). Perhaps you meant #1/0, but that is only good for 300 A.

I'd be more concerned about the thermal rating than the short circuit rating. The 300 kVA transformer has a 480V current of 360A at its rating.

 

[mpparent]

dpc,

The ampacities you are quoting are in conduit. In the poster's 1st note, he said #1 in "free air". That ampacity would be per 310.17, which would be 195A per cable, which is ok.

Mike

Also, my assumption is that the cables are OEM jumpers, so, I think as long as it's UL labeled it's ok, but I could be wrong on that one.

 

[BigRedfishCatcher]

I have never used this forum before and I would like to thank you all for your help. I have the answer I been looking for. The cable is #1 AWG. The AWN is an appliance rated cable.
asper info written on the cable.

 

[mpparent]

BigRed..

So those are OEM cables then??

Mike