Transformer cable sizing

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rand

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Hello,

I have a question regarding transformer cable sizing per the NEC. We have an interesting scenario where I work where we have a transformer which feeds a single load. Please note that no additional load will be added to the transformer in the future. The transformer is oversized for the load. Downstream from the secondary side of the transformer, there is a circuit breaker which protects the load. The circuit breaker's protection settings are properly set to protect the load side cables. The transformer has its own differential and feeder protection relays.

My question is, per the NEC, can we size the primary and secondary side cables of the transformer for the load (x1.25) instead of 1.25 x nameplate rating of the transformer due to our set-up? Can we match the circuit breaker load side cables in conductor size?

I have looked through NEC articles 450, 210, 215, etc., but could not find anything related to this. Any feedback is greatly appreciated. Let me know if you have any questions.

Thank you.
 
rand said:
Hello,

I have a question regarding transformer cable sizing per the NEC. We have an interesting scenario where I work where we have a transformer which feeds a single load. Please note that no additional load will be added to the transformer in the future. The transformer is oversized for the load. Downstream from the secondary side of the transformer, there is a circuit breaker which protects the load. The circuit breaker's protection settings are properly set to protect the load side cables. The transformer has its own differential and feeder protection relays.

My question is, per the NEC, can we size the primary and secondary side cables of the transformer for the load (x1.25) instead of 1.25 x nameplate rating of the transformer due to our set-up? Can we match the circuit breaker load side cables in conductor size?

I have looked through NEC articles 450, 210, 215, etc., but could not find anything related to this. Any feedback is greatly appreciated. Let me know if you have any questions.

Thank you.
Click to expand...

Yes you can do this - but I would advise to size the primary to the full KVA of the XFMR and size the secondary to the specific load; as long as the CB is sized to protect the cable.


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Isaiah said:
Yes you can do this - but I would advise to size the primary to the full KVA of the XFMR and size the secondary to the specific load; as long as the CB is sized to protect the cable.


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Click to expand...

Thank you for your reply! The CB will be properly set to protect the cable. Is there a reason why you are recommending that the primary side be sized to the nameplate of the XFMR? In addition, do you know of a specific NEC article which states I can size per load instead of transformer nameplate?
 
I would guess that Isaiah was thinking about inrush current when the transformer is first energized.
 
rand said:
Thank you for your reply! The CB will be properly set to protect the cable. Is there a reason why you are recommending that the primary side be sized to the nameplate of the XFMR? In addition, do you know of a specific NEC article which states I can size per load instead of transformer nameplate?
Click to expand...

If you size the primary for the full rating you can always add another secondary cable for a future load. Even though the probability may be very remote I would consider it anyway. The NEC values reflect maximum values. You can always downsize as long as windings and cable is protected


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rand said:
... specific NEC article which states I can size per load instead of transformer nameplate?
Click to expand...

Read article 450 carefully. When discussing protective device sizing it uses the word 'maximum', therefore you are allowed to use a smaller value.
 
rand said:
In addition, do you know of a specific NEC article which states I can size per load instead of transformer nameplate?
Click to expand...

215.2 states that you can size the conductors for the load.

There is no Code section that states that the conductors must be sized per the transformer nameplate.
 
david luchini said:
215.2 states that you can size the conductors for the load.

There is no Code section that states that the conductors must be sized per the transformer nameplate.
Click to expand...


Thank you for the reply! I was thinking that was the case. It didn't really make sense for me to size the cables to xfmr nameplate when the load itself is significantly less.
 
infinity said:
I would guess that Isaiah was thinking about inrush current when the transformer is first energized.
Click to expand...

Thanks for the feedback! If you don't mind me asking, how would inrush current play into this? If the inrush current lasts for a few milliseconds (~30 cycles) and is MANY times higher than the nominal current draw of the xfmr, doesn't the cable have a short circuit current rating which should be able to handle the inrush for that short a period?
 
The 'inrush' current often lasts long enough to be in the instantaneous pickup region of protective devices' time current curves.
This is one reason that article 450 allows primary side devices to be as large as 250% of FLA.

The inrush current is more dependent on the size of the transformer, and its core construction, than on the amount of load connected to the transformer.
There is some anecdotal evidence that lightly loaded transformers may have higher inrush than heavily loaded ones.
 
As Jim stated you can go well above the rating of the transformer primary current but the caveat is that if you do choose to go with a 250% OCPD then you will also need conductors to match.
 
There are tricks to avoid transformer inrush, essentially 'pre-energizing' circuits. However if the transformer is really oversized for the load, then it might be a false economy to try to use it. Transformers burn watts 24x7, and an oversized transformer will have oversized core losses.

-Jon
 
rand said:
Thanks for the feedback! If you don't mind me asking, how would inrush current play into this? If the inrush current lasts for a few milliseconds (~30 cycles) and is MANY times higher than the nominal current draw of the xfmr, doesn't the cable have a short circuit current rating which should be able to handle the inrush for that short a period?
Click to expand...

Back in the good 'ol days, massive inrush current was a major cause of nuisance tripping on primary side CB's.
With today's energy efficient transformers however, that has changed. For example an Eaton Cutler-Hammer 45kVA, 3Ph, 480-120/208V (#V48M28T4516) only pulls about 3 times FLA (165A) upon energization - and only lasts a few cycles.

You have to thoroughly review your particular transformer data sheet(s). I would still tend to size the primary side OCPD at 125% of transformer FLA.
 
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