Transformer Secondary Cable Confusion

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Electriman

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Folks,

I am looking for your help to solve this problem.

I have a 30 kVA 480/208-120 V lighting transformer with a 50 A primary OCPD. The secondary of the transformer is terminated in a lighting panel with an incoming breaker of 220 A. The actual lighting load is not more than 10 kW.

Everything is outdoor.

What is the minimum size of the cable between transformer secondary and lighting panel?


The transformer and lighting panel are next to each other.


I am not sure if I have to size it based on the actual load (#8) or based on 225 A (breaker size of lighting panel) (3/0) or 50A (primary OCPD) (#1).

Please support your replays with NEC article number.
 
I don't deal with transformer that much but it needs to be based on the 225A OCPD
 
Electriman said:
Folks,

I am looking for your help to solve this problem.

I have a 30 kVA 480/208-120 V lighting transformer with a 50 A primary OCPD. The secondary of the transformer is terminated in a lighting panel with an incoming breaker of 220 A. The actual lighting load is not more than 10 kW.

Everything is outdoor.

What is the minimum size of the cable between transformer secondary and lighting panel?


The transformer and lighting panel are next to each other.


I am not sure if I have to size it based on the actual load (#8) or based on 225 A (breaker size of lighting panel) (3/0) or 50A (primary OCPD) (#1).

Please support your replays with NEC article number.
Click to expand...
Take a look at table 450.3 (B). You may need to reconsider the rating of the CB on the secondary side.
 
It being a 3 phase 4 wire secondary, the secondary conductors can not be considered protected by the primary OCP device so they would have to be sized to your secondary protection, in this case your 220 (??) amp breaker.
One solution, if warranted, would be to install a smaller breaker in your secondary panel and back-feed it.
 
Electriman said:
I have a 30 kVA 480/208-120 V lighting transformer with a 50 A primary OCPD. The secondary of the transformer is terminated in a lighting panel with an incoming breaker of 220 A. The actual lighting load is not more than 10 kW.
Click to expand...

There is more than one way to do this and I am certainly not an engineer but here is how I would do it.

30kVA*1,000=30,000 VA

30,000VA/480/1.73=36 amps.

NEC Table 450.3(B) shows our primary protection can be up to 250% of the current.

36 amps * 2.5 = 90 amps.

Our max primary breaker size is 90 and the conductors between that breaker and the transformer would be based on the breaker size used. In your case a 50 amp so you would need at least 6 AWG copper.

Table 450.3(B) also shows us the secondary overcurrent protection can be up to 125% of the current.

30,000VA/208V/1.73=83 amps.

83 amps * 1.25 =103

Note 1 to table 450.3(B) allows us to round up to the next standard size breaker which happens to be a 110 amp breaker.

So the maximum size secondary over current protection is 110 amps. The wire size would be based on this overcurrent device size.

Keep in mind that in most cases you can only run 10' of conductor from the transformer terminals to the overcurrent device terminals. See 240.21(C) for more details and options.

The 225 breaker has to be downsized or you can add another overcurrent device ahead of it.
 
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iwire said:
...

The 225 breaker has to be downsized or you can add another overcurrent device ahead of it.
Click to expand...
The 225A breaker doesn't have to be downsized, but then the secondary conductors must have an ampacity not less than that rating... and because a 225A rated OCPD exceeds the 125% permitted for secondary protection, the primary only protection scheme would be required.

30,000VA/480/sqrt3 * 125% = 45.1A

So a 50A OCPD meets the 125% requirement per Note 1.
 
Smart $ said:
The 225A breaker doesn't have to be downsized, but then the secondary conductors must have an ampacity not less than that rating... and because a 225A rated OCPD exceeds the 125% permitted for secondary protection, the primary only protection scheme would be required..
Click to expand...

iwire said:
There is more than one way to do this and I am certainly not an engineer but here is how I would do it.
Click to expand...

Seemed clear to me. :)
 
The secondary conductors of a transformers are sized and protected by the rules in Article 240, most notably those in 240.21(C). Article 450 does not apply to the secondary conductors.
The secondary protection can be used to choose the primary protection per 450.3.

So you would need 4/0 conductors feeding your '225A' main breaker panel, unless you install a different breaker as Iwire suggested.
 
240.21 Location in Circuit.
***
(C) Transformer Secondary Conductors.
***
(1) Protection by Primary Overcurrent Device.
***
Single-phase (other than 2-wire) and multiphase (other
than delta-delta, 3-wire) transformer secondary conductors
are not considered to be protected by the primary overcurrent
protective device.
Click to expand...
I don't see how the secondary conductors of this particular xfmr can be protected by the xfmr primary side OCPD.
 
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