Transformer Secondary conductors Not over 25ft

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mpwasco

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looking for someone with clarification of Secondary Transformer conductors not over 25ft long referencing NEC2017 code article 220.21 (C).(6).(1).
I am confused exactly how that article reads..... i come up with a sum based on my interpretation that doesn't seem to be correct to me.
Here is my situation, looking for the size of my secondary conductors based off of the NEC.
I have a 75KVA 480-208/120v Transformer with a primary protection of 125amps. looking to feed a 225 amp MLO panel board 20ft away. my question is....
based on 2017 NEC 220.21(C).(6).(1) what size would my secondary conductors have to be sized to?
please provide your answer with written results. Thank You
 
mpwasco said:
looking for someone with clarification of Secondary Transformer conductors not over 25ft long referencing NEC2017 code article 220.21 (C).(6).(1).
I am confused exactly how that article reads..... i come up with a sum based on my interpretation that doesn't seem to be correct to me.
Here is my situation, looking for the size of my secondary conductors based off of the NEC.
I have a 75KVA 480-208/120v Transformer with a primary protection of 125amps. looking to feed a 225 amp MLO panel board 20ft away. my question is....
based on 2017 NEC 220.21(C).(6).(1) what size would my secondary conductors have to be sized to?
please provide your answer with written results. Thank You
Click to expand...
You can't do this. Your panelboard needs a main device someplace between its bussing and the transformer, 408.36. Also 240.21(C) basically requires secondary conductors to terminate in a single protective device.
 
jim dungar said:
You can't do this. Your panelboard needs a main device someplace between its bussing and the transformer, 408.36. Also 240.21(C) basically requires secondary conductors to terminate in a single protective device.
Click to expand...
Thanks for the reference. lets say i had a single device, would you know the size of conductors needed?
 
The secondary conductors would need to be sized to your device. If it's a 225 breaker, 4/0 Cu or 300 AL
 
mpwasco said:
Thanks for the reference. lets say i had a single device, would you know the size of conductors needed?
Click to expand...
If the transformer primary side protection does not exceed125% of its FLA, see 450.3, then your secondary side protection can be any size you want as long as it can supply the load and protect the conductors.
 
augie47 said:
The secondary conductors would need to be sized to your device. If it's a 225 breaker, 4/0 Cu or 300 AL
Click to expand...
this would still hold true if they were extended over 20ft to the first over current device being a transformer secondary feeder? i wouldn't have to upsize them at all? per article 220.41(C)(6)(1)
 
mpwasco said:
this would still hold true if they were extended over 20ft to the first over current device being a transformer secondary feeder? i wouldn't have to upsize them at all? per article 220.41(C)(6)(1)
Click to expand...
In general the maximum length of the transformer secondary conductor is 25'. The provisions of 240.21(C)(6)(1) typically only come into play where you have multiple sets of secondary conductors running to small OCPDs.
If you are trying to supply the 225 amp panel at or near its ampacity, that rule will not be an issue.
You will size the conductor to the load and make sure that the ampacity of the conductor is equal to or greater than the rating of the OCPD at the load end of the secondary conductors. Note that the provisions of 240.4(B) do not apply to transformer secondary conductors.
 
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