Transformer Secondary OCPD

[Vlietracer64]

Am i correct that i can run over 25ft with secondaries if primary OCPD is less than 125% of primary full load current.

I am looking to install a temp 100-KVA transformer 208V primary 277/480V secondary to feed a 480v 600-amp panel feed with parallel 300MCM copper.
If i fuse primary 208v feed at 300-Amp im less then 125%, can i supply the seconday 125' to panel without over current protection?

Where the primary overcurrent protective device does not exceed 125 percent of the primary full load current, secondary overcurrent protection for the transformer is not required. An allowance is made to go to the next larger standard size of fuse or circuit breaker where 125 percent of transformer full load current does not match any of the standard sizes shown in 240.6.

 

[david]

Am i correct that i can run over 25ft with secondaries if primary OCPD is less than 125% of primary full load current.

I am looking to install a temp 100-KVA transformer 208V primary 277/480V secondary to feed a 480v 600-amp panel feed with parallel 300MCM copper.
If i fuse primary 208v feed at 300-Amp im less then 125%, can i supply the seconday 125' to panel without over current protection?

Where the primary overcurrent protective device does not exceed 125 percent of the primary full load current, secondary overcurrent protection for the transformer is not required. An allowance is made to go to the next larger standard size of fuse or circuit breaker where 125 percent of transformer full load current does not match any of the standard sizes shown in 240.6.

Article 240 would provide overcurrent protection for the circuit conductors, you need both the transformer and the secondary conductors to have overcurrent protection

 

[Vlietracer64]

Article 240 would provide overcurrent protection for the circuit conductors, you need both the transformer and the secondary conductors to have overcurrent protection

Even with max output being 120-amps 150 at 125% and its feeding 600-amp cable and panel board

 

[david]

When you have a (slash) secondary voltage 480 /277 the primary overcurrent protection for the transformer cannot provide overcurrent protection for the secondary conductors

 

[Vlietracer64]

When you have a (slash) secondary voltage 480 /277 the primary overcurrent protection for the transformer cannot provide overcurrent protection for the secondary conductors

Thank you that make sense, so if delta to delta 208 to 480 i can use primary OCPD to protect secondary

 

[augie47]

Thank you that make sense, so if delta to delta 208 to 480 i can use primary OCPD to protect secondary

Yes.
Keep two other things in mind: 240.21(C) tells you the transformer secondary conductors cannot be sized per 240.4(B) (next size up) so your 300 kcmil would be u8ndersized and 408.36 states the panel must have a main.

 

[Vlietracer64]

my mistake took the 90*C column it would be 350 mcm rated at 310

 

[augie47]

my mistake took the 90*C column it would be 350 mcm rated at 310

:thumbsup:

 

[Vlietracer64]

Yes.
Keep two other things in mind: 240.21(C) tells you the transformer secondary conductors cannot be sized per 240.4(B) (next size up) so your 300 kcmil would be undersized and 408.36 states the panel must have a main.

I see in 408.36(B) calls for protection on secondary side for panel-boards with an exception, 240.21(C)(1) single phase 2 wire and delta to delta 3 wire can be protected by primary side, not the case above but nice to know.

*If i get 240.21(C)(6) correct and i meet
1. conductor ampacity not less than valve od pri to sec ratio X 1/3 primary OCPD
2. Terminant to single OCPD
3. Protected from physical damage

and my conductors are less than 25' than i can use the main breaker in panel as protection of the secondary side of transformer.

If over 25 than i would install OCPD within 10' of transformer but then can feed a main lug panel provided panel and conductors meet rating of OCPD