Sizing conductors to load or OCPD

[Lightninarrester]

Guys, 1500KVA, 13.8kv delta - 480Y/277V Transformer. Primary OCPD is sized for 600% of 62.7A, use next Breaker rating 1200A. Secondary OCPD 125% of 1804A, 2255A, use next size 2500A Breaker. Question, do you size your conductors to the load or to your OCPD? , i.e. on secondary , you size them for 1804 plus 125% or to the rating of the secondary Breaker 2500A?

 

[yoopersup]

Re: Sizing conductors to load or OCPD

Breaker

 

[Lightninarrester]

Re: Sizing conductors to load or OCPD

Can you please reference where in the NEC says you side the conductors to your breaker (OCPD) rating? Thanks

 

[bob]

Re: Sizing conductors to load or OCPD

You size the conductors for the load but they must be protected against overcurrent by the breaker. Read section 240 in its entirety and especially 240.4. What else did you have in mind?

[ June 24, 2005, 05:16 PM: Message edited by: bob ]

 

[lauraj]

Re: Sizing conductors to load or OCPD

240.4

 

[tx2step]

Re: Sizing conductors to load or OCPD

Originally posted by Lightninarrester:
Guys, 1500KVA, 13.8kv delta - 480Y/277V Transformer. Primary OCPD is sized for 600% of 62.7A, use next Breaker rating 1200A.

I think that a primary OCPD CB rating of 1200A might be a tad high?

 

[iwire]

Re: Sizing conductors to load or OCPD

Originally posted by tx2step:
I think that a primary OCPD CB rating of 1200A might be a tad high?

Table 450.3(A) seems to support that rating.

 

[sandsnow]

Re: Sizing conductors to load or OCPD

Originally posted by tx2step:

I think that a primary OCPD CB rating of 1200A might be a tad high?

I agree. 62.7 X 6.0 = 376.2 So 400A c/b
Assuming you're allowed 600% Note 1 ampacity
What am I missing?

 

[iwire]

Re: Sizing conductors to load or OCPD

Originally posted by sandsnow:

Originally posted by tx2step:

I think that a primary OCPD CB rating of 1200A might be a tad high?

I agree. 62.7 X 6.0 = 376.2 So 400A c/b
Assuming you're allowed 600% Note 1 ampacity
What am I missing?


I did not bother to do the math I was focused on the 600%.

 

[Lightninarrester]

Re: Sizing conductors to load or OCPD

I think what you are missing is that there is no standard 13.8kv, 400A breaker .

 

[rbalex]

Re: Sizing conductors to load or OCPD

Originally posted by Lightninarrester:
I think what you are missing is that there is no standard 13.8kv, 400A breaker .

This is true; however a 15kV class circuit breaker isn't an "inverse time circuit breaker" either and the "standard" sizes in 240.6 don't apply. For 15kV class, the 1200A rating only refers to the continuous current rating of the breaker. You must begin to apply both 240, Part IX and 450.3(A) which refers to "settings" as well as "ratings."

Many other things are necessary to consider, including the transformer mechanical bracing, cooling characteristics, impedance, etc. to properly protect both the transformer and conductors.

As we would in similar installation threads, this time I recommend getting a qualified engineer to determine the settings/sizing.

[ June 25, 2005, 01:48 PM: Message edited by: rbalex ]

 

[sandsnow]

Re: Sizing conductors to load or OCPD

Originally posted by Lightninarrester:
I think what you are missing is that there is no standard 13.8kv, 400A breaker .

That just shows how much experience I have in Over 600 volts. Not much.