75kva conductor sizing

[journeyman0217]

i know this is back to the basics but i'm a little confused.

3 phase 75kva transformer
480v primary

Primary conductor size:
75000/830=90amps
90amps x 1.25=113 amps

2 awg good for 115amps.
Next standard breaker size is 125amp
Can I put #2's on the 125 or do I have to go to 1awg?

I have a feeling i'm not interpreting the next higher standard size breaker rule correctly.

 

[SceneryDriver]

i know this is back to the basics but i'm a little confused.

3 phase 75kva transformer
480v primary

Primary conductor size:
75000/830=90amps
90amps x 1.25=113 amps

2 awg good for 115amps.
Next standard breaker size is 125amp
Can I put #2's on the 125 or do I have to go to 1awg?

I have a feeling i'm not interpreting the next higher standard size breaker rule correctly.

Look at table 450.3(B). It will give you permissible OCPD values for transformers. You want to upsize the primary breaker to prevent nuisance trips on powerup.


SceneryDriver

 

[augie47]

240.21(C) will not allow you to use the 240.4(B) next size up rule for transformer secondary conductors.
I think his question relates to the primary. Once you select the OCP device you wish to use for the primary protection can you use a conductor employing the next size up rule ?

 

[tortuga]

Look at table 450.3(B). It will give you permissible OCPD values for transformers.

I dont think 450 covers the primary feeder conductors themselves, just the protection of the transformer.
OP is still allowed to round up per the general rule in 240.4(B), primary feeder still has to meet 215.2.

 

[augie47]

I dont think 450 covers the primary feeder conductors themselves, just the protection of the transformer.
OP is still allowed to round up per the general rule in 240.4(B), primary feeder still has to meet 215.2.

True, but in the case of a transformer, it can operate at an overload so, if you sized your primary breaker at 125% would you have to size the conductor to match and not use the next size up rule ?

 

[herding_cats]

I was in this exact situation 2 months ago on a grocery store build out. I upsized to #1.

 

[journeyman0217]

I always thought you could round up on the primary but not the secondary. It through me off when I entered the transformer info into the transformer calculator on mike holt website. It says primary conductor #1.

 

[winnie]

I think @wwhitney gave a good answer to this question last year:

Transformer primary conductor sizing

For a 225 kVA transformer 480V delta primary, what size conductors are required with a primary OCPD at 350A (125% of FLA = 338 amps) fed from a distribution panel upstream. Assuming noncontinuous loads, the transformer feeder minimum size would need to be sized at 100% of the transformer FLA =...

forums.mikeholt.com

In a nutshell, after you select the primary breaker (for operation and protection of the transformer), you must select a primary conductor that is protected by that breaker, but you can use the 'next size up' rule to determine if the primary breaker is acceptable.

 

[petersonra]

i know this is back to the basics but i'm a little confused.

3 phase 75kva transformer
480v primary

Primary conductor size:
75000/830=90amps
90amps x 1.25=113 amps

2 awg good for 115amps.
Next standard breaker size is 125amp
Can I put #2's on the 125 or do I have to go to 1awg?

I have a feeling i'm not interpreting the next higher standard size breaker rule correctly.

You have to protect both the transformer and the conductors. Normally you use the same OCPD for this.