Transformer Conductors and OCPD Sizing

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304sparky

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Hey guys,

My boss gave me a 30kva transformer, delta to wye, 3 phase, 480v to 120/208v. It’s feeding a breaker panel with a 60A main. My boss says to feed the transformer with 10awg conductors on the primary protected by 30A fuses. On the secondary to feed the panel with 6awg conductors with a 60A main breaker in the panel. Is this correct?

How and when do I choose to downsize the primary in conjunction with the secondary?

My thoughts are to feed the primary with 8awg and protected by 50A fuses. And feed the 60A main breaker with 4awg conductors.
 
In reality you can feed a transformer with just about any thing you want. The NEC simply lists the values you cannot exceed for the OCPD.

Start with determining your transformer full load values. In your case 36A on the primary and 83A on the secondary.
Now choose your secondary conductors and protection. Your boss choose 60A equipment which is less than 83A so you are good to go. Use 240.21(C) to determine where the OCPD is located.
A 30A protective device was chosen for the primary. Looking at 450.3(B) you can see this is less than 125% of the primary, so again you are good to go.

Note
If the primary protection was larger than 45A (125%) only then do you need to be concerned with the secondary protection size.

Whenever you use such a small primary device, you need to be concerned with nuisance tripping when the transformer is energized. I would have chosen 40A equipment for the primary.
 
Last edited:
304sparky said:
My boss gave me a 30kva transformer, delta to wye, 3 phase, 480v to 120/208v. It’s feeding a breaker panel with a 60A main. My boss says to feed the transformer with 10awg conductors on the primary protected by 30A fuses. On the secondary to feed the panel with 6awg conductors with a 60A main breaker in the panel. Is this correct?
Click to expand...
Sounds good to me unless you're secondary conductors are NM cable.
 
jim dungar said:
In reality you can feed a transformer with just about any thing you want. The NEC simply lists the values you cannot exceed for the OCPD.

Start with determining your transformer full load values. In your case 36A on the primary and 83A on the secondary.
Now choose your secondary conductors and protection. Your boss choose 60A equipment which is less than 83A so you are good to go. Use 240.21(C) to determine where the OCPD is located.
A 30A protective device was chosen for the primary. Looking at 450.3(B) you can see this is less than 125% of the primary, so again you are good to go.

Note
If the primary protection was larger than 45A (125%) only then do you need to be concerned with the secondary protection size.

Whenever you use such a small primary device, you need to be concerned with nuisance tripping when the transformer is energized. I would have chosen 40A equipment for the primary.
Click to expand...
Thank you for explaining! Definitely cleared things up!
 
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