OCPD Transformer over protect?

[thewire]

Understandable table 450.3(B) protection method can be

1) Primary protection only 125%
2) Primary and Secondary protection 250%

over 9A
480-240/120 1phase
Lets say I have a 10kVA transformer.

Ip=10k/480 = 20.8A

My primary protection should be 20.8X1.25 =26A so the next size up is 30A. What happened if the electrician installed a 50A breaker? is that acceptable for primary protection? It's that exceed the 1.25% requirement per table 450.3B.

Also does the wire needs to size to what size? 30A or 50A breaker?

 

[david luchini]

Understandable table 450.3(B) protection method can be

1) Primary protection only 125%
2) Primary and Secondary protection 250%

over 9A
480-240/120 1phase
Lets say I have a 10kVA transformer.

Ip=10k/480 = 20.8A

My primary protection should be 20.8X1.25 =26A so the next size up is 30A. What happened if the electrician installed a 50A breaker? is that acceptable for primary protection? It's that exceed the 1.25% requirement per table 450.3B.

Also does the wire needs to size to what size? 30A or 50A breaker?

The 50A primary breaker meets the 250% requirement for the primary OCPD for "primary and secondary protection."

The primary feeder should be sized for the 50A breaker.

 

[thewire]

The 50A primary breaker meets the 250% requirement for the primary OCPD for "primary and secondary protection."

The primary feeder should be sized for the 50A breaker.

So 26A *2.5 is 65A. So it's technically 2.5. What's the rule on the breaker size if we are following 1.25 and 2.5. Can they sized over 2.5, like 3X times or size between 1.25-2.5 is suffice?

 

[david luchini]

So 26A *2.5 is 65A. So it's technically 2.5. What's the rule on the breaker size if we are following 1.25 and 2.5. Can they sized over 2.5, like 3X times or size between 1.25-2.5 is suffice?

20.8A * 2.5 is 52A. The primary breaker cannot be larger than 52A, so a 50A breaker is the largest breaker that can be used on the primary.

 

[thewire]

20.8A * 2.5 is 52A. The primary breaker cannot be larger than 52A, so a 50A breaker is the largest breaker that can be used on the primary.

yea. So they can't be larger than 2.5. it can be anything from 1.25-2.5. What If the breaker is 40A? Is this applicable? I am trying to understand how the 2.5 and 1.25 rules plays

Does it have to be either 1.25 or 2.5? can't be anything in between?

 

[david luchini]

yea. So they can't be larger than 2.5. it can be anything from 1.25-2.5. What If the breaker is 40A? Is this applicable? I am trying to understand how the 2.5 and 1.25 rules plays

The primary breaker can be anything 50A or smaller, as long as it is larger enough to support the load. A 40A breaker would be fine.

If the primary breaker is larger than 125% (or the next size up ~ 30A in this example) then the secondary breaker must not be larger than 125% of the secondary rated current (or the next size up.)