"under" size the transformer breaker

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badgers

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75KVA step down transformer
480V to 120/208Wye

208FLA
X 1.25% =260A

next standard size breaker is 300A

I would like to make the secondary breaker 225, because that is a cheaper panel and main.

I do not expect inrush currents to be an issue and I can set the Instantaneous setting to 10X on the main.

is this a violation?

thank you for your time and have a good day
 
A 225 amp OCPD on the secondary side would be fine, as far as protection of the transformer is concerned. What is the calculated load that will be on the secondary side of the transformer?

450.3(b) also gives you the option to protect the transformer on the primary side only. This method only protects the transformer. It does nothing for your secondary conductors as far as protection is concerned. See 240.21(c) for your conductor protection requirements.

The calculated load on the secondary is what will drive the size of your secondary OCPD.

Pete
 
Most design and build electricians in this part of the US only provide a 200A breaker and panel. Most consulting firms use a 225A breaker and panel.

In 30yrs, I can't remember any time someone used more than a 225A breaker on a 75kVA transformer.

The part I disagree with is when they want to use a 100A breaker on the primary.
 
You can always go smaller. Keep in mind that you can only load the panel to 80% of the rating of the main breaker, (Unless it's 100% rated) so you will be limiting yourself to 180A. If you need the full 200A (i.e. 208A) to meet your project requirements, this could be a problem.
 
kingpb said:
You can always go smaller. Keep in mind that you can only load the panel to 80% of the rating of the main breaker, (Unless it's 100% rated) so you will be limiting yourself to 180A. If you need the full 200A (i.e. 208A) to meet your project requirements, this could be a problem.
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I believe that your 80% number is incorrect except in a few instances such as with continuous load. Can you tell me more about where you get the 80% figure?

Jim T
 
Jim,

What do you normally specify for the primary OCPD on a 75kva transformer? I am used to seeing, not that it's right or wrong, either a 90 or 100 amp OCPD.

Pete
 
pete m. said:
Jim,

What do you normally specify for the primary OCPD on a 75kva transformer? I am used to seeing, not that it's right or wrong, either a 90 or 100 amp OCPD.

Pete
Click to expand...

It depends on the application, but I don't hesitate to use a 110 or a 125 amp breaker. That will often keep the transformer primary feeder wire to a manageable size, although code would allow a much larger breaker if I needed one.

The point of my previous post is I can get more than 180 amps out of a 225 amp breaker.

Jim T
 
Thanks Jim,

I do have another question in relation to transformer protection;

450.3(b), using the primary and secondary protection application would allow a person to use, for a 75kva 480 to 120/208 transformer, a maximum of a 225 amp OCPD on the primary and a maximum of a 300 amp OCPD on the secondary. Keeping in mind that there is a definite relationship between the primary and secondary, why would anyone size the primary to the maximum when it would seem that there is no instance where the primary OCPD would see that much current due to the size of the secondary OCPD?

Any and all replies welcome.

Pete
 
pete m. said:
Thanks Jim,

I do have another question in relation to transformer protection;

450.3(b), using the primary and secondary protection application would allow a person to use, for a 75kva 480 to 120/208 transformer, a maximum of a 225 amp OCPD on the primary and a maximum of a 300 amp OCPD on the secondary. Keeping in mind that there is a definite relationship between the primary and secondary, why would anyone size the primary to the maximum when it would seem that there is no instance where the primary OCPD would see that much current due to the size of the secondary OCPD?

Any and all replies welcome.

Pete
Click to expand...

You can often run 2 75 kva transformers off a single breaker and feeder if you use the 250% rule with primary and secondary protection. I had that very instance at a motel the other day.

Jim T
 
If you have a properly sized secondary OCPD, it will prevent the transformer from being overloaded (same concept as motor overloads). In this case the NEC allows the primary OCPD to be upsized as it only has to protect against short circuits (again just like motors).
 
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