XMFR Sizing

[Zyb]

Hi,

I'm trying to understand how to size transformer and breaker size on the primary. I know Primary (V x I) = Secondary (V x I),
1.) does efficiency, load losses and PF affects sizing of transformer and breaker on the primary side? are there other factors affects sizing the breaker and transformer
2.) am i correct 38,400W / 0.9725(efficiency) = 39,486 < 40KVA, use 40KVA
3.) is 100A on the primary side suffice

Thank you

 

[jim dungar]

Efficiency is not part of the sizing criteria, at all.
The NEC does not have guidance for selecting the actual transformer size, similar to how it doesn't size motors.

 

[Zyb]

Efficiency is not part of the sizing criteria, at all.
The NEC does not have guidance for selecting the actual transformer size, similar to how it doesn't size motors.

does this means that if I have 200A/240V with max 160A load on secondary I can then use 100A/480V on primary?

 

[petersonra]

Transformer sizing is a design decision.

Conductor ampacity and ocpd rating is controlled by the NEC but there is wide latitude given.

Efficiency has nothing to do with the kva rating of a transformer. The transformer can put out 100% of its rated VA continuously, under the specified conditions.

If you are electing to take advantage of the transformer ocpd ratings allowed for transformers with both primary and secondary ocpd, the secondary ocpd cannot exceed 125% of the rated secondary current, subject to the next size up allowance.

The rating of the primary ocpd cannot exceed 250% of the rated primary current.

The rating of the primary and secondary ocpds are not directly related.

In your case a 40kva xfmr could have ocpd ratings of 200 amps on both the primary and secondary. These are design decisions.

You could also have a 100 amp rated CB on the secondary and a 200 amp on the primary if the load calculations on the panel board justified it.

 

[jim dungar]

does this means that if I have 200A/240V with max 160A load on secondary I can then use 100A/480V on primary?

The NEC has rules for the conductors and the OCPDs.
The primary side OCPD must be sized no larger than 125% of the transformer FLA and the secondary device can be any size.
The primary side OCPD can be as large as 250% as long as the secondary device is no larger than 125%.

Yes you can use a 100A breaker on the primary, but that is not all you need to consider. Your load requires a 38kVA transformer which is not a standard size.

A standard 45kVA transformer might work with a 100A breaker but probably will not handle the inrush/start up current.

 

[wwhitney]

Transformer sizing is a design decision.

The NEC does have some requirements indirectly.

With 160A @ 240V of load on the secondary, the secondary OCPD must be at least 160A--if we restrict to standard sizes, that means 175A. And as that load becomes 80A on the primary, the primary OCPD must be at least 80A (a standard size).

Therefore the minimum transformer size would be one with a rated primary current of 71A/125% or a rated secondary current of 151A/125% (due to Table 450.3(B) including Note 1). The former is clearly smaller, so the minimum transformer size is 71A * 480V /125% = 27 kVA (not a standard size, I imagine).

Cheers, Wayne

 

[petersonra]

The NEC does have some requirements indirectly.

With 160A @ 240V of load on the secondary, the secondary OCPD must be at least 160A--if we restrict to standard sizes, that means 175A. And as that load becomes 80A on the primary, the primary OCPD must be at least 80A (a standard size).

It appears to me that what the OP did was multiply the 200 A rating of the main CB in the PB by 80% to come up with 160 Amps. That's not an actual load calculation. Its not an unreasonable way to make such a determination, but it does not replace the required load calculation that is what actually defines the minimum OCPD feeding the PB.

I might be in error, but I do not recall anyplace in the code that requires any minimum rating for primary OCPD on a transformer. There are practical reasons you might select certain ratings, but only the maximum rating seems to be a code requirement.

I am not sure there would be any prohibition on adding another OCPD between the secondary and the PB that was say 60 Amps and using that to determine the minimum transformer rating, especially if the load calculation supported doing so.

Maybe not such a good idea, but code compliant.

 

[wwhitney]

It appears to me that what the OP did was multiply the 200 A rating of the main CB in the PB by 80% to come up with 160 Amps. That's not an actual load calculation.

Sure, but for my example, I assumed it was. Alternate values could be substituted.

I might be in error, but I do not recall anyplace in the code that requires any minimum rating for primary OCPD on a transformer.

215.3 applies to the primary side feeder.

Cheers, Wayne

 

[petersonra]

Sure, but for my example, I assumed it was. Alternate values could be substituted.

215.3 applies to the primary side feeder.

Cheers, Wayne

215.3 refers one to article 240.

Where in 240 does it state what you are claiming.

 

[jim dungar]

Therefore the minimum transformer size would be one with a rated primary current of 71A/125% or a rated secondary current of 151A/125% (due to Table 450.3(B) including Note 1). The former is clearly smaller, so the minimum transformer size is 71A * 480V /125% = 27 kVA (not a standard size, I imagine).

The OP asked for the secondary to be 38,400A, (160a @240v), the why is irrelevant but it looks like he is planning on a continuous load.

The transformer needs to be a minimum of a 38.4kVA, nothing else matters, which was his question.
After a standard size transformer is selected, only then you can work on the primary breaker rating.

 

[wwhitney]

215.3 refers one to article 240. Where in 240 does it state what you are claiming.

215.3 has a second sentence in addition to the first sentence pointing you to Article 240 Part I [which sentence I believe violates the style manual and should be deleted or changed to an Informational Note.] Namely for the 2017 NEC:

"Where a feeder supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load."

That gives a minimum OCPD size. That applies to the primary OCPD size for the transformer, with respect to the Article 220 load on those primary conductors.

Cheers, Wayne

 

[wwhitney]

The OP asked for the secondary to be 38,400VA . . . The transformer needs to be a minimum of a 38.4kVA

What NEC section says that the transformer rating must be at least the calculated load on the transformer?

Suppose the calculated load is 38,400 VA but you know that the actual current will not exceed the rated current of a 30 kVA transformer. What NEC section would prohibit a 30 kVA transformer with primary and secondary OCPD and conductors sized for 38,400 VA (as required by 215.2 and 215.3)?

Cheers, Wayne

 

[petersonra]

215.3 has a second sentence in addition to the first sentence pointing you to Article 240 Part I [which sentence I believe violates the style manual and should be deleted or changed to an Informational Note.] Namely for the 2017 NEC:

"Where a feeder supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load."

That gives a minimum OCPD size. That applies to the primary OCPD size for the transformer, with respect to the Article 220 load on those primary conductors.

Cheers, Wayne

only seems to apply if there are continuous loads present.

 

[wwhitney]

only seems to apply if there are continuous loads present.

Negative. "Any combination of continuous or noncontinuous loads" covers all the possible cases: both continuous and non-continuous loads; all continuous loads; and all non-continuous loads.

Cheers, Wayne

 

[jim dungar]

What NEC section says that the transformer rating must be at least the calculated load on the transformer?

There isn't one, so it is a design decision.

Go back and read the OP. The design parameters were to provide 160A.
Go ahead tell the customer they are wrong, and you will quote and provide what you want to.

 

[wwhitney]

There isn't one, so it is a design decision.

Yes, to some extent. 450.3 plus 215.3 do impose a lower bound on transformer rating based on the calculated load. In the example I worked out in my first post in this thread, it was about 70% of the calculated load.

If you try to use a 25 kVA transformer for a 38.4 kVA load on a 480V : 240V single phase transformer, 450.3 says that either the secondary protection must be no more than 150A, or the primary protection must be no more than 70A. While 215.3 says that the secondary protection must be at least 160A, and the primary protection must be at least 80A. No way to choose OCPD sizes to satisfy both 450.3 and 215.3.

Cheers, Wayne

 

[jim dungar]

If you try to use a 25 kVA transformer for a 38.4 kVA load on a 480V : 240V single phase...

Then you have not met the design criteria.

No way to choose OCPD sizes to satisfy both 450.3 and 215.3.

Based on you interpretation, you should be having a discussion with the designer.

I am going back to my original post and say the minimum size transformer is 38.4kVA, with the minimum standard size being a 45kVA in which case the selected primary device of 100A should be evaluated as it is likely to nuisance trip.

 

[wwhitney]

Then you have not met the design criteria.

Why do the design criteria need to include that the transformer rating is at least the NEC calculated load?

I am going back to my original post and say the minimum size transformer is 38.4kVA, with the minimum standard size being a 45kVA

The panel is labeled 120/240V, so the OP looks like single phase to me.

Cheers, Wayne

 

[jim dungar]

Why do the design criteria need to include that the transformer rating is at least the NEC calculated load?

The designer asked for 160A or 38.4kVA, it has nothing to do with what the NEC says or does not say.

OP looks like single phase to me.

So then the smaller standard design would be 37.5kVA and the next larger 50kVA.
Time to talk to the designer.

 

[wwhitney]

The designer asked for 160A or 38.4kVA

OK, if that's the scenario you've been discussing.

As I hope I've indicated, my comments have been based on the NEC calculated load being 38.4 kVA, and then the question is what the designer may or may not wish to specify, and what the NEC requires.

Cheers, Wayne