45 kva to 75 kva transformer - keep the same primary feeders

[rsmith115]

Situation in a nutshell:

Customer wants to replace a 45 kva transformer with a 75 kva transformer to feed a specific piece of equipment that doesn't require the full output that a 75 kva transformer can provide. After doing a bunch of math, I've come to the conclusion (don't know if it's right) that the current feeder for the 45 kva transformer is still sufficient enough to carry the additional current required by the new piece of equipment.

Details:

The current setup:

3 phase 480V -- 60A breaker -- #6 THHN feeding 45 kva 480v delta 120/208 wye transformer -- secondary feeds 125A breaker disconnect.

Customer is requesting that we:

Keep the same primary (cost savings). 60A breaker with #6 feeders to a 75 kva transformer. The secondary will then feed a 150A breaker disconnect with 1/0 conductors.

Here are the specs for the new equipment that were given to me.

Recommended service amps: 150
FLA: 102
PF: .95
FL KVA: 34.79
FL KW: 33.05

And here's my math.

New equipment FLA 102 x 125% (continuous load) = 127.5A
Secondary math: 127.5A x 208V x 1.73 = 45.8 kva
Primary math: 45.8kva/480v/1.73 = 55.25A (which the 60A breaker and #6 will be sufficient for)

If that math is wrong, and I should actually be using the secondary breaker disconnect rating instead since the conductors should be protected all the way up to 150A, then:

secondary disconnect: 150A x 208V x 1.73 = 53.9 kva
primary math: 53.9kva/480v/1.73 = 65A (which is more than the 60a primary breaker but still good with #6)
The FLA is 102A which is well short of the 150A secondary breaker so I don't think that would be an issue. If there was a problem with the equipment, I would think it will actually trip the 60A primary side breaker before the 150A secondary breaker, and since it's well below the transformer rating, the transformer will still be protected.

Here's my issue:

Any transformer I've ever installed has been to the full rating of the transformer. A primary breaker and conductor sized appropriately and a secondary breaker and conductor sized appropriately for the full use of the transformer. In this situation, the piece of equipment needs slightly over what the 45kva transformer can provide, but not the full nut that a 75kva transformer can provide (which according to the FLA specs, I honestly think it will still work with the 45kva, but they're pushing for the 75kva to be installed). So my question is, is it code compliant to still feed the 75kva with a 60A breaker and #6 conductors even though it's really rated to be fed with 90A. My thoughts are that it's ok because the conductors and equipment are all protected appropriately by it's corresponding breaker. Who cares if I'm not using the transformer to it's full potential as long as what I have installed is protected by a breaker? But, speaking with a fellow contractor, he also has never been in this situation, and his thoughts are that if the primary conductor is undersized according to what's available by the transformer, someone down the road might think it's ok to increase the secondary load assuming the primary feeder is sufficient. Which is very possible, but at that point, the primary 60A breaker should still trip before the #6 gets damaged, right? This might be a common thing for others, but for me and my other contractor friend, we've never had this situation so just want to hear from anyone else if this allowed. I've also seen a few things online about an inrush current when turning the transformer on which can cause the primary breaker to trip if not sized properly. So if that's an issue that people have seen I'll have to keep that in mind as well.

 

[petersonra]

What someone might do down the road is not a code issue. If the installation as it is when you leave is code compliant you are good.

I would have probably used a 100 A primary breaker on the original system. I don't like nuisance trips.

BTW, the 125% is for wire sizing not ocpd sizing.

 

[rsmith115]

What someone might do down the road is not a code issue. If the installation as it is when you leave is code compliant you are good.

I totally agree with that, but I’m asking if what I’m being asked to do is code compliant. The reason I included what might be done down the road is because sometimes for me it helps me understand why some things are codes, knowing what the potential problems that may be caused down the road.

 

[david luchini]

I totally agree with that, but I’m asking if what I’m being asked to do is code compliant.

Appears to be code compliant. For a 102A load, I'm not sure why they wouldn't just keep the 45kVA transformer and increase the secondary to 150A, if that's what they're looking for.

 

[petersonra]

Increasing the size of the transformer means it will have a larger power-up current, especially if the load is energized. It might cause nuisance tripping.

I think what you are doing is code compliant but maybe not necessary. 102 Amps @ 208 V is only about 37 kVA. I'd be inclined to just leave the 45 kVA xfmr in place.

 

[jim dungar]

Increasing the size of the transformer means it will have a larger power-up current, especially if the load is energized. It might cause nuisance tripping.

Yep.

The inrush will always be that of a 75kVA transformer regardless of the load on the transformer. I would not be using anything less than a 100A breaker.

Transformers are rated for continuous loading, this is why their protective devices are allowed to be 125% above their FLA.

 

[petersonra]

Who is agitating to increase the transformer rating? It seems adequate to me. Maybe they don't understand very well. Are they trying to accomodate the 150 A service suggestion? It seems to me that on the secondary side it is not a service anyway. It is a feeder or branch circuit. The service would be somewhere upstream of the transformer primary.

 

[rsmith115]

BTW, the 125% is for wire sizing not ocpd sizing.

NEC 210.20 (A) - where a branch circuit supplies continuous loads the rating of the overcurrent device shall be 125% of the continuous load

The FLA is 103 Amps, it's a continuous load so to size my conductors, I'd do 103A x 125% = 128.75A.

There's no 128A breaker, the next size up is 150A. I think that's where they are getting the thought that they need a 150A breaker disconnect ahead of this equipment.

If something happens to this equipment and the current goes up to 150A for some reason, the math is 208V x 150A x 1.73 = 53,976 VA which is greater than the rating of a 45 kva transformer, may potentially damage it.

The 75 kva transformer will be able to easily handle 53,976 VA which is the max VA it will see in the event the current on the machine rises up to the 150A secondary breaker threshhold.

The reason I'm thinking I can get away with the 60A primary breaker with the #6 conductors, is 480V x 60A x 1.73 = 49,824 VA. This primary breaker would trip before the 150A breaker would. Everything is protected. Under normal conditions, this equipment at the most should only draw 103A.

I just don't know if that's ok to do code-wise. I'm basically restricting the full capacity of a 75 kva because I only need 50kva. And again the thought of the inrush current of a 75 kva tripping a 60A breaker would obviously not be good.

Thanks all for the input.

 

[rsmith115]

Who is agitating to increase the transformer rating? It seems adequate to me. Maybe they don't understand very well. Are they trying to accomodate the 150 A service suggestion? It seems to me that on the secondary side it is not a service anyway. It is a feeder or branch circuit. The service would be somewhere upstream of the transformer primary.

And yes, the "recommended 150A service" throws me off. That's what the spec sheet says that the customer forwarded to me, which I'm going to question tomorrow. 150A service to you and me means, well... a service. Not a branch circuit like this will be. Maybe the equipment specs recommends that the service feeding the breaker should be at least a 150A service that way it can accomodate a 125A branch breaker. It's weird language.

 

[jim dungar]

NEC 210.20 (A) - where a branch circuit supplies continuous loads the rating of the overcurrent device shall be 125% of the continuous load

The FLA is 103 Amps, it's a continuous load so to size my conductors, I'd do 103A x 125% = 128.75A.

There's no 128A breaker, the next size up is 150A. I think that's where they are getting the thought that they need a 150A breaker disconnect ahead of this equipment.

If something happens to this equipment and the current goes up to 150A for some reason, the math is 208V x 150A x 1.73 = 53,976 VA which is greater than the rating of a 45 kva transformer, may potentially damage it.

The 75 kva transformer will be able to easily handle 53,976 VA which is the max VA it will see in the event the current on the machine rises up to the 150A secondary breaker threshhold.

The reason I'm thinking I can get away with the 60A primary breaker with the #6 conductors, is 480V x 60A x 1.73 = 49,824 VA. This primary breaker would trip before the 150A breaker would. Everything is protected. Under normal conditions, this equipment at the most should only draw 103A.

I just don't know if that's ok to do code-wise. I'm basically restricting the full capacity of a 75 kva because I only need 50kva. And again the thought of the inrush current of a 75 kva tripping a 60A breaker would obviously not be good.

Thanks all for the input.

The NEC does not care what the loading on a transformer is. Overloading or unloading it is between you and the manufacturer.
The NEC does care about the maximum rating of the transformer primary protective device.

 

[paulengr]

With motors (the FLA number) the general sizing is convert to kw. A conservative estimate is the sum of the kw with a 0.85 PF plus 5 times the largest motor (assuming ATL starting). A rule of thumb version of this that is pretty close is assume HP=kVA and sum HP plus 5 times the largest motor. Mostly useful for sizing MCCs assuming most motors are roughly similar size.

If you can estimate %Z using the infinite bus assumption and actual LRC you can dial this in for say no more than 10% voltage drop (conservative) although you can usually successfully start with as much as 20% VD. This is not Code just outright engineering. There are good %Z estimates if you don’t know. I built a spreadsheet for this that I use frequently. You can’t just use name plate because transformers are highly tolerant of temporary overloads, but if the bus voltage gets so low your contactors drop out, it’s too small.

This gets deeper into the obvious question...are you going to run into VD issues with 45 kVA or not?

 

[rsmith115]

With motors (the FLA number) the general sizing is convert to kw. A conservative estimate is the sum of the kw with a 0.85 PF plus 5 times the largest motor (assuming ATL starting). A rule of thumb version of this that is pretty close is assume HP=kVA and sum HP plus 5 times the largest motor. Mostly useful for sizing MCCs assuming most motors are roughly similar size.

If you can estimate %Z using the infinite bus assumption and actual LRC you can dial this in for say no more than 10% voltage drop (conservative) although you can usually successfully start with as much as 20% VD. This is not Code just outright engineering. There are good %Z estimates if you don’t know. I built a spreadsheet for this that I use frequently. You can’t just use name plate because transformers are highly tolerant of temporary overloads, but if the bus voltage gets so low your contactors drop out, it’s too small.

This gets deeper into the obvious question...are you going to run into VD issues with 45 kVA or not?

Yes thanks for bringing this to light as well. I was just looking into this about an hour ago but decided to put it to bed and come back to it tomorrow. Since you brought it up it just reinforces that this may be a problem

 

[rsmith115]

This is a copy and paste from an EC&M article.


9 amps or more
When a transformer has a primary current of 9 amps or more, Table 450.3(B) requires this value to be multiplied by 125 percent to obtain the size of the primary OCPD. For example, the OCPD for a 480-volt, three-phase, 50-kilovolt-ampere transformer having a FLC of 60 amps per Table 450.3(B) must be increased by multiplying this value by 125 percent (60 × 125 percent = 75A). A 75 amp standard-size OCPD is not listed in 240.6 (A). Therefore, the rounding-up rule of Note 1 to the table should be exercised. Based on the application of this rule, the size of the OCPD selected for the primary side is 80 amps.

so if this transformer was feeding a panel, I can totally see why you’d take the rated current which would be 90A for a 75kva and use that for all calculations. You might fully load the panel fed off the secondary at some point. But in my application, it’s feeding a specific piece of equipment, not a distribution panel, so to have to take the full current rating might not be necessary. All that code article says is if it has a current of 9 or more. It doesn’t say current rating or predicted current draw.

I think I actually talked myself out of thinking feeding the 75 with a 60A breaker is ok while posting this.

 

[winnie]

The only problem with using a transformer with lower OCPD than allowed is the possibility of tripping on inrush current. If you install a 75 kVA transformer with OCPD sized for 45kVA and the primary breaker holds, then you are kosher.

With a calculated load of less than 65A, you can use a 70A breaker to protect the #6 conductors. That would help with tripping on inrush if there is a problem.

Jon

 

[topgone]

This is a copy and paste from an EC&M article.


9 amps or more
When a transformer has a primary current of 9 amps or more, Table 450.3(B) requires this value to be multiplied by 125 percent to obtain the size of the primary OCPD. For example, the OCPD for a 480-volt, three-phase, 50-kilovolt-ampere transformer having a FLC of 60 amps per Table 450.3(B) must be increased by multiplying this value by 125 percent (60 × 125 percent = 75A). A 75 amp standard-size OCPD is not listed in 240.6 (A). Therefore, the rounding-up rule of Note 1 to the table should be exercised. Based on the application of this rule, the size of the OCPD selected for the primary side is 80 amps.

so if this transformer was feeding a panel, I can totally see why you’d take the rated current which would be 90A for a 75kva and use that for all calculations. You might fully load the panel fed off the secondary at some point. But in my application, it’s feeding a specific piece of equipment, not a distribution panel, so to have to take the full current rating might not be necessary. All that code article says is if it has a current of 9 or more. It doesn’t say current rating or predicted current draw.

I think I actually talked myself out of thinking feeding the 75 with a 60A breaker is ok while posting this.

For a 45 kVA 480/208-120V with FLA = 54A, use a 150A breaker. With a 75 kVA transformer (FLA=90A), we use a 250A. Better still, you can get a better sizing choice using the published TCC of available circuit breakers and comparing the curves to your transformer inrush and full load plots! There's no problem using a bit smaller CB, you just have to bear the inconveniences of nuisance trips!

 

[winnie]

For a 45 kVA 480/208-120V with FLA = 54A, use a 150A breaker. With a 75 kVA transformer (FLA=90A), we use a 250A. Better still, you can get a better sizing choice using the published TCC of available circuit breakers and comparing the curves to your transformer inrush and full load plots! There's no problem using a bit smaller CB, you just have to bear the inconveniences of nuisance trips!

The primary OCPD must protect both the transformer and the supply conductors.

With secondary protection used, the primary OCPD can be quite large, as you note. This is great for preventing nuisance trips. But the OP has that pesky #6 feeder that also needs protection.

If the total length of the primary and secondary conductors are short enough, tap rules apply. But I doubt that is the situation here, since keeping the #6 feeder is a significant cost issue.

IMHO the OP is limited to a 70A supply breaker. Of course the #6 feeder will also act to limit inrush current. I'd bet a beer that there will be no problems with inrush current tripping this undersized breaker on a 75 kVA transformer.

Jon

 

[hillbilly1]

IMHO the OP is limited to a 70A supply breaker. Of course the #6 feeder will also act to limit inrush current. I'd bet a beer that there will be no problems with inrush current tripping this undersized breaker on a 75 kVA transformer.

Jon

You may be buying a lot of beers! LOL! I’ve had issues with 125 amp breakers tripping with a 75 kva transformer. The lucky thing was most of the 125 amp and larger breakers have a magnetic trip adjustment, and setting it to maximum cured the problem. Setting without load may hold, but the issues I was having was transferring back from generator to utility under load. Until the generator was installed, it was never turned off once on.

 

[kwired]

With motors (the FLA number) the general sizing is convert to kw. A conservative estimate is the sum of the kw with a 0.85 PF plus 5 times the largest motor (assuming ATL starting). A rule of thumb version of this that is pretty close is assume HP=kVA and sum HP plus 5 times the largest motor. Mostly useful for sizing MCCs assuming most motors are roughly similar size.

If you can estimate %Z using the infinite bus assumption and actual LRC you can dial this in for say no more than 10% voltage drop (conservative) although you can usually successfully start with as much as 20% VD. This is not Code just outright engineering. There are good %Z estimates if you don’t know. I built a spreadsheet for this that I use frequently. You can’t just use name plate because transformers are highly tolerant of temporary overloads, but if the bus voltage gets so low your contactors drop out, it’s too small.

This gets deeper into the obvious question...are you going to run into VD issues with 45 kVA or not?

And starting of a larger motor across the line is where there is likely to be the most VD issues, and could be a reason to want 75 vs 45 kVA transformer. However that may also tend to give you primary protection trips if you leave that protection as it was on the 45.

 

[winnie]

You may be buying a lot of beers! LOL! I’ve had issues with 125 amp breakers tripping with a 75 kva transformer. The lucky thing was most of the 125 amp and larger breakers have a magnetic trip adjustment, and setting it to maximum cured the problem. Setting without load may hold, but the issues I was having was transferring back from generator to utility under load. Until the generator was installed, it was never turned off once on.

*grin* I'd bet at most a six pack.

The situation you describe (switching from utility to generator and back under load) is likely the worst case for inrush.

There are switching tricks that can be used to reduce inrush, but they are likely not worth the effort.

The whole reason that the OP wants to use a reduced size breaker with a 75 kVA transformer is the #6 feeder. If this feeder is very short, then the answer is 'replace it', or at best 'try the existing feeder and if it doesn't work, replace it'. If this feeder is very long, then the resistance of the feeder itself will reduce any inrush current problems.

If the reason for moving to a 75kVA transformer is to reduce impedance and voltage drop, then the impedance of the feeder should also be a consideration for its replacement.

-Jon

 

[topgone]

The primary OCPD must protect both the transformer and the supply conductors.

With secondary protection used, the primary OCPD can be quite large, as you note. This is great for preventing nuisance trips. But the OP has that pesky #6 feeder that also needs protection.

If the total length of the primary and secondary conductors are short enough, tap rules apply. But I doubt that is the situation here, since keeping the #6 feeder is a significant cost issue.

IMHO the OP is limited to a 70A supply breaker. Of course the #6 feeder will also act to limit inrush current. I'd bet a beer that there will be no problems with inrush current tripping this undersized breaker on a 75 kVA transformer.

Jon

I beg to disagree! You will let the primary circuit breaker protect the transformer for faults and let the secondary protection protect the whole transformer from overcurrent (that includes the protection of the primary lines as there will be no overcurrent in the primary if the secondary breaker trips!