transformer secondary feed

[mull982]

I have a 1500kVA 4.16kV to 480V Delta-Wye transformer. I have calculated the secondary full load current of this transformer to be 1806A.

The secondary of this transformer is feeding a 2000A bus on a 480V MCC. The breaker (Siemens RL) on the secondary of this transformer feeding the MCC bus is set to 2000A. In reviewing this it appears that the transformer secondary breaker is set to feed a 2000A bus when the avaliable full load on the secondary is only capable of 1806A. This arrangement would appear to exceed the avaliable current from the secondary of the transformer and possibly cause damage if MCC bus is fully loaded.

Does this arrangement appear to be correct? Is there a particular NEC article that I can cite when I bring up this topic to the engineers?

I am reviewing a power systems study for the first time, which is the reason for alot of posts recently. I appreciate the help and responses that people have been providing. This is a GREAT Forumn!!!

 

[chaterpilar]

I don't see any problem in using a 2000 amps breaker, as all breakers have adjustable trip settings.

You can set it to 1800 amps, or even less, depending on actual load conditions.(just ensure that your load does not exceed 1800 amps)

Cheers.

 

[charlie b]

See Note 1 to Table 450.3(A). You can protect the secondary of the transformer at 125% of its full load current rating. That means you can protect the transformer secondary windings with a main circuit breaker setting up to 2256 amps. But since the MCC rating is only 2000, then you must set the breaker no higher than 2000 amps. Please note that if you choose that breaker setting, then the secondary conductors (from transformer to MCC) must also have a rating of at least 2000 amps.

 

[mull982]

I don't see any problem in using a 2000 amps breaker, as all breakers have adjustable trip settings.

You can set it to 1800 amps, or even less, depending on actual load conditions.(just ensure that your load does not exceed 1800 amps)

Cheers.

The breaker is a 2000A breaker and is set for 2000A not 1800. That was the reason for my concern. If we loaded the bus all the way up to 2000A then the breaker would not trip b/c it is set for 2000A however the transformer secondary was only rated for 1800A and we would be putting it at possible risk.

See Note 1 to Table 450.3(A). You can protect the secondary of the transformer at 125% of its full load current rating. That means you can protect the transformer secondary windings with a main circuit breaker setting up to 2256 amps. But since the MCC rating is only 2000, then you must set the breaker no higher than 2000 amps. Please note that if you choose that breaker setting, then the secondary conductors (from transformer to MCC) must also have a rating of at least 2000 amps.

The breaker being used has CT's used for the pickup value, therefore the pickup value can be set to any value, 1800 if needed. However this breaker is set for a pickup of 2000A which according to 450.3(A) is withing the 125% limit of the transformer secondary. By setting this protection to 125% are we risking damage to the transformer? Why is a FLA value of 1804 given on the transformer if we are allowed to exceed this? Does this mean then we can load the 480V MCC bus all the way up to its full rating of 2000A?

The cables for this feed are (6) sets of 500MCM which will handle the given 2000A capacity.

 

[charlie b]

All good questions.

By setting this protection to 125% are we risking damage to the transformer? Why is a FLA value of 1804 given on the transformer if we are allowed to exceed this?

You are not risking damage to the transformer. At worst, you are risking a reduction in its useful lifetime (e.g., it might only last 20 years, instead of 40). It?s all about heat. If you run a transformer above its rating, it will get hotter. A rule of thumb is that every 10 degrees C you add to the operating temperature (meaning that you keep that 10 degrees continuously throughout the life of the component) will cut the useful lifetime in half.

Does this mean then we can load the 480V MCC bus all the way up to its full rating of 2000A?

Tough question. My answer is no. I think you need to limit the ?calculated load? (not necessarily the same as ?connected load?) to the full load rating of the transformer. But I cannot cite an NEC article that requires this. Anyone able to help me on this issue?

The cables for this feed are (6) sets of 500MCM which will handle the given 2000A capacity.

Yes indeed, that will answer the need.

 

[weressl]

I have a 1500kVA 4.16kV to 480V Delta-Wye transformer. I have calculated the secondary full load current of this transformer to be 1806A.

The secondary of this transformer is feeding a 2000A bus on a 480V MCC. The breaker (Siemens RL) on the secondary of this transformer feeding the MCC bus is set to 2000A. In reviewing this it appears that the transformer secondary breaker is set to feed a 2000A bus when the avaliable full load on the secondary is only capable of 1806A. This arrangement would appear to exceed the avaliable current from the secondary of the transformer and possibly cause damage if MCC bus is fully loaded.

Does this arrangement appear to be correct? Is there a particular NEC article that I can cite when I bring up this topic to the engineers?

I am reviewing a power systems study for the first time, which is the reason for alot of posts recently. I appreciate the help and responses that people have been providing. This is a GREAT Forumn!!!

Please note that many trasnformers are designed for overloading.

Ex. liquid filled forced air cooled transformer has the following loading parameters at OA/FA 55/65C* rating:

1500kVA=1804A OA55C*
1725kVA=2075A FA55C*
1680kVA=2021A OA65C*
1932kVA=2324A FA65C*

 

[jghrist]

It?s all about heat.

And if you load the tranformer to 125% for an hour and the rest of the time it is loaded at less than 100%, it may not get above the rated temperature because it takes a while to heat up.

 

[zog]

The breaker is a 2000A breaker and is set for 2000A not 1800. That was the reason for my concern. If we loaded the bus all the way up to 2000A then the breaker would not trip b/c it is set for 2000A however the transformer secondary was only rated for 1800A and we would be putting it at possible risk.



The breaker being used has CT's used for the pickup value, therefore the pickup value can be set to any value, 1800 if needed. However this breaker is set for a pickup of 2000A which according to 450.3(A) is withing the 125% limit of the transformer secondary. By setting this protection to 125% are we risking damage to the transformer? Why is a FLA value of 1804 given on the transformer if we are allowed to exceed this? Does this mean then we can load the 480V MCC bus all the way up to its full rating of 2000A?

The cables for this feed are (6) sets of 500MCM which will handle the given 2000A capacity.

A Siemens RL breaker has CT's (2000A:0.5A in this case) that as you said are set values, but the trip unit (Static Trip III I am guessing) has LTPU settings of .5, .55, .6,.65, .7, .75, .8, .85, .9, .95, or 1.0 times the current sensor
rating. So you can set it a 2000A, 1900A, 1800A, etc..

 

[mull982]

A Siemens RL breaker has CT's (2000A:0.5A in this case) that as you said are set values, but the trip unit (Static Trip III I am guessing) has LTPU settings of .5, .55, .6,.65, .7, .75, .8, .85, .9, .95, or 1.0 times the current sensor
rating. So you can set it a 2000A, 1900A, 1800A, etc..

This is true. The coordination study that I recieved is telling me to set the LTPU at a value of 1 corrosponding to the 2000A I mentioned.

 

[zog]

This is true. The coordination study that I recieved is telling me to set the LTPU at a value of 1 corrosponding to the 2000A I mentioned.

I see, and you are questioning the setting on the study? Lots of times the study uses factory defaluts, shouldnt be that way but sometimes they are like that, you are right to question it.