Transformer Burning Up

[eeee]

I have been told by my colleague that my transformer will burn up in the configuration it is, although the facility designer does not think so. I have a 300KVA, 208Y/120Yvolt, 3 phase pad mounted transformer. That equates to 833amps for the transformer rating. I have a 45amp fuse for the primary of the transformer I calculate to be sufficient for this transformer. The primary originates from the substation.

I have two secondaries emanating from this pad mounted transformer; one going to a part of the facility that has a 1200amp fuse in its main circuit breaker. I have another secondary that goes to another part of the facility that has a 800amp fuse in a main switchboard.

My colleague says the two fuses attached to both secondaries add to 2000 amps, i.e. (800amps+1200amps), therefore the transformer will burn up since it is only rated for 800Amps.

It seems to me that the secondary fused at 1200 amps will pull more current than the transformer is rated for provided the load for that secondary exceeds 800amps. This will in fact burn up the transformer in my opinion.

The other secondary is fused at 800amps, which is what the transformer is rated for, so I do not see any connected loads from this secondary causing the transformer to burn up.

Please Advise??

 

[jtester]

Re: Transformer Burning Up

If your installation is governed by the NEC, you may have a problem. Table 450.3(A) requires that the sum of secondary protection ratings be equal or lesser than that allowed for a single OCPD.

If I understand your question properly, you may have at least two problems, one is too high a sum of OCPD's and the other is that all secondary OCPD's should be grouped at one location.

Jim T

 

[bob]

Re: Transformer Burning Up

eeee

It seems to me that the secondary fused at 1200 amps will pull more current than the transformer is rated for provided the load for that secondary exceeds 800amps. This will in fact burn up the transformer in my opinion.

It will definitely burn the transformer up if indeed the loads requires 1200 amps.

The other secondary is fused at 800amps, which is what the transformer is rated for, so I do not see any connected loads from this secondary causing the transformer to burn up.

I'm not sure what you mean by this statement but you have a potential total load of
1200 amps + 800 amps = 2000 amps.

What is the primary voltage?

[ November 10, 2005, 04:12 PM: Message edited by: bob ]

 

[sceepe]

Re: Transformer Burning Up

and the other is that all secondary OCPD's should be grouped at one location.

Can't you have remote disconnects that are shunt tripped to one location?

Also, if you are sizing the POCO xfmr then its up to you. I provide the POCO with undiversified (connected load) broken down by load types. In my experience all POCO's will diversify the load differently, and often different POCO's choose different size xfmrs. Not uncommon to see a 3000 kva connected load fed from a 1500 kva xfmr. If I were in your shoes I would want to see a detailed connected load summary from the engineer.

If you are sizing a dry type xfmr, then I agree with the previous posts.

 

[john m. caloggero]

Re: Transformer Burning Up

It is necessary to know the primary voltage to provide an accurate answer.

 

[john m. caloggero]

Re: Transformer Burning Up

Sorry an after thought, does the transformer qualify as supervised?

 

[charlie b]

Re: Transformer Burning Up

The primary voltage is certainly over 600V, based on the assertion that a 45 amp primary fuse is acceptable. My guess is that the primary is 12,470 volts. That puts us into Table 450.3(A). We can't go any further without knowing whether the facility is a "supervised location," as John has pointed out. But if this is a supervised location, then the secondary protection can be as high as 250% of the rated secondary current, or 2083 amps. That would mean a 1200 amp fuse and an 800 amp fuse would be acceptable.

 

[eeee]

Re: Transformer Burning Up

The primary is 12,470volts. I assume that you mean a supervised location is a location that has people near it. This is true. There is a maintenance crew that is located nearby.

It is interesting Charlie that you say the 800Amps+1200Amps=2000Amps is ok here; you state 2083amps permitted.

When I read the table 450.3(A), it indicated that there is a certain amount of overloading that could not be exceeded and Bob expressed this also as it relates to one overall circuit breaker governing this configuration. This information you have provided indicates that I am not overloaded with circuit breakers.

I was next concerned that I could not have two circuit breakers under this configuration not grouped at one location as Bob indicates and you seem to indicate that I can given the overloading considerations that apply here.

 

[charlie b]

Re: Transformer Burning Up

You are starting to concern me now. Since you have read Table 450.3(A), perhaps you should take a closer look at its notes.

Originally posted by eeee: I assume that you mean a supervised location is a location that has people near it. This is true. There is a maintenance crew that is located nearby.

That is not what is meant by "supervised location." Read Note 3.

Originally posted by eeee: It is interesting Charlie that you say the 800Amps+1200Amps=2000Amps is ok here; you state 2083amps permitted.

I didn't state that it's "OK" here. I interpreted the table to mean that the secondary protection can be as high as 250% in a supervised location. I would not protect the secondary at 250% without knowing, from other information sources, that the transformer will not be overloaded. Specifically, I would want to see the results of the load calculation, and I would like to know about the large motor loads.

Originally posted by eeee: This information you have provided indicates that I am not overloaded with circuit breakers.

Your original question said you were using fuses. Which is it?

Originally posted by eeee: I was next concerned that I could not have two circuit breakers under this configuration not grouped at one location as Bob indicates and you seem to indicate that I can given the overloading considerations that apply here.

You need to read Note 2.

 

[eeee]

Re: Transformer Burning Up

Yes, it is fuses that are the main service diconnecting means for the circuit breaker panel.

The maintenance personnel are qualified, so it is a supervised location. I did not recall note 3 last time I read Table 450.3(A), but should have noted it. I appologize.

What concerns me is that the NEC permits me to put in 250% of overload protection apparently as I try to interpret the notes below the table, but yet the transformer secondary rated at 833amps is relatively small compared to the 2083amps of fused overload protection per my interpretation. It is now being reported that in fact my transformer can overload and burn up if the demand load is sufficient (demand load and not connected load being key here in my opinion), even though the NEC permits 2083amps worth of fusing as I interpret the last sentence of note 2. Although this is somewhat ambigous when it says "if both circuit breakers and fuses" (in that I suspect they are referring to main disconnects being comprised of fuses and circuit breakers both in existance as part of the configuration). Not so here since my main disconnects are fuses only.

The only estimate we have for demand load presently is a transformer reading we took in June on a not so hot day (required for maximum demand load), but this reading was not done per the NEC tabulating 1 months worth of data. Also we did not have a grasp on all the loads that should have been verified as on when we took the reading such as Computer Room Air conditioning unit and bldg. air handlers. Calculating the loads emperically is an unfathomable project to discern the power requirementr for all the networking equipment, etc.

I have suggested we take the power reading from the transformer next July in order to verify we are not overloading the transformer. We don't have the manpower or equipment to take a full months worth of readings per the NEC. We may be able to hire a company to do this provided the funds are acquired, but that is not my decision.

 

[charlie b]

Re: Transformer Burning Up

Originally posted by eeee:The maintenance personnel are qualified, so it is a supervised location.

That is still not what it means to have a "supervised location." The fact that your maintenance people are qualified is not the point. The point is that there needs to be administrative controls, written procedures, and perhaps even locked doors, so that nobody but them can gain access to, or can do any monitoring or servicing, of the transformer installation.

 

[charlie b]

Re: Transformer Burning Up

Originally posted by eeee:What concerns me is that the NEC permits me to put in 250% of overload protection . . . .

The point of allowing 250% protection is to permit large motors to start without blowing the fuses. But the total load on the transformer should be no more than its rated value.

Are you contemplating some new project that will add load or add that second set of fuses? Or is this just an academic question, a concern over the existing configuration? If you are adding load, then a 30 day load test is your best bet. There are companies that can do this for you, and it is not too expensive.

Back to the earlier discussion: What about Note 2 of the Table? What about the requirement that the "up to six" disconnects must be grouped in one location?

 

[eeee]

Re: Transformer Burning Up

Yes,

I am adding a serious amount of load. Presently the bldg. is powered by two transformers and this does not meet the NEC. We are eliminating one 75KVA transformer and putting the whole bldg. power on the larger 300KVA transformer in order to meet the requirements of one service entrance per the NEC.

I believe I have an understanding of what you are implying as related to note 2 now. My secondary fuses are located in two separate mechanical rooms located 150 feet away from each other. Therefore they are not grouped together in one location and must be in violation of the NEC code. I guess a person must interpret what is meant by one location. Two separate mechanical rooms is probable not what is meant by one location.

I have no large motor loads that would justify a circuit breaker above the rating of the secondary amps of the transformer(at first glance, but will need to check out the specifications of the units to see if they will be causing a start to exceed the 833amps for the collective loads of the bldg.), except two separate chillers that share the support of a two story 24,000 square foot bldg (and are respectively sized for this) and two separate 30-45hp air handler motors that do the same. The remaining motors are smaller.

I would suggest dropping the fuse sizes down to something such as 425amps and 425amps, so they add to 850amps within range of the 833Amp secondary rating of the 300KVA pad mounted transformer that will now support the whole bldg.

Problem is that my fuses will still not be located in the same area. Maybe I need to move my circuit breaker panels with fuses in to the same mechanical room to meet 450.3(A), note 2??

 

[charlie b]

Re: Transformer Burning Up

Originally posted by eeee:I am adding a serious amount of load.

Then you need to calculate the load in accordance with 220.16 (B) - this takes you back to 220.12 and 14 - or you need to determine the existing load in accordance with 220.87. You cannot add load without knowing that your existing system has the capacity to add load.

 

[eeee]

Re: Transformer Burning Up

I understand that I must calculate the load and have plans to do this per the NEC chapters you indicate. I hope to hire a company to take power readings for a month in July and then choose fuses based on these readings. The power readings will be taken from each secondary (i.e. one power reading for each secondary). The fuse for each secondary will be selected based on this outcome.

I am concerned that my fuses may not be in the same area and that this may violate the NEC as discussed in note 2 of Table 450.3(A). They are located 150 feet apart and are located in separate mechanical rooms. As I interpret note 2, it does not matter whether I have fuses or circuit breakers for disconnects, they must be grouped in the same area and I think this means the same mechanical room.

 

[thinfool]

Re: Transformer Burning Up

Who is providing the xfmr? If you are, then all these calcs are necessary....if it is the POCO, they will need a 'Load Sheet' from the customer and they will size the xfmr as they please, regardless of the equipment the customer is using.
As I think about it...who owns the two transformers that you are replacing?
Here is why I ask....many industrial locations take a primary service (12470-13200) and serve several transformers with it. The location I have in mind feeds 4 unit-subs from the incoming primary. Each unit-sub has a primary switch (12470), a secondary main (480) and then a plethora of feeder breakers to various location of the plant. So, from one point of view, this building is served by four transformers. However, it only has one meter...on the primary coming in, and four 'main disconnects' (six disconnect rule), one on each primary feeder to each unit-sub.

 

[eeee]

Re: Transformer Burning Up

I wanted to reply to the previous respondent. I am actually installing an external switch next to the transformer so I only have one secondary (I unfortunately forgot this fact), but still traverse to two main circuit breaker panels from the external switch servicing two functionally different parts of the bldg.

I plan to put in a rated circuit breaker or fuse on the load side of the external switch and have a company or myself do a load calculation for the whole building so as to size the fuse properly for the one secondary emanating from the transformer that feeds the whole bldg. This will settle the issue.