MV transformer requirements

[Mr. Chippers]

Customer is looking to replace a medium voltage transformer that is well past it's end of life date. Transformer gets extremely hot. It is installed in a supervised location. I was provided as-built drawings for reference. My question is, is the existing transformer even sized properly for the amount of load on the secondary?

Information is as follows:

Transformer: 2000kVA, 13.2kV-480Y/277V,3ph,4w, Z:5.62%
Primary OCPD: 150E fuse
Primary feeder size unknown
Secondary feeder consists of (8) 4"C, 4#535kCMIL & 1#4/0gd (diesel locomotive cable), type RHW, 90degC which goes into a tap box and connects to a 3000A busway to the switchboard
Switchboard: 3000A-3P LSIG MCB, 3000A swbd
Switchboard Load: Connected load = 6192.3A, Demand load = 2753.7A

Based on the transformer size, I calculated the rated currents as Ip=87.48A and Is=2405.63A.

Using NEC 450.3(A):
1. The primary fuse is within 300% requirement.
2. The secondary breaker is within the 250% requirement.

How can the transformer rated current be less than the demand load? I feel this could be why the transformer is hot all the time. Is this to code?

 

[petersonra]

2000 KVA at 480 V is about 2400 Amps. I could see how the transformer OCPD might not ever trip at 2750 Amps since it has a 3000 A CB.

I don't think there is any code language that specifies how to size a transformer.

I am not sure what you mean by demand load. Is this a load you measured?

What do you mean by connected load? Is that the load calculated according to the code requirements for feeder calculations?

Keep in mind the table is for transformer protection and you also have to have conductor protection.

A transformer can be quite warm to the touch and well within its safe operating range.

IMO, the ground wire is way undersized.

 

[Mr. Chippers]

2000 KVA at 480 V is about 2400 Amps. I could see how the transformer OCPD might not ever trip at 2750 Amps since it has a 3000 A CB.

I don't think there is any code language that specifies how to size a transformer.

I am not sure what you mean by demand load. Is this a load you measured?

What do you mean by connected load? Is that the load calculated according to the code requirements for feeder calculations?

Keep in mind the table is for transformer protection and you also have to have conductor protection.

A transformer can be quite warm to the touch and well within its safe operating range.

IMO, the ground wire is way undersized.

The connected load is the FLA of all loads connected to the switchboard. The demand load is the calculated load based on NEC demand load percentages, and what the transformer sees in reality.

These loads were given on a set of prints, and were calculated by an engineer from the past job that was done.

These are just the existing conditions I was given to work with.

 

[Elect117]

You need the transformer's nameplate, transformer type (padmount, substation style, etc.) and the cooling method/oil liquid insulation method.

Usually the nameplate will indicate a temp rise and typically, it is good for 130% to 150% overload. The secondary though will melt the paddles if overloaded so try to make sure the number of runs suffice. You can see on the picture below, it is rated for 130% for 3 hours (but can probably handle a lot more than that).

There are some IEEE papers on it somewhere.

 

[petersonra]

The connected load is the FLA of all loads connected to the switchboard. The demand load is the calculated load based on NEC demand load percentages, and what the transformer sees in reality.

These loads were given on a set of prints, and were calculated by an engineer from the past job that was done.

These are just the existing conditions I was given to work with.

the NEC calculated loads tend to be very conservative. if you measure the actual load I would not be surprised if it was less than half the calculated load.

I have a rule of thumb about transformers. If you can put your thumb on the outside skin of the transformer and count to three without getting scalded, it is not too hot, even if it is uncomfortably warm.

 

[David Castor]

Is this demand load a measurement or just a calculated value? I wouldn't be overly concerned by the loading you have given. But if you are buying a new one, why not go up to 2500 kVA? You would have to evaluate the increase short-circuit current of course. Is this a pad-mounted xfmr?

If you haven't looked into lead times for transformers recently, you're in for a shock. For a new transformer, I'd guess you are looking at 18 to 24 months. You might find a refurbished one, but that's a crap shoot.

 

[petersonra]

Is this demand load a measurement or just a calculated value? I wouldn't be overly concerned by the loading you have given. But if you are buying a new one, why not go up to 2500 kVA? You would have to evaluate the increase short-circuit current of course. Is this a pad-mounted xfmr?

If you haven't looked into lead times for transformers recently, you're in for a shock. For a new transformer, I'd guess you are looking at 18 to 24 months. You might find a refurbished one, but that's a crap shoot.

Dry type xfmrs are easier to come by.

 

[Elect117]

Dry type xfmrs are easier to come by.

They make MV dry types?

 

[petersonra]

They make MV dry types?

I hope so. I got a quote on one not that long ago for a project that sadly did not come to fruition. I don't recall what brand. Customer wanted oil filled but they were all 60 weeks plus but dry ones were available in a few months.

 

[Elect117]

Thats dope!

I didn't know that.

If you don't mind me asking, what was the price like in comparison to Padmounts or substation style transformers?

 

[petersonra]

Thats dope!

I didn't know that.

If you don't mind me asking, what was the price like in comparison to Padmounts or substation style transformers?

Don't know. Don't have a lot of experience to compare them.

I looked it up. Eaton. The transformer and switchgear totaled just under $250k. 3500 KVA. 4160/480-277. Price was not broken out.

 

[jim dungar]

Dry type xfmrs are easier to come by.

But they are extremely larger than comparable liquid filled units.

 

[petersonra]

But they are extremely larger than comparable liquid filled units.

But you can actually acquire them. Oil filled units are really hard to come by.

 

[David Castor]

I'd be surprised if you could get a dry-type MV transformer in a "few months" at the present time. They use the same transformer steel and that is one of the components in short supply. It's really tough right now.

 

[petersonra]

I'd be surprised if you could get a dry-type MV transformer in a "few months" at the present time. They use the same transformer steel and that is one of the components in short supply. It's really tough right now.

My quote was over a year ago so you might be right. We get some specialty inductors made now and then. The company that makes them for us seems to have a really tough time getting the steel. Not sure what is do special about it but they have told me Tesla is buying it all up to make battery cases.

 

[electrofelon]

I will just add that humans aren't very good at assessing what "hot" is for non human things. Too hot to touch can be barely warm for a motor or transformer. Perhaps some actual temperature measurements would be prudent. Maybe throw a logger on there too (if you haven't already, still a little confused about what your demand load is).

 

[petersonra]

Maybe throw a logger on there too (if you haven't already, still a little confused about what your demand load is).

I wish people would not use terms that are poorly defined. Using a term like nec calculated load tells the reader exactly what is meant.

 

[electrofelon]

I wish people would not use terms that are poorly defined. Using a term like nec calculated load tells the reader exactly what is meant.

Yeah this statement is confusing:

"The demand load is the calculated load based on NEC demand load percentages, and what the transformer sees in reality."

IMO in general there should be two types of loads: An NEC load calc, and an actual measured, logged load, typically 15 minute demand.

 

[petersonra]

Yeah this statement is confusing:

"The demand load is the calculated load based on NEC demand load percentages, and what the transformer sees in reality."

IMO in general there should be two types of loads: An NEC load calc, and an actual measured, logged load, typically 15 minute demand.

The nec calculated load is not typically anywhere close to what the real load is so I didnt get what was the meaning.

 

[Mr. Chippers]

Sorry for the confusion. The NEC demand load is based on calculated load, not measured load.

If the demand load is 2753.7A, and a 2000kVA TX can only supply 2405A on the secondary, is that code compliant? Wouldn't they have to upsize to a 2500kVA?