Installing a bigger tranformer than needed

[de2]

I calculated and I need 45kva 480/208 V transformer to feed my panel.
I put a OCPD on the primary of the transformer for 60A and , OCPD on the secondary for 150A.

I found a deal on 75kVA transformer, cheaper than 45kva!.

If I go with 75kVA and not change the conductor or OCPD I think I am in violation of NEC 450.3(B).

I just want to know what would you do if you find a cheaper and higher rated transformer that your need? Now facing with reality of increasing cable size for a bigger transformer?

 

[david luchini]

If I go with 75kVA and not change the conductor or OCPD I think I am in violation of NEC 450.3(B).

No violation of 450.3(B) to install the larger transformer. You may have issues with inrush tripping the primary breaker, though.

Why not change the primary feeder? They'd then have the capacity to install a second panel on the secondary at a later date.

 

[kwired]

I don't know what differences in transformer losses might be, but something to think about if you don't need the extra capacity.

 

[dmsv10]

Here's a quick xref that covers the breaker sizing.

When installing a transformer, use the transformer secondary rules in 240.21(C)(1) through (C)(6). 


Where installed in accordance with one of these six rules, a set of conductors feeding a single load, or each set of conductors feeding separate loads, can be connected to a transformer secondary without overcurrent protection at the secondary [240.21(C)]. There is an important informational note under this section that references 450.3. Section 450.3 covering overcurrent protection of transformers, and tables 450.3(A) and (B) provide the maximum rating or setting of overcurrent protection for transformers. Table 450.3(A) is for transformers over 600 volts (V), and Table 450.3(B) is for transformers 600V and less. 


Sometimes there is confusion when looking at these tables. For example, a 75-kilovolt-ampere (kVA), three-phase transformer will be installed in a small industrial plant. The primary side voltage will be 480V, and the secondary side voltage will be 208Y/120V. A three-phase, fused disconnect with 125-ampere (A) fuses will be installed on the primary side of this transformer. Is secondary overcurrent protection required for this transformer?

This transformer is rated 75,000 volt-amperes (75 kVA × 1,000 = 75,000). The primary side current will be 90A (75,000 ÷ 480 ÷ 1.732 = 90.2 = 90). In accordance with the top row or Table 450.3(B), the maximum rating for the primary overcurrent protection is 125 percent. After multiplying the primary current by 125 percent, the ampacity is 113A (90 × 125% = 112.5 = 113). Note 1 under Table 450.3(B) states, where 125 percent of this current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, a higher rating that does not exceed the next higher standard rating shall be permitted.*



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[kwired]

Sometimes there is confusion when looking at these tables. For example, a 75-kilovolt-ampere (kVA), three-phase transformer will be installed in a small industrial plant. The primary side voltage will be 480V, and the secondary side voltage will be 208Y/120V. A three-phase, fused disconnect with 125-ampere (A) fuses will be installed on the primary side of this transformer. Is secondary overcurrent protection required for this transformer?

I won't say you must always have overcurrent protection on that secondary, but if anything you typically will want to protect each secondary conductor from being loaded beyond the capacity of the transformer winding they are connected to or at least a certain percentage. 75kVA is approximately 208 amps per line. Without secondary protection it is possible to load one line to neutral to more than 208 amps and not trip the primary protection.

This transformer is rated 75,000 volt-amperes (75 kVA × 1,000 = 75,000). The primary side current will be 90A (75,000 ÷ 480 ÷ 1.732 = 90.2 = 90). In accordance with the top row or Table 450.3(B), the maximum rating for the primary overcurrent protection is 125 percent. After multiplying the primary current by 125 percent, the ampacity is 113A (90 × 125% = 112.5 = 113). Note 1 under Table 450.3(B) states, where 125 percent of this current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, a higher rating that does not exceed the next higher standard rating shall be permitted.*

But nothing prohibits less than 90 amp primary overcurrent device either, mostly comes down to whether or not what you have selected will hold when energizing.

 

[dmsv10]

I won't say you must always have overcurrent protection on that secondary, but if anything you typically will want to protect each secondary conductor from being loaded beyond the capacity of the transformer winding they are connected to or at least a certain percentage. 75kVA is approximately 208 amps per line. Without secondary protection it is possible to load one line to neutral to more than 208 amps and not trip the primary protection.

But nothing prohibits less than 90 amp primary overcurrent device either, mostly comes down to whether or not what you have selected will hold when energizing.

So if you have existing Breakers and existing feeders for the smaller Transformer and are considering putting in the 75, you would be required to put in the properly sized breaker and wire for the 75.

If you're concerned with inrush current on a 75 KVA transformer for the first time, you could always connect it to a separate or different 480 volt source, hook up all your meters, measure amps, inrush current, etc. and typically once the Transformer is saturated there will be very low inrush the next time you energize it.

We used to do that for the larger 200 KVA Transformers that had sat for a year before energizing.

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[de2]

I am happy that I am not the only one with confusion
Customer trying to save money for not installing bigger conductor for 75kva transformer and not to pay MEP engineer to re-do drawings and seal cost.

 

[david luchini]

So if you have existing Breakers and existing feeders for the smaller Transformer and are considering putting in the 75, you would be required to put in the properly sized breaker and wire for the 75.

Code would not require changing the feeders.

 

[zbang]

Something else to consider is that 10 (5? 3?) years down the road somebody will want to add load to the "lightly loaded transformer", find out that the primary supply wire & breaker are "undersized", and curse the names of everyone involved with the original install.

 

[kwired]

So if you have existing Breakers and existing feeders for the smaller Transformer and are considering putting in the 75, you would be required to put in the properly sized breaker and wire for the 75.

If you're concerned with inrush current on a 75 KVA transformer for the first time, you could always connect it to a separate or different 480 volt source, hook up all your meters, measure amps, inrush current, etc. and typically once the Transformer is saturated there will be very low inrush the next time you energize it.

We used to do that for the larger 200 KVA Transformers that had sat for a year before energizing.

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Properly sized is anything under maximum allowed yet will still hold when energizing.

I have uses 25kVA (480 x 120/240) single phase for temporary power before where it was what was available though a 10 kVA would have been sufficient. "proper" primary protection for many would have been 60 amps, this one worked just fine on an existing 30 amp fused disconnect that wasn't in use at the time.

Inrush current is going to be dependent on impedance of the supply, meaning the supply itself as well as conductors between the unit and the supply. Smaller conductors will provide some current limiting to that inrush so what works on same primary in one location might not work in another.

 

[dmsv10]

I am happy that I am not the only one with confusion
Customer trying to save money for not installing bigger conductor for 75kva transformer and not to pay MEP engineer to re-do drawings and seal cost.

Well if you're the one that's going to be there 5 years from now as mentioned by others I would make the case to do the upgrade and change the drawings although we know the customer doesn't always understand that future planning pays off.

So if he has a set of drawings with all the specs for the smaller Transformer and they don't want to spend any more money than you're stuck putting it in as engineered.

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[sameguy]

80-90% loading for best efficiency.

 

[kwired]

80-90% loading for best efficiency.

Why not 100%?

If you don't have a fixed load it becomes more difficult to select at any specific load level as being the most efficient for the application though.

 

[texie]

I calculated and I need 45kva 480/208 V transformer to feed my panel.
I put a OCPD on the primary of the transformer for 60A and , OCPD on the secondary for 150A.

I found a deal on 75kVA transformer, cheaper than 45kva!.

If I go with 75kVA and not change the conductor or OCPD I think I am in violation of NEC 450.3(B).

I just want to know what would you do if you find a cheaper and higher rated transformer that your need? Now facing with reality of increasing cable size for a bigger transformer?

The owner will pay a price for the way oversized transformer loses that will far exceed the lower purchase price of the larger transformer over its life span.

 

[jim dungar]

80-90% loading for best efficiency.

DOE started to govern transformer efficiency as far back as 2005 (Public Law 109-28 of the Energy Policy Act). Roughly, at that time the lowest efficiency was for a 3-phase 15kVA unit at 97.0%. New rules raised most efficiencies in 2016, so the lowest went up to 97.7%.
But these values are not determined at full load.

I recall that these efficiencies are based on 35% loading.

These rules apply to most general purpose ventilated dry type power transformers. This is the 'law of the land' so of course there are exceptions to it.

 

[kwired]

The owner will pay a price for the way oversized transformer loses that will far exceed the lower purchase price of the larger transformer over its life span.

Kind of thought so myself.

Majority of current at little to no load is reactive current, but there is still real power lost, the thing wouldn't give off any heat if there wasn't any losses. More heat it gives off the more energy was wasted, though if indoors and during heating season is not exactly a complete loss.

 

[sameguy]

DOE started to govern transformer efficiency as far back as 2005 (Public Law 109-28 of the Energy Policy Act). Roughly, at that time the lowest efficiency was for a 3-phase 15kVA unit at 97.0%. New rules raised most efficiencies in 2016, so the lowest went up to 97.7%.
But these values are not determined at full load.

I recall that these efficiencies are based on 35% loading.

These rules apply to most general purpose ventilated dry type power transformers. This is the 'law of the land' so of course there are exceptions to it.

Thanks Jim I was going by ~1980s learning, when I was sent to multiple manufacturers for training or done in house.
Kwired iirc core losses and heat putting life vs efficiency per dollar. As I said long ago.

 

[kwired]

DOE started to govern transformer efficiency as far back as 2005 (Public Law 109-28 of the Energy Policy Act). Roughly, at that time the lowest efficiency was for a 3-phase 15kVA unit at 97.0%. New rules raised most efficiencies in 2016, so the lowest went up to 97.7%.
But these values are not determined at full load.

I recall that these efficiencies are based on 35% loading.

These rules apply to most general purpose ventilated dry type power transformers. This is the 'law of the land' so of course there are exceptions to it.

If I understand correctly that would mean a 15kVA unit @ 35% load would be 5250 VA, and if you are allowed say 3% inefficiency at that level you could expect about 158 watts of losses. If that were constant load year round you lose 1384 kWhr a year depending on your rates that is maybe $130 - 150 per year in energy cost. I would assume the losses only increase as load increases - loaded transformer gives off more heat than an unloaded one. Seems like bad design to go too large if you know you will never need the capacity and especially if you are expecting to need to use it for 10+ years

 

[jim dungar]

The owner will pay a price for the way oversized transformer loses that will far exceed the lower purchase price of the larger transformer over its life span.

Remember there are two set of losses in a transformer. You need to look at both of them independently. For the most part the efficiency is a useless number.

Core losses are a function of the steel that makes up the transformer. These losses are constant 24hrs/day regardless of the loading. The core losses will be larger for a 75kVA than for a 45kVA.
Conductor, or winding, losses are a direct result of the percentage loading of the transformer. For a given load you could expect that the conductor losses of a 75kVA transformer would be lower than those of a 45kVA unit.

You need to have a loading profile of the transformer in order to truly evaluate the combination of these losses. However, I would not be surprise to find there is a point where 75kVA has fewer losses than the 45kVA, even though its % efficiency may be lower.

 

[de2]

I am happy that I am not the only one with confusion
Customer trying to save money for not installing bigger conductor for 75kva transformer and not to pay MEP engineer to re-do drawings and seal cost.

David, can you explain, why code do not require bigger conductor for bigger xfmr, I want to say because code allows conductor based on the load connected?
Can you point out the article # is possible?