Transformer sizing

[jim dungar]

If there are more exceptions in the code, please let me know. i am always up for learning. Also, how much of a loss are talking about between a 45 and 75kva transformer? I have never done the math, but I would presume its negligible.

Oversized transformers are a significant waste of energy, this was a driving force behind the DOE transformer energy efficiency ratings being based on 35% average loading for 'small' power and lighting transformers.

When protected by the NEC requirements, of 125% maximum, you would not be able to overload the 45kVA, in your example, even if the connected load greatly exceeded its nominal rating. Are you saying an AHJ would not routinely accept a properly protected design?

 

[Knightryder12]

Oversized transformers are a significant waste of energy, this was a driving force behind the DOE transformer energy efficiency ratings being based on 35% average loading for 'small' power and lighting transformers.

When protected by the NEC requirements, of 125% maximum, you would not be able to overload the 45kVA, in your example, even if the connected load greatly exceeded its nominal rating. Are you saying an AHJ would not routinely accept a properly protected design?

Jim, it seems I am wasting time arguing with you on this subject. You design the way you design and I will design the way I want to. Never had any issues in the last 23 years. And we both know that a thermal mag breaker can and will carry more load than what its rated for for a extended (a very extended) period of time.

And until you can show me in code, have a good day.

 

[jim dungar]

Jim, it seems I am wasting time arguing with you on this subject. You design the way you design and I will design the way I want to. Never had any issues in the last 23 years. And we both know that a thermal mag breaker can and will carry more load than what its rated for for a extended (a very extended) period of time.

And until you can show me in code, have a good day.

Yes, to each their own.
My point is that the NEC never once provides assistance in sizing transformers and motors. It considers these design decisions.

 

[wwhitney]

My point is that the NEC never once provides assistance in sizing transformers and motors. It considers these design decisions.

Seems to me the combination of 450.3 and 215.3 means that the transformer rating needs to be at least 80% of the NEC calculated load, even if you know the operating load will be much lower.

Cheers, Wayne

 

[electrofelon]

Isn't that what the NEC has us do? By your logic, its ok to have 160a connected load on a 150a rated panel. That does not make sense.

That is different. 408.36 requires a panelboard to be protected at its rating by an OCPD. There is no such rule for a transformer.

But as I have said before, show me in the code that allows that

That is not how the code works. You need to provide a code showing something is a violation. You could use the catch all 110.3:

110.3 Examination, Identification, Installation, Use, and Listing (Product Certification) of Equipment.
(A) Examination. In judging equipment, considerations such as the following shall be evaluated:
(1) Suitability for installation and use in conformity with the provisions of this Code
(2) Mechanical strength and durability, including, for parts designed to enclose and protect other equipment, the adequacy of the protection thus provided
(3) Wire-bending and connection space
(4) Electrical insulation
(5) Heating effects under normal conditions of use and also under abnormal conditions likely to arise in service
(6) Arcing effects
(7) Classification by type, size, voltage, current capacity, and specific use
(8) Other factors that contribute to the practical safeguarding of persons using or likely to come in contact with the equipment

There is also 110.3(B), but the problem with both of those is they dont say specifically how to determine if the equipment's capacity is being met. Many would assume an article 220 load calculation, but they dont spell that out.

 

[wwhitney]

That is different. 408.36 requires a panelboard to be protected at its rating by an OCPD. There is no such rule for a transformer.

Sure there is, 450.3 is similar. It says for most cases that you either have to provide primary protection of at most 125% of the transformer rating, or you have to provide secondary protection of at most 125% of the transformer rating.

While 215.3 gives you minimum sizes of both primary and secondary OCPD based on the NEC calculated load.

The net result is that the smallest transformer you can use has a rating of 80% of the NEC calculated load, for the case that all the load is non-continuous, or you are using 100% rated OCPD.

Cheers, Wayne

 

[electrofelon]

Sure there is, 450.3 is similar. It says for most cases that you either have to provide primary protection of at most 125% of the transformer rating, or you have to provide secondary protection of at most 125% of the transformer rating.

While 215.3 gives you minimum sizes of both primary and secondary OCPD based on the NEC calculated load.

The net result is that the smallest transformer you can use has a rating of 80% of the NEC calculated load, for the case that all the load is non-continuous, or you are using 100% rated OCPD.

Cheers, Wayne

No disagreement there. Depends on how you want to put it. 450.3 lets you overload a transformer but has limits. There is no protection requirements beyond that, unless you try to evoke something in the 110's

 

[jim dungar]

No disagreement there. Depends on how you want to put it. 450.3 lets you overload a transformer but has limits. There is no protection requirements beyond that, unless you try to evoke something in the 110's

When I was a sales engineer for a transformer manufacturer, we worried about the operational loading of the transformer, its ambient temperature, and its insulation temperature rating as well as the length of time the transformer was loaded.

This is a heat transfer issue. Effectively if you can keep the ultimate 'hot spot' temperature below 220°C, you can load a dry type transformer as much as you want.
For awhile, at least one manufacturer said their 80°C units could be continuously overload by 30%. This would allow the previous example 45kVA unit to carry the connected load of 47kVA.

Sizing transformers is a design choice.
Protecting a chosen transformer is an NEC requirement.

 

[wwhitney]

Protecting a chosen transformer is an NEC requirement.

But do you agree that it is impossible to satisfy both NEC 215.3 and 450.3 unless the chosen transformer has a "rated current" at least 80% of the NEC calculated load?

Because 450.3 places an upper bound on OCPD size based on the transformer's "rated current", while 215.3 place a lower bound on the OCPD size based on the NEC calculated load?

Cheers, Wayne

 

[jim dungar]

But do you agree that it is impossible to satisfy both NEC 215.3 and 450.3 unless the chosen transformer has a "rated current" at least 80% of the NEC calculated load?

Because 450.3 places an upper bound on OCPD size based on the transformer's "rated current", while 215.3 place a lower bound on the OCPD size based on the NEC calculated load?

Cheers, Wayne

The protection sizing on the secondary side of the transformer is only dependent on being able to at least supply the connected load, per 215.3.

There is no direct NEC relationship between the secondary OCPD and the transformer rating itself.
450.3 requires the transformer primary OCPD to be sized at 125% of the transformer rating, there is no mention of the connected loading. There is an allowance to increase the size of the primary device if the secondary is limited to 125% of the transformer. These 125% ratings are not dependent on if the protective devices are 100% rated or not.

And yes, it can be argued that the primary side device must be able to carry the load calculated by 215.3 which could then indirectly affect the transformer sizing.

 

[wwhitney]

And yes, it can be argued that the primary side device must be able to carry the load calculated by 215.3 which could then indirectly affect the transformer sizing.

It directly affects the transformer sizing.

If the calculated load on the transformer primary is 100A, then if you choose to use maximum 125% primary protection under 450.3, your transformer rated primary current needs to be at least 80A. If you know that the NEC calculated load is conservative by a factor of two, and you would like to select a transformer with a rated primary current of 50A, you are out of luck; using a 60A primary OCPD would violate 215.3.

For the case where you instead choose to use higher primary OCPD, you are required to limit the secondary OCPD to 125% of the transformer rated current. The net result is the same as above, just with all the currents multiplied by the primary to secondary voltage ratio, since they are on the secondary side.

Cheers, Wayne

 

[petersonra]

Isn't that what the NEC has us do? By your logic, its ok to have 160a connected load on a 150a rated panel. That does not make sense. Again, if i can prove that certain loads are no coincidental, I will use that to my and the owners advantage. But as I have said before, show me in the code that allows that other than heat/cool and receptacle load.
I have been doing design for a long time and have been trained by some very knowledgeable Electrical engineers. I do not know of any other diversities we can take on loading/service/panel/transformer sizing other than the ones I mentioned.
I have had the owner provide a letter to the AHJ to state that certain equipment will never be used at the same time and have decent luck to have the AHJ accept that. But not all will.

If there are more exceptions in the code, please let me know. i am always up for learning. Also, how much of a loss are talking about between a 45 and 75kva transformer? I have never done the math, but I would presume its negligible.

Where does the code prohibit having 160 amps of connected load on a 150 amp panelboard.

 

[electrofelon]

We

Where does the code prohibit having 160 amps of connected load on a 150 amp panelboard.

408.30 General. All panelboards shall have a rating not less than the minimum feeder capacity required for the load calculated in accordance with Part III, IV, or V of Article 220, as applicable.

IF you can find a way to have an NEC load calc be 150 amps but the actual load be 160 amps, then yeah you are free to do it!

 

[jim dungar]

It directly affects the transformer sizing.

If the calculated load on the transformer primary is 100A, then if you choose to use maximum 125% primary protection under 450.3, your transformer rated primary current needs to be at least 80A. If you know that the NEC calculated load is conservative by a factor of two, and you would like to select a transformer with a rated primary current of 50A, you are out of luck; using a 60A primary OCPD would violate 215.3.

For the case where you instead choose to use higher primary OCPD, you are required to limit the secondary OCPD to 125% of the transformer rated current. The net result is the same as above, just with all the currents multiplied by the primary to secondary voltage ratio, since they are on the secondary side.

Cheers, Wayne

I did not talk about whether the NEC load calculations are conservative or not. I was discussing whether a transformer was sized per the NEC connected loads or by the loads that were actually in operation. The examples given included primary and backup pumps and motors that are on VFDs and not intended to run at full speed. Again, a given example was could an NEC connected load of 47kVA be put on a 45kVA transformer or would a 75kVA be needed.

 

[mehdi1351]

Gents,
Many thanks for the time answering my question. It was much appreciated.
Another question I have is:
Does the future solar energy makes any effects on the stepping down transformer which I am going to buy now?
I am guessing it may required some more overheating protection, BUT what about the sizing? Should I consider any precautions while I selecting the transformer while there is an opportunity to add Solar energy into the plant?

 

[kwired]

The heating and cooling thing I get and that is in the NEC. But we get no other major diversity in the NEC. Everything we do is over sized. I know pretty much every service I design is usually way over sized.
I do a lot of pharmaceutical work where the heat is used for dehumidification of the space, so the heat and cool will run at the same time. But typically I am not feeding that from a dry type transformer.

I dare you to submit a set of calculations for permit where you have, lets say, 47KVA connected load on a 45Kva transformer. AHJ will reject that.

I've been around dairy processing plants or similar since ~1988 when I was a very green apprentice. There are normal production loads as well as some loads that never run during production - particularly for cleaning pipelines, vessels, etc. No good reason to count any that will never run at same time toward a total load on a feeder or service. Yes there are some production pumps that certainly do run while washing to move cleaning water through the lines, you do have to consider those. But one example is a dryer in a particular plant that has like 200 to 250 HP worth of motors that most are blowers and only one is a feed pump bringing liquid product to the dryer. absolutely none of those ever run when washing the dryer. A few smaller loads for handling the dried product at end of the process never run while washing either. Maybe only 4 or 6 fractional HP motors there.

The cleaning system generally has it's own supply and possibly multiple return pumps. Sometimes a product pump in a line being cleaned gets run during cleaning. But in general we kind of don't even do feeder load calculations with the cleaning loads unless the feeder itself would be mostly limited to cleaning loads as the cleaning load is almost always way less than the production load.

Same plant I was talking about also had an evaporator for condensing product - a 250 HP blower motor was the biggest load on that thing and next biggest loads were all 15 HP or less but probably only half dozen or so that run during production. That 250 HP never run during cleaning operations either some the other pumps maybe did along with cleaning system pumps, probably one supply pump and one return pump each likely being 10 HP or less.

 

[Knightryder12]

I did not talk about whether the NEC load calculations are conservative or not. I was discussing whether a transformer was sized per the NEC connected loads or by the loads that were actually in operation. The examples given included primary and backup pumps and motors that are on VFDs and not intended to run at full speed. Again, a given example was could an NEC connected load of 47kVA be put on a 45kVA transformer or would a 75kVA be needed.

And what happens if said VFD's have a by-pass on them?

 

[jim dungar]

And what happens if said VFD's have a by-pass on them?

Then you need to size it based on the full voltage bypass, assuming that is a probable actual load. Specs often call for full voltage bypass but many processes are never intended to run at full speed, otherwise a VFD may not have been required in the first place.