TRANSFORMER LOAD

[TMMB]

PLAESE CAN HELPING ME , I KNOW THAT ANY TRANSFORMER LOAD MUST BE 80% OF ITS RATING , IS THAT BASED ON NEC? WHICH ARTICLE?

i mean if i have 1000KVA transformer, then the maximum load must be 800KVA max.

Please advise

 

[iwire]

The NEC maximum allowable continuous load on a transformer is 100% of its rating.

The NEC will allow a 1000kVA transfomer to be loaded 24/7 at 1000kVA

 

[TMMB]

can you tell me which article no. said that, please advise

 

[iwire]

can you tell me which article no. said that, please advise

No, I can not but more importantly no one can point to an NEC article limiting a transformer to 80%.

The '80% rules' have to do with over current devices and conductors but none have to do with transformers.

Keep in mind 80% is the inverse of 125%

II. Branch-Circuit Ratings
210.19 Conductors — Minimum Ampacity and Size.
(A) Branch Circuits Not More Than 600 Volts.
(1) General. Branch-circuit conductors shall have an ampacity
not less than the maximum load to be served. Where
a branch circuit supplies continuous loads or any combination
of continuous and noncontinuous loads, the minimum
branch-circuit conductor size, before the application of any
adjustment or correction factors, shall have an allowable ampacity
not less than the noncontinuous load plus 125 percent
of the continuous load.

Exception: If the assembly, including the overcurrent devices
protecting the branch circuit(s), is listed for operation
at 100 percent of its rating, the allowable ampacity of the
branch circuit conductors shall be permitted to be not less
than the sum of the continuous load plus the noncontinuous
load.

215.2 Minimum Rating and Size.
(A) Feeders Not More Than 600 Volts.
(1) General. Feeder conductors shall have an ampacity not
less than required to supply the load as calculated in Parts
III, IV, and V of Article 220. The minimum feeder-circuit
conductor size, before the application of any adjustment or
correction factors, shall have an allowable ampacity not
less than the noncontinuous load plus 125 percent of the
continuous load.

215.3 Overcurrent Protection. Feeders shall be protected
against overcurrent in accordance with the provisions of
Part I of Article 240. Where a feeder supplies continuous
loads or any combination of continuous and noncontinuous
loads, the rating of the overcurrent device shall not be less
than the noncontinuous load plus 125 percent of the continuous
load.

230.42 Minimum Size and Rating.
(A) General. The ampacity of the service-entrance conductors
before the application of any adjustment or correction
factors shall not be less than either 230.42(A)(1) or
(A)(2). Loads shall be determined in accordance with Part
III, IV, or V of Article 220, as applicable. Ampacity shall be
determined from 310.15. The maximum allowable current
of busways shall be that value for which the busway has
been listed or labeled.

(1) The sum of the noncontinuous loads plus 125 percent
of continuous loads

 

[ceb58]

can you tell me which article no. said that, please advise

It has been said here many times. The NEC is a permissive document. Unless it states you cannot then you can.(baring local amendments)
Your mfg. instruction is what you would go by, if it states 100% continuous load then that is what it is. If the mfg. instruction states to load it to 80% and you load it to 85% then you would fall into a code violation of 110.3 (B) for not following mfg. instruction.

 

[Dennis Alwon]

I agree with the others. A transformer is designed to be used at 100%. I have never seen anything in the NEC that says otherwise.

 

[infinity]

In general, isn't the Kva rating of a transformer it's maximum continuous load or is that number derived in some other way?

 

[jim dungar]

In general, isn't the Kva rating of a transformer it's maximum continuous load or is that number derived in some other way?

Transformer loading is dependent on heat transfer. There are many transformers that are designed with either fan cooling or 'low heat rise' which allow them to be run, continuously, above its nominal kVA rating.

For example; a transformer built with 150?C insulation may have a nameplate rating based on an 80?C temperature rise. If you do not mind the extra heat output, these transformers can often be run with a continuous 30% overload above their nameplate.

 

[kwired]

If transformer rated output was 100 amps that means if 125% overcurrent protection is needed then you could supply 100 amp continuous load protected by 125 amp overcurrent device.

 

[weressl]

Transformer loading is dependent on heat transfer. There are many transformers that are designed with either fan cooling or 'low heat rise' which allow them to be run, continuously, above its nominal kVA rating.

For example; a transformer built with 150?C insulation may have a nameplate rating based on an 80?C temperature rise. If you do not mind the extra heat output, these transformers can often be run with a continuous 30% overload above their nameplate.

...and if you don't mind shortening the transformers life.

 

[jim dungar]

...and if you don't mind shortening the transformers life.

Transformer life may not be affected if there is sufficient cooling between the periods of overloading, or if the ambient temperature is low enough.

Another problem with overloading a transformer is the increase in losses (i.e. a 110% overload can yield 121% of conductor losses).

 

[templdl]

Transformer life may not be affected if there is sufficient cooling between the periods of overloading, or if the ambient temperature is low enough.

Another problem with overloading a transformer is the increase in losses (i.e. a 110% overload can yield 121% of conductor losses).

Jim, you?re pretty sharp on this stuff. What's your opinion on this?
Transformer life is relative. Would you agree that if it were based upon a fully loaded transformer 27/7 at 30degC ambient it may be 5-7 years or so? At and average loading a 30% it may last for a very long time, 20, 30 years would be a wild guess?
Then there is the subject off "overloading" a transformer. The transformer conductors are based upon the transformer rating. To overload the transformer would mean overloading those conductors. The transformer NP tells a lot. A DTDT with an insulation class of 220deg with a 150degC rise really doesn't have any head room as an AA rated transformer loaded at 100% at its rating. But, there are transformer damage curves often available. As an example I have a damage curve in my file that shows that a 300kva transformer that is normally loaded at 50% (150kva) and want to overload it by 50% (450kva) I can operate it for 1-3/4 hours before returning is to the 150kva. This can be done once per day w/o damage to the transformer. This curve that I have reference to is not a publish curve that I am aware of but one as provided to me by a transformer design engineer.
Then you have the 115degC and 80degC rise transformers that provided additional head room. Should the transformer conductor be sized sufficiently a 115degC rise transformer has an additional capacity of 15% of its rating and the 80degC an additional 30% which brings if equal to a 150degC rise without exceeding the insulation class based upon 40degC ambient and a 30degC winding hotspot allowance. As such overload an 80degC transformer 50% would be a lot more forgiving than a 150degC transformer. and of course if the transformer ahde a NP with FFA it would have been designed for adding fans for an additional 30% capacity.

Understanding that you start out with a 40degC ambient +150degC rise + 30degC hotspot allowance+220degC insulation class knowing that you have common ambient temperatures of 25-30degC provides some forgiveness when overloading a dry type transformer.

But the fact still remains are you overloading the bus and/or conductors? What about the Pri and Sec OCPDs?

As such all of this may be considered when weighing the issues of overloading a transformer.

 

[jim dungar]

Would you agree that if it were based upon a fully loaded transformer 27/7 at 30degC ambient it may be 5-7 years or so? At and average loading a 30% it may last for a very long time, 20, 30 years would be a wild guess?

Yep.

I remember something put out by Sorgel Transformers in the mid-late 70's (when they were promoting 80?C temperature rise) that had the 'half-life' of a transformer as being 3-1/2 years at maximum temperature.

But the fact still remains are you overloading the bus and/or conductors? What about the Pri and Sec OCPDs?
As such all of this may be considered when weighing the issues of overloading a transformer.

You are correct, the loading of the transformer is only one item, which must not be considered independent of other factors.

 

[weressl]

Transformer life may not be affected if there is sufficient cooling between the periods of overloading, or if the ambient temperature is low enough.

That is incorrectly stated. Thermal damage is not reversible and it is cummulative.

The difference between two identically sized, but different construction transformers - less copper, less iron, etc. - would be the total mass involved in the thermal accumulation and disspation, given identical insulation systems. An example would be two sets of coils, one immersed in oil the other is relying air convection. Given the same overloads that slope of the temperature rise in the case of air cooled transformer would be steeper and would reach thermal equilibrium quicker. In cyclic loading thermal equilibrium is not necessarily occurs, but in general the air cooled transformer would experience higher temperatures than the liquid cooled one.

Magnitude of temperature is one determinant and the other one is I^2t a way to measure cummulative damaging effects. (I^2 is the more influental in the formula.) In protective formulas that do not tuse the actualthermal modle of the equipment, the I^3t or even the I^4t element helps to resolve the thermal latency of the mass.

Since transfomers are not subject to the same violent overloading conditions as single motors the practice of installing RTD's in the windings does not offer same level of close thermal profiling as with motors.

 

[jim dungar]

That is incorrectly stated. Thermal damage is not reversible and it is cummulative.

Yes the thermal constant of the transformer is another issue that must be addressed.

My statement was a simply an interpretation of the IEEE Red book section 10.4.3: "Transformers have certain overload capabilities, varying with ambient temperature, preloading, and overload duration."

 

[Hv&Lv]

That is incorrectly stated. Thermal damage is not reversible and it is cummulative.

The difference between two identically sized, but different construction transformers - less copper, less iron, etc. - would be the total mass involved in the thermal accumulation and disspation, given identical insulation systems. An example would be two sets of coils, one immersed in oil the other is relying air convection. Given the same overloads that slope of the temperature rise in the case of air cooled transformer would be steeper and would reach thermal equilibrium quicker. In cyclic loading thermal equilibrium is not necessarily occurs, but in general the air cooled transformer would experience higher temperatures than the liquid cooled one.

Magnitude of temperature is one determinant and the other one is I^2t a way to measure cummulative damaging effects. (I^2 is the more influental in the formula.) In protective formulas that do not tuse the actualthermal modle of the equipment, the I^3t or even the I^4t element helps to resolve the thermal latency of the mass.

Since transfomers are not subject to the same violent overloading conditions as single motors the practice of installing RTD's in the windings does not offer same level of close thermal profiling as with motors.

I don't mean to sidetrack this thread, but can you tell me anything about the levels where voltage regulation in a transformer will start to fall off? I have been trying to find some charts showing the loading versus voltage drop in an XF. I have seen many 25 kVa XF's doubled for a couple of hours and do fine, and even close to triple the nameplate rating before the voltage will drop significantly.

 

[weressl]

Yes the thermal constant of the transformer is another issue that must be addressed.

It is not 'another issue' but fundamental element of the thermal design, which is the determinant of the transformers life expectancy.

 

[hurk27]

I don't mean to sidetrack this thread, but can you tell me anything about the levels where voltage regulation in a transformer will start to fall off? I have been trying to find some charts showing the loading versus voltage drop in an XF. I have seen many 25 kVa XF's doubled for a couple of hours and do fine, and even close to triple the nameplate rating before the voltage will drop significantly.

You think thats bad, I have a 10kva at a trailer park, it's in the old potable water treatment plant which is fed via a 240 volt grounded "B" delta which the transformer is used for 120/240 volt loads, well after they got city water, the only thing using this little transformer is the office and plant lights, office was added to it, as was several large street lights over the years, it now has a continuous load of over 120 amps, on a transformer rated barely 41.6, you could fry an egg on it, I gave them a price on installing a 200 amp single phase service and eliminate the 240 delta as it is no longer needed, but never heard back, they say that transformer has been in service over 30 years.

 

[T.M.Haja Sahib]

You think thats bad, I have a 10kva at a trailer park, it's in the old potable water treatment plant which is fed via a 240 volt grounded "B" delta which the transformer is used for 120/240 volt loads, well after they got city water, the only thing using this little transformer is the office and plant lights, office was added to it, as was several large street lights over the years, it now has a continuous load of over 120 amps, on a transformer rated barely 41.6, you could fry an egg on it, I gave them a price on installing a 200 amp single phase service and eliminate the 240 delta as it is no longer needed, but never heard back, they say that transformer has been in service over 30 years.

That transformer with that much over load may have only five or six years at most remaining for its survival,

Here is a thumb rule:

(Transformer actual life)/(Transformer rated life )=Square of Transformer rated current/Square of transformer actual current.

Let the subject 10 KVA transformer have a rated life of, say,45 years.Then using the above thumb rule for the present loading of 120 amps,the actual life of the transformer is 45x0.12=5.4years only.