# Overloading transformers



## guitarboyled (Jun 22, 2009)

Do transformers have some form of safety factor?
Let’s say my total load is 77 kVA, can I still use a 75 kVA tranny or do I have to move up to a 112.5 kVA?


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## jza (Oct 31, 2009)

125%. 

26-240 to 26-268.


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## guitarboyled (Jun 22, 2009)

Does it also apply to the NEC

Is it a good pratice? Don't transformers last less longer when they are constantly overloaded?


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## micromind (Aug 11, 2007)

It depends on the type of Xfmr; a dry type cannot take much of an overload for very long while an oil filled one can be grossly overloaded for quite some time. 

Ambient temperature makes a difference too. 

Running any transformer at high winding temperature will shorten its life, just as it will with motors.


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## Meadow (Jan 14, 2011)

Is this load temporary or permanent? Dry or oil filled? How much life do you want out of the unit?


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## guitarboyled (Jun 22, 2009)

meadow said:


> Is this load temporary or permanent? Dry or oil filled? How much life do you want out of the unit?


Are oïl filled much more expensive than dry?

The load is permanent from 8 to 5 daily


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## Big John (May 23, 2010)

Transformer capacity is determined by heat dissipation. If you can keep the transformer cool enough, you can overload the heck out of it. It's why you often see multiple load ratings on one distribution transformer, depending on how many methods are being combined together to remove heat.

Putting 77kVA continuous isn't even a 3% overload; it strikes me as unlikely to do much damage. But the rule of thumb is that for every 10°C above the maximum hotspot temperature, the insulation service life is reduced by 50%. 

That said, IEEE C57 dictates that dry-type transformers are designed with intermittent overload curves. They have to be designed to handle a certain percentage more than their nameplate capacity for so many minutes without winding insulation damage.


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## guitarboyled (Jun 22, 2009)

I'm guessing there would be a voltage drop on the secondary?


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## EB Electric (Feb 8, 2013)

Overheating the windings and constantly overloading your tx will brake down the insulation on your windings. This is not good obviously. Usually in oil filled xfmr's an oil sample can detect dissolved gases from off gassing from faults and provide a fairly accurate picture on the condition of the transformer. More to the tune of your question on sizing, you do not want to undersize the tx as it will only supply so much current. If it is being overloaded with more load than it is capable of handling, based on your kVA or MVA rating, not only will the equipment be subject to the possibility of damage, the efficiency and power factor of your transformer will suffer. Keep in mind that transformers operate most efficiently at full load! This goes the same with motors too, they operate on the same principle really. Sometimes salesman will try and up sell a custome. For example say there is a grain elevator with a 75 hp 600v motor to drive the auger. It gets replaced or whatever they want a new motor irrelavent.... the salesman decides he has more margins and better profit on a slightly bigger 100HP motor.....sounds bigger and better right. However, if it was sized and spec'd for a 75hp (FLA roughly 75A) which was drawing for fun say 65A, and then the salesman sells them a 100HP instead just to make some more commission, the motor will not run anywhere close to the 100A of full load current. Sure the motor is capable of handling more load, but it is going to run only half load its entire life , at lower efficiency and a poorer power factor than if the proper sized 75HP was put in. In the end it only costs the customer more. The mailman could deliver mail in 6.0l v8 chev 2500hd if he wants to, but obviously its not going to be as efficient on fuel as a small say ford focus which would be more then adequate to transport 100 envelopes. Same idea. Sorry for the rambling.


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## Paulusgnome (Mar 28, 2009)

Two thoughts : 1) while loading to just a hair over 100% is unlikely to kill the transformer, it will run warm and so a good supply of cooling air would seem prudent; 2) If you don't already have the transformer, can you see if one is available with a temp sensor embedded in the windings? This would give you the means to close-monitor the transformer temp and perhaps shunt-trip the breaker if the temp started getting too high.


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## Meadow (Jan 14, 2011)

guitarboyled said:


> Are oïl filled much more expensive than dry?
> 
> The load is permanent from 8 to 5 daily



Depending on the application. Under 600 volts dry type is cheaper and the norm. Since the load is daily I would just round up. Unlike oil units that use the oil as a thermal buffer dry types are poor and have little if any over load capability.


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## Big John (May 23, 2010)

guitarboyled said:


> ...The load is permanent from 8 to 5 daily


 This table lists values from the overload curve I mentioned in my previous post. It's from IEEE "Guide for Loading Dry Type Distribution and Power Transformers":


There are other tables, I just assumed you had a 150° rise transformer and picked worst-case 40° ambient. To be able to tolerate an 8 hour overload of 103%, the continuous load immediately prior to and after your overload condition can not be greater than 70%.

Personally, I hesitated to post this, because I think it's a poor design to push a regular transformer right to the hilt instead of just buying the next size up. I don't know about your codes, but here it would probably be a 110.3 violation even if it may be technically acceptable.


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## dmxtothemax (Jun 15, 2010)

guitarboyled said:


> Do transformers have some form of safety factor?
> Let’s say my total load is 77 kVA, can I still use a 75 kVA tranny or do I have to move up to a 112.5 kVA?


 
If the load is solid and constant ?
Then I would go for the next size up !
If its just occasional short term peaks then you could get away with 75.
It all comes down to expected life span !


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