Transformer 5 amps over on one leg

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AC\DC

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So I calculated load on a circuit for a new air handler was within the 45kva of my transformer.
Close but it calculated out.
I put on my fluke amp logger on after I wired it up. Wanted to verify it.
I am 5 amps over on peak for one phase.
Average amp load is 25 amps. But that really does not matter.

Hvac installer pulled out 5kw of the heat strip. After I pulled off the logger and saw that
Doing a favor for the hvac installer, ended up backfiring on me.

Trans are sensitive to heat. Room is kept cool. But I don’t know how to justify for inspector that the reduction in heat will help transformer operate higher for short periods
 

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I would be surprised if anyone has a problem with it. You are less than 5% over the nameplate kva and the full load is likely short duration.
 
a 45 kVA transformer has a lot of metal in it. It's continuous high current that will overheat it.
 
How you justify this to the inspector depends upon what sort of documentation is required.

General experience should be sufficient in this case. You have a momentary <5% overload followed by operation at less than 20% of normal rating. This will not be a problem. It wouldn't surprise me if some energy code somewhere required a _smaller_ transformer (which would have a greater transient overload) to improve overall efficiency.

If an explicit calculation is required, then the general approach is to calculate how quickly the transformer temperature rises during the overload (the rate of temperature change), and determine if the duration of the overload would cause an excessive temperature. You would need information on the duration of the startup overload and the number of starts per hour (worst case). Unloaded the transformer will be cool (not cold, because there are magnetization losses), and under normal 20% load the transformer will also be cool. Each time the load starts and the transformer gets overloaded, the temperature will go up a bit. Between starts the temperature would go down again. Have enough of an overload and you will notice the temperature change.


For comparison, motors of similar power rating, starting cold, can easily handle 2x rated power for tens of minutes at a time. During this 100% overload the motor is heating up, and after 10 or 20 minutes has reached the maximum rated insulation temperature.

The 5% overload for a few seconds on a 45kVA transformer? It would be difficult to even measure the temperature change.

-Jon
 
The safety of transformer was not really what I was worry about its dealing with inspector like winni pointed out.
Thanks guys.
So hypothetically there’s got to be a cut off point to the transformer overcurrent for let’s say start up on motors.
Or does sizing the breaker basically cover all situations of that??

But I guess it’s all temperate and longevity effected for overcurrent problems
 
AC\DC said:
The safety of transformer was not really what I was worry about its dealing with inspector like winni pointed out.
Thanks guys.
So hypothetically there’s got to be a cut off point to the transformer overcurrent for let’s say start up on motors.
Or does sizing the breaker basically cover all situations of that??

But I guess it’s all temperate and longevity effected for overcurrent problems
Click to expand...
Not exactly apples to apples, but think of the POCO transformers. How often do you see them sized to our NEC calculations for load? Would never come close to reality?
 
The OCPD requirements for transformers imply that a 125% continuous overload would be allowed by the breaker. I expect that if you really wanted to get up close and personal to 100% loading of a transformer that breakers would not be accurate enough.

IMHO if you follow article 220 calculation requirements to determine the load on the transformer, and the transformer is larger than the results of the calculation, you are well in the clear. (And likely the transformer will be significantly underutilized.)

-Jon
 
In principal you can energize a transformer with inrush well below the normal current rating, if you have proper circuit closure timing or use some sort of 'pre-charge' circuit. This is rarely (if ever) done because simply closing properly sized breakers and accepting the inrush commonly works.

But if you really have a problem with inrush know that there are ways around it.

-Jon
 
AC\DC said:
So I calculated load on a circuit for a new air handler was within the 45kva of my transformer.
Close but it calculated out.
I put on my fluke amp logger on after I wired it up. Wanted to verify it.
I am 5 amps over on peak for one phase.
Average amp load is 25 amps. But that really does not matter.

Hvac installer pulled out 5kw of the heat strip. After I pulled off the logger and saw that
Doing a favor for the hvac installer, ended up backfiring on me.

Trans are sensitive to heat. Room is kept cool. But I don’t know how to justify for inspector that the reduction in heat will help transformer operate higher for short periods
Click to expand...
My question is, how would the inspector know that you were 5% over during startup only? Will you have to show him the results of the load test that you did? Or do you just turn on the equipment and say, see Mr. Inspector, everything is running?
 
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