Running Transformer at 70% Capacity?

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Machiavelli999

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I have hooked up some equipment to a 112.5KVA transformer and the load is turning out to be more than I expected. On a 112.5KVA transformer, we are measuring an average load of 76KVA and a peak load of up to 85KVA.

Everything is working fine but how concerned should I be at running the transformer at 70-75% of its rated capacity? Any thoughts?
 
Machiavelli999 said:
I have hooked up some equipment to a 112.5KVA transformer and the load is turning out to be more than I expected. On a 112.5KVA transformer, we are measuring an average load of 76KVA and a peak load of up to 85KVA.

Everything is working fine but how concerned should I be at running the transformer at 70-75% of its rated capacity? Any thoughts?
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I would not be worried until I started running it at 110% or so. At 75%, it will probably barely be warm. if you can hold your hand on it, it is fine.
 
Machiavelli999 said:
I have hooked up some equipment to a 112.5KVA transformer and the load is turning out to be more than I expected. On a 112.5KVA transformer, we are measuring an average load of 76KVA and a peak load of up to 85KVA.

Everything is working fine but how concerned should I be at running the transformer at 70-75% of its rated capacity? Any thoughts?
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Unless the "some equipment" generates high harmonic content current. Is the xfm hot to touch?

ice
 
don_resqcapt19 said:
Even if you can't it may be fine. The surface temperature of some transformers can exceed 140?F when fully loaded.
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I must admit I am curious just how oversized the OP thinks a transformer ought to be.

I am a fan of building in some room to grow but a transformer that is only 70% loaded has almost 50% spare capacity.
 
petersonra said:
I must admit I am curious just how oversized the OP thinks a transformer ought to be.

I am a fan of building in some room to grow but a transformer that is only 70% loaded has almost 50% spare capacity.
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Interesting comment. I can't comment on the OP's thoughts. However, I do exclusively industrial applications and I would say the initial loading is entirely design dependent.

Double ended subs are never over 50%, unless there is a loadshed scheme - which is really rare. Normal every day dry types up to 500KVA are rarely loaded over 80%. Special stuff, like 100kva 480/277 grounded, or 225kva (or smaller) 208/120 are rarely loaded over 75%.

So, I must admit I am curious, just how undersize do you think a transformer ought to be? :)

ice
 
iceworm said:
It.

Double ended subs are never over 50%, unless there is a loadshed scheme - which is really rare. Normal every day dry types up to 500KVA are rarely loaded over 80%. Special stuff, like 100kva 480/277 grounded, or 225kva (or smaller) 208/120 are rarely loaded over 75%.



ice
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Your application is design dependent loading above 50% would defeat your redundancy.

In Washington DC (and most places I have worked) a typical office building design has a transformers operating at or less than 30%
 
temp rating should be on nameplate

temp rating should be on nameplate

don_resqcapt19 said:
Even if you can't it may be fine. The surface temperature of some transformers can exceed 140?F when fully loaded.
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Nameplate will tell temp rise rating; it varies based on design. 150C (300F) is common design max temp for full load temp altho I hate to see it that high. Most xfmrs today use 220C insulation system. Most xfmrs of this size we sell probably sit at 140F idle.
 
Besoeker said:
I have a notion that Jim Dungar might not agree.
From memory, I think he came up with a figure of 30% loading being the target best efficiency point for some transformer directive/guide.
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Science not withstanding, my opinion does not matter.

The US government enacted a law (Public Law 109-58 of the Energy Policy of 2005) that required transformers after 2007, to be designed with specific minimum efficiencies determined based on: (1) no-load losses at a temperature of 25?C and (2) load losses at a temperature of 75?C and 35 per cent of nameplate load.

The industry refers to these transformers as being per NEMA TP-1
 
jim dungar said:
Science not withstanding, my opinion does not matter.

The US government enacted a law (Public Law 109-58 of the Energy Policy of 2005) that required transformers after 2007, to be designed with specific minimum efficiencies determined based on: (1) no-load losses at a temperature of 25?C and (2) load losses at a temperature of 75?C and 35 per cent of nameplate load.

The industry refers to these transformers as being per NEMA TP-1
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Yes. I should have checked then I wouldn't have dropped the 30% goolie.
At least I got your name right.
Does that go some way to mitigating my gaffe?
:)
 
jim dungar said:
Science not withstanding, my opinion does not matter.

The US government enacted a law (Public Law 109-58 of the Energy Policy of 2005) that required transformers after 2007, to be designed with specific minimum efficiencies determined based on: (1) no-load losses at a temperature of 25?C and (2) load losses at a temperature of 75?C and 35 per cent of nameplate load.

The industry refers to these transformers as being per NEMA TP-1
Click to expand...

But these are newer transformers?
 
jim dungar said:
Science not withstanding, my opinion does not matter.

The US government enacted a law (Public Law 109-58 of the Energy Policy of 2005) that required transformers after 2007, to be designed with specific minimum efficiencies determined based on: (1) no-load losses at a temperature of 25?C and (2) load losses at a temperature of 75?C and 35 per cent of nameplate load.

The industry refers to these transformers as being per NEMA TP-1
Click to expand...

of course general purpose distribution xfmrs built after 1/1/2007 must meet these specs, but it does not change physics: it is seldom that MAX efficiency happens at such a low 35% load level. so do not think 35% load level is now the default highest efficiency point - it is not. also seldom does max efficiency happen at 100% load - most of the time is is around 75-80% load. but the difference is seldom more than a couple % at most from no load to full load so of little concern.
 
Besoeker said:
Yes. I should have checked then I wouldn't have dropped the 30% goolie.
At least I got your name right.
Does that go some way to mitigating my gaffe?
:)
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30-35% who's counting. The real point is that it is a federal law, not a matter of an ideal world, designating the design criteria.
 
jim dungar said:
The real point is that it is a federal law, not a matter of an ideal world, designating the design criteria.
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But look at it, it just says that the efficiency at 35% load must meet some number ... and I'd bet that the TIME AVERAGED load on a general purpose transformer is in that range ...

It does not say (from what was written, I did not dig into the actual law) that the efficiency must be maximum at a 35% load.
 
iceworm said:
Interesting comment. I can't comment on the OP's thoughts. However, I do exclusively industrial applications and I would say the initial loading is entirely design dependent.

Double ended subs are never over 50%, unless there is a loadshed scheme - which is really rare. Normal every day dry types up to 500KVA are rarely loaded over 80%. Special stuff, like 100kva 480/277 grounded, or 225kva (or smaller) 208/120 are rarely loaded over 75%.

So, I must admit I am curious, just how undersize do you think a transformer ought to be? :)

ice
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I don't personally care how loaded it is. It is a design decision. It just seemed odd that the OP seemed to be worried that it was loaded at 70%. I would not give that a second thought.
 
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