delectric123
Senior Member
- Location
- South Dakota
if the load consists mostly of non-linear loads, there is no way the transformer will be capable of 170 KW without overheating.
Transformer Loading to 125%?
- Thread starter thecrazyezel
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thecrazyezel
Member
Couple things here. We are talking about a dry-type transformer and the 170 kW is the calculated demand load on a panel connected to the secondary side. From reading manufacturer product installation sheets, it is clear that a 150 kVA transformer is designed and UL listed to carry a maximum continuous load of 150 kVA all day and all night as its insulation is rated to do so provided it is in a properly ventilated area. That is not to say that it can't be overloaded but everything I have found states that overloading a transformer is a) not a good idea and b) only good for a short amount of time before excessive damage is done to the transformer's insulation.
So here is my take. I agree with iwire and 110.3 (B) because it jives with what the manufacturer is saying it can do and what UL says it is listed to do. For the primary and secondary OCPD, you follow the transformer table and as long as your breakers are in line you are good to go. As far as overloading the transformer is concerned, I think it is code legal but not good design practice from an engineering standpoint because you are going to ruin the insulation. I also see a calculated load of 170 kW much more differently than an actual load of 170 kW. If it was an actual load of 170 kW I would put my foot down and say no-way. If it is a calculated load, then I am a lot more lenient because I know the actual load is probably 50 - 67% of the calculated load and all we are doing is loading up the transformer to run at a higher efficiency which is a good design practice.
Thoughts?
So here is my take. I agree with iwire and 110.3 (B) because it jives with what the manufacturer is saying it can do and what UL says it is listed to do. For the primary and secondary OCPD, you follow the transformer table and as long as your breakers are in line you are good to go. As far as overloading the transformer is concerned, I think it is code legal but not good design practice from an engineering standpoint because you are going to ruin the insulation. I also see a calculated load of 170 kW much more differently than an actual load of 170 kW. If it was an actual load of 170 kW I would put my foot down and say no-way. If it is a calculated load, then I am a lot more lenient because I know the actual load is probably 50 - 67% of the calculated load and all we are doing is loading up the transformer to run at a higher efficiency which is a good design practice.
Thoughts?
mike_kilroy
Senior Member
- Location
- United States
I agree it comes down to this question. And I think it is easy to answer: ASK THE XFMR MFGR.
We sell xfmrs. We will CLEARLY tell you that if you run 170kva load on a 150kva xfmr for over 3 thermal time constants, the xfmr is allowed to overheat and fail. Nature of the beast. As much as we would like to, we are not in the business of giving away free capacity - it would bankrupt us. If you buy a 170kva rated xfmr from us we will make it so it can run 170kva all day long, 24/7. YOU may believe no one will ever REALLY load it this high, but that is not a valid argument for undersizing it this way IMHO.
I would also be concerned about the actual load balance involved in the 170 kw figure.
A 150 transformer would normally indicate 3 phase. How are the loads distributed ? More than likely not evenly so you will end up with more load on one set of windings.
A 150 transformer would normally indicate 3 phase. How are the loads distributed ? More than likely not evenly so you will end up with more load on one set of windings.
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mike_kilroy
Senior Member
- Location
- United States
right! load is listed @170kw, xfmr is rated 150kva.... since kw=kva*power factor, is it possible the power factor is anywhere near 1.0? Probably not. So the ACTUAL KVA xfmr size required for 170kw is PROBABLY closer to 200-225kva anyway!
also some 'hope' may have been given by a previous post showing some xfmrs are built with as low as 80c rise so can be overloaded by 30%. So true, BUT NOT IN 150KVA SIZES! we build custom xfmrs. I cannot recall in 33 years of working with them ever supplying a 150kva xfmr with this low rise. the additional cost would be horrendous. the low rise is done to keep the xfmr COOL for some application reason. these xfmrs are mostly in <3KVA sizes like control xfmrs in packed machine tool cabinets. imagine trying to dissipate that 3% loss at 150kva (swag 5000 watts) in that 2'x3'x3' transformer! You can get there on a small 6"x6"x6" unit by making surfaces larger to convect it away, but bet you could not get there on a 150kva size unit - unless its size became 20'x20'x30'!
also some 'hope' may have been given by a previous post showing some xfmrs are built with as low as 80c rise so can be overloaded by 30%. So true, BUT NOT IN 150KVA SIZES! we build custom xfmrs. I cannot recall in 33 years of working with them ever supplying a 150kva xfmr with this low rise. the additional cost would be horrendous. the low rise is done to keep the xfmr COOL for some application reason. these xfmrs are mostly in <3KVA sizes like control xfmrs in packed machine tool cabinets. imagine trying to dissipate that 3% loss at 150kva (swag 5000 watts) in that 2'x3'x3' transformer! You can get there on a small 6"x6"x6" unit by making surfaces larger to convect it away, but bet you could not get there on a 150kva size unit - unless its size became 20'x20'x30'!
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- Location
- Wisconsin
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- PE (Retired) - Power Systems
And yet it seems every major manufacturer of low voltage general purpose dry type transformers lists 80?C rise units in their catalog as standard up to 500kVA. Although none of them appear to be marketed for their increased capacity.
For example 150kVA 480-208Y/120: Schneider Electric EE150T3HB, General Electric 9T40G0006G82, and Jefferson Electric 423-7264-085
heavyaslead
Member
XFR load design
XFR load design
There are many ways to 'design' transformer loads and much depends on reactance of the loads and diversity.
The questioner states 170kW; this is odd to me as an engineer as most all my demand calcs are in kVA
The transformer (depending on manufacturer) can routinelly be loaded in excess of its rating even up to 150% for some short peroid of time before damage is possible.
The questioner states the load is demand, which means this will be more or less a continuous expected load with diversity figured in.
We actually need more transformer data and the type loads served to make a proper assessment of wether this will work or not.
I would be interested to see what the connected load is in relation to the demand.
XFR load design
There are many ways to 'design' transformer loads and much depends on reactance of the loads and diversity.
The questioner states 170kW; this is odd to me as an engineer as most all my demand calcs are in kVA
The transformer (depending on manufacturer) can routinelly be loaded in excess of its rating even up to 150% for some short peroid of time before damage is possible.
The questioner states the load is demand, which means this will be more or less a continuous expected load with diversity figured in.
We actually need more transformer data and the type loads served to make a proper assessment of wether this will work or not.
I would be interested to see what the connected load is in relation to the demand.
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