Transformer Loading to 125%?

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I had a question with an electric engineer at my office today so I thought I would pose the question to the group for their thoughts.

The engineer is showing a new 150 KVA transformer and showing 170 demand kW on the panel schedule attached to this transformer. I state that he has overloaded the transformer and that you can't show more than 150 for the demand kW on that transformer. The engineer states that you are allowed to go up to 125% of the transformer rating and that we have not gone over the 600A OCPD for the panel so we are ok. I counter that this is something that utilities routinely do but they are not bound by the NEC but we are. We consult the NEC, specifically section 450 but neither of us can find anything relating to how much load we can show on the transformer.

As engineers, we both know that the transformer will probably see half of the 170 kW peak demand and that even if it is run to 125% (187,500 KVA) it will still operate.

So the question becomes, is this code legal and what codes are involved here?
 

petersonra

Senior Member
Location
Northern illinois
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engineer
There are requirements for primary and secondary protection, and primary and secondary conductor protection, but no requirements I am aware of that call for any particular size of the transformer itself. That would seem to make it a design decision.
 

GoldDigger

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There are requirements for primary and secondary protection, and primary and secondary conductor protection, but no requirements I am aware of that call for any particular size of the transformer itself. That would seem to make it a design decision.
If the situation were egregious enough that the OCPDs did not adequately protect the windings of the undersized transformer, I could see it being an NEC issue.
It depends on exactly how the transformer is specified in terms of allowed amps on primary and secondary, and how this related to the OCPD chosen.
 

templdl

Senior Member
Location
Wisconsin
How can you even address this issue when nothing is know about the transformer's nameplate rating nor what the ambient temperature is. You're shooting at shadows. These are necessary to have an idea of what the transformer's capabilities are. Even the duty cycle is not know, that is how long it is in overload and the magnitude, what the length of time and level of the reduced load. All of this affects transformer life.
One place where they kick the butt of dry type transformers is on oil drilling rigs where real estate is limited which courses then to max out transformer capacity. Transformer life often ends up to be very limited making it necessary to keep spares on hand to be flown in.
 

jumper

Senior Member
How can you even address this issue when nothing is know about the transformer's nameplate rating nor what the ambient temperature is. You're shooting at shadows. These are necessary to have an idea of what the transformer's capabilities are. Even the duty cycle is not know, that is how long it is in overload and the magnitude, what the length of time and level of the reduced load. All of this affects transformer life.
One place where they kick the butt of dry type transformers is on oil drilling rigs where real estate is limited which courses then to max out transformer capacity. Transformer life often ends up to be very limited making it necessary to keep spares on hand to be flown in.

The question is: Where does it state that I cannot kick the butt of a tranny and overmax it?
 

GoldDigger

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I would agree if I could find an adequate and definitive code reference......:(

Bob/iwire will now shortly show up and quote the section he thinks applies.:)
Let me try to beat iwire to the punch. Since this transformer is governed by NEC rather than NESC, I would start with 450.3 and in particular Table 450.3(B). I think that is where the engineer got his 125% from.

The OCPD is boxed in by two different requirements. One is that it has to be able to carry the calculated load, with some fudge factors, and the other that it be able to protect the secondary conductors of the transformer with another fudge factor (leaving out the whole mess of primary protection instead.)
Because the OCPD is in the middle between the two, this creates a relationship between calculated load and the transformer rating. But possibly with some slack in between.
But if the breaker has to carry 125% of the load and also protect at 125% of the transformer rating, there is no slack and the transformer cannot be undersized. (It can be oversized as much as you want.) :)


Going back to primary protection, if you use only primary side protection and the current is over 9 amps, you cannot go over 125% of the transformer rating on the primary side and the box relating load to transformer rating remains effectively the same, although there may be a little extra room from standard sizes in this case.

To get away with this you would have to argue that the main CB on the secondary side is just there to protect the panel and not to protect the transformer.
 
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petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Let me try to beat iwire to the punch. Since this transformer is governed by NEC rather than NESC, I would start with 450.3 and in particular Table 450.3(B). I think that is where the engineer got his 125% from.

The OCPD is boxed in by two different requirements. One is that it has to be able to carry the calculated load, with some fudge factors, and the other that it be able to protect the secondary conductors of the transformer with another fudge factor (leaving out the whole mess of primary protection instead.)
Because the OCPD is in the middle between the two, this creates a relationship between calculated load and the transformer rating. But possibly with some slack in between.
But if the breaker has to carry 125% of the load and also protect at 125% of the transformer rating, there is no slack and the transformer cannot be undersized. (It can be oversized as much as you want.) :)


Going back to primary protection, if you use only primary side protection and the current is over 9 amps, you cannot go over 125% of the transformer rating on the primary side and the box relating load to transformer rating remains effectively the same, although there may be a little extra room from standard sizes in this case.

To get away with this you would have to argue that the main CB on the secondary side is just there to protect the panel and not to protect the transformer.

where does it actually say this?

think about what it actually says and not what you think it says.
 

jumper

Senior Member
Let me try to beat iwire to the punch. Since this transformer is governed by NEC rather than NESC, I would start with 450.3 and in particular Table 450.3(B). I think that is where the engineer got his 125% from.

The OCPD is boxed in by two different requirements. One is that it has to be able to carry the calculated load, with some fudge factors, and the other that it be able to protect the secondary conductors of the transformer with another fudge factor (leaving out the whole mess of primary protection instead.)
Because the OCPD is in the middle between the two, this creates a relationship between calculated load and the transformer rating. But possibly with some slack in between.
But if the breaker has to carry 125% of the load and also protect at 125% of the transformer rating, there is no slack and the transformer cannot be undersized. (It can be oversized as much as you want.) :)


Going back to primary protection, if you use only primary side protection and the current is over 9 amps, you cannot go over 125% of the transformer rating on the primary side and the box relating load to transformer rating remains effectively the same, although there may be a little extra room from standard sizes in this case.

To get away with this you would have to argue that the main CB on the secondary side is just there to protect the panel and not to protect the transformer.

Gold, the primary OCPD has to protect the transformer and the conductors, show me where to size the tranny. Common sense and good wiring practices aside-you are not gonna find it.

As far as the secondary OCPD, it protects the conductors also.:)
 

mbrooke

Batteries Included
Location
United States
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Technician
Thermo-dynamics and transformer life trade off apply hear. The engineer may or may not know what he is doing. If the overload is brief and right after a light load period it may be fine without shortening the transformers life. As long as the maximum hot sopt temperature is not exceeded no life loss will occur. Of course the engineer may be choosing to overload for the trade off of life.

If this is an oil-filled transformer the overload can last a long time no problem (heat will be absorbed by the oil until it becomes as hot as the core/windings), and if fans are thrown in the overload can last indefinitely. However, with dry type units you are very limited (usually to minutes rather than hours), when it comes to overloads. The ocpds are more for conductor protection rather than winding protection, I have seen dry type units fail to a crisp without any breakers tripping.

Bigger question is, what type of load are you feeding?
 

GoldDigger

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Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Gold, the primary OCPD has to protect the transformer and the conductors, show me where to size the tranny. Common sense and good wiring practices aside-you are not gonna find it.

As far as the secondary OCPD, it protects the conductors also.:)

What I am saying is this: The secondary OCPD (or the primary OCPD too) has to protect the transformer. Agreed? That is in the table, as long as there is a secondary OCPD in the first place.

So if I undersize the transformer, I have to undersize the secondary OCPD too.
If that same OCPD is the main for the panel, it also has to be sized to carry the calculated load, so there is a limit on how small it can go.

I guess if you are splitting hairs, as we so often do for entertainment here, you could have a large main breaker in the panel and a smaller breaker next to the transformer and still be code compliant. But there would be a whole lot of nuisance trips. (As you say, common sense and not Code.)
This would be the same situation as if you undersized the tranny, used a small OCPD such that the secondary could not deliver the required load current and then brought it to a large main breaker in the panel.

What you cannot do is actually overwork the tranny. You can undersize it to the extent that the real load is less than the calculated load as long as you do not share the secondary OCPD and main breaker as the same device. If you assert that the main breaker is not acting as a secondary OCPD because none is required, I will would take even odds on whether the typical/average inspector would accept that. :)
If you provide primary protection only, then you can undersize the tranny to the extent that the real load is less than the calculated load.
 

templdl

Senior Member
Location
Wisconsin
The question is: Where does it state that I cannot kick the butt of a tranny and overmax it?

From what I have read from these posts so far there appears to be a lack of knowledge of the design for transformers. Since it is most likely that the transformers in question will be dry type distribution transformers.
You see the modern day transformer has coils that are wound with wire that has 220degC insulation. Since no nameplate information has been provided I will start a common transformer with a temperature rise of 150degC. The kva as a transformer is based open that 150degC + 40degC ambient + 30degC winding hot spot allowance = 220degC.
From that, what do you have to work with that would allow for you to overload the transformer?
You have that 40degC. Your ambient may be 25degC which would give you 15degC to work with.
At 40degC you would be allowed to load the transformer to 100%. At 50degC About 92%,
60degC 84% without exceeding the insulation rating.***
There are also 115degC and 80degC transformers that also have 220degC insulation. Since the 115degC rise KVA is based upon 115 + 40 + 30 = 185degC. Thus there is a 35degC margin which allows for a 15% overload.
Likewise an 80degC rise transformer has a 30% overload capacity.
In addition how is an overload defined? Would it be continuous are for some limited period of time? The shorter the overload the less time that there is to accumulate heat. The longer the over load the more cooking time at a lower load that is required.
There is nothing with regard to 125% load.
Should the NP of the transformer include the designation as AA/FFA that means the the transformer has been designed to add Fans for additional capacity. A transformers capacity can not be increased by simply adding fan unless it is designed as such.
If you have calculated you full load and no load amps, sized you conductors based upon that' then selected you OCPD to protect the cable, then sized the transformer to supply the load accordingly the transformer should be protected.
As such how are you going to beat up the transformer? What must take place in order to "kick its butt?"
mbrooke did address the transformer's design somewhat.
 

robbietan

Senior Member
Location
Antipolo City
I had a question with an electric engineer at my office today so I thought I would pose the question to the group for their thoughts.

The engineer is showing a new 150 KVA transformer and showing 170 demand kW on the panel schedule attached to this transformer. I state that he has overloaded the transformer and that you can't show more than 150 for the demand kW on that transformer. The engineer states that you are allowed to go up to 125% of the transformer rating and that we have not gone over the 600A OCPD for the panel so we are ok. I counter that this is something that utilities routinely do but they are not bound by the NEC but we are. We consult the NEC, specifically section 450 but neither of us can find anything relating to how much load we can show on the transformer.

As engineers, we both know that the transformer will probably see half of the 170 kW peak demand and that even if it is run to 125% (187,500 KVA) it will still operate.

So the question becomes, is this code legal and what codes are involved here?

we had a case where there was a demand of 501 kW placed on a 500 kVA transformer. lights flickered, sensitive UPS units were activated, tempers flared, utility personnel called in. no overheating as the transformer was in a very comfy air conditioned room. source of overload is the on/off cycle of a water pump motor.
 

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
we had a case where there was a demand of 501 kW placed on a 500 kVA transformer. lights flickered, sensitive UPS units were activated, tempers flared, utility personnel called in. no overheating as the transformer was in a very comfy air conditioned room. source of overload is the on/off cycle of a water pump motor.

But fortunately no code violation, right?
Otherwise it might have been really bad. :)
 

JoeStillman

Senior Member
Location
West Chester, PA
The question I would ask is; how did he come up with 170 kW? Assuming the secondary voltage is 208 3?, this is suspiciously close to exactly 80% of the 600A panel rating. Could it be an arbitrary number? :?

It's hard to prove there is nothing in The Code that says how to size transformers, but I've been looking since the 1978 edition and haven't found it yet. All we know is how to protect them (article 450).
 
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