Class 155 Insulation for transformers

[WastefulMiser]

I am wanting to install a 150kVA, 4160D:208Y/120, Al, dry-type transformer in a building. Code says I need Class 155 Insulation unless I enclose it in a 1 hour fire rated room. Is this custom or do transformer manufactures typically make this insulation class as a standard offering?

The transformer manufacture I was looking at (Hammond) apparently has Class 220 as a standard offering which I am assuming is worse than Class 155. Although the code says "or higher", but I figure the code is referring to a insulation which will not get hotter than Class 155 ...

Also, it says in Exception #2: "completely enclosed". I am also assuming that a NEMA 3R enclosure would satisfy this requirement, yes?

I always enjoy it when people post the code section they are referring to:

NEC 2008 450.21(B):
(B) Over 1121⁄2 kVA. Individual dry-type transformers of
more than 1121⁄2 kVA rating shall be installed in a transformer
room of fire-resistant construction. Unless specified
otherwise in this article, the term fire resistant means a
construction having a minimum fire rating of 1 hour.

Exception No. 1: Transformers with Class 155 or higher
insulation systems and separated from combustible material
by a fire-resistant, heat-insulating barrier or by not less
than 1.83 m (6 ft) horizontally and 3.7 m (12 ft) vertically.

Exception No. 2: Transformers with Class 155 or higher
insulation systems and completely enclosed except for ventilating
openings.

 

[ed downey]

I haven't fully looked into it but I had this same situation last week and I called the Engineer who is a friend and he told me that all standard transformers manufactured today fall under the exception and therefor edo not require the 1 hour rating of the room.
-Ed

 

[Mike01]

220 Degree C

220 Degree C

I may be incorrect on this but I believe all I should say most transformers are designed with 220Degree c insulation (low voltage transformers) now in your situtation you metioned 4160-208 I am not sure how the medium voltage is designed.

 

[WastefulMiser]

I haven't fully looked into it but I had this same situation last week and I called the Engineer who is a friend and he told me that all standard transformers manufactured today fall under the exception and therefor edo not require the 1 hour rating of the room.
-Ed

It would be wonderful if that was the case. Hammond only sales them in NEMA 3R (or 4 or 12) so I kind of guessed that they are intending for the transformers to be installed outdoors, but maybe they figured it was simpler to just make everything N3R and not worry about the indoor/outdoor applications.

From my research (Google) I obtained that Class 155 was better than Class 220 as it would cause the insulation to not get as hot. It is very possibly that I have it backwards ... I don't know. I figured I'd ask here first and call some transformer reps sometime next week.

Also, I enjoy when fellow members point out other / potential violations and experience with the subject matter.

 

[wireguru]

from federal pacific for their MV transformers

? Free-Standing Indoor Ventilated Enclosure
? Optional NEMA 3R Outdoor Enclosure
? Optional 80? C or 115? C Temperature Rise
? Standard Aluminum or Optional Copper Windings

 

[steve66]

The "Class" of insulation has nothing to do with how hot the transformer gets. The actual temperature of the transformer is determined by the "Temperature Rise".

So Class 220 insulation can get a lot hotter without damage than Class 155 insulation. Therefore, Class 220 is better than 155.

For Temp. Rise, an 115 deg rise would get hotter than a 80 deg rise, so the 80 deg rise would be better.

However, note that the temp. rise is not the hottest part of a transformer, so you can't directly compare the temp. class to the insulation class without doing some elaborate equations.

Steve

 

[jim dungar]

The "Class" of insulation has nothing to do with how hot the transformer gets. The actual temperature of the transformer is determined by the "Temperature Rise".

Absolutely.

The code wants the insulation to be able to get hot without starting on fire.

Perversely (?) the code does not limit how hot the transformer can get, which is why it has added restrictions on ventilation and 'fire-proof' construction.

 

[Mike01]

As stated earlier:

As stated earlier:

Transformers are designed with 220 oC insulation systems. The insulation system classification represents the maximum temperature permitted in the hottest spot in the winding when operated in a 40 oC maximum ambient. The hotspot temperature is determined by adding the maximum value for each of the following:
40degree C maximum ambient
+150degree C maximum average winding rise
+30degree C maximum hot spot in winding
=220degree C ultimate temperature at hot spot

The temperature rise commonly associated with transformers is the temperature of the conductor inside the coil and does not apply to the outside surface. The wiring compartment is ventilated and cooled. When 220 oC insulation is specified with 80degree C or 115degree C rise you will get better overloaded capability, better efficiency, and longer life. These transformers are designed to operate with a lower rise see the following example at 80degree C rise
40degree C maximum ambient
+ 80degree C maximum average winding rise
+ 30degree C maximum hot spot in winding
+ 30degree C thermal overload 30% (this will vary depending on 80degree c rise, 115degree c rise, or 150degree c rise)
= 220degree C ultimate temperature at hot spot

So the transformer if designed to NEMA / TP-1 standards should have 220degree insulation, the rest is just how it's specified for it's intended use.

 

[WastefulMiser]

The "Class" of insulation has nothing to do with how hot the transformer gets. The actual temperature of the transformer is determined by the "Temperature Rise".

So Class 220 insulation can get a lot hotter without damage than Class 155 insulation. Therefore, Class 220 is better than 155.

For Temp. Rise, an 115 deg rise would get hotter than a 80 deg rise, so the 80 deg rise would be better.

However, note that the temp. rise is not the hottest part of a transformer, so you can't directly compare the temp. class to the insulation class without doing some elaborate equations.

Steve

Couldn't you look at it as if a Class 155 Insulation cannot exceed 155*C and Class 220 Insulation cannot exceed 220*C? Thus Class 155 transformer will have an overall lower temperature as its insulation is not rated as high. I assume that's the intentions of the code: to keep the overall temperature down, eh? Or am I wrong all together in my thought process?

 

[WastefulMiser]

The temperature rise commonly associated with transformers is the temperature of the conductor inside the coil and does not apply to the outside surface. The wiring compartment is ventilated and cooled. When 220 oC insulation is specified with 80degree C or 115degree C rise you will get better overloaded capability, better efficiency, and longer life. These transformers are designed to operate with a lower rise see the following example at 80degree C rise
40degree C maximum ambient
+ 80degree C maximum average winding rise
+ 30degree C maximum hot spot in winding
+ 30degree C thermal overload 30% (this will vary depending on 80degree c rise, 115degree c rise, or 150degree c rise)
= 220degree C ultimate temperature at hot spot

So the transformer if designed to NEMA / TP-1 standards should have 220degree insulation, the rest is just how it's specified for it's intended use.

I don't know how that equals 220*C.

Hmm. So a transformer designed to NEMA / TP-1 standard must have minimum 220*C? I wonder why that conflicts with NEC which requires Class 155 (assuming Class 155 is a worse insulation).

 

[steve66]

Couldn't you look at it as if a Class 155 Insulation cannot exceed 155*C and Class 220 Insulation cannot exceed 220*C? Thus Class 155 transformer will have an overall lower temperature as its insulation is not rated as high. I assume that's the intentions of the code: to keep the overall temperature down, eh? Or am I wrong all together in my thought process?

No, that was my point. Let me try to explain it another way:

All other factors equal, a Class 155 and Class 220 transformer would both run at the same temperature. However, we would expect the Class 220 to last longer since the actual temp. of the transformer isn't as close to the insulation rating. The class 220 would also fair better in high ambient temps., and would probably handle overloads much better.

Steve

 

[WastefulMiser]

No, that was my point. Let me try to explain it another way:

All other factors equal, a Class 155 and Class 220 transformer would both run at the same temperature. However, we would expect the Class 220 to last longer since the actual temp. of the transformer isn't as close to the insulation rating. The class 220 would also fair better in high ambient temps., and would probably handle overloads much better.

Steve

Thank you for your patience. So in conclusion, a Class 220 transformer is acceptable to be installed indoors without being installed in a 1 hour fire rated room.

 

[steve66]

Thank you for your patience. So in conclusion, a Class 220 transformer is acceptable to be installed indoors without being installed in a 1 hour fire rated room.

Yes.

Steve