Figure 4-1. Insulation System Ratings

UFC 3 -520-01

June 10, 2002

Figure 4-1. Insulation System Ratings

30C

Hot Spot

200 C

Allowance

30C

Hot Spot

Allowance

30C

Hot Spot

150 C

Allowance

Temperature

Rise

115 C

Temperature

10 Hot Spot

C

Rise

100 C

Allowance

80 C

Temperature

Rise

55 C

Temperature

Rise

Ambient

Temperature

Allowance

40 C

4 0 C

40 C

0 C

Class 105C

Class 150 C

Class 185 C

Class 220 C

4-1.3.3 The kVA rating and the insulation system rating are related. Select the

desired kVA rating or insulation s ystem based on the following considerations:

Relative loading--transformers loaded at or close to their kVA ratings will operate

hotter than transformers that are lightly loaded. A higher kVA rating can be selected

just to ensure that the transformer operates cooler to avoid long -term thermal

damage.

Duty cycle--the duty cycle might have the transformer fully loaded most of the time

or lightly loaded most of the time. The transformer kVA rating has to be capable of

supplying the system full -load current, but the capacity margin can be lower for

lightly loaded duty cycles.

Ambient temperature--the average and maximum ambient temperatures at the

installation location must be determined (or estimated) as part of the selection

process. If necessary, increase the kVA rating or insulation system class to reduce

the degree of thermal damage at higher temperatures.

4-1.3.4 Unless there are special application or environmental requirements,

transformers rated 15 kVA or greater should have a Class 220 insulation system.

Transformers rated less than 15 kVA should have a Class 185 insulation system.

4-3