Low Temperature Rise Transformers
A standard ventilated transformer will be rated for a 150°C rise at full load with a 220°C insulation
system. Please note that smaller products including potted, control, and line reactors use insulation
systems with lower temperature ratings and have different temperature rises. For this discussion we
will concentrate on magnetics using a 220°C insulation system. The 220°C insulation system allows for
a 150°C rise, a 40°C ambient temperature and a 30°C hot spot. Lower temperature rise units maintain
the 220°C insulation system, but lower the temperature rise, typically to 115°C or 80°C. These units
have a longer life, lower Mean Time Between Failure because a lower temperature rise is easier on
insulation.
This extra capacity (these figures only apply to a 220°C insulation system) can be used as a safety
margin, can be available for future load additions, or used for periodic times of overload. A low
temperature rise unit, with no K-factor rating used on a highly harmonic load, can still have heating
problems. K-rated units with low temperature rises should be specified to get the full advantage if
harmonics are present.
There are three common methods manufacturers use to achieve lower temperature rise units:
• Better efficiency
• De-rating a larger unit
• Fan Cooling
One common misconception is that low temperature rise units are more efficient. While they usually
have better efficiency at full load, it doesn’t guarantee that this will be the case at lower loads.
Efficiency regulations typically use an average load of 35% to 50% when specifying efficiency. A low
temperature rise transformer with more mass and surface area, but running with a low load may be
less efficient and produce more heat than a standard transformer, while still maintaining a low overall
temperature rise. Core loss for low temperature rise units are higher than a transformer with the same
kVA rating, but a higher rise (present whenever unit is energized). Transformer temperature rise and
efficiency should be regarded as two separate issues. If a high efficiency unit is desired, the TP1 (DOE
10 CFR Part 431), C802.2 (Canada) or NEMA Premium® energy efficient specifications should be
noted. These regulations are all compatible with low temperature rise transformers.
Canada Sales Office: 595 Southgate Dr., Guelph, ON, Canada N1G 3W6
Toll free in Canada: 1-888-798-8882
USA Sales Office: 1100 Lake St., Baraboo, WI, USA 53913-2866
Toll free in USA: 1-866-705-4684
www.hammondpowersolutions.com
Transformer Insulation Systems
Insulation
Average Winding
Hot Spot
Maximum Winding
Rating
Temperature Rise Temperature Rise
Temperature
Class 105
55°C
65°C
105°C
Class 150 or 130
80°C
110°C
150°C
Class 180
115°C
145°C
180°C
Class 200
130°C
160°C
200°C
Class 220
150°C
180°C
220°C
NOTE: The maximum acceptable temperature rise based on an average ambient of 30°C during any 24 hour
period and a maximum ambient of 40°C at any time.
Canada Sales Office: 595 Southgate Dr., Guelph, ON, Canada N1G 3W6
Toll free in Canada: 1-888-798-8882
USA Sales Office: 1100 Lake St., Baraboo, WI, USA 53913-2866
Toll free in USA: 1-866-705-4684
www.hammondpowersolutions.com