TechTopics No. 09

Heat generation estimation for type GM38 switchgear (up to 38 kV)

We are often asked to provide estimated heat generation data for our equipment. This issue of TechTopics provides information that allows calculation of approximate heat generated by the type GM38 switchgear under assumed loading conditions.


The heat generation data given in the table is based on fullrated continuous current. Actual heat generation calculations must take into account the true loading of the equipment.


The amount of heat generated is related to the square of the current, so a circuit breaker operating at one-half rated current will have heat generation only one-quarter of that at full-rated continuous current. Because the effect of the square relationship is very significant, it is overly conservative to estimate heat generation based on the assumption that all sections and all circuit breakers each carry their rated continuous current at all times. Air conditioning systems sized based upon such estimates will be much larger than the real operating conditions will require.

Table 1: Approximate full load heat generation (in watts (W)) for type GM38 switchgear (up to 38 kV)
Continuous current - circuit breaker (rows 1-3) or main bus (row 4)
Category rated current
1,200 A
2,000 A
3,000 A
Category actual current
1,200 A
2,000 A
3,000 A
Circuit breaker cell with circuit breaker
461 W
738 W
1,443 W
Vertical section with main bus
211 W
275 W
452 W
Space heaters per vertical section
1,000 W
1,000 W
1,000 W
Voltage transformer (VT) stationary
250 W
250 W
250 W
Control power transformer (CPT) (stationary)
4% of CPT kVA rating
4% of CPT kVA rating
4% of CPT kVA rating
Relaying and instrumentation per circuit breaker cell microprocessor type
50 W
50 W
50 W
Relaying and instrumentation per circuit breaker cell electromechanical non-complex
100 W
100 W
100 W
Relaying and instrumentation per circuit breaker cell electromechanical complex
200 W to 300 W
200 W to 300 W
200 W to 300 W

Notes on assumptions:

  1. Space heaters, when provided, are not normally controlled by a thermostat. Hence, their load is represented as a continuous load. The purpose of space heaters is to prevent condensation, and this is not limited by the absolute temperature. Even when a thermostat is used to control the heaters, it is set to shut the heaters off at a temperature of approximately 110º F. Therefore, in an airconditioned room, the heaters would be energized continuously.
  2. Heat generated by current transformers is ignored as it is usually insignificant and varies according to the CT ratio as well as the loading.
  3. The CPT heat generation estimate is very conservative and assumes the CPT is operated at full-rated capacity. If normal loading is at less than full rating, heat generation may be adjusted by the square of the percent loading.
  4. Relaying and instrumentation heat generation estimates are very approximate and are normally estimated on the basis of the number of circuit breaker cells. Extensive relaying and instrumentation may warrant additional conservatism in the estimation of associated heat generation.
  5. Conversion factor: watts x 3.415179 = BTU/hour.

To estimate the heat generated under actual loading conditions, determine the component heat generation for each of the components indicated in Table 1. Estimated heat generation for circuit breakers should be adjusted for actual loading based on the ratio of the squares of the actual current and the rated current.


To be precise, this adjustment should also be made for the actual current loading of the main bus for each individual vertical section, but this is frequently ignored in the interest of simplification. Instead, the main circuit breaker loading is usually assumed to be equal to the main bus loading in all vertical sections.


Example: Assume a lineup with five vertical sections, one 2,000 A main circuit breaker (loaded to 1,650 A), three 1,200 A feeder circuit breakers (loading 850 A, 200 A and 600 A), 2,000 A main bus and space heaters. The lineup includes one set of VTs, one 15 kVA CPT and microprocessor relaying, and instrumentation. The calculations would be as described in Table 2.


If true loading were not considered (for example, all calculations performed on the basis of full-rated current), the calculations would yield a heat generation of 9,546 W.

Table 2: Calculations
Heat generation
2,000 A circuit breaker at 1,650 A = 738 x (1,650/2,000)2
502 W
1,200 A circuit breaker at 850 A = 461 x (850/1,200)2
231 W
1,200 A circuit breaker at 200 A = 461 x (200/1,200)2
13 W
1,200 A circuit breaker at 600 A = 461 x (600/1,200)2
115 W
Total heat generation for circuit breaker cells
861 W
Vertical sections with 2,000 A bus at 1,650 A = 5 x 275 x (1,650/2,000)2
936 W
Space heaters for three vertical sections = 5 x 1,000 =
5,000 W
Set of VTs = 1 x 250 = 
250 W
CPT = 1 x 5% x 15 kVA = 
600 W
Relaying and instrumentation = 4 x 50 = 
200 W
Total estimated heat generation under assumed loading conditions
7,847 W

Should you have any questions about this issue of TechTopics or any of our products, solutions, or services, please contact your local Siemens sales representative for more information.

Mailing address

Siemens Industry, Inc.

7000 Siemens Road

Wendell, North Carolina 27591

United States of America