 # Busbar ampacity calculation

##### Sources ：wenhui　　Date：2020-07-24 10:57

Busbar ampacity calculation

I4=2.45Id

In the formula, I4 is the current carrying capacity of the four-layer copper busbar (A).

Note: Two-layer and above copper bars mean that there is an air gap between the two-layer busbar equal to the thickness of the busbar.

The conversion relationship between the bus at an ambient temperature of 40℃ and an ambient temperature of 25℃ is: IAl=ICu/1.3

In the formula, ICu is the current carrying capacity of the copper bus; IAl is the current carrying capacity of the copper bus.

For example, according to the above formula, the current carrying capacity of the TMY100×10 bus is:

Single layer: 100×18.5 = 1850A. The search manual is 1860A. This data is based on the “High Voltage Distribution Device Regulations” issued by the Ministry of Water and Electricity of China Industry Press.

Double layer: 2 (TMY100×10) current carrying capacity is:

1850×1.58=2923(A); [Check manual is 2942A];

Three layers: 3 (TMY100×10) current carrying capacity is:

1850×2=3700A [Check the manual for 3780A]

All the above calculations are quite close to the data in the manual, and the busbar ampacity can be calculated according to the above formula.

3.4 Description

(1) The above calculation formula is applicable to bus bars of 120×12 and below specifications.

(2) For the current-carrying capacity of the shaped bus, it is recommended to use economic current density for calculation.

4 Thermal and electrodynamic effects of busbars

4.1 Thermal effect of bus

4.1.1 The thermal effect of the bus bar refers to the thermal effect that the bus bar can carry under the specified conditions due to the flow of current. In switching equipment and control equipment, it refers to the rated short-time withstand current (IK) carried by the busbar within the specified time under the specified use and performance conditions.

4.1.2 Determine the minimum cross section of the bus according to the rated short-time withstand current

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According to the formula in GB3906-1991 "3.6-40.5kV AC metal-enclosed switchgear and control equipment" [Appendix F]: S=(I/a)(t/θ)1/2 to determine the minimum cross section of the bus bar.

In the formula:

S—Minimum cross section of bus bar, mm2;

I-- rated short-time withstand current, A;

A—Material coefficient, copper is 13, aluminum is 8.5;

T-- rated short-circuit duration, s;

Θ—Temperature rise (K), generally 180K for bare conductors and 215K for 4s duration.

For the 31.5kA/4S system, the minimum cross-sectional area of the copper busbar is:

S=(31500/13)×(4/215)1/2=330 mm2

The relationship between the minimum cross-sectional area of the aluminum bus and the minimum cross-sectional area of the copper bus is:

SAl=1.62SCu

In the formula, SAl is the minimum cross-sectional area of the aluminum bus; SCu is the minimum cross-sectional area of the copper bus.

For the 31.5kA/4S system, the minimum cross-sectional area of the aluminum bus is:

SAl=1.62×330 =540 mm2

According to the provisions of 7.4.3 in DL404-1997 "Technical Conditions for Ordering Indoor AC High Voltage Switchgear", the grounding busbar and the cross-section of the conductor connected to it should be able to calculate 87% of the rated short-circuit breaking current of the nameplate. The minimum cross-sectional area of the ground bus under the short-circuit capacity of the system (short-circuit time is 4S).