BUSBAR CALCULATIONS FOR MDB'S

You given the equation for current carriying capacity of busbar and not the SC Current.

SC Current is calculated by knowing the Fault MVA of the system connected to the Busbar. The factor 0.8 will be PF or any thumb rule factor

Could you please explain more on your statement since iam a novice. What is the difference between the SC calculation and a current withstand capability of busbar. I thought they were the same.My requirement is for the given the system, i have to check whether the busbar selected(for dimensions and material) is an optimum one.

0.8 is a safety factor.

I agree with electrical@wonderla.com. The equation provided is for current carrying capacity of busbar and not the SC Current withstand rating. As to the 0.8 factor, it may be an uncertainty factor or temperature correction factor.

Calculating the short current withstand capability of busbar has to take into consideration the spacing between the bus bars and mechanical bracing of them.

Short Circuit current is calculated depending on the percentage Impedance and line voltage. Line voltage( V _L ) is multiplied by 1.1 to take carte of the 10 % rise in voltage and Z .

As an example , In 3 phase 1000KVA Transformer ( single ), 22/ 0.415 kV,

the % Z = 5.25 and

Short Circuit Current

...... _{1.1 x V L}

I = -----------

......... ^{√ 3 x Z}

The Impedance Z should be calculated in ohms

........→ _{% Z x 10 x kV²}

Z = ----------------- say in this case 0.009 (calculated)

............→ ^kVA

..................................→ _{1.1 x 415}

Short circuit current I = ----------------- = 29.3 kA

. ................................^{√3 x 0.009}

The 0.8 given is a derating factor depending on the heat dispensing of the enclosure considering the panel is installed in indoor in well-ventilated room. .

Look at the Copper Development Association website - it has a book called "Copper for Busbars" for free with everything you need to know. This is the definitive reference.

Please provide a link to that book..

Thanks.

hi yaar

0.8 is noyhing but it is a derating factor.

.8 is the current per mm

it is the correction or derating factor

I = Size buss * 160/SQRT(TIME_breaker_OFF) => 1/SQRT(TIME_breaker_OFF) = 0.8