Busbar Size Calculation

The four main factors to take in to account are;the temperature rise in the bars that you can accept, the mechanical forces that will be generated in the event of a short circuit, the expansion due to temperature changes and any certification that will be required.

I am going to assume that you have a three phase system which would be more common for a transformer of this size. The creepage distances for the insulation will be determined by your voltage, but the phase separation is more likely to be determined by the terminations at each end of your busbars.

Electrical current is mostly carried by the outside surface of a conductor. As you are not using the centre of the conductor, you don't pay for it. For heavy currents it is common to use tubular conductors (easier if you are using forced cooling with a refrigerated gas), or two right angle sections mounted to form a diamond shape with a slot at the top and bottom to release heat from the inside (easy for clamping to the top of porcelain insulators and attaching expansion compensating flexible braids). Other shapes are toe to toe channel sections, and simple vertical bars, single or in groups.

To arrive at a figure for temperature rise you need to know voltage, current, length, route (the calculation for vertical bars is different to horizontal bars, sloping bars are different again), resistance of the bars, how much heat is dissipated from the configuration of bars you are using, how much heat is dissipated by the enclosure, how much natural or forced cooling you have, your target temperature rise, and the worst case ambient temperature. To work out the heat dissipation you must decide whether the enclosure will be in the form of a common duct (all three phases run in the same space), segregated phase (a common duct with metal segregation between the phases), isolated phase (each phase in it's own duct) or mounted in free air (not possible if this poses an electrocution risk). There are a few more parameters but it is nearly forty years since I did this for a living and my memory is not what it was. Now you assign values to all the variables, making educated guesses for the ones you don't know, (like the size of the bars), and put them into a formula. Run the formula to get an initial result which will probably be wrong but it will tell you whether to adjust your guessed figures up or down. Run a second iteration of the formula with the adjustments incorporated. After five or six iterations you should have a fairly accurate estimate for the size of your bars. In the late 1960s I wrote a programme to do the iterations and ran it on a IBM207, the worlds first desk top computer. It took several hours to run by feeding in the program punched out on a pack of individual cards. Taping the cars together so that one would pull the next card through the slot was a major innovation. Hay I might of invented punch tape. Later I used to run the calculations on an IBM1900 series, and it only took half an hour (We typically did ten iterations). Today you could probably find an ap on your mobile phone to do it.

Now work out your short circuit current and from the length and spacing of your bars you will be able to calculate to side forces imposed by the electromagnetic repulsion in the event of a a short circuit. Use this to select the insulators and mounting configuration for the busbar. This could affect the size of your duct, so you may have to go back and do your heat calculations again.

You have calculated the temperature rise and you know the route lengths so you can find how much the bars will expand during normal working conditions. You will need to cater for this expansion by inserting flexible links in the form of braided or laminated sections. Make sure that the braids carry the same (or more) current as your bars, or you will need to do the heat calculations yet again.

Finally you may need the system certified by a body like ASTA before you can run it. Bear in mind that a one off system will be very expensive and time consuming to obtain certification.

My advice would be to buy a pre-certified bus bar system from somebody who knows what they are doing.