Current carrying capacity of aluminium

http://www.ems-industrial.com/main/aluminum_bus_bar.asp

or Google AL bus bar ampacity

Not sure what you mean exactly by the term "flat." Assume this is one of those King's English terms that doesn't translate exactly across the Pond. But in general if you are using wire of any type to conduct electricity from one point to another, you have to have the following data in hand before you can answer your question fully. You need to know how much potential or tension drop or power loss you can stand, and you need to know how long the conductor will have to be. 840 Amps sounds more like busbar than wire, but the principle is the same. The equation that governs this is R= pl/A, where R is resistance in Ohms, and I have used p for the Greek symbol rho, which is the resistivity of the material in question in Ohm-cm or Ohm-meters, l is the conductor length in cm or meters and A is the cross-sectional diameter in sq cm or sq meters. The cross-sectional area is of course related to the wire gauge you are looking for, and the resistance term is the allowable potential or tension drop across the length of the conductor divided by the amperage to be carried. Once you have this data, you can look up the wire gauge in a table, as suggested by the other respondent. My old 1953 Reference Handbook for Radio Engineers even has a table of required wire gauge for various amperages and lengths of conductor based on 2% voltage drop. But it's for copper wire - I believe gauges are different for aluminum.

Sizing conductors be they wire or bus bar is not the simple exercise most people believe it to be. You need additional information. Length is needed to determine voltage drop. Ambient temperature is needed, Is the structure outside? How much solar heating is expected? Latitude? How about cooling? Is it to be forced air cooled? Is it located in a windy environment? Is is buried? Is it inside housing? What color? The design of these is best left to personnel or companies with experience in their design. We have had good performance from Powell Electrical Systems Inc. at www.powellind.com

you can use 12.5mm x 112.5 mm size of alumnium busbar subject to the ambient temp is not more than 50 and working tem is not more than 60 degree c.

For preliminary calculations current density for aluminium bus bar ≈120A/cm²

Cross sectional area = 840/120=7cm² accordingly size may be arrived from mfg catalouges