Minimum Power for Spark in Electrical Cable

John has given the answer, but if you need an example then the 'crack' you get sometimes when you get out of your car is a good one, it's a high voltage spark but with very little power, which is why it just tickles your finger rather than flattening you.
Del

Incendive sparks cannot be produced by an EEx ia compliant hazardous area installation with two faults applied.

http://en.wikipedia.org/wiki/Electrical_equipment_in_hazardous_areas

A current of about 300mA can create a spark in air.

Normally it is considered that a voltage of about 700 volts is capable to produce a spark in air, still the major consideration are moisture content of air, ambient temperature, shape and material of the electrodes and most importantly the air gap between the electrodes.

Sharp pointed electrodes separated less than one mm when used in a laboratory experiment will produce a spark when voltage approaches 700 v.

It really depends on the application, scale and what you define a spark to be (do you mean an electrical breakdown in air across a fixed air gap, an electrical breakdown in air across a gradually increasing air gap, an electrical discharge in the form of light and heat when two conductive surfaces meet creating a circuit, etc).

It also depends on what standard this is referenced to (for example the intrinsically safe electrical standard for hazardous areas)

Can you please provide more information.

As others mentioned or alluded to, the query is somewhat pointless without context. With a large enough magnifying glass and a dark enough room I can show you a "spark" that can be created off of a 1-1/2V AAA cell battery. But that would hardly be considered dangerous in just about any environment. Dangerous "sparks" are those that cause OTHER things to happen, such as the ignition of volatile gasses or melting of protective insulation that then propagates a short circuit. So to determine the level of danger, you must first explore the nature of the threat.