How long can a video clip be when recording with the D800/D800E?
You may have noticed there is a recording limit for video on the Nikon D800/D800E. The limit in high quality is 20 minutes and in normal quality 30 minutes (minus exactly one second!), regardless of resolution and frame rate.
At the highest quality and resolution, a 20-minute file is a bit more than 3.5GB, coming close to the file system limitation of 4GB per file. However, it's technically possible to seamlessly start a new file every time the 4GB boundary is met. Moreover, it does not explain the limit on lower settings.
The limit of 29 minutes and 59 seconds is actually not a software or hardware limitation at all. Devices that can record 30 minutes or longer at a time qualify as video recorders by European law and are therefore subject to a 5.4% higher import duty. The original idea behind this is aimed at video recorders that can make -possibly illegal- copies of television shows or movies. The 5.4% is used to compensate the theoretical loss of royalties to copyright holders. While this hardly makes sense for cameras, the law does apply to them.
For some manufacturers this law is beneficial as it protects their line of video cameras, but this goes not for Nikon. Still, Nikon has this limit built in for all cameras worldwide. In fact, all DSLR brands have this limit. I would have gladly paid the extra 5.4%, but I'm afraid we will have to await a firmware hack.
If you are serious about video there is a solution: the external HDMI recorder that can plug in to the D800. If you bought the recorder in Europe, you can bet the 5.4% was included in the price. The fixed artificial time limit does not apply with an external recorder, but you still cannot shoot indefinitely. When the sensor becomes too warm, the camera will stop end live view as an overheating protection measure. In practice, you should be able to shoot about an hour at a time, depending on the ambient temperature.
The EN-EL15 battery will probably last no more than an hour, so I would suggest the AC power supply.