I have set an upload limit of 30kB/s, but sometimes it is uploading 100kB/s. I have tested uTorrent and BitTorrent and they have the same problem. I disabled the setting "use more upload slot if upload < 90%". I have set 30kB/s limit both in the current torrent preferences and in the general preferences.
Why does it still use more bandwidth?
EDIT. Found the solution, enable "limit local peer bandwidth, under bittorrent tab"