MacBook discharging while plugged in under load

I have the latest Retina MacBook Pro MacBookPro11,2

I quite often have the CPU at about 400%-500% usage (e.g. 4-5 cores at 100%) and also do GPU intensive tasks. While this is happening, the battery can actually discharge even though it is plugged in. This has happened since I first got the MacBook and is not something which has developed over time.

I have the model without a dedicated GPU (Intel Iris Pro only), and I have an 85 watt power supply which came with the MacBook

This is very irritating for me, is this a fault or normal behaviour?

Solution 1:

It is a known problem: https://discussions.apple.com/thread/3069428 Someone has calculated the max wattage draw of a MBPr, but I can't find that a.t.m.