I've seen people ask this question on several online communities, but responders just quote the manual, and don't say anything helpful. Usually something like "that's not a supported configuration". Lets get something straight. This machine has a list of supported configurations, but supported in this case just means Dell tested those configurations, and confirmed they work. It does not mean other configurations aren't valid, and the manual provides instruction for helping you create your own valid unsupported configurations. The closest thing to an answer about exceeding the 72GB limit that I could find was seeing some refurbishers online selling T5500s with 144GB of RAM, but you never know if these small scale refurbishers really know their stuff.
I decided to do a test of my own in the hopes of answering this question once and for all. I don't have 144GB of RAM dimms available, but I do have 100GB (six 16GB dimms, and two 2GB dimms). Following the manual's instructions for correct order and placement I was able to place 100GB of RAM in the system. The system booted. The BIOS saw all the RAM. Windows 10 saw all the RAM. Lubuntu 18.04 saw all the RAM. However, my PCI-e USB 3.0 card ports no longer worked. The ports were providing power still as far as I could tell, but no data. At first I thought the issue might be a bad RAM dimm, but after doing a lot of taking dimms in and out (enough for a lifetime), and a some memory testing, I discovered the issue only seemed to occur when exceeding the 72GB limit listed in the manual. While not a conclusive answer it appeared that exceeding 72GB may cause issues with other hardware components. Since refurbishers often don't equip systems with any pci cards other than a graphcis card, it would explain why they were able to exceed the limit, but I was not. Because, I only had issues with my USB 3.0 PCI-e card, and this card was only recently installed, I was concerned the card might be the problem. So I decided to perform a second test. I downgraded the system to 68gb, and over the next couple weeks I kept an eye on the PCI-e USB 3.0 card. It experienced the same issue randomly. Rebooting always solved the issue. It appeared my original test might have been skewed by the card. Since rebooting solves the issue decreasing the ram quantity could have appeared to solve the issue when it was actually the rebooting solving it. To better rule out the PCI-e USB 3.0 card as the problem I installed another made by a different brand. I verified the card worked, and re-upgraded the ram to 100GB. The ram showed up in my BIOS, it showed up in Windows 10, it showed up in Lubuntu 18.04, and all my hardware worked fine.
These limited tests are not a conclusive answer, but they do strongly suggest that it is possible to exceed the 72GB limit without issue. These tests do confirm that 16GB dimms work in this system.
TL;DR. The Dell Precision T5500 officially supports a maximum of 72GB of RAM, but it is possible to exceed this using 16GB dimms.
License:Comprehensible Open License 3.0
Comprehensible Open License 3.0
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