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If you are using two R9-390X GPU cards you will need a stronger PSU than 1000 watts. According to this website: PSU REQUIREMENTS - RealHardTechX the minimum total system wattage for a PSU for your GPU Cards in Crossfire is 1100 watts.
How to configure AMD Crossfire : How to Configure AMD CrossFire™ Using AMD Radeon™ Settings
How do I setup AMD CrossFire™ on my system?
If you would like to setup AMD CrossFire between two or more discrete graphics cards (PCIe socket cards) correctly, please read the following steps:
1. Please make sure that the power supply unit meets the recommended requirements of the complete AMD CrossFire setup. There are free 3rd party tools available online that can determine the power requirements of your system.
2. Check with your motherboard manufacturer that your motherboard is AMD CrossFire compatible with the required number of PCIe x16 slots available (running at a minimum of PCIe x8 speed).
3. Check the AMD CrossFire Compatibility Charts to verify that the cards you are using are recommended for AMD CrossFire.
4. Please make sure that you are properly grounded and the system is powered off before installing the graphics cards.
5. Connect the preferred card into the primary PCIe slot on the motherboard (check motherboard manual to determine the primary PCIe slot).
6. Connect additional cards to the secondary PCIe slots on the motherboard. Note: AMD CrossFire only supports up to 4 GPUs.
7. If any AMD CrossFire bridge cables were included with the graphics cards, please connect them now.
8. After starting up the system ensure that you install the latest compatible driver for the graphics cards.
9. Go to the AMD CrossFire option in the AMD Catalyst™ Control Center and make sure that the check box for "Enable AMD CrossFire" is checked.
Note: For AMD Dual Graphics setup with an APU and a single discrete graphics card connected to the PCIe slot please see the AMD Dual Graphics FAQ.
Sorry for the late response
I have punched in my details in the calculator. It shows that the rec is 855 watt not 1100 like the chart says for my cards. Can someone explain that charts logic?
I'd like to double check before I order a 1100 watt+ psu.