Mac Pro Molex Power

hartleymartin

macrumors regular
Original poster
Jul 15, 2016
207
47
Sydney, Australia
What is the available power through the Molex connector on a 2008 Mac Pro? I have both PCIe power cables powering a Radeon 7950 (non-Mac version) and I would like to power the 8800GT card to give me a boot screen on a secondary display until I get around to installing Windows, etc to flash the 7950.

Can I use a Molex to PCIe adaptor cable to power the 8800GT card?
 

zeeimpulsenine

macrumors newbie
May 27, 2016
14
2
Can I use a Molex to PCIe adaptor cable to power the 8800GT card?
I am currently using a twin SATA (from the 5.25 bay) to PCIe 8 Pin connector to power the additional 8 pin on my 980ti if that helps at all. the SATA connections supply 75W each i believe.
 

hartleymartin

macrumors regular
Original poster
Jul 15, 2016
207
47
Sydney, Australia
In my 2008 Mac Pro, there are no SATA power connections in the 5.25" bay that I can find, though I did manage to locate the two SATA-data ports.
 

zeeimpulsenine

macrumors newbie
May 27, 2016
14
2
In my 2008 Mac Pro, there are no SATA power connections in the 5.25" bay that I can find, though I did manage to locate the two SATA-data ports.
My 5,1 has combined SATA data/power so i just tapped the power part. I'm pretty sure molex connections carry the same power as the SATA.
 

CapnDavey

macrumors 6502
Apr 11, 2015
345
87
If you look a PC's that use these adapters you need 2 drive power cables for one power connection on a video card hope that helps.
 

ColdCase

macrumors 68040
Feb 10, 2008
3,001
152
NH
My 3,1 (2008) has two molex power connector in the optical (5.5) drive bay for the IDE optical drive(s) (they may be split from one feed). I run SSDs off them through molex to SATA power adapters, But I dunno how much power is available there.
 
Last edited:

elf69

macrumors 68020
Jun 2, 2016
2,333
486
Cornwall UK
try to use different feed lines from psu if you can.

spread the load.
on some psu if use same feed line can overload psu.

some psu only supply so many watts per line.
others do not, i suspect apple psu be better quality.
 

slughead

macrumors 68040
Apr 28, 2004
3,104
236
I am currently using a twin SATA (from the 5.25 bay) to PCIe 8 Pin connector to power the additional 8 pin on my 980ti if that helps at all. the SATA connections supply 75W each i believe.
You realize SATA power plugs are not rated for that kind of 12 volt power. That is very risky. The older Mac Pro models used a standard 4 pin Molex.

To answer your question (or not), OP: Molex connectors can carry enough amps to power the card. It will not get hot, melt, or catch fire. It will likely not damage any components. However, I have had issues in the past hooking too much stuff on there on a Mac Pro 1,1. If it draws too much amps from whatever rail on the PSU, the PSU will simply shut off. I think I had 3-4 hard drives in a RAID hooked up to the molex once and it wouldn't boot. It'd start, get somewhere into the boot process, and it would click and turn off to protect the PSU.
 

h9826790

macrumors G5
Apr 3, 2014
14,271
6,866
Hong Kong
I am currently using a twin SATA (from the 5.25 bay) to PCIe 8 Pin connector to power the additional 8 pin on my 980ti if that helps at all. the SATA connections supply 75W each i believe.
Standard SATA only rated up to ~55W (there are three 12V pins inside the SATA port, and each of them can supply up to 1.5A, so total 54W).

So, in your case, 2x SATA rated up to 108W, and you use it to supply a 150W rated 8pin input.

From other threads, it seems the 980Ti usually only draw around 120W from the 8pin. So, it's just slightly over the limit. However, if it really draw 150W from the 8pin, it may kill your SATA port.
 

orph

macrumors 68000
Dec 12, 2005
1,884
391
UK
It's not to power a secondary display it's to power a 8800GT.

why do you need boot display?
you only get boot screen on the primary GPU, so if you set you 8800GT as secondary you wont get boot screen & if you set it as primary then all the graphics work will be done on the 8800GT.
(i think thats correct)

vary few app's are able to see more than the primary GPU so setting the 7950 as secondary GPU may give you some problems.

iv not had boot display since 2013 ruining a gtx 660 i think only once or twice i have bothered to plug back in the original mac GPU, with at the mo 4 installs of osx and a windows 7 install which i can switch between with no problems.

why not just plug in the 8800GT when you need it instead of all the time?
 

carpsafari

Suspended
Sep 13, 2015
277
56
the Netherlands
Dump the crappy 8800GT!!

I really don't understand why some make such a fuzz over bootscreens and they probably also don't (want to) know about the Startup-Disk option in Sysytem Preferences.

When TS wants to proceed, I still dump the crappy 8800GT and got myself a GT120, it doesn't need additional power.
 

slughead

macrumors 68040
Apr 28, 2004
3,104
236
why do you need boot display?
Boot selecting? Disk Repair? OSX upgrades? There are lots of reasons

I resisted getting a GT120 for literally 10 years, I just got one and needed it immediately. Feels good to not have to pay the MacVidCards tax to flash my GTX980 (Sorry MVC, I bought your 7970 though and it was great!).
 

--AG--

macrumors member
Dec 20, 2012
36
14
I did exactly this on my MP3.1 with one nvidia k5000 (appx 122W, one 6-pin power) and one tesla K20 (appx 225W, two six-pin power). In total three 6-pin pci-power needed and the MP have two. I added one 6-pin using the molex and connected to the k5000 card. Worked just fine (running ubuntu). I disconnected the dvd to save some power.
 

orph

macrumors 68000
Dec 12, 2005
1,884
391
UK
im on a gtx660, i can still boot from recover disc etc it's only boot select that i cant do (from option restart i can from system prefs).
it's only the GTX 9XX cards that are the real problem on the nvida side as they only run after the OS has loaded the web drivers.

i dont know the ATI cards so well but if it wont let you do OSX installs and recovery boot then just slap in the GT8800 when you need it, it's not hard to do and you'll only need it once a year or so.

plus that things going to make noise, heat and use power just to bug you.

edit
re-read your post yep your using a GTX 9xx card so you will have problems, instead of a gt120 you can just use a GTX6xx card there cheeper and work fine (no boot display but everything else works fine like installing osx and recovery boot) or there's a bunch of ATI cards that are cheep and can be flashed.
lots of people use gt610/620 for this or tasks that have low GPU need
 
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