Power draw limit on molex?

G

Gorilla

Supreme [H]ardness
Joined
Sep 14, 2007
Messages
7,341
My PSU has one set of molex connectors coming out of it (the rest are modular and not really accessible because of the position of my radiator). From that set I am powering my fan controller and 7 120mm fans, 3 HHDs and 1 SSD, and my X20 750 pump, 2 12" cold cathodes. I want to add 6 RGB LEDs that should have a draw of about 120ma.
Is there a limit to how much that single 4 pin molex can provide? I have no idea what the draw of all the items is. I'm not even sure if I'm expressing myself clearly.

Thanks

Update: I was wrong, the disc drives are powered by sata.
 
Last edited:
Plugging in an LED is insignificant next to the power of the Force. But yes, 10-11A would be it.
 
SolidBladez said:
More like 5A per line.

http://www.playtool.com/pages/psuconnectors/connectors.html#peripheral

The connector itself has a maximum threshold of 13A. Maybe try putting some of those other devices such as the cold cathodes on a separate connector as they tend to draw more power.
Click to expand...

The Molex 8981 Series connection handles 11A per pin.

http://en.wikipedia.org/wiki/Molex_connector

Molex's own specs say 10A, odd discrepancy, but whatever. It seems it's more dependent on the size of the wire, rather then the connector.

http://www.molex.com/pdm_docs/ps/PS-8981-4M4P.pdf

Would I ever recommend pulling that much from it? No. They're way too prone to sketchy connections and arcing. That is however the spec.
 
So the next question how much power are all of those devices actually pulling.
 
*Mr. Plinkett* Never trust a person who can't white balance their photographs.
 
fans depending on what model usually are .4a or so per, personally I would never load as much as you have on a single connector, and listing the things you have, well you need to be a bit more specific as just saying 120mm fan says very little.

I will do quick maths for you :)
7x120mm(average).4a=2.8
HDDx3 1.1a(or thereabouts per)=3.3a
SSD 2a(max I could see but generally much lower=2a
dvd drive unsure=~1.2a
BD drive again unsure=~1.2a
========
10.5a, again without knowing exactly what model of things you are using(most everything has a sticker on it to tell you) it is really hard to give exact numbers, what I do know, the more you draw, the hotter the lines get, so they are less efficient and cannot give out quite as much or work harder to provide it.

I think you are pushing the limit of what 1 molex can safely provide, 11a per pin(per cable) is what most agree with though in theory a standard power supply can give some 16a to the molex the connector simply cannot handle it(probably melt/burn out)
 
dragonstongue said:
fans depending on what model usually are .4a or so per, personally I would never load as much as you have on a single connector, and listing the things you have, well you need to be a bit more specific as just saying 120mm fan says very little.

I will do quick maths for you :)
7x120mm(average).4a=2.8
HDDx3 1.1a(or thereabouts per)=3.3a
SSD 2a(max I could see but generally much lower=2a
dvd drive unsure=~1.2a
BD drive again unsure=~1.2a
========
10.5a, again without knowing exactly what model of things you are using(most everything has a sticker on it to tell you) it is really hard to give exact numbers, what I do know, the more you draw, the hotter the lines get, so they are less efficient and cannot give out quite as much or work harder to provide it.

I think you are pushing the limit of what 1 molex can safely provide, 11a per pin(per cable) is what most agree with though in theory a standard power supply can give some 16a to the molex the connector simply cannot handle it(probably melt/burn out)
Click to expand...

Thanks for your help, but I'm wondering if your estimates might be high. I'm not an expert but people say that Amps = Watts/Volts.
For HHDs, looking at some tests of 7200rpm HDDs, the highest watts used seemed to be around 3.5 when reading. 3.5/12 is about .3A. If it's actually using 5V then its 3.5/5 = .7A but still nowhere near 1.1A. The watts for my Samsung 830 according to some sites is less than 1W when being tested, which means that it's much less than 2A.

The one thing that I don't know is whether the voltages being used are 5V or 12V as molex provides both. I know that my pump uses the 12V wire. Some reviews of my pump suggest that it uses a max of .55A or so.

Am I using Ohm's law correctly?

The other thing is that the fans are set so that they are just barely spinning (silence and low temperatures, the beauty of water cooling).

Edit: looking up my exact HDDs it sounds like usage is higher than I stated previously, but still under 2A for all 3 drives under load.
 
Last edited:
JapaneseGorilla said:
Thanks for your help, but I'm wondering if your estimates might be high. I'm not an expert but people say that Amps = Watts/Volts.
For HHDs, looking at some tests of 7200rpm HDDs, the highest watts used seemed to be around 3.5 when reading. 3.5/12 is about .3A. If it's actually using 5V then its 3.5/5 = .7A but still nowhere near 1.1A. The watts for my Samsung 830 according to some sites is less than 1W when being tested, which means that it's much less than 2A.

The one thing that I don't know is whether the voltages being used are 5V or 12V as molex provides both. I know that my pump uses the 12V wire. Some reviews of my pump suggest that it uses a max of .55A or so.

Am I using Ohm's law correctly?

The other thing is that the fans are set so that they are just barely spinning (silence and low temperatures, the beauty of water cooling).

Edit: looking up my exact HDDs it sounds like usage is higher than I stated previously, but still under 2A for all 3 drives under load.
Click to expand...

As far as HDDs go, you get a high amp draw on start up, thats why enterprise level HDD controllers have spin up groups. Once spinning they aren't too bad.

That is the only time I have ever had an issue with a PSU/wiring get pissed. I am assuming that you aren't going to have a double digit HDD count spinning on a single feed though...
 
most things use 12v most fans do, as do modern hard drives, you are correct in that they provide both, but most use 12v these days.

The power P in watts (W) is equal to the current I in amps (A), times the voltage V in volts (V):
P(W) = I(A) × V(V)
So watts are equal to amps times volts:
watt = amp × volt
or
W = A × V

and yes, hard drives use X more power to "start" then to maintain, this is an electric motor after all, and your fans should say dc12v at x a or x watt, very very rarely do fans use 5v as most will be underpowered and not spin properly, they can be devolted by resistors and the like, but are still fed via 12v or 110(ac fans)

so for your fans, I know a lot of fan controllers say not to use high powered fans on them which is 1a(max per channel) or you can/will burn them out, same with motherboard headers.

Point is, if you know the specific parts it is much easier to figure out if you are "safe" to load more on it, and as for the hard drives, most will use 1a or less start phase or around 9w or less(caviar black 1TB uses around this mark, and I know of some older hard drives that can use more) if you take it in the watts scale, 132w is what a molex should be rated for. i.e 11A x 12v=132w

Seagate Barracuda ST3300831A claims that their ‘cudas also take 2.8 amps of +12V to spin up.(I will not provide link to this source as it is an exploit)

and if all molex connectors leading off your power supply are from the same "source" then overdrawing all of them is as bad as overdrawing one of them, it is "split" to the cable, e.g if 5 cables come from the single connection(on the psu) then even though each cable can handle this amount of power, if loading all of them to max, you will generally take a chance on burning the circuit(rail) giving the power to them.

I know that some makers of gpu and such used to bundle a dual molex to single 6 pin or 8 pin pci-e power for those who did not have the proper connectors available, however, alot of folks also burned out molex connectors for this reason i.e not load balanced as all connectors originated from the same source instead of multiple sources(2 molex leading to 2 pins or at least 2 separate power sources from the psu)

Im confusing the issue lol.

Yes it might be overestimated, its better to be over then under, be nothing worse then having parts fail or worse yet burnout/catch fire cause they get to hot, not enough stable power etc. Most folks will put 5 hard drives max(or equivalent) per lead to be on the safe side, some say its 10a, spec is 11, and some say 13.

I would look at worst case power draw, not average, then you know you are playing safe.
 
Thanks for help everyone. I ended up just giving up on really figuring it all out and plugging the led strip into molex. There were sparks when I plugged my cold cathodes back in which was concerning. My HDDs spun down. But once the cold cathodes were plugged all the way in everything worked. Weird and concerning.

The interesting thing is that the whole reason that I was even trying to power the led strip off of molex is that I can't power it with an ac adapter. I'm heading to LanFest NW this Friday and there is a strict 2 outlet 3A limit. So I was trying to get around the 2 outlet limit by powering an led light sign from my psu. Now that I've learned a little more about power consumption, the 3A limit sounds really really low as your monitor counts towards that limit.
 
Jorona said:
Thats only 360w... Thats a REALLY tiny frame to work in.
Click to expand...

I know. Here's hoping that we don't pull too much as they have said that they will go around with kill a watts and check everyone if they have problems. I would probably be screwed. It's not like I can even cut my usage. My 3 HDDs are in raid 0 so I can't just unplug one, plus I would lose access to all of my games. SSD has my OS on it. Can't turn off my pump or my system will over heat in a minute or so. I could not OC my cpu, but that's about it. Turn the cathodes and leds off I guess. Heck, my gpu is very efficient. What about the guys with gtx 480s in their systems? Anyway, hopefully things work out.
 
JapaneseGorilla said:
I know. Here's hoping that we don't pull too much as they have said that they will go around with kill a watts and check everyone if they have problems. I would probably be screwed.
Click to expand...
You would think so, but LAN parties never have the man power or training for this kind of micromanagement. I'm not sure why they pick an arbitrary number and think anybody will know what it means and take actual precautions to not exceed it. The fact is every attendee with a desktop is going to need probably +800W with their monitor. A lot of people bring laptops too, so there's always enough power to go around. They simply don't want people bringing in mini-fridges, microwaves, AC cooling units, floor fans, neon signs, vacuum cleaners, massage chairs, fish aquariums, four monitors, and whatever else people would think of to erect their mobile public mancave. The forms probably even say not to bring these things, but people still do. Again, the man power and expertise usually isn't there to deny them from walking in with it. As long as nobody is an asshole there won't be problems, "just my gaming PC... online magazines and forums told me to get this stuff, I didn't mean to incite nuclear assistance"
 
Auzner said:
You would think so, but LAN parties never have the man power or training for this kind of micromanagement. I'm not sure why they pick an arbitrary number and think anybody will know what it means and take actual precautions to not exceed it. The fact is every attendee with a desktop is going to need probably +800W with their monitor. A lot of people bring laptops too, so there's always enough power to go around. They simply don't want people bringing in mini-fridges, microwaves, AC cooling units, floor fans, neon signs, vacuum cleaners, massage chairs, fish aquariums, four monitors, and whatever else people would think of to erect their mobile public mancave. The forms probably even say not to bring these things, but people still do. Again, the man power and expertise usually isn't there to deny them from walking in with it. As long as nobody is an asshole there won't be problems, "just my gaming PC... online magazines and forums told me to get this stuff, I didn't mean to incite nuclear assistance"
Click to expand...

I wouldn't be so sure about that. First, this isn't a giant lan. 120 people I think. Second, they have had power problems in the past. One of the admins said that they actually have had to cut the number of seats in the past and only have been able to reintroduce them recently. They are very conscious of the power limits in that they have tried to improve them during building renovations. They have specifically said that you can't plug in your own power strip, and it's not that hard for them to just look at the central strip for a table to see if people are using more than 2 outlets. It's also not tough to look at a rig and see that it's probably using too much power.
 
You must log in or register to reply here.
Back
Top